development of criteria for selection of components …

DEVELOPMENT OF CRITERIA FOR SELECTION OF COMPONENTS FOR

FORMULATION OF AMINE BLENDS BASED ON STRUCTURE AND ACTIVITY

RELATIONSHIPS OF AMINES, AND VALIDATION OF FORMULATED BLENDS IN

A BENCH SCALE CO2 CAPTURE PILOT PLANT

A Thesis

Submitted to the Faculty of Graduate Studies and Research

In Partial Fulfillment of the Requirements

For the Degree of

Master of Applied Science

In

Process Systems Engineering

University of Regina

By

Jessica Narku-Tetteh

Regina, Saskatchewan

September 2017

Copyright 2017: J. Narku-Tetteh

UNIVERSITY OF REGINA

FACULTY OF GRADUATE STUDIES AND RESEARCH

SUPERVISORY AND EXAMINING COMMITTEE

Jessica Narku-Tetteh, candidate for the degree of Master of Applied Science in Process Systems Engineering, has presented a thesis titled, Development of Criteria for Selection of Components for Formulation of Amine Blends Based on Structure and Activity Relationships of Amines, and Validation of Formulated Blends in a Bench Scale CO2 Capture Pilot Plant, in an oral examination held on August 29, 2017. The following committee members have found the thesis acceptable in form and content, and that the candidate demonstrated satisfactory knowledge of the subject material. External Examiner: Dr. Fanhua Zeng, Petroleum Systems Engineering

Supervisor: Dr. Raphael Idem, Process Systems Engineering

Committee Member: Dr. Hussameldin Ibrahim, Process Systems Engineering

Committee Member: Dr. Teeradet Supap, Adjunct

Committee Member: Dr. David deMontigny, Process Systems Engineering

Chair of Defense: Dr. Abu Bockarie, Faculty of Education

i

ABSTRACT

Due to modernisation and industrialisation, an increase in the global energy demand is

inevitable. Nuclear, fossil fuels, renewables, hydro and biomass are the major sources of energy.

However, considering the current energy framework, fossil fuels appear to be the most reliable

and stable energy source. As a result, emphasis on the reduction of emissions of carbon dioxide

(CO2, a major type of greenhouse gas (GHG)) is very crucial because almost all fossil fuel

activities lead to generation of this environmentally harmful GHG. Scientists have shown that the

average global temperature has increased by up to about 1 degree over the last century. Thus, if

this issue is left unabated, it will have long lasting consequences both on human lives and the

environment. Extreme weather conditions, heat waves, sea level rise, wild fires, health problems

are glaring repercussions of global warming and climate change. Various strategies such as use

of alternative energy, energy conservation or fossil fuel-energy coupled with carbon capture and

sequestration (CCS) are all attempts to mitigate this problem. However, CCS stands out to be the

anchor technique due to its compatibility with existing energy infrastructure in conjunction with

the reliability of fossil fuel-based energy itself. Post combustion capture which uses a regenerable

liquid sorbent, appears to be the predominant technology used and has proven to be successful in

most industrial applications. However, this technology is still far from being perfect. My thesis

research addresses the imperfections and challenges identified with this technique from the

context of sorbent formulation. Optimum sorbent performance cannot be achieved with one

single amine sorbent. It is therefore essential to develop new amine sorbent systems by blending

and combining their individual strengths to achieve an optimum performance sorbent. Most

approaches used to solve this problem use indirect means whereas we need the type of studies

that will directly link the chemical structure of the amine sorbent to its performance since this

will provide the key in unlocking the rationale in selecting the blend components. For this reason,

my research objective focuses on coming up with a rational way to use a fundamental chemical

structure – activity relationship study to develop and formulate an optimum sorbent blend. This

novel blend is validated in a bench scale pilot plant to ensure that the developed criterion leads

to a blend that is practical and implementable.

The effects of the chemical structure, namely, side chain structure and number of hydroxyl

groups in primary, secondary and tertiary amines as well as the alkanol chain length in primary

alkanolamines and the alkyl chain length in secondary and tertiary alkanolamines on amine

ii

activities such as CO2 absorption and desorption kinetics, equilibrium loading, heat duty, cyclic

capacity, heat of absorption and pKa were studied and used to develop rational criteria for

selecting components to formulate an optimum amine blend. Based on the criteria, amines that

had a combination of high CO2 absorption parameter and high CO2 desorption parameter were

selected. Their mixing ratios and concentrations were varied to obtain the best overall

performance. The optimum blend was then validated in a bench scale pilot plant and compared

with the benchmark 7M MEA-MDEA solvent blend. The role of a solid acid catalyst in aiding

CO2 desorption and further enhancing the performance of the developed novel blend was tested

and, again, compared to the benchmark 7M MEA-MDEA blend.

The results of this study showed that, in comparison with their straight-chain analogues,

steric hindrance present in branched-chain alkanolamines resulted in much faster desorption rate,

higher equilibrium CO2 loading and cyclic capacity, much lower heat duty for solvent

regeneration, but just a slight decrease in CO2 absorption rate. The effect of chain length studies

also showed that, longer alkanol chain lengths of primary alkanolamines and longer alkyl chain

lengths of secondary and tertiary alkanolamines led to higher equilibrium CO2 loading and pKa.

However, the influence of mass transfer limitations on these positive effects resulted in a

maximum trend for initial rate of CO2 absorption for secondary and tertiary alkanolamines. On

the other hand, the increase in the chain lengths also caused the generation of larger amounts of

bicarbonate ions which resulted in higher CO2 desorption rates and cyclic capacity, but lower

heat duty. However, the longer chain alkanolamines also had high viscosities which adversely

modified their performance by also introducing mass transfer limitations. The developed criteria,

in terms of absorption parameter and desorption parameter, resulted in formulating an excellent

bi-solvent aqueous amine blend (comprising 2M BEA + 2M AMP), which had an outstanding

desorption characteristics/heat duty as well as very good absorption characteristics. In addition,

this work developed a new non-trial-and-error procedure to determine the heat of CO2 absorption

based on Gibbs-Helmholtz equation. Also, the pilot plant studies showed that the novel blend,

4M BEA-AMP blend showed outstanding performance in absorber efficiency, heat duty and

cyclic capacity over the 7M MEA-MDEA blend, implying that it is a good potential solvent for

post combustion CO2 capture thereby validating the developed selection criteria that yielded the

2M BEA + 2M AMP blend. The addition of catalyst in the process led to tremendous

improvements in all the performance indicators of the two solvent blend systems.

iii

ACKNOWLEDGEMENTS

I would like to show my appreciation to my supervisor, Dr. Raphael Idem, for the privilege

he gave me to work under his supervision. His direction, intellectual contributions and close

guidance throughout the course of my research are very much appreciated. I would also like to

thank him for grooming me to think outside the box in order to come up with innovative ideas

that have helped me tremendously in shaping what has now become my thesis research work. I

would also like to thank my co-advisors, Dr. Teeradet Supap and Dr. Chintana Saiwan, for their

valuable contributions and assistance in my work.

I would also like to thank all members of the CO2 research group in Clean Energy

Technologies Research Institute (CETRI) for their support, contributions and ideas offered during

our weekly progress research meetings. Special thanks also go to the CO2 research group at the

Petroleum and Petrochemical College, Chulalongkorn University, Bangkok, Thailand for their

informative ideas and support during the bi-weekly Skype research meetings between CETRI,

University of Regina, Canada and Petroleum and Petrochemical College, Chulalongkorn

University, Thailand.

Furthermore, I would like to thank the Natural Science and Engineering Research Council

of Canada (NSERC) as well as to Canada Foundation for Innovation (CFI) for their financial

support through research grants to my supervisor, Dr. Raphael Idem. In addition, I also want to

thank the Clean Energy Technologies Research Institute (CETRI), and Faculty of Graduate

Studies and Research (FGSR), University of Regina for other supports. Finally, I am grateful to

the Government of Saskatchewan for giving me the Saskatchewan Innovation Scholarship Award

to enable me to successfully complete my MASc degree program in Process Systems

Engineering.

iv

TABLE OF CONTENTS

ABSTRACT ................................................................................................................................... i

ACKNOWLEDGEMENTS ......................................................................................................... iii

TABLE OF CONTENTS ............................................................................................................. iv

LIST OF FIGURES .................................................................................................................... xii

LIST OF TABLES ...................................................................................................................... xv

NOMENCLATURE .................................................................................................................. xvi

CHAPTER ONE: INTRODUCTION ........................................................................................... 1

1.1 Why CO2 Capture? .............................................................................................................. 1

1.2. Emission of CO2 ................................................................................................................. 1

1.3 Carbon Capture and Storage (CCS) .................................................................................... 3

1.3.1 Carbon Capture Technologies ...................................................................................... 4

1.3.1.1 Pre-combustion Capture ............................................................................................ 4

1.3.1.2 Oxyfuel Combustion ................................................................................................. 4

1.3.1.3 Post Combustion Capture .......................................................................................... 4

1.4 Chemical Absorption using Amines ................................................................................... 7

1.5 Amine Solvents used in CO2 capture .................................................................................. 7

1.6 Thesis Objective and Outline ............................................................................................ 10

1.6.1 Specific Objectives ..................................................................................................... 10

1.7 Scope of Thesis ................................................................................................................. 11

1.8 Organisation of Thesis ...................................................................................................... 12

CHAPTER TWO: LITERATURE REVIEW ............................................................................. 14

2.1 Solvent Properties ............................................................................................................. 14

2.1.1 Absorption Kinetics .................................................................................................... 14

v

2.1.2 pKa.............................................................................................................................. 15

2.1.3 Desorption Kinetics .................................................................................................... 15

2.1.4 CO2 equilibrium loading ............................................................................................. 16

2.1.5 Amine cyclic capacity ................................................................................................ 16

2.1.6 Heat of CO2 absorption (Habs) ................................................................................... 16

2.1.7 Heat of regeneration ................................................................................................... 17

2.1.8 Amine corrosiveness................................................................................................... 17

2.1.9 Amine degradation ..................................................................................................... 17

2.2 Significance of Solvent Properties .................................................................................... 18

2.2.1 CO2 Absorption Rate .................................................................................................. 18

2.2.2 CO2 Desorption rate.................................................................................................... 18

2.2.3 Cyclic capacity ........................................................................................................... 19

2.2.4 Amine basicity (pKa).................................................................................................. 20

2.2.5 Absorption capacity .................................................................................................... 20

2.2.6 Heat of CO2 reaction................................................................................................... 20

2.2.7 Heat of Regeneration .................................................................................................. 21

2.2.8 Amine corrosiveness................................................................................................... 21

2.2.9 Degradation of amines ................................................................................................ 22

2.3 CO2-Amine Reaction Mechanisms ................................................................................... 23

2.3.1 Primary and secondary amines ................................................................................... 23

2.3.2 Tertiary amines. .......................................................................................................... 23

2.4 Structure – Activity Relationships of Amine-Based Solvents: Effect of Molecular

Structure on Solvent Properties ............................................................................................... 24

2.4.1 Carbamate stability and bicarbonate formation .......................................................... 24

2.4.2 Sterical hindrance ....................................................................................................... 25

vi

2.4.3 Electron density .......................................................................................................... 25

2.4.3.1 Inductive Effect ....................................................................................................... 25

2.4.4 Structure – activity relationship studies...................................................................... 26

2.4.5 Criteria for selecting amines ....................................................................................... 28

2.4.6 Amine Blending .......................................................................................................... 29

2.4.6.1 Blended Amine Chemistry ...................................................................................... 34

2.2.7 Pilot Plant Validation.................................................................................................. 34

CHAPTER THREE: EXPERIMENTAL SECTION .................................................................. 36

3.1 Overview of Screening Experiments ................................................................................. 36

3.2 Amines Studied ................................................................................................................. 36

3.3 Materials and Equipment .................................................................................................. 38

3.4 Absorption Experiment ..................................................................................................... 39

3.5 Desorption Experiment ..................................................................................................... 41

3.6 Heat Duty Determination .................................................................................................. 41

3.7 Acid Dissociation Constant (pKa) ..................................................................................... 42

3.8 Equilibrium CO2 Solubility ............................................................................................... 43

3.9 Heat of Absorption ............................................................................................................ 46

3.10 CO2 Loading Test ............................................................................................................ 46

3.11 Viscosity Measurement ................................................................................................... 47

3.12 Heat Capacity Determination .......................................................................................... 47

3.13 Pilot Plant Runs ............................................................................................................... 48

3.13.1 Materials and equipment .......................................................................................... 48

3.13.2 Pilot plant continuous flow steady state experiments ............................................... 48

3.13.3 Typical pilot plant experimental run ........................................................................ 49

3.13.4 Heat duty calculations .............................................................................................. 53

vii

CHAPTER FOUR: RESULTS AND DISCUSSION OF SCREENING TESTS OF THE

EFFECTS OF THE AMINE CHEMICAL STRUCTURE ON HEIR CARBON DIOXIDE

CAPTURE ACTIVITIES ........................................................................................................... 54

4.1 Effect of side chain and number of hydroxyl groups in an alkanolamine molecule ......... 54

4.1.1 Initial CO2 Absorption Rate for Various Amines ....................................................... 54

4.1.2 Viscosity of Dilute Single Solvent Systems and their Effects on Initial CO2

Absorption Rate ................................................................................................................... 61

4.1.3 Acid Dissociation Constant (pKa) .............................................................................. 65

4.1.4 Equilibrium CO2 Solubility ........................................................................................ 70

4.1.5 CO2 Desorption Rate .................................................................................................. 70

4.1.6 Heat Duty for Solvent Regeneration .......................................................................... 80

4.1.7 Cyclic Capacity........................................................................................................... 80

4.1.8 Heat of CO2 Absorption ............................................................................................. 80

4.1.8.1 Validation of the New Procedure for Determination of Heat of Absorption .......... 80

4.1.8.2 Heat of Absorption for alkanolamines..................................................................... 91

4.1.9 Correlations between Different Activities .................................................................. 93

4.1.9.1 Rate of CO2 Absorption versus Heat of CO2 Absorption ........................................ 93

4.1.9.2 Heat Duty for Solvent Regeneration Versus Heat of CO2 Absorption .................... 96

4.2 Effect of alkyl and alkanol chain length of alkanolamines ............................................... 98

4.2.1 Acid Dissociation Constant (pKa) .............................................................................. 98

4.2.1.1 Primary Alkanolamines ........................................................................................... 98

4.2.1.3 Tertiary Alkanolamines ......................................................................................... 103

4.2.2 Equilibrium CO2 Solubility ...................................................................................... 103

4.2.2.1 Primary Alkanolamines ...................................................................................... 104

4.2.2.2 Secondary Alkanolamines .................................................................................. 104


viii

4.2.3 Initial Rate of CO2 Absorption ................................................................................. 107

4.2.3.1 Primary Alkanolamines ......................................................................................... 107

4.2.3.2 Secondary Alkanolamines ..................................................................................... 111


4.2.4. Initial CO2 Desorption Rate ..................................................................................... 118

4.2.4.1 Primary Alkanolamines ...................................................................................... 118



4.2.5 Cyclic Capacity......................................................................................................... 119

4.2.5.1 Primary Alkanolamines ......................................................................................... 124


4.2.5.3 Tertiary Alkanolamines ...................................................................................... 124

4.2.6 Heat Duty for Regeneration for Primary, Secondary and Tertiary Alkanolamines . 124

CHAPTER FIVE: DEVELOPMENT OF SELECTION CRITERIA USING THE

STRUCTURE AND ACTIVITY RELATIONSHIP STUDIES OBTAINED FROM THE

SCREENING ANALYSIS ....................................................................................................... 132

5.1 Criteria for Amine Component Selection for Blended Amine Solvents ......................... 132

5.1.1 Rate of CO2 Absorption Versus pKa ........................................................................ 132

5.1.2 CO2 Equilibrium Solubility Versus pKa .................................................................. 134

5.2 Absorption-Desorption Parameters ................................................................................. 134

5.2.1 Viscosity of Concentrated Single and Blended Solvent systems ............................. 136

5.2.2 Evaluation of Absorption Parameter and Desorption Parameter for Blended Amines

........................................................................................................................................... 139

5.2.2.1 CO2 Absorption Rate for Blended Amines ............................................................ 139

5.2.2.2 CO2 Desorption Rate for blended amines .............................................................. 143

5.2.2.3 Cyclic Capacity for blended amines ...................................................................... 143

ix

5.2.2.4 Equilibrium CO2 Solubility for blended amines.................................................... 147

5.2.3 Determination of Optimum Amine Solvent Blend Using an Absorption Parameter-

Desorption Parameter Diagram ......................................................................................... 147

CHAPTER SIX: PILOT PLANT VALIDATION .................................................................... 152

6.1 Role of Catalyst ............................................................................................................... 152

6.2 Absorber Efficiency ........................................................................................................ 153

6.3 CO2 concentration and temperature profiles ................................................................... 156

6.3.1 CO2 concentration profile ......................................................................................... 156

6.3.2 Temperature profile .................................................................................................. 156

6.4 Cyclic Capacity ............................................................................................................... 156

6.5 Heat Duty ........................................................................................................................ 162

6.5.1 Calculation of the heat duty terms ............................................................................ 164

6.5.2 Comparison of the Heat Duty Terms for Catalytic and Non-Catalytic CO2 Desorption

for BEA-AMP and MEA-MDEA Solvent Blends ............................................................ 166

6.6 Analysis of amine cost .................................................................................................... 168

CHAPTER SEVEN: CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK

................................................................................................................................................... 172

7.1 Conclusions ..................................................................................................................... 172

7.2 Recommendations ........................................................................................................... 174

7.2.1 Expanding the Amine Structure – Activity Relationships as a Criterion for Selection

........................................................................................................................................... 174

7.2.2 Catalyst Development/Improvement for CO2 Absorption and CO2 Desorption in

Relation to Selected Solvent Blend ................................................................................... 175

7.2.3 Confirmation or Elimination of Untested Assumptions ........................................... 175

7.2.4 Viscosity Studies in relation to CO2 capture performance ....................................... 176

REFERENCES ......................................................................................................................... 177

x

APPENDICES .......................................................................................................................... 189

APPENDIX A: Safety Precautions taken during experiments .............................................. 189

Appendix A1: Solution Concentration determination ........................................................... 189

Appendix A2: CO2 loading Determination ........................................................................... 190

Appendix A3: Initial absorption rate determination ............................................................. 190

Appendix A4: Initial Desorption Rate .................................................................................. 191

Appendix A5: Calculation of heat transfer rate, q ................................................................ 192

Appendix A6: Heat duty determination ................................................................................ 193

Appendix A7: pKa determination ......................................................................................... 193

Appendix B: Calculation of experimental data from pilot plant studies .................................. 196

Appendix B1: A typical Process flow diagram (PFD) - (LABVIEW SOFTWARE) ........... 196

Appendix B2: Packed Column Experimental data ................................................................ 197

Appendix B3: CO2 absorbed calculation from the gas side. ............................................... 199

Appendix B4: loading CO2 production ................................................................................ 199

Appendix B5: Mass Balance Error ...................................................................................... 199

Appendix B6: Absorber Efficiency calculation .................................................................... 200

Appendix B7: Reboiler Duty ................................................................................................. 201

Appendix B8: Heat Duty ....................................................................................................... 201

Appendix B9: Sensible Heat ................................................................................................. 201

Appendix B10: Heat of vaporisation ..................................................................................... 205

Appendix B11: Heat of desorption ........................................................................................ 205

xi

LIST OF FIGURES

Figure 1.1: Breakdown of global CO2 emissions by activity sector in 2005. Source: OECD/IEA

(2008) .............................................................................................................................................2

Figure 1.2: Diagram showing the three capture technologies (Figueroa et al., 2008) ...................6

Figure 1.3: Post Combustion Capture Process (Clean Energy Technologies Research Institute) .8

Figure 2.1a: PCC alternatives solvents as a plot of the second order reaction rate at 313 K, log

k2, and absorption heat ( Ha) (Liu et al., 2016) .........................................................................30

Figure 2.1b: PCC alternatives solvents as a plot of the second order reaction rate at 313 K, log

k2, and absorption heat ( Ha) (Liu et al., 2016) .........................................................................31

Figure 2.2: The trade-off between reaction heat and CO2 absorption rate (Chowdhury et al.,

2013) ............................................................................................................................................32

Figure 2.3: pKa versus Heat of reaction of tertiary and cyclic amines (Rayer et al., 2014) ........33

Figure 3.1: Simplified diagram of absorption and desorption set up ...........................................40

Figure 3.2: Experimental setup of equilibrium solubility according to Maneeintr et al., 2009 ...44

Figure 3.3: Validation of solubility data for 5M MEA by comparison with Aronu et al. (2011)

......................................................................................................................................................45

Figure 3.4: Schematic representation of the experimental set-up for CO2 removal (Srisang et al.,

2017) ............................................................................................................................................50

Figure 3.5: Column Packing and catalyst bed arrangement (Srisang et al., 2017) ......................51

Figure 4.1-1: CO2 Absorption Profile for Alkanolamines ...........................................................55

Figure 4.1-2: CO2 Absorption Profile for Alkyl amines and MEA ............................................56

Figure 4.1-3: Effect of side chain position on initial CO2 absorption rates of primary

alkylamines ..................................................................................................................................57

Figure 4.1-4: Initial CO2 absorption rate of primary, secondary and tertiary alkanolamines ......59

Figure 4.1-5: Effect of number of hydroxyl group substituents on the initial CO2 absorption rate

of 1o, 2o and 3o unhindered amines ..............................................................................................62

Figure 4.1-6: Comparison of the initial CO2 absorption rates of 1o, 2o and 3o hindered amines .63

Figure 4.1-7: Effect of number of hydroxyl group substituents on the pKa of 1o, 2o and 3o

amines ..........................................................................................................................................67

Figure 4.1-8: Comparison of the pKa of primary, secondary and tertiary alkanolamines ...........68

Figure 4.1-9: Effect of side chain position on pKa of primary alkylamines ................................69

xii

Figure 4.1-10: Effect of side chain position on equilibrium CO2 solubility of primary

alkylamines at 15% CO2 partial pressure @ 40oC ......................................................................71

Figure 4.1-11: Comparison of the equilibrium CO2 solubility of primary, secondary and tertiary

alkanolamines @ 40oC and 15% CO2 partial pressure ................................................................72

Figure 4.1-12: Equilibrium solubility of butylamines at different partial pressures and

temperatures (A: secButylamine (SEC); B: Butylamine (BUTYL); C: isoButylamine(ISO)). ...73

Figure 4.1-13: Equilibrium solubility of alkanolamines at different partial pressures and

temperatures (A:2M MEA; B: 5M MEA; C: 4-A-1-B; D: AMP; E: tBEA; F: BEA; G: BDEA;

H: tBDEA) ...................................................................................................................................74

Figure 4.1-14: Comparison of the initial CO2 desorption rates of primary, secondary and tertiary

alkanolamines ..............................................................................................................................75

Figure 4.1-15: Comparison of the initial CO2 desorption rates of 1o, 2o and 3o hindered amines

......................................................................................................................................................78

Figure 4.1-16: CO2 Desorption Profile for Alkanolamines ........................................................79

Figure 4.1-17: Heat Duties of primary, secondary and tertiary alkanolamines ...........................81

Figure 4.1-18: Cyclic Capacities of primary, secondary and tertiary alkanolamines ..................82

Figure 4.1-19: Equilibrium Solubility data of 5M MEA at 313,323,333,353 and 363K ............86

Figure 4.1-20: Plot of lnPCO2 and 1/T at 0.40 CO2 loading .........................................................87

Figure 4.1-21: Plot of lnPCO2 and 1/T at 0.45 CO2 loading .........................................................88

Figure 4.1-22: Heat of CO2 absorption values for 1o, 2o and 3o alkanolamines ..........................92

Figure 4.1-23: Absorption Rate versus Heat of absorption .........................................................94

Figure 4.1-25: Heat Duty for Solvent Regeneration versus Heat of CO2 Absorption .................97

Figure 4.2-1: Effect of alkanol chain length of primary alkanolamines on their pKa values (2 is

2-Amino-1-ethanol (MEA); 3 is 3-Amino-1-propanol; 4 is 4-Amino-1-butanol) .....................101

Figure 4.2-2: Effect of alkyl chain length in secondary alkanolamines on their pKa values (0 is

MEA; 1 is MMEA; 2 is EMEA; 3 is PMEA; 4 is BEA) ...........................................................102

Figure 4.2-3: Effect of alkyl chain length in tertiary alkanolamines on their pKa values (MEA

and MMEA included for comparison) .......................................................................................105

Figure 4.2-4: Equilibrium CO2 loading for primary alkanolamines ..........................................106

Figure 4.2-5: Equilibrium CO2 loading for secondary alkanolamines and MEA ......................108

Figure 4.2-6: Equilibrium CO2 loading for tertiary alkanolamines, MEA and MMEA ............109

xiii

Figure 4.2-7a: Absorption profile for Primary amines ..............................................................113

Figure 4.2-7b: Absorption profile for secondary amines ...........................................................113

Figure 4.2-7c: Absorption profile for tertiary amines ................................................................114

Fig 4.2-8: Effect of alkanol chain length on the initial absorption rate of primary alkanolamines

....................................................................................................................................................115

Figure 4.2-9: Effect of alkanol chain length on the initial absorption rate of secondary

alkanolamines compared with MEA ..........................................................................................116

Figure 4.2-10: Initial Absorption rate for tertiary amines ..........................................................117

Figure 4.2-11a: Desorption profile for primary alkanolamines .................................................120

Figure 4.2-11b: Desorption Profile for Secondary Alkanolamines ...........................................120

Figure 4.2-11c: Desorption profile for tertiary amines ..............................................................121

Figure 4.2-12: Effect of alkanol chain length on the initial desorption rate of primary

alkanolamines ............................................................................................................................122

Figure 4.2-13: Effect of alkyl chain length on the initial desorption rate of secondary

alkanolamines and MEA ............................................................................................................123

Figure 4.2-14: Effect of alkyl chain length on the initial desorption rate of tertiary

alkanolamines, MEA and MMEA .............................................................................................125

Figure 4.2-15: Effect of alkanol chain length on the cyclic capacity of primary amines ..........126

Figure 4.2-16: Effect of alkyl chain length on the cyclic capacity of secondary alkanolamines

and MEA ....................................................................................................................................127

Figure 4.2-17: Effect of alkyl chain length on the cyclic capacity of tertiary alkanolamines,

MEA and MMEA ......................................................................................................................128

Figure 4.2-18: Effect of alkanol chain length on the heat duty of primary alkanolamines .......129

Figure 4.2-19: Effect of alkyl chain length on the heat duty of secondary alkanolamines ........130

Figure 4.2-20: Effect of alkyl chain length on the heat duty of tertiary alkanolamines ............131

Figure 5.1: CO2 Absorption Rate – pKa Relationship ...............................................................133

Figure 5.2: pKa-Solubility Relationship ....................................................................................135

Figure 5.3: Absorption versus Desorption Parameter in Selection Criteria ...............................137

Figure 5.4: Absorption profiles for blended amines ..................................................................140

Figure 5.5: Initial Absorption Rates of Blended Amines ..........................................................141

Figure 5.6: Desorption Profile for blended amines ...................................................................144

xiv

Figure 5.7: Initial Desorption Rates of Blended Amines ...........................................................145

Figure 5.8: Cyclic capacities of Blended Amines ......................................................................146

Figure 5.9: Equilibrium loadings of Blended Amines ...............................................................148

Figure 5.10a: Absorption versus Desorption Parameter for all amines (using loading in mol

CO2/mol amine) .........................................................................................................................149

Figure 5.10b: Absorption versus Desorption Parameter for all amines using loading in (mol

CO2/L.soltn) ...............................................................................................................................149

Figure 6.1: Non-catalytic and Catalytic CO2 absorber efficiencies for BEA-AMP and MEA-

MDEA blends .............................................................................. Error! Bookmark not defined.

Figure 6.2: Effect of catalyst on the absorber efficiency of blended system .............................155

Figure 6.3a: Non-catalytic CO2 concentration profiles for BEA-AMP and MEA-MDEA blends

....................................................................................................................................................157

Figure 6.3b: Catalytic CO2 concentration profiles for BEA-AMP and MEA-MDEA blends ...157

Figure 6.4a: Effect of catalyst on the CO2 concentration profile of MEA-MDEA system .......158

Figure 6.4b: Effect of catalyst on the CO2 concentration profile of BEA-AMP system ...........158

Figure 6.5a: Non-catalytic temperature profile of BEA-AMP and MEA-MDEA blends .........159

Figure 6.5b: Catalytic temperature profile of BEA-AMP and MEA-MDEA blends ................159

Figure 6.6a: Effect of catalyst on the temperature profile of BEA-AMP system ......................160

Figure 6.6b: Effect of catalyst on the temperature profile of BEA-AMP system ......................160

Figure 6.7: Catalytic and Non-catalytic cyclic capacities of BEA-AMP and MEA-MDEA

blends .........................................................................................................................................161

Figure 6.8: Effect of catalyst on the cyclic capacity of blemded systems . Error! Bookmark not

defined.

Figure 6.9: Catalytic and Non-catalytic heat duties of BEA-AMP and MEA-MDEA blends. .163

Figure 6.10: Effect of catalyst on the heat duty of blended systems ......... Error! Bookmark not

defined.

Figure 6.11: Schematic Illustration for Calculation of Sensible Heat .......................................165

Figure A1: Initial Absorption Rate profile.................................................................................191

Figure A2: Initial Desorption Rate Profile.................................................................................192

Figure B1: Process flow diagram (PFD) (Akachuku, 2017) ......................................................196

Figure B2: Schematic Illustration for Calculation of Sensible Heat …………………………204

xv

LIST OF TABLES

Table 3.1: Operating conditions used in the pilot plant experiments ..........................................52

Table 4.1-1: CO2 activities for all alkylamines and MEA ...........................................................58

Table 4.1-2: CO2 activities for all single alkanolamines studied .................................................60

Table 4.1-3: Viscosity (mPa.s) data for primary, secondary and tertiary alkanolamines @ 2M and

40oC..............................................................................................................................................64

Table 4.1-4: Comparison of experimental pKa values with literature values ..............................66

Table 4.1-5: The equations of lines of best fit for CO2 partial pressure-equilibrium loading

relationships and their corresponding degrees of correlation for various solvents at different

temperatures: ................................................................................................................................84

Table 4.1-6: Partial pressure and corresponding temperatures obtained at selected loadings .....89

Table 4.1-7: Comparison of heat of absorption of 5M MEA at specific loadings with literature

......................................................................................................................................................90

Table 4.2-1: Effect of chain length on CO2 absorption–desorption performance of primary,

secondary and tertiary amines ......................................................................................................99

Table 4.2-2: Validation of pKa values .......................................................................................100

Table 4.2-3: Viscosities of primary, secondary and tertiary alkanolamines at 40oC .................110

Table 5.1: Viscosities (mPa.s) of amine blends and single amines at higher concentrations ....138

Table 5.2: CO2 activities for amine blends ................................................................................142

Table 6.1: A summary of the sensible heat, heat of vaporisation and heat of desorption of the

blends .........................................................................................................................................167

Table 6.2: Summary of amine cost/kg CO2 removed ................................................................170

Table A1: pKa calculated data ...................................................................................................195

Table B1-a: Experimental data for typical run (150g catalyst, 60ml/min amine floe rate BEA-

AMP system) .............................................................................................................................197

Table B1-b:Temperature and concentration profiles of BEA-AMP system(150g catalyst, 60

mL/min)………………………………………………………………………………………198

xvi

NOMENCLATURE

Solvent Notation

2-A-1-E 2-amino-1-ethanol or monoethanolamine

3-A-1-P 3-Amino-1-propanol

4-A-1-B 4-amino-1-butanol

5-A-1-P 5-amino-1-pentanol

AMP 2-amino-2-methyl-1-propanol

BEA = BEA Butyl ethanolamine = Butylmonoethanolamine

BDEA Butyldiethanolamine

BUTYL Butylamine

EDEA Ethyldiethanolamine

EMEA Ethylmonoethanolamine

ISO isoButylamine

MDEA Methyldiethanolamine

MEA Monothanolamine

MMEA Methylmonoethanolamine

PMEA Propylmonoethanolamine

SEC secButylamine

tBDEA tertButyldiethanolamine

tBEA tertButylethanolamine Parameter Notation A cross-sectional area normal to the direction of heat flow (m2) Ac conversion constant (22.4 mol/L)

C1 amine concentration (mol/L)

Cp heat capacity (kJ/kgoC)

Cp,hw heat capacity of heating medium (kJ/kgoC)

dt temperature difference (K)

xvii

dt/dx temperature gradient (K/m)

dx wall thickness (m)

q rate of heat transfer (J/s)

Gin volumetric flow rate of inlet feed gas (SLPM)

Gout volumetric flow rates of outlet off gas (SLPM)

H+ = H3O+ proton

Hdes heat of desorption (GJ/tonne)

Hsens sensible heat (GJ/tonne)

Hvap heat of vaporisation (GJ/tonne)

k thermal conductivity (W/Mk)

Kamine amine dissociation constant

mCO2 mass flow rate of CO2 produced (kg/min)

mhw mass flow rate of heating medium (kg/min)

nHCl number of moles of HCl

n0 initial moles

PCO2 partial pressure of CO2

Qreg heat duty (J/mol)

R molar gas constant (J/molK)

T temperature (K)

Thw,in inlet temperature of heating medium

Thw,out outlet temperature of heating medium

V1 amine sample volume (ml)

Vtotal total liquid volume after titration (ml)

XCO2,in CO2 composition in feed gas stream (mol/mol)

XCO2out CO2 composition in off gas stream (mol/mol)

mCO2,l amount of CO2 in liquid phase (kg/l.soltn)

mg1 CO2 in gas phase in stream entering heater (kg/l.soltn)

mg2 CO2 in gas phase in stream leaving heater (kg/l.soltn)

ml1 mass of liquid solution entering heater (kg/l.soltn)

xviii

ml2 mass of liquid solution leaving heater (kg/l.soltn)

T1 amine solution temperature at heater inlet

T2 amine solution temperature at heater outlet

Greek and other Symbols Notation

CO2 loading (mol CO2/mol amine)

Habs heat of absorption (kJ/mol)

[ ] Concentration of species (mol/dm3)

1o Primary amines

2o Secondary amines

3o Tertiary amines

1

CHAPTER ONE: INTRODUCTION

1.1 Why CO2 Capture?

In this fast-moving world where industrialisation is on the upswing, emphasis on the

reduction of greenhouse gas emissions has become very crucial. The ever-increasing global

energy demand due to commercialisation has made CO2 mitigation very necessary as most, if not

all, industrial activities lead to the generation of this harmful gas. Global warming is as a result

of the increasing concentrations of CO2 and other anthropogenic GHGs like methane, and nitrous

oxide in the atmosphere. The scientific evidence of the global warming effect, for example,

melting of the polar land ice, bushfires, extreme weather conditions and sea level rise are all

glaring repercussions of global warming and climate change. The United Nations –

Intergovernmental Panel on Climate Change (IPCC) and the IPCC projects a further global

warming of 1.8 to 4°C in this century, and in the worst-case scenario, 6.4°C is expected. The

European Union (EU) agreed in 2008 to reduce GHG emissions by 20% below the 1990 levels

by the year 2020. Canada amongst other countries recently at the 21st Conferences of Parties,

COP 21, Paris Summit (2016) came to an agreement to combat climate change, which aims at

keeping the global temperature rise below 2oC. Predictions show that in order to stabilise the CO2

atmospheric concentration at no more than 50% above its current level, CO2 emissions may need

to be reduced by more than 60% by 2100. Considering the current energy framework, fossil fuels

are the most reliable and most stable energy source which accounts for about 75% of the

anthropogenic CO2 emissions (IPCC, 2001c).

1.2. Emission of CO2

Figure 1.1 shows the contribution of the various sectors to CO2 emissions. We can see that

electricity generation, transportation and industrial sectors contribute to about 80% of the CO2

emissions. According to the OECD/IEA, 2009, nearly 66% of the world’s electricity generation

is from fossil fuels. As such, due to the ever-increasing global demand, this trend is not going to

change in the next decades. Fossil fuels will most likely satisfy the world’s ever increasing energy

needs. IEA is of the view that no technology singly can reduce the impact of CO2 emissions from

fossil fuel combustion. In this light, IEA promotes the development of various technologies

together with other alternatives such as renewable energy, nuclear energy, etc.

2

Figure 1.1: Breakdown of global CO2 emissions by activity sector in 2005. Source: OECD/IEA (2008)

3

Carbon capture and sequestration or storage (CCS) is one of the technologies generally

adopted to reduce these emissions. The IPCC report ranks CCS to reduce CO2 emissions by up

to 40% during this century (IPCC, 2005). From the IEA perspective, CCS is not only limited to

power generation but to other industrial sectors like iron and steel, as well as petrochemicals and

cement manufacturing industries. The International Panel on Climate Change 5th Assessment

Report (2014) emphasises the incapability of many models to limit warming to below 2oC if CCS

and bioenergy and the combination are limited.

1.3 Carbon Capture and Storage (CCS)

Carbon capture and Storage is a combination of several technologies that deal with CO2

capture, transportation and storage. Carbon capture can be applied to large point sources like

large fossil fuel or biomass energy facilities, major CO2-emitting industries, natural gas

production, synthetic fuel plants and fossil fuel-based hydrogen production plants (IPCC, 2005).

Carbon capture is divided into three different technologies. These are: Pre-Combustion, Post

Combustion, and Oxy-fuel Combustion Capture. Pre-Combustion refers to removing CO2 from

fossil fuels before the combustion process. Post combustion deals with separation of CO2 from

flue gases after combustion. Oxyfuel deals with the burning of fossil fuels in an oxygen-enriched

stream instead of air to produce a CO2 rich gas stream. Large amounts of CO2 are transported

through pipelines over distances of up to 1000 km. For smaller amounts (less than a few million

tonnes of CO2 per year) or for longer distances, ships have proven to be economically feasible.

Conveying dry CO2 is very crucial as any moisture present in the lines will corrode the pipelines.

As such, all moisture is removed from the CO2 before transportation through the pipelines. Over

40 metric tonnes of CO2 produced in the USA has been transported through pipeline over

distances of about 2500 km for enhanced oil recovery (IPCC, 2005). The transported CO2 is then

injected into storage reservoirs mainly for enhanced oil recovery (EOR). Geological formations

have proven to be economically feasible currently for storage and/or sequestration typically in

terms of enhanced oil recovery (EOR), compared with other potential reservoirs like deep ocean,

ocean sediments, and minerals (in terms of CO2 mineralisation).

4

1.3.1 Carbon Capture Technologies

1.3.1.1 Pre-combustion Capture

Pre-combustion capture is typically applied in Integrated Gasification Combined Cycle

power plants. In this process, the coal is first gasified to produce a synthesis gas composed of

carbon monoxide (CO) and hydrogen (H2). The CO by-product undergoes a water-gas shift

reaction with water to produce CO2 and more H2. This process inherently captures the CO2. The

H2 product is sent to a turbine to produce electricity as shown in Figure 1.2. The pre-combustion

capture process could be employed where natural gas is used as the primary fuel.Here, the natural

gas is reacted with steam to produce H2 and CO2. The pre-combustion capture process has the

benefit of producing an almost pure CO2 rich stream and promotes the deployment of efficient

separation methods like pressure-swing adsorption, separation membranes, and electric swing

adsorption since the CO2 stream is at an elevated pressure.

1.3.1.2 Oxyfuel Combustion

In the Oxy fuel combustion process, the fuel is not directly contacted with air but instead,

the fuel is burned in the presence of an oxygen-enriched stream as shown in Figure 1.2. This

process employs an air separation unit which separates N2 form the air mixture to produce a clean

oxygen stream which is used for the fuel combustion to produce a stream mainly CO2 and water.

The air separation is done using a low temperature separation process/distillation process. The

disadvantage of this technology is the high capital cost associated with the Air Separation Unit.

Also, this technique makes it difficult to retrofit into existing plants.

1.3.1.3 Post Combustion Capture

In post combustion capture the CO2 is separated for the flue gas produced after

combustion of the fuel as shown in Figure 1.2. The typical CO2 concentration in the flue gas

stream is around (3-15% by volume) in which nitrogen from air is the main constituent (IPCC,

2005). The separation is done using various techniques like adsorption, cryogenics, membrane

separation and absorption. The adsorption process uses solid materials like zeolites and activated

carbon to adsorb CO2 from the flue gas, typically based on pressure or temperature swing

5

adsorption. The disadvantage of this technique is the low availability of CO2 selective- and high

capacity-adsorbents.

6

Figure 1.2: Diagram showing the three capture technologies (Figueroa et al., 2008)

7

In cryogenics, the flue gas is liquefied via cooling and condensation, and the liquefied

CO2 is then separated from the gas phase. The cryogenic technique is very expensive due to the

cost of energy associated with cooling and condensation. The membrane technology uses a semi-

permeable membrane which allows selected components to pass through the barrier based on a

pressure or concentration gradient acting as the driving force. The absorption process is the most

mature and viable option amongst these techniques. The absorption process typically uses a liquid

solvent to chemically or physically absorb CO2 from the flue gas. The liquid solvents include

reactive solvents like chilled ammonia, ionic liquids, amino slats, amines and physical solvents

like selexol. Of all the PCC techniques listed, the literature has shown that significant research,

deployment and development of work has been done in CO2 capture using chemical reactive

solvents, which has led to its maturity over time compared to other technologies (Feron et al.,

2009; Abu-Zahra et al., 2013).

1.4 Chemical Absorption using Amines

In the chemical absorption process, the flue gas is first cooled and desulfurized before

coming into contact with the solvent. The flue gas enters the bottom of the absorber unit (typically

around 40-60oC) and contacts the down flowing amine counter currently. As the liquid contacts

the gas it chemically absorbs the CO2 from the flue gas and leaves the bottom of the absorber as

rich amine. The flue gas is washed to remove water and solvent remnants before leaving the top

of the absorber. The rich amine is preheated through the lean-rich heat exchanger before entering

the desorber (regenerator) which is usually done between 100 and 140oC. The reboiler generates

steam from the bottom of the desorber which is used to strip the CO2 from the rich amine. The

stripped gas is a mixture of H2O vapor and CO2 which leaves the top of the desorber. The water

is recovered by condensation, and the CO2 is compressed and stored. The lean amine leaves the

bottom of the desorber, and is recycled back to the absorber for further CO2 absorption. The

process diagram is shown in Figure 1.3.

1.5 Amine Solvents used in CO2 capture

Amine solvents have been used for CO2 capture since 1930 (Rochelle, 2009). The solvents

used in capture are typically classified into primary, secondary and tertiary amines.

8

Figure 1.3: Post Combustion Capture Process (Liang et al., 2013)

9

The primary amines have two hydrogen atoms attached to the nitrogen atom, while secondary

amines have one H attached, whereas the tertiary amines have no H group attached to the nitrogen

central atom. The most common examples of amine solvents used are monoethanolamine (MEA),

diethanolamine (DEA), methyldieathanolamine (MDEA) and 2-amino-2-methyl-1-propanol

(AMP).

Primary and secondary amines are generally known to form carbamates when they react with

CO2, whereas tertiary amines form bicarbonates in their reaction with CO2 (Kohl and Nielsen,

1997). Before an amine solvent is identified for use in CO2 capture, there are some important

characteristics that have to be looked at. These include the absorption and desorption rate, the

energy of regeneration, the absorption and desorption capacity, and amine stability (Peeters et al.,

2007). The absorption and desorption kinetics determine the sizes of the absorber and desorbers.

The capital costs accounts for about 40% of the total costs of the amine based CO2 capture process

(IEA GHG, 2004). Although post combustion capture using amine is the most mature and has

received the most attention, there are still challenges that need to be addressed some of which are

related to cost and efficiency of the process. Consequently, studies are still ongoing to improve

this technology. Improvements in efficiency are currently based on process optimization and

solvent technology improvement. In the latter case, it is well known that the individual

conventional amines used such as monoethanolamine (MEA, primary amine), diethanolamine

(DEA, secondary amine) and methyldiethanolamine (MDEA, tertiary amine) each has its own

limitations, including high energy requirement, low solvent stability, corrosiveness, slow

kinetics, low absorption capacity and low thermal stability. Consequently, an optimum solvent

needs to be developed in order to achieve improved CO2 capture performance through the

improvement of the solvent technology despite the individual limitations of each single amine.

Blending of different amines has been suggested to address these shortcomings (Chakravarty et

al., 1985), and has recently received considerable attention (Nwaoha et al., 2016; Idem et al.,

2006; Sakwattanapong et al. 2005). In addition, several studies have been reported in the literature

on solvent performance based on kinetics and mass transfer of CO2 absorption in the solvent.

However, this is unlikely to provide a fundamental understanding of the chemical structural

features in the amine that are directly responsible for amine performance. The lack of this

information implies that further improvement on amine solvent technology may be limited.

Recently, the amine structure-activity relationships in CO2 absorption/desorption has received

10

considerable attention. Structural differences such as side chain, alkanol chain length of primary

alkanolamines, alkyl chain length of secondary amines, number of functional groups, alkyl group

position, etc. in the amine have been seen to possess a significant effect on an amine activity

performance (Singh and Versteeg, 2008; Singh et al., 2009; Singh et al., 2007; Singh, 2011).

However, studies have not covered all classes of amines and some CO2 capture activities such as

the heat duty. Thus, the focus of this work is to establish a structure – activity relationship of all

classes of amine and use this to develop a criterion for solvent selection and test the selected

potential solvent in a bench scale pilot plant.

1.6 Thesis Objective and Outline

The overall objective of this research is to develop a criterion for selection of

components for formulation of amine blends based on structure – activity relationships of

different types of amines, and to validate the formulated blends in a bench scale CO2 capture pilot

plant.

1.6.1 Specific Objectives

The specific objectives have been divided into three phases as described below:

Phase 1: Study and establish structure and activity relationship trends of primary secondary and

tertiary amines. In the literature, the structure and activity studies do not cover all the possible

CO2 capture activities that impact the solvent performance. For example, the heat of regeneration

and heat of CO2 absorption was not studied by Singh et al. (2009), Singh et al. (2007) and Singh

et al. (2008) in their work. In addition, not all classes of amine like the secondary and tertiary

amines have been studied in the literature. As such the aim is to fill in the gaps by establishing

trends that will aid researchers in combining the different aspects of the structure – activity

relationships of all classes of amine to formulate an efficient solvent system.

Phase 2: To develop a criterion for solvent component selection in a blend. In the literature,

different strategies have been formulated to represent the criteria for selecting components to

make an amine blend. These are based mostly on selecting any two pairs of activities at a time

and then deciding which of the pairs can be used to select the best components in a solvent blend.

This is not easily achievable since the pairs of activities typically yield contrasting results. Thus,

11

we are developing a strategy that involves all the amine performance activities. These are: CO2

equilibrium solubility (i.e. amine capacity to hold CO2), acid dissociation constant (i.e.

alkalinity), initial CO2 absorption rate (which determines the size of absorber), initial CO2

desorption rate (which affects the size of the desorber), CO2 cyclic capacity (which determines

the maximum amount of CO2 that can be produced per cycle), heat duty for solvent regeneration

(which affect the operating costs in terms of energy penalty), and heat of absorption (which

indicates how much heat is produced when CO2 is absorbed in an amine solvent).

Phase 3: To test and validate the formulated solvent in a bench scale CO2 capture pilot plant. In

order to achieve this, the formulated solvent was tested in a pilot plant and compared with the

conventional MEA and MEA-MDEA blend system. Before testing in the pilot plant, a

preliminary test was done in a semi-batch scale to obtain the optimum mixing ratio and then

translated to the more practical bench scale CO2 capture pilot plant.

1.7 Scope of Thesis

The structure and activity relationship studies focused mainly on the follow structural properties:

1. Effect of side/branched chain on primary, secondary and tertiary amines on their CO2

capture activities.

2. Effect of the alkanol chain length of primary alkanolamines and the alkyl chain length of

secondary and tertiary alkanolamines on their CO2 capture activities

3. Effect of the number of hydroxyl group of primary, secondary and tertiary amines on their

CO2 capture activities.

2-amino-2-methyl-1- propanol, a primary amine which has a branched chain, has been seen

to possess good desorption properties. This work focused on studying the effect of side chain as

the desorption features of an amine play a very important role in affecting both the operational

and capital cost associated with the capture process. The alkanol chain length of primary amines

has been seen to impact the CO2 absorption and desorption capacity. This property is desirable

as the absorption and desorption capacity also impacts the sizes of the absorber, desorber, heat

exchangers and amine pumps. The presence of the hydroxyl group has been seen to impact the

absorption rate. Amine reactivity with CO2 is very essential as this determines the size of the

absorption column, and consequently affects the capital costs.

12

The structure – activity relationship studies were performed by first doing a solvent screening

tests in a semi batch scale. In the solvent screening experiments, the mentioned structural aspects

were tested on their influence on all the CO2 capture activities; namely, the absorption and

desorption rates, the absorption and desorption capacities, the pKa, the heat of CO2 absorption

and the heat duty (heat of regeneration).

The selection criteria development focused on identifying the relevant activities that have

specific related impacts on the CO2 capture performance and using the information to categorise

the activities into parameters that affect absorption and those that affect desorption. The solvents

selected based on the criteria were combined in a blend (and their mixing ratios varied) and

further tested on their influence on the afore-mentioned capture activities to obtain the optimum

blend.

The pilot plant testing focused on evaluating the energy efficiency of the optimum blend

and comparing the performance with the conventional MEA and MEA-MDEA blend for both

catalyst-aided and non-catalyst-aided CO2 capture processes. The results for all the various

aspects are presented and discussed in this thesis.

1.8 Organisation of Thesis

This thesis is written in a standard thesis format as outlined below:

Chapter 1: An overall introduction into the CO2 Capture technology, its associated problems and

the importance of research work are discussed.

Chapter 2: A comprehensive review of the literature on solvent properties, structure and activity

relationships, CO2 reaction mechanism, and solvent chemistry of single and blended amines.

Chapter 3: Experimental Procedure of Screening tests and Pilot plant tests

Chapter 4: Results and Discussion of Screening Tests

Chapter 5: Development of Selection criteria using the Structure and Activity relationship studies

obtained from the screening analysis.

Chapter 6: Validation of selected amine blends in a bench scale pilot plant

13

Chapter 7: Major conclusions and recommendations for future work

14

CHAPTER TWO: LITERATURE REVIEW

2.1 Solvent Properties

There are several solvent properties that are commonly used to evaluate the performance

of the amine-based CO2 capture process. These are the amine basicity (pKa), absorption rate,

desorption rate, CO2 equilibrium loading, amine cyclic capacity, heat of CO2 absorption,

corrosiveness, amine degradation, and heat of regeneration. These properties are measured using

different techniques under specified conditions. The different measurement methods used in the

literature are reported and described below.

2.1.1 Absorption Kinetics

The kinetics of CO2 absorption is very important because it indicates how fast the reaction

is. The literature reports several techniques used in measuring and studying the absorption

kinetics of CO2 in liquid solvents.

Puxty et al. (2009) reported the study of the absorption of CO2 into aqueous ammonia and

MEA solutions using a wetted wall column. The setup used allowed a thin liquid film to contact

a flowing gas stream in a counter-current fashion. The wetted wall technique provides a bigger

surface area and can be used for a wide range of solvents with varying reaction kinetics.

Henni et al. (2008) studied the reaction kinetics of CO2 in aqueous 1-Amino-2-propanol, 3-

Amino-1-Propanol and Dimethylmonoethanolaamine solutions using the stopped flow technique.

In this technique, amine and CO2 gas are both in aqueous solutions and so does not truly represent

the liquid-gas phase reaction. Also, the liquud-gas interfacial area of contact is not known. The

stopped flow technique, however, allows for only dilute solutions and is used for small volumes

of amine and is a quick screening method.

Tippayawong and Thanompongchart (2010) used a packed column which comprised a

Pyrex glass cylinder randomly packed with bioballs to study the kinetics of CO2 and H2S removal

from a biogas feed using MEA, NaOH and Ca(OH)2 as liquid solvents. The packed column

provides a large surface area and can be used for a wide range of solvents irrespective of their

unique kinetics.

The stirred cell reactor was employed by Vaidya et al. (2009) and was operated batch-

wise to study the kinetics of CO2 absorption in N-Ethylethanolamine and N, N-

15

Diethylethanolamine solutions. The reactor used was a double-cell stirred reactor made of glass

with inner diameter 103.8 mm and height 130.5 mm. This technique allows measurement of

liquids with a single known composition. This technique provides a large surface area and can be

used for all types of amines.

Edali et al. (2009) studied the kinetics of CO2 absorption into mixed aqueous solutions of

MEA and MDEA using a laminar jet absorber. The laminar jet technique is suitable for fast

reacting amines, real amine concentrations and varying temperatures. However, it is not useful

for slow kinetics and inaccurate for low CO2 partial pressure.

Singto et al. (2016) studied the kinetics of CO2 absorption with a set of newly synthesised

amines using a semi batch technique. The apparatus consisted of a three-necked round bottomed

flask which was used as the reaction vessel. This technique generally allows measurements of

liquids with known composition and allows for realistic amine concentration and CO2 partial

pressures. The area of contact between liquid and gas is however unknown in this technique.

2.1.2 pKa

Amines are basic solvents. The pKa of amines indicate their basic strengths. The literature

reports techniques that have been used to evaluate the pKa of amines. Rayer et al. (2014) used

the potentiometric titration technique developed by Albert and Searjeant (1984) to study the pKa

of tertiary and cyclic amines and their dependence on temperature. Shi et al. (2012) also used the

titration technique to predict the dissociation constant of a novel amine species. There are other

computational softwares that are used in the literature to estimate and predict the pKa of amines.

For example, Singh (2011) and Puxty et al. (2009) used the ACD/pKa Phys. Chem. software by

ACD/Labs to estimate the pKa of different amine solvents. This software uses the Hammett

equation to estimate pKa of different amine based solvents.

2.1.3 Desorption Kinetics

Desorption kinetics is another important parameter in the CO2 capture process as it

influences the sizes of the desorber and heat exchanger as well as the temperature of desorption.

Desorption studies normally employ the batch, and continuous technique to determine the rate of

CO2 desorption. Singh et al. (2007) used the batch method to do the regeneration studies. The

16

apparatus consisted of a reaction vessel containing the pre- loaded amine solution (saturated with

CO2) which is heated to the desired desorption temperature. The amount of CO2 desorbed per

time is then diluted by a CO2 free nitrogen stream and monitored by a CO2 analyzer. Singto et al.

(2016) also used the batch technique to measure the desorption rates of amines. However, the

method of CO2 analysis was different. They used the liquid sample analysis technique by which

they took samples and analysed the amount of CO2 still remaining in the liquid phase.

2.1.4 CO2 equilibrium loading

CO2 equilibrium loading can be measure by using the solubility apparatus described by

Maneeintr et al. (2009) or by using any of the techniques mentioned in determining the absorption

kinetics. The CO2 loading (usually for liquid phase analysis) is also determined using the

apparatus developed by Horwitz (1975), the Gas Chromatograph or the CO2 analyzer (usually for

gas phase analysis).

2.1.5 Amine cyclic capacity

The amine cyclic capacity, also known as the effective solvent loading, is determined by

the difference between the rich and lean loading as reported in the literature (Singto et al., 2016;

Srisang et al., 2017a, b)

2.1.6 Heat of CO2 absorption (Habs)

The heat of CO2 reaction is measured using the Gibbs Helmholtz equation or via the

calorimetric method. Several works (Liu et al., 2015; 2016; Singto et al., 2016; Liang et al., 2015;

Zhang et al., 2016) have employed the Gibbs-Helmholtz equation to predict the heat of CO2

absorption. The use of the Gibbs-Helmholtz equation to predict heat of reaction implies that the

Habs is differential in loading, but not in temperature. The calorimetry technique has received a

lot of attention in predicting the Habs. Kim et al. (2014) used a Differential Reaction Calorimeter

(DRC) to predict the Habs of amines with multi amino groups. Kim and Svendson (2007)

proposed a method to predict Habs (of alkanolamine solutions) that was differential in

temperature and semi-differential in loading using a reaction calorimeter.

17

2.1.7 Heat of regeneration

The energy of regeneration is determined usually from pilot plant studies where the

reboiler heat duty can be obtained. Sakwattanapong et al. (2005) used a bench scale gas stripper

and solvent regeneration system to study the regeneration behavior of single and blended

alkanolamines. Srisang et al. (2017a, b) also used a bench scale pilot plant unit to study the energy

of regeneration for MEA and MEA-MDEA blended amine solutions. Idem et al. (2006) also

employed the use of a full cycle demonstration plant (Boundary Dam CO2 Capture Demonstration

Plant) to evaluate the heat duty performance of single and blended amine solvents. Singto et al.

(2016) used the batch scale technique for desorption kinetics to obtain the heat duty from the heat

supplied for regeneration by the heating medium.

2.1.8 Amine corrosiveness

The typically used techniques for measuring corrosion in amine based solvents are the

Static weight loss measurement and the electrochemical corrosion measurement. The former

allows the measurement of weight loss of specimen before and after immersion in amine solution.

The specimen is usually made of a material that is corrosion prone like carbon steel. This

technique can be used for a batch scale, semibatch or a continuous full cycle plant. The

disadvantage with this method is that there is possible error in the weight measurements. The

latter technique measures current densities of anodic and cathodic reactions from which it obtains

the corrosion rate. Martin et al. (2012) used the weight loss method to evaluate the corrosiveness

of different amines. On the other hand, Nainar and Veaweb (2009) performed corrosion studies

on blended MEA and Piperazine solutions using the electrochemical technique. Zhao et al. (2011)

also employed the electrochemical technique for amine corrosiveness studies.

2.1.9 Amine degradation

Degradation studies done in the literature typically use the stirred cell reactor or the

stainless steel batch reactor which allows for accelerated degradation evaluation of amine

solvents. The laboratory scale CO2 absorption unit can also be used for degradation studies;

however, this is suitable for long term degradation evaluation of amines. Lepaumier et al. (2009;

2010) used the stainless steel batch reactor to evaluate the degradation kinetics of amines. Supap

18

et al. (2009) also used a stainless-steel batch reactor for the study of the kinetics of SO2 and O2

induced degradation of MEA. Knudsen et al. (2009) used a pilot plant CO2 capture unit to

evaluate the formation of heat stable salts due to amine degradation over long periods of

continuous operation.

2.2 Significance of Solvent Properties

2.2.1 CO2 Absorption Rate

The CO2 absorption rate which measures how fast is the reaction between CO2 and the

amine is a very important characteristic to consider in the amine based CO2 capture process. The

size of the absorber strongly depends on the rate of reaction. This implies that the slower the

kinetics, the longer the residence time which consequently determines the absorber size. A fast

reaction implies a shorter absorber column, whereas a slow reaction implies a relatively tall

column. About 40% of the total costs of the plants make up the capital cost of the absorber (IEA

GHG, 2004), implying that in order to cut down the cost associated with the amine process, a fast

reacting amine is desirable. The costs and energy requirements of the flue gas coolers are

determined by the temperature of absorption. The closer it is to the desorption temperature, the

more preferable it is as this will decrease the cost of the solvent heat exchanger and flue gas

coolers (Peeters et al., 2007). Aside the absorption temperature and the inherent solvent

characteristic itself, there are other factors like viscosity, amine concentration and CO2 partial

pressure that affect the rate of CO2 absorption. Higher CO2 partial pressures favor higher rate of

absorption because there is a bigger driving force with smaller resistance in the gas phase. Higher

concentrations and viscosity slows down the absorption rate due to increased resistance in the

liquid phase.

2.2.2 CO2 Desorption rate

The CO2 desorption rate shows how fast it takes to reverse the amine-CO2 forward

reaction. In other words, this is a measure of the ease with which amine-CO2 bonded species can

break down. The product species of CO2 absorption are carbamate, bicarbonate or carbonate ions,

depending on the amine solvent. In CO2 desorption, the breakdown of the mentioned species are

affected by factors like the desorption temperature, the chemical binding energy, carbamate

19

instability which is strongly dependent on the amine structure, amine bulkiness (also depends on

amine structure) and amine basicity. All these determine how fast the CO2 desorption will be.

The chemical binding energy shows how strong the CO2 amine bond is. The stronger the bond

is, the higher the binding energy implying that more energy is required to break the bond. A low

chemical binding energy implies a lower energy required to reverse the CO2-amine reaction,

which is desirable for amine desorption. However, this may negatively affect the absorption rate

(Peeters et al., 2007).

The desorption temperature determines the temperature (as well the pressure) of steam

extracted from the steam cycle. A higher temperature of desorption favors the desorption rate;

however, this strongly impacts the energy requirement of the plant which determines the

operational costs associated with the capture process. Lowering the temperature of desorption

will imply a higher lean loading for some particular solvents thus reducing their cyclic capacity.

An alternative way to improve the desorption kinetics while lowering the desorption temperature

is to use other alternative amine solvents like sterically hindered amines (fast desorbing amines).

The rate of CO2 desorption will significantly impact the size of the desorber which has a direct

bearing on the capital costs of the CO2 capture plant.

2.2.3 Cyclic capacity

The cyclic capacity is the difference between the rich and lean loadings. The cyclic capacity

depends on the combined effect of absorber and desorber temperatures, solvent concentration and

the amine type. The lower the absorber temperature is, the higher will be the rich loading. On the

other hand, a higher desorption temperature will cause a very low lean loading. Keeping the

absorber and desorber temperatures constant, the cyclic capacity of amines is an inherent property

of the amine solvent which is determined by the amine structure. A higher cyclic capacity implies

a higher absorber efficiency or a lower solvent circulation rate for a fixed absorber efficiency,

which will reduce the diameter of the absorption column. Not only will a high cyclic capacity

affect the absorber size, but also, it will influence the dimensions of the solvent heat exchangers,

the amine pumps, reboiler, and the amine piping equipment. In addition, it will also translate into

lower electricity consumption of the amine pumps and the energy required for solvent heating

(Peeters et al., 2007).

20

2.2.4 Amine basicity (pKa)

The acid dissociation constant (pKa) of an amine is the first indicator of its reactivity

towards CO2 (McCann et al., 2011). A high pKa indicates a strongly basic amine solvent. In the

CO2 reaction mechanism, the removal of proton from the intermediate zwitterions formed during

the reaction with CO2 is a very important step. As such, when the basic strength is reduced, the

tendency to remove a proton from the intermediate zwitterions becomes difficult. The pKa itself

is an inherent property of the solvent which is strongly influenced by the amine structure.

2.2.5 Absorption capacity

The absorption capacity is the capacity of amine to absorb CO2. In other words, it shows

how much CO2 an amine can absorb. The absorption capacity is a strong factor of the absorption

temperature and the intrinsic amine structural property. The lower the absorption temperature is,

the higher will be the absorption capacity. The absorption capacity affects the solvent circulation

rate. A high capacity will lead to a lower solvent circulation rate, which will lower the size of the

absorber, heat exchangers, amine pumps (and its power consumption), and amine piping

equipment. These will translate to lower capital costs associated with this equipment.

2.2.6 Heat of CO2 reaction

The reaction between CO2 and amine is an exothermic reaction which leads to an increase

in temperature. The reaction heat gives a reflection of the amine affinity towards CO2. In simple

terms, the heat of reaction is a measure of the force of interaction between the amine and CO2. If

the force is large, the heat released is very high. Several studies in the literature have made the

assumption that the heat of reaction is the same as the heat of desorption. Simply, they assume

that the heat released during the forward reaction is the same heat required to reverse the reaction.

As such, several studies have been done on evaluating the heat of reaction of different amines

and used this to predict the heat required to regenerate the amines (Liu et al., 2016; Chowdhury

et al., 2013; Rayer et al., 2014) In most cases, a high heat of reaction will lead to a high heat duty

21

for solvent regeneration (known as regeneration heat). The heat of reaction depends on the amine

type, temperature and CO2 loading (Kim and Svendson, 2007).

2.2.7 Heat of Regeneration

The heat of regeneration, also known as the heat duty, is the energy required to regenerate

the liquid solvent. It shows how much external heat is needed for solvent regeneration. The

energy of regeneration is one of the important parameters that accounts for a major part of the

CO2 capture operational costs. In addition, the steam that is extracted from the steam cycle and

used for solvent regeneration leads to a decrease in the thermal efficiency of the coal fired power

plant (Srisang et al., 2017a, b; Liu et al., 2015). Therefore, the integration of a capture process

with the power plant becomes a difficult option due to the parasitic energy loss. Thus, numerous

studies are focused on improving and optimising the capture process in order to reduce the high-

energy penalty (Idem et al., 2017; Shi et al., 2014). According to Rochelle (2009), about 50% of

the minimum theoretical energy required for the CO2 capture process is used for regeneration,

while the remaining 50% is used for CO2 compression for transport and sequestration. The heat

duty is a function of the desorption temperature and the amine type. The heat duty comprises the

sensible heat, heat of vaporisation and the heat of desorption. The sensible heat depends on the

heat capacity of the solvent, and solvent concentration. The heat of vaporisation for amines may

vary only slightly but will depend on the desorption temperature (Liu et al., 2016; Chowdhury et

al., 2013). The heat of desorption depends on the chemical binding energy (which is an inherent

amine property), as mentioned earlier.

2.2.8 Amine corrosiveness

Corrosion is an electrochemical process that involves oxidation and reduction reactions

which occur on the surface of a metal when it comes into contact with a solution. This reaction

eats up the metal causing it to wear away. Corrosion can occur in typical locations of the capture

plant like at the bottom of the absorber, regenerator, heat exchanger, reboiler bundles, amine

cooler and condenser (Kittel and Gonzalez, 2014). Corrosion in amine based CO2 capture process

is one of the operational problems associated with the amine process which reduces the plant

efficiency due to plant failure, unplanned downtime and loss of equipment. Amine corrosiveness

22

depends on the type of amine, the amine concentration, O2 concentration, temperature, presence

of heat stable salts, CO2 loading and SO2 concentration. Work done by Kladkaew et al. (2009)

showed that corrosion rate increases with an increase in the aforementioned parameters.

According to Srinivasan et al. (2013), the use of a corrosion inhibitor is the most

economical and flexible strategy as compared to other methods since it can be applied to the

existing process without requiring any major process modification. The use of specific materials

for the construction of certain parts of the CO2 capture plant was proposed by Billingham et al.

(2011). The benefit is that it will allow the use of cheap carbon steel for the construction of the

pipes, vessels and units that are not prone to corrosion, and only uses specific corrosion resistant

materials for areas that are prone to corrosion, thereby preventing the occurrence of corrosion in

those areas.

2.2.9 Degradation of amines

Degradation simply means the loss of active amine due to chemical changes in the amine

solvents. The chemical change may be due to temperature or chemical reaction of amines with

impurities like fly ash, O2, SOx and NOx. Degradation breaks down the amine molecules to

inactive non-CO2 absorbing species. Degradation is a very severe operational problem in the

capture process since it affects the overall efficiency of the capture plant. Degradation can also

translate to corrosion problems because the products formed due to the degradation are corrosive

(Chakravarti et al., 2001). Degradation reduces the absorption capacity of the amines. Some

degradation products release toxic emissions which makes it an environmental concern. Solvent

reclaiming is done to remove the degradation products using techniques like distillation, vacuum

distillation, ion exchange or electro-dialysis. Thermal reclaiming is very energy intensive and

increases the energy consumption of the capture plant, leading to an increase in the operational

costs. Due to this, some preventive techniques have been proposed to control degradation of

amines. These include removal of all flue gas impurities and using solvents that are not

susceptible to degradation. However, this preventive option is costly, difficult and most often

time consuming or impractical. Thus, chemical additives that reduce the rate of amine

degradation like inhibitors have been used (Sexton et al., 2009).

23

2.3 CO2-Amine Reaction Mechanisms

2.3.1 Primary and secondary amines

The absorption reaction between CO2 and primary or secondary amine was first

introduced by Caplow et al. (1968) and was later reintroduced into the chemical engineering

literature by Danckwerts et al. (1970). These researchers described the reaction as a zwitterion

mechanism, which is a two-step reaction comprising of the reaction of aqueous CO2 with

primary/secondary amine to form zwitterion followed by the removal of a proton by any base in

the system to produce the carbamate. Later, the termolecular mechanism was introduced by

Crooks and Donellan et al. (1989). The two step reaction is as follows:

Step 1:

2 + 2( ) 2+ - 2.1

Step 2:

R 2+ + + + 2.2

where B represents a base molecule which can be water, a hydroxyl group or the amine

functionality (Blauwhoff et al., 1984 ).

The overall reaction is then simplified as:

2 2 + 2 2 RNH- + 2+ 2.3

The single step termolecular mechanism proposed by Crooks and Donellan et al. (1989) is as

follows:

B + R1R2NH + CO2 R1R2NCOO¯ + BH+ 2.4

B can be any base molecule. This mechanism considers that the reaction between amine and CO2

and the proton transfer occurs simultaneously.

2.3.2 Tertiary amines.

Since tertiary amines lack a free proton, they are not able to react directly with CO2 to

form carbamate (Blauwhoff et al., 1984). Hence, tertiary alkanolamines act as a base and catalyze

24

the hydration of CO2, leading to the formation of bicarbonate (Donaldsen and Nguyen 1980;

Rinker et al. 1995). The reaction mechanism is as shown:

CO2 (aq) + H2O H2CO3 2.5

CO2 (aq) + OH ¯ HCO3¯ 2.6

CO2 + R1R2R3N + H2O R1R2R3N+H + HCO3¯ 2.7

2.4 Structure – Activity Relationships of Amine-Based Solvents: Effect of Molecular Structure

on Solvent Properties

The structure – activity relationships show the relationships that exist between the

chemical structure of amines and their solvent properties. Several efforts have been made in the

literature to show the influence of the various structures of amines and their impact on their CO2

capture activities. Since the significance of the solvent properties has already been discussed, it

is important to now focus on finding the factors that impact these characteristics. Factors like

process conditions influence the CO2 performance of amines. However, the backbone of the

amine behavior towards CO2 is the chemical structure of the amine, which makes them behave

the way they do in the CO2 capture process.

2.4.1 Carbamate stability and bicarbonate formation

Carbamates, usually formed from the CO2 reaction with primary and tertiary amines, can

undergo hydrolysis to form bicarbonate. The degree of this hydrolysis will depend on the

chemical stability which is influenced by factors like temperature and sterical hindrance (will be

discussed later). The hydrolysis reaction is shown as follows:

R1R2NCOO¯ + H2O R1R2NH + HCO3¯ 2.8

CO2 can also directly react with water to form bicarbonate according to the reactions below:

CO2 (aq) + H2O H2CO3 2.9

CO2 (aq) + OH ¯ HCO3¯ 2.10

H2CO3 + OH ¯ HCO3¯ + H2O 2.11

25

Carbamate stability is an important characteristic that influences the desorption kinetics, and

absorption capacity.

2.4.2 Sterical hindrance

Satori and Savage (1983) describe a sterically hindered amine structurally as a primary

amine in which the amino group is attached to a tertiary carbon atom, or a secondary amine in

which the amino group is attached to a secondary or a tertiary carbon atom. Some sterically

hindered amines include 2-Amino-2-methyl-1- propanol (AMP), 2-Piperidine ethanol, 2-

Piperidine methanol and 2-amino-2-hydroxymethyl-1,3-propanediol (Satori and Savage, 1983;

Park et al., 2003; Singh, 2011). Park et al. (2003) performed studies on 2-amino-2-

hydroxymethyl-1,3-propanediol (APHD). From their studies, the results showed a higher CO2

loading for APHD than for the conventional amine, MEA, at CO2 partial pressures above 4kPa

but lower loadings at lower partial pressures. NMR studies performed by Hook (1997) to

determine the carbamate, bicarbonate and carbonate concentration in many CO2-loaded amine

solutions showed the level of sterical hindrance of different amines. It was found that a higher

number of methyl groups present at the -carbon position resulted in lower carbamate stability,

but the effect of one methyl group present at the -carbon was not strong enough to induce full

conversion of carbamate to bicarbonate.

2.4.3 Electron density

The electron density is simply a measure of the probability of finding an electron in a

particular location. An atom or group with higher electron density means a part of the molecular

structure is shifting negative charge towards itself in the molecule, whereas an atom or group

with lower electron density means that a part of the molecular structure is shifting negative charge

away. In the CO2 reaction mechanism, the zwitterion formation requires electrons for that step of

reaction to occur. According to Danckwerts (1978), this reaction is the rate determining step

meaning that it is the slowest reaction step. Because this reaction requires electrons and is the

rate determining step, electron density around the nitrogen molecule becomes very essential in

the CO2 reaction. Electron density can be increased or reduced through inductive effects.

2.4.3.1 Inductive Effect

26

When electron donating or electron withdrawing group located somewhere in a molecule

have an effect on the electron density in another part of the molecule, the effect is known as an

inductive effect. An electron withdrawing group is an atom or group that withdraws electrons

from neighbouring atoms towards itself. As a consequence, there is reduced electron density

around the atom that wants to share its electrons. Electron withdrawing groups include halogens

(like -F, -Cl, -Br, -I), and other groups like -COOH, -CN. An electron donating group is an atom

or group that releases electron to the neighbouring atoms from itself. This increases the electron

density around the atom that wants to share its electrons. Examples of electron donating groups

are -alkyl, -COO¯ and -O¯.

Inductive effects depend on three factors:

1. Electronegativity of the group

The electronegativity of an atom is its ability to attract electrons to itself in a covalent bond. If

the atom that provides the electron pair is highly electronegative it tends to resist giving the

electrons away; but rather attract any electrons available to itself.

2. Number of electron withdrawing or electron donating groups

The number of electron withdrawing or electron donating groups determines if the inductive

effect will be -I effect (that is a decrease in the electron density) or +I effect (an increase in the

electron density). The effect depends on which groups dominate (that is which ones have the

higher numbers).

3. Distance from the group

The distance of the group also plays a key role in determining what kind of inductive effect will

result. If one particular group is closer to the electron density site than the other group, its effect

will be more dominant. For example, if the electron withdrawing group is close than the electron

donating group, its effect dominates, resulting in an -I effect (reducing electron density).

2.4.4 Structure – activity relationship studies

Several works have been reported in the literature showing how one or more structural

property can affect certain specific chosen activities. The structural properties commonly looked

27

at include effect of cyclic amine, effect of chain length, effect of poly amines, number of hydroxyl

groups, position of hydroxyl groups and functional group substitution. Singh et al. (2009)

investigated the effect of side chain position of different amines on the absorption capacity and

initial absorption rate. From their work, it was seen that the introduction of a substituent at the -

carbon position decreased the initial absorption rate for primary amines but increased the

absorption capacity when compared to a substituent at the -carbon position. This behavior was

attributed to sterical hindrance created by the -carbon substitution. According to Sartori and

Savage (1983), sterical hindrance reduces the stability of the carbamate. Consequently, the

carbamate ion can easily undergo hydrolysis to form the bicarbonate ion. These researchers

also studied the effect of increasing the number of amino groups on the aforementioned activities.

Their results showed an increase in the initial absorption rate with the increase in the number of

amino groups from 0 to 2 but, no further increase in rate when the amino groups were increased

from 2 to 3. The effect of this structural difference showed a general increase in absorption

capacity from 0 groups to 3 amino groups. The effect of the substitution of methyl and amino

groups on cyclic amines on the absorption rate and capacity were also studied by Singh et al.

(2009). Results showed that substitution with an amino group showed a higher increase in the

rate than with a methyl group. An increase in the number of methyl groups, however, further

increased the rate.

Work done by El Hadri et al. (2017) showed the impact of the type of -substituent (be it

an alkyl group or an alcohol group) on the absorption capacity. An alkyl group substituent was

found to enhance the absorption capacity whereas the AOH group negatively affected the CO2

absorption. Chowdhury et al. (2013) investigated synthetic and commercial amine based

absorbents by modifying their chemical structures. The results showed that the placement of

functional groups within the amino group affects CO2 absorption-regeneration performances.

Singh et al. (2008) studied the effects of chain length side chain, and functional groups in

alkanolamines, alkylamines and diamines, as well as the effect of functional groups in cyclic

amines on their desorption performance. The substitution of a methyl group at the alpha carbon

position showed a positive improvement in the desorption performance than a substitution at the

beta carbon position. This was attributed to steric hindrance effect on the side chain methyl alpha

carbon substituent. An increase in the desorption capacity and rate was observed as the number

of amino groups were increased. The effect of chain length in alkanolamines, diamines and

28

alkylamines were not as straight forward as seen in the functional groups and side chain effect.

The interpretation of the anomalies was that desorption is a complex interaction of mass transfer,

kinetics and equilibrium.

Bonenfant et al. (2003) studied the CO2 absorpiton and regereation of some amines

including 2-(2-aminoethylamino) ethanol (AEE), and N-(2-aminoethyl)-1,3-propanediamine

(AEPDNH2). Their results showed the potential of AEE and AEPDNH2 as CO2 absorbents due

to their good absorption and desorption performance. The high number of amino groups present

in these amines was claimed to be responsible for their performance. Lepaumier et al. (2009)

studied the effect of structural properties on amine degradation. The replacement of the alcohol

function by an amine function showed that tertiary amines were slightly more stable than primary

and secondary amines. Also, it was concluded that steric hindrance (AMP) also decreases

degradation by avoiding highly volatile compound formation due to dealkylation. Lepaumier et

al. (2010) also studied the impact of alkyl chain length between amino groups on polyamines

degradation. These studies revealed that amines will degrade depending on the ring closure

formed. If the molecule easily formed five or six membered rings, then degradation will be high.

However, if three, four or more than six membered rings were formed easily, then the amine will

be more stable.

2.4.5 Criteria for selecting amines

Different strategies have been formulated to represent the criteria for selecting amines as

potential absorbents for CO2 capture. These are based mostly on selecting any two pairs of

activities at a time and then deciding which of the pairs can be used to select the best components.

This may however pose the issue of bias in the selection as some of the activities may yield

contrasting results. For example, in Liu et al. (2016), two activities, namely, the absorption

kinetics in terms of the second order rate constant and the heat of absorption were used to

represent the criteria for selection. These researchers used the absorption heat to represent the

desorption performance with the assumption that the heat of desorption (an important parameter

in determining the energy requirement) will follow the same trend as that of the absorption heat.

As such, the ideal amines are those with high log k values and low heat of absorption. Also,

another plot of log k2 values verses heat of protonation (Hpro) was generated to show a possible

selection criteria. This is shown in Figure 2.1a and 2.1b. These researchers attributed a low

29

protonation heat to favorable regeneration since it was shown in their work that the Hpro

contributes mainly to a higher desorption capacity and lower energy of regeneration.

Chowdhury et al. (2013) studied the performance of different amines and generated a plot

of absorption rate versus reaction heat as shown in Figure 2.2. From the plot, the preferred target

shows amines with high absorption rates and low reaction heats. Recently, Liu et al. (2015)

performed solubility, absorption heat and mass transfer studies on 1-dimethylamino-2-propanol

(1-DMA2P), a tertiary amine. The justification of their selection was based on earlier absorption

kinetic studies done by Kadiwala et al. (2012) whose work showed that 1DMA2P was a potential

solvent due to its better absorption kinetics compared to other tertiary amines solvents. Rayer et

al. (2014) studied the pKa of tertiary and cyclic amines. A plot of the heat of reaction versus pKa

was generated to represent a possible selection strategy. The plot is shown in Figure 2.3. From

their plot, amines with higher basicity but lower reaction heats were identified to be potential

amines for CO2 capture.

2.4.6 Amine Blending

Blending of amines was first proposed by Chakravarty et al. (1985) who suggested that

the individual strengths of amines can be optimized by blending them. This has led to the blending

of amines with individual attractive solvent characteristics like high absorption capacity, good

absorption and fast desorption kinetics by several researchers. Some of these works are

mentioned below. For example, Idem et al. (2006) blended MEA and MDEA in the 4:1 molar

ratio and compared the performance to the conventional 5M MEA. The results showed that the

heat duty for the blend was much less than that of the single solvent system, implying that it is

economically feasible to employ such a system in industrial applications for CO2 capture.

Bruder et al. (2011) tested the performance of blended solutions of AMP and Piperazine

(PZ) in their work. The blended systems were found to have higher absorption capacities and

cyclic capacities than the conventional 5M MEA single solvent system.

30

Figure 2.1a: PCC alternatives solvents as a plot of the second order reaction rate at 313 K, log k2, and absorption heat ( Ha) (Liu et al., 2016)

31

Figure 2.1b: PCC alternatives solvents as a plot of the second order reaction rate at 313 K, log k2, and absorption heat ( Ha) (Liu et al., 2016)

32

Figure 2.2: The trade-off between reaction heat and CO2 absorption rate (Chowdhury et al., 2013)

33

Figure 2.3: pKa versus Heat of reaction of tertiary and cyclic amines (Rayer et al., 2014)

34

Blending of more than two solvents has also proven to be successful as this has been

shown to increase the absorption capacity, cyclic capacity and allowed the application of higher

amine concentrations (Nwaoha et al.2016). Sakwattanapong et al. (2005) suggested that the

mixing ratio used for blending will impact the success of a blend. For instance, bi-solvent blends

of AMP and PZ in the molar ratios of 3:1.5 and 4:1 showed that the 3:1.5 ratio had higher

equilibrium loading and cyclic capacity in mol CO2/mol amine than the 4:1 ratio. All these works

among several others (Maneeintr et al., 2010; Bruder et al., 2012) have proven the benefits of

amine blending.

2.4.6.1 Blended Amine Chemistry

The reaction mechanism of blended systems depends on the type of amine components used with

regard to primary, secondary and tertiary amines. The mechanism for blended amines is treated

in the same way as single amines (Glasscock et al., 1991). That is if the blend consists of primary

and secondary amines their mechanism will be the same as mentioned for those classes of amines.

However, the presence of a tertiary amine in a blend will add to the number of bases that can

deprotonate the zwitterion formed by the carbamate forming amines.

2.2.7 Pilot Plant Validation

Pilot plant studies are used for a wide variety of purposes. For example, it has been used

to demonstrate the practicality of post combustion CO2 capture, test the stability of solvents in

terms of degradation and corrosiveness, and to test solvent activities like kinetics, mass transfer,

heat duty and the comparison of the performance of different solvents. A few of these tests done

to show the operability of the PCC have been reported in this section.

Kittel et al. (2009) performed corrosion studies using two different pilot plants under MEA

operation. Their studies revealed the various locations of the pilot plant susceptible to corrosion.

Knudsen et al. (2009) employed the use of a pilot plant unit to test the stability of different amine

solvents. The tests were done to show the formation of heat stable salts due to amine degradation

over long periods of continuous operation. Idem et al. (2006) also tested the chemical stability

and the energy requirement of two different solvent systems using a CO2 capture demonstration

plant and a technology development CO2 capture pilot plant. Results showed the benefit of using

35

MEA-MDEA solvent over MEA due to a higher stability and lower energy of regeneration of the

blended system. Also, Wilson et al. (2004) conducted a series of tests on the Boundary Dam CO2

capture pilot plant to show the practicability of the PCC process with regards to absorber

performance, solvent regeneration, solvent stability (degradation) and amine corrosiveness. Abu-

Zhara et al. (2007) performed pilot plant parametric studies of the performance of MEA showing

possibilities of process optimisation. Process parameters like lean solvent loading, amine

concentration and stripper operating pressure were found to impact the realization of energy

savings. Mangalapally et al. (2009) also performed parametric studies of the performance of

MEA and two other solvents using a pilot plant.

Pilot plant studies can also be applied to explore new technologies. For example, Srisang

et al. (2017a, b), Decardi-Nelson et al. (2016; 2017), Osei et al. (2017) and Akachuku (2016)

were the first to implement the amine-based catalyst-aided CO2 capture process patented by Idem

et al. (2011) in a pilot plant. Different aspects, namely, heat duty (Srisang et al., 2017a, b),

desorption mass transfer studies (Osei et al., 2017), desorption kinetics (Akachuku, 2016) and

model development (Decardi-Nelson et al., 2016; 2017) of the catalyst-aided process were

studied. Singh (2011) validated a newly formulated solvent from a preliminary screening tests

performed using structure – activity relationships in a continuously operated conventional pilot

plant to prove its operability. All the aforementioned tests done by researchers have shown the

importance of pilot plant studies. Hence, in the current study, a bench scale pilot plant will be

used to validate the selected potential solvents as well as investigate the role of catalyst in further

improving the overall efficiency of the process.

36

CHAPTER THREE: EXPERIMENTAL SECTION

3.1 Overview of Screening Experiments

The screening for various CO2 capture activities was performed at the same conditions for all

the amines studied. The activities were: absorption and desorption kinetics, amine basicity,

equilibrium solubility, heat of CO2 absorption, cyclic capacity, and heat duty for amine

regeneration. The absorption and desorption kinetics were expressed in terms of the initial

absorption rate and initial desorption rates, respectively. The absorption experiment was done

using a reaction vessel operating in the semi batch mode. This technique allowed for easy

screening of amines with known composition, and allowed for realistic amine concentration and

CO2 partial pressures. The desorption experiment, which also operated in the semi-batch mode,

allowed for quick and easy screening of various amines. The CO2 analysis was done by measuring

the CO2 in the liquid phase. The basicity was measured in terms of pKa. This was done using the

titration technique which provides a more deterministic way of measuring pKa without making

estimates. Equilibrium solubility data was obtained from the solubility apparatus which allowed

for measurements at varying partial pressures and temperatures. Cyclic capacity was obtained

basically from the rich and lean loadings. Heat of absorption was done based on the Gibbs-

Helmholtz equation, which is useful for screening purposes. The heat duty was calculated based

on the heat supplied and the total amount of CO2 desorbed in the regeneration experiment. The

selected amines from the screening tests were then blended and screened further under the same

experimental conditions to obtain the optimum blend.

3.1.1 Safety Precautions

Safety measures were taken during the running of experiments. Protective clothing, eye goggles,

hand gloves and boots were worn to protect the body from harm. Details are provided in

Appendix A.

3.2 Amines Studied

The amines studied for the effect of side chain and number of hydroxyl groups were

Primary Alkylamines: Butylamine (BUTYL), secButylamine (SEC) and isoButylamine (ISO),

Primary Alkanolamines:4-amino-1-butanol (4-A-1-B) and 2-amino-2-methyl-1-propanol (AMP)

as its analogue branched chain amine, Secondary Amines: Butylethanolamine (BEA) and its side

37

chain analogue as tert-Butylethanolamine (t-BEA) and for Tertiary Amines: Butyldiethanolamine

(BDEA) and tert_Butyldiethanolamine (t-BDEA). For the effect of alkanol chain length and alkyl

chain length, Primary Amines: 2-amino-1-ethanolamine or Monoethaolamine (MEA), 3-amino-

1-Propanol (3-A-1-P), 4-amino-1-butanol (4-A-1-B) and 5-amino-1-pentanol (5-A-1-P) were

studied, for Secondary amines: Methylmonoethanolamine (MMEA),

Ethylmonoethanolamine(EMEA), Propylmonoethanmolamine (PMEA) and

butylmonoethanolamine(BEA);for Tertiary Amines, Methyldiethanolamine (MDEA),

Ethyldiethanolamine (EDEA) and Butyldiethanolamine (BDEA). The chemical structures of all

these amines are shown below:

OH

NH

3-Amino-1-Propanol (3-A-1-P) Methyl monoethanolamine (MMEA)

OH

NH2 4-Amino-1-Butanol (4-A-1-B) Monoethanolamine (MEA or 2-A-1-E)

NH2 OH OH

NH

5-Amino-1-Pentanol (5-A-1-P) Ethyl monoethanolamine (EMEA) OH

NH

OH

NH

Propyl monoethanolamine (PMEA) Butyl monoethanolamine (BEA)

N

OHOH

NOHOH

Ethyl diethanolamine (EDEA) Butyldiethanolamine (BDEA

NH2

OH

NH2 OH

38

NH2

Methyl diethanolamine (MDEA) Butylamine (BUTYL)

NH2

OHNH2

sec-Butylamine (SEC) iso-Butylamine (ISO) 2-amino-2-methyl-1-propanol(AMP)

OHNH

OHN

OH

tert-Butylethanolamine tert-Butyldiethanolamine

3.3 Materials and Equipment

BEA ( 98%), MEA or 2-A-1-E (> 99%), 5-A-1-P (> 95%), MMEA (> 98%), EMEA (>

98%), PMEA (> 98%), MDEA (> 99%), EDEA (> 98%), AMP (> 99%), SEC, (99%) ISO (99%),

BUTYL (99.5%) , tBDEA (97%) and 1N hydrochloric acid (HCl) were purchased form Sigma-

Aldrich Co., Canada. BDEA (98%) and 3-A-1-P (> 99%) were purchased from Acros Organics

while 4-A-1-B (> 96%) was purchased from Pure Chemistry Scientific Inc. and from tBEA

purchased from Oakwood Chemicals from U.S.A. The premixed gases used for the solubility

experiment, (3, 8, 15, 30 and 100% CO2 (N2 balance) were supplied by Praxair Inc., Regina,

Canada.

The water bath used in the solubility experiment with model number: 12112-12, SN:

G29972, which was equipped with a temperature controller (-20 to 200 oC capacity with ± 0.01

NH2

N

OHOH

39

oC accuracy) was obtained from Cole-Palmer, Canada. A mass flow meter (Electronic

AALBORG GFM171 with a range of 0 – 500 ml/min with accuracy of ±1%, and a hot plate from

Fischer Scientific Company, were used. Viscosities were measured using the Lovis 2000 M/ME

from Anton Paar with an accuracy of 0.5% for viscosity and 0.02 oC for temperature, as well as

with a Size 50 Cannon-Fenske Opaque tube in a KV3000 heating bath (manufactured by Koehler

Instruments) purchased form Fischer Scientific, Toronto.

3.4 Absorption Experiment

The absorption experiment was done at 40 ± 1 oC based on the method used by Singto et

al. (2015) as shown in Figure 3.1. In order to screen as many solvents as possible, including

tertiary amines and compare them on the same basis, a concentration of 2 M was selected and

used. The concentrations of the prepared amine solutions were confirmed by titration with 1 N

HCl. A stirring speed of 600 rpm was maintained for all the test runs. All amines used were

soluble in water. An average absorption time of 6 h was allowed. The volume of amine sample

used was maintained at 100 mL for all experiments. The apparatus basically comprised of a three-

necked round bottomed flask with a condenser installed at the middle neck, a thermometer at one

neck for amine solution temperature measurement, and a gas dispersion tube on the other neck

for feeding the gas. At the start of the experiment the flask containing a known volume of solution

was immersed in a preheated oil bath and allowed to reach the desired absorption temperature.

Then, a premixed gas (15% CO2 and 85% N2) at a flow rate of 200 mL/min (±2 accuracy) was

bubbled into the solution through the dispersion tube. Samples were then taken at time intervals

of 10 min (starting from 0 min after reaching the desired temperature) for the first hour, and then

at 30 min interval after then in order to measure the CO2 loading using the Chittick apparatus

until equilibrium was reached. A plot of loading versus time was generated and the initial

absorption rate was determined by finding the slope of the linear part of the absorption profile.

The absorption experiment was repeated at least twice and the repeatability was recorded as 1%.

40

Figure 3.1: Simplified diagram of absorption and desorption set up

41

3.5 Desorption Experiment

A known volume (76 mL) of the equilibrium loaded solution from the absorption tests

was used for the desorption experiment. The same setup in Figure 3.1 was used for this

experiment. However, in this test the gas dispersion tube was removed and the neck sealed off.

At the beginning of the experiment, the flask was totally immersed in the preheated oil bath and

allowed to reach the desorption temperature of 90 oC. The heating time was approximately 5 min.

A sample was taken from the flask at 4, 5, 8, 12 and 20 min. After that, a sample was taken at

intervals of 10 min until equilibrium was reached at 90 oC. Since the rate of desorption slowed

down after 5 min, and gradually reaching equilibrium, the initial desorption rate was calculated

by determining the slope of the points from time 0 min to time = 5 min as most of the removable

CO2 had been removed within this heating time. The initial time of zero min was used also to

reflect the practical situation in the regeneration unit where desorption begins all the way from

the cross-heat exchanger as the rich amine solution is preheated even before it finally enters the

desorber. The desorption experiment was repeated at least twice and the average repeatability

was 1%.

3.6 Heat Duty Determination

The heat duty was calculated by determining the ratio of the steady state heat transfer to

the amount of CO2 removed during desorption over a 5 min period (the linear portion of the

desorption kinetics profile). Fourier’s equation of thermal conduction was used to calculate the

heat supplied from the oil bath as shown in Eq. 3.1

q = 3.1

where q is the rate of heat transfer at steady state in J/s, k is the thermal conductivity of the Pyrex

glass used for the flask material in W/m K, A is the cross-sectional area normal to the direction

of heat flow in m2, dT/dx is the temperature gradient (Km-1). The temperature difference, dT was

taken as the difference between the oil temperature and the inner wall temperature of the flask,

while dx was the glass wall thickness. The heat duty was then calculated using Eq. 3.2.

Qreg = 3.2

42

The amount of CO2 removed was obtained by determining the cyclic capacity of the amine in 5

min at 90 oC using the linear portion of the desorption profile. The repeatability is recorded as

2%.

3.7 Acid Dissociation Constant (pKa)

The dissociation constant was determined using the titration technique described by Shi

et al. (2012). A known volume of 100 mL of 0.05M of amine solution was prepared and titrated

stepwise with 1.0 N HCl until methyl orange endpoint was achieved. After every 0.5 ml addition

of HCl, the pH was measured. After obtaining the concentration of H+ from the pH meter, the

pKa was then calculated using equations 3.3 – 3.7. Specifically, the protonation reaction of the

amine in aqueous solution is given in equation 3.3:

Amine + H+ AmineH+ 3.3

By assuming that the solution is ideal (activity coefficient is equal to 1), as suggested by

Kent et al. (1976) and Horwitz (1975), the Amine H+ dissociation constant K can be calculated

by:

3.4

Hence, pKa is determined using:

pKa = -log(Ka) = -log ( 3.5

Since there is a reduction in the concentration of H+ during titration as a result of its reaction with

the amine to form AmineH+ as shown in equation (3), the concentration of AmineH+ can be

calculated using the mass balance of protons in the equation:

3.6

The concentration of free amine can be calculated using:

3.7

where, is the number of moles of HCl added during the titration, Vtotal is the total liquid

volume after titration, and is the initial number of moles of Amine, which can be

determined by titration with 1.0 N HCl until the methyl orange end point. The repeatability of the

43

pKa experiment was less than 1%. Also, pKa values were compared to literature values for

validation and the percentage deviation was less than 2%.

3.8 Equilibrium CO2 Solubility

The equilibrium CO2 solubility tests were done using the procedure described by

Maneeintr et al. (2009) at the temperature range of 40-90oC for high boiling point amines and 20-

40oC for low boiling point amines, for CO2 partial pressures in the range of 3-100 kPa. The

experimental set up shown in Figure 2 was used and it comprised of a water bath installed with a

temperature controller with an accuracy of ± 0.01 oC. Prior to the experiment the water bath was

set at the desired temperature and a known volume of amine solution (20 mL) was loaded into

the absorption reactor, and placed in the bath. To recover amine and water carry over, a condenser

was installed at the outlet of the reactors. After the solution had attained the desired temperature,

the desired premixed gas at flow rate of 350 mL/min was bubbled first though a water saturation

cell and then finally into the reactor through gas dispersion tubes. To ensure that equilibrium was

attained, the process was kept in operation for an average time of 18 – 20 hours. To confirm that

equilibrium was reached, samples were taken and analyzed for loading at a time interval of two

hours in order to have two consecutive readings that showed just a slight difference of not greater

than 5%. Samples were analyzed for CO2 loading using the Chittick apparatus (Horwitz, 1975).

The CO2 loading measurements were repeated twice and the average taken. The error of the

measurements was typically within ± 0.01. The solubility data obtained for the current work were

validated by comparing the solubility data for 5 M MEA obtained from this work with those in

the literature (Aronu et al., 2011). The equilibrium solubility experiment repeatability was around

1%. Also, the solubility data for 5M MEA were compared with literature and validated as being

reproducible.

44

Figure 3.2: Experimental setup of equilibrium solubility according to Maneeintr et al., 2009

45

Figure 3.3: Validation of solubility data for 5M MEA by comparison with Aronu et al. (2011)

46

3.9 Heat of Absorption

The heat of absorption was estimated using the Gibbs-Helmholtz equation:

RH

TP

absCO

)d(1/)d(ln

2 3.8)

Typically, equilibrium solubility data obtained at a minimum of three different temperatures and

different CO2 partial pressures are needed in order to calculate the heat of absorption. When the

same coinciding CO2 loading of at least three different temperatures and partial pressures are

obtained, the Gibbs-Helmholtz equation is used to determine the value of the heat of absorption

by generating a plot of ln PCO2 versus 1/T; and the slope of the line gives the term Habs/R where

R is the molar gas constant in J/mol-K. This method is very tedious as it involves trial and error

experiments in order to obtain the same coinciding CO2 loading at different temperatures. In

addition, the loading that is attained may be higher than the actual equilibrium loading of the

amine, thereby making it impractical, and also including the values of heat of physically absorbed

CO2. Consequently, in this work, a new method has been developed to address these limitations,

and which requires no trial and error. Equilibrium solubility data at 5 different partial pressures

ranging from 3-100 kPa and at least four temperatures was obtained. A line of best fit equation

was determined and used to describe each curve at each temperature. With this implemented, any

practical loading and temperature range can be used. By substituting the desired loading in the

equation of the line of best fit, the PCO2 at any of the selected temperatures can be obtained by

interpolation. The PCO2 and temperature values are then used in equation (8) to determine the heat

of absorption. In order to validate this method, the equilibrium solubility data obtained in this

work for 5M MEA at different temperatures were used to validate the developed procedure

against values of heat of absorption obtained by the calorimeter method by Bruder et al. (2011).

3.10 CO2 Loading Test

The CO2 loading tests were performed with the Chittick apparatus. Sample volumes of

2ml were first pipetted into a 250 mL Erlenmeyer flask, before adding 2 to 3 drops of methyl

orange indicator. Approximately 12 mL of distilled water is then added to dilute the sample. The

sample flask filled with solution is then properly connected and fitted with a rubber cock

connected to the Chittick apparatus and sealed with parafilm tape to avoid any gas leakage. The

47

1M HCl acid contained in the burette is then added dropwise into the flask until the end point is

reached. The loading is obtained using equation below:

3.9

Where is the loading in mol CO2/mol amine, Vgas is the volume (mL) of displaced solution in

the gas measuring tube, Vacid is the volume of acid titrant in mL, P is barometric pressure (mmHg),

T is room temperature (K), C1 is amine concentration (mol/L) V1 is the amine solution sample

volume (mL) and Ac is conversion constant (22.41 L/mol). Loading measurements were repeated

twice and the repeatability was recorded as less than 2%.

3.11 Viscosity Measurement

For the amines that were studied for effect of side chain and number of hydroxyl groups,

the Digital Anton Par micro viscometer model (Lovis - 2000 M/ME) was used to measure the

viscosities in mPa·s. This micro viscometer works by using the rolling ball principle which

employs three inductive sensors to measure the time it takes for the ball to move through the

liquid filled capillary. High quality deionized water was used to calibrate the equipment after

each run. Prior to each measurement, the sample was kept inside the viscometer until the set

temperature reached equilibrium condition. Each experiment was repeated twice and the data

reported was the average value. The accuracy was within 0.5% for viscosity and ± 0.02oC for

temperature.

For amines studied for the effect of alkanol chain length and alkyl chain length, the

viscosity measurements were done by Petroleum Analytical Laboratories using a Size 50

Cannon-Fenske Opaque tube in a KV3000 heating bath (manufactured by Koehler Instruments)

purchased form Fischer Scientific, Toronto. The analysis was done using the ASTM D445

method.

3.12 Heat Capacity Determination

The heat capacities of lean amine solutions of the two blend systems: BEA-AMP and

MEA-MDEA were measured following the procedure reported by Pouryousefi et al. (2016). This

employs the liquid thermal conductivity meter (THW-LAMBDA, Therm Test Inc, Ontario)

Ac

48

which uses a thin platinum wire whose heat resistance profile with respect to time is measured

when immersed into the sample. The temperature-time profile of the amine sample was generated

and these plots were then used to determine thermal conductivity, thermal diffusivity, and heat

capacity as outlined by Pouryousefi et al. (2016).

3.13 Pilot Plant Runs

3.13.1 Materials and equipment

The solvents used: butylethanolamine ( 98%), 2-amino-2-methyl-1-propanol ( 99%),

monoethanolamine ( 99%), and methyldiethaolamine ( 99%) were purchased from Sigma-

Aldrich Co., Canada. Titrations were performed using 1 N hydrochloric acid (HCl) which was

also purchased from Sigma-Aldrich.100% CO2. N2 gas was purchased from Praxair Inc., Regina,

Canada. CO2 gas analyzer was purchased from (NOVA analytical system Inc.). Gas analyzer

calibrations were done using 15% CO2 (N2 balance) also acquired from Praxair Inc., Regina,

Canada. HZSM-5 catalyst was purchased from Zibo Yinghe Chemical Company Limited.

Stainless steel LDX Sulzer structured packing with outside diameter of 0.047 m was provided by

Sulzer Chemtech Ltd. Temperatures were recorded using J-type thermocouples from Cole

Parmer, Canada.

3.13.2 Pilot plant continuous flow steady state experiments

A bench scale pilot plant using hot water as the heating medium was employed in this

work. A detailed diagram of the experimental set up is shown in Figure 3.4. The set up basically

consisted of two lagged stainless-steel columns (an absorption column and a desorption column)

each measuring 3.5 ft (1.067 m) in height and having an internal diameter of 2 inches (0.0508

m), a saturator, a hot water heat exchanger, an amine storage tank with capacity of 4L, amine

pumps, lean-rich heat exchanger and flow meters.

The absorption column was designed with 6 gas sampling points at equal intervals of 0.15

m on one side for measurement of the CO2 concentration profile along the column, and six

thermocouple channels located on the exact opposite side of the absorber column for temperature

profile measurement along the column. In between these channels (ports) were installed, 5.08 cm

LDX sulzer structured packings. The desorber column on the other had had a different packing

49

arrangement, to make provision for the catalytic bed which was mixed with smaller inert marbles.

Larger marbles were used as support for the catalytic bed. The whole arrangement is shown in

detail in Figure 3.5.

3.13.3 Typical pilot plant experimental run

At the beginning of each run, an amine solution, with a known concentration and flow

rate, was pumped from the storage tank via the variable-speed gear pump to the top of the

absorber. Meanwhile the hot water bath is switched on and set to the desired setpoint temperature

to generate hot water for rich amine heating prior to entry into the desorber. Once amine solvent

circulation was set, a mixture of CO2 and N2 gas at the appropriate CO2 partial pressure and

concentration of 15% was then introduced to the bottom of the absorber column through the gas

flow meter which controlled the gas flows individually. This allowed for the gas to contact the

down flowing liquid amine in a counter current manner. Treated gas leaves the top of the column

while the rich amine solvent leaves the absorber bottom and is pre heated by the hot lean amine

stream (coming from the bottom of the desorber) before entering the hot water heater and finally

entering the desorber at a temperature of about 87 oC.

Upon contacting the catalytic desorber bed, further desorption is enhanced by the catalyst

bed and the lean amine leaves the bottom of the desorber, is cooled by the condenser and fed into

the absorber column for the cycle to continue. The CO2 product gas at the top of the desorber is

cooled by the condenser to remove any entrained water and is dry when measured by the

rotameter. When the system had reached steady state, both the rich and lean amines samples were

taken for CO2 loading analysis by the titration method. The CO2 concentrations in the gas phase

and temperature profile readings were taken along the height of the column using the IR gas

analyzer and thermocouples, respectively. A mass balance error calculation of the liquid and gas

phase CO2 amounts was done to determine the validity of each run. A value of 10% or less was

considered a valid run. Table 3.1 shows the operating conditions used in the pilot plant

experiments.

50

Figure 3.4: Schematic representation of the experimental set-up for CO2 removal (Srisang et al., 2017)

51

Figure 3.5: Column Packing and catalyst bed arrangement (Srisang et al., 2017)

Absorber Desorber

LDX Sulzer

0.96 m

0.05 m

0.18 m LDX Sulzer

0.55 m Catalytic bed Catalysts + inert

0.025 m marbles

0.05 m

0.18 m LDX Sulzer

0.025 m marbles

52

Table 3.1: Operating conditions used in the pilot plant experiments

Condition Value

Solvent used 5M MEA, 7M MEA/MDEA, 4M BEA/AMP

Solvent flowrate 60 mL/min

Feed Gas flow rate 15 SLPM

CO2 in feed gas 15%

Desorber amine inlet Temperature 87oC

Desorber Catalyst HZSM-5 (Si/Al =19)

Desorber Catalyst weight 150 g

53

3.13.4 Heat duty calculations

The heat duty, q (heat input / CO2 produced) and absorber efficiency were obtained using

the following equations:

Absorber efficiency = * 100% 3.10

q = 3.11

where Gin and Gout are the volumetric flow rates of inlet feed gas and outlet off gas (SLPM); XCO2

in and XCO2 out are the CO2 compositions in the inlet and outlet gas respectively (mol CO2 /mol);

mhw is mass flow rate of heating medium (kg/min); Cphw heat capacity of heating medium

(kJ/kgoC); Thw in and Thw out inlet and outlet temperatures of heating medium (oC) mCO2 mass flow

rate of CO2 produced (kg/min).

54

CHAPTER FOUR: RESULTS AND DISCUSSION OF SCREENING TESTS OF THE

EFFECTS OF THE AMINE CHEMICAL STRUCTURE ON HEIR CARBON DIOXIDE

CAPTURE ACTIVITIES

This chapter presents the results and discussion of the screening tests performed to

evaluate the effect of the chemical structure of the amines on various CO2 capture activities,

namely, initial CO2 absorption rate, initial CO2 desorption rate, pKa, equilibrium solubility, heat

of CO2 absorption, cyclic capacity, and heat duty for amine regeneration. The chemical structures

evaluated in this work were: (i) the existence of a side chain and the number of hydroxyl groups

in the amine molecule, and (ii) the alkyl and/or alkanol chain length in the alkanolamine

molecule.

4.1 Effect of side chain and number of hydroxyl groups in an alkanolamine molecule

4.1.1 Initial CO2 Absorption Rate for Various Amines

The CO2 absorption profiles for all the alkanolamines and primary alkylamines used in

this study are shown in Figure 4.1-1 and Figure 4.1-2, respectively. The initial CO2 absorption

rates calculated from the linear section of these profiles are shown in Figure 4.1-3 as well as in

Table 4.1-1 These results show that for primary alkylamines, the initial absorption rate depended

on the type of side chain, and decreased in the order: isobutylamine (ISO) > secbutylamine (SEC)

> butylamine (BUTYL) > MEA implying that the position of the substituents at the amino group

is important. It is important to note that all the alkylamines had higher rates than MEA due which

can be attributed to the presence of alkyl groups which tend to increase the electron density

around the amine group thereby increasing their reactivity as compared with MEA. This trend

confirmed the results obtained by Singh et al. (2009) who attributed the effect of the position of

alkyl group subtitution on the absorption rate to sterical hindrance. For primary amines, 4-A-1-B

> AMP, for secomdary amines, BEA > tBEA, and for tertiary amines, BDEA > tBDEA (Figure

4.1-4 and Table 4.1-2). This confirms the impact of sterical hindrance on the absorption rate

created by the -carbon substitution as obtained in the work of Singh et al., 2009.

55

Figure 4.1-1: CO2 Absorption Profile for Alkanolamines

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 100 200 300 400 500 600

load

ing,

mol

CO

2/mol

am

ine

Time,min

MEA BEA tBEA BDEA tBDEA BUTYL 4-A-1-B AMP

56

Figure 4.1-2: CO2 Absorption Profile for Alkyl amines and MEA

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 50 100 150 200 250 300

load

ing,

mol

CO

2/mol

am

ine

Time,min

BUTYL SEC ISO MEA

57

Figure 4.1-3: Effect of side chain position on initial CO2 absorption rates of primary alkylamines

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

MEA BUTYL SEC( -carbon position) ISO( -carbon position)

Initi

al C

O2

Abs

orpt

ion

rate

,*10

-2m

ol/l.

min

58

Tabl

e 4.

1-1:

CO

2 act

iviti

es fo

r all

alky

lam

ines

and

MEA

AM

INE

Hea

t dut

y,kJ

/mol

CO

2

deso

rbed

Initi

al C

O2

Abs

orpt

ion

rate

,*10

-2

mol

/l.m

inpk

a

Initi

al C

O2

Des

orpt

ion

rate

*10-2

mol

/l.m

in

Cycl

ic

Capa

city

,mol

CO

2

Equi

libriu

m

load

ing,

mol

C

O2/m

ol

amin

eM

EA45

0.66

1.14

9.33

4.08

0.3

0.55

BUTY

LN

A1.

2410

.66

NA

NA

0.61

SEC

NA

1.3

10.5

3N

AN

A0.

72

ISO

NA

1.52

10.6

NA

NA

0.65

391.

21

1.1

2

9.33

4

.7

0.33

59

Figu

re 4

.1-4

: Ini

tial C

O2 a

bsor

ptio

n ra

te o

f prim

ary,

seco

ndar

y an

d te

rtiar

y al

kano

lam

ines

0

0.2

0.4

0.6

0.81

1.2

1.4

ME

A4-

A-1

-BA

MP

BE

AtB

EA

BD

EA

tBD

EA

Initial CO2 Absorption rate,*10-2 mol/l.min

1o

2o

3o

60

Tabl

e 4.

1-2:

CO

2 act

iviti

es fo

r all

sing

le a

lkan

olam

ines

stud

ied

AM

INE

Hea

t dut

y,kJ

/mol

CO

2

deso

rbed

Hea

t of

abso

rptio

n,kJ

/mol

C

O2 a

bsor

bed

Initi

al C

O2

Abs

orpt

ion

rate

,*10

-2

mol

/l.m

in

pka

Initi

al C

O2

Des

orpt

ion

rate

*10-2

mol

/l.m

in

Cyc

lic

Cap

acity

,mol

C

O2

Equi

libriu

m

load

ing,

mol

C

O2/m

ol a

min

e

MEA

450.

6690

.28

1.14

9.33

4.08

0.3

0.55

4-A

-1-B

835.

7762

.75

1.09

102.

20.

220.

56

AM

P15

2.71

88.8

40.

979.

7812

.04

0.8

0.7

BEA

200.

2979

.94

1.22

10.1

39.

180.

640.

64

tBEA

247.

1388

.71

0.7

10.4

47.

440.

620.

78

BDEA

294.

6649

.109

0.44

8.89

6.24

0.46

0.31

tBD

EA27

4.43

64.1

690.

388.

846.

70.

60.

42

391.

21

90.2

8

1.1

2

9

.33

4

.7

0.33

0

.56

537.

63

72.3

7

1.1

2

10.1

2

3

.42

0

.36

0

.63

200.

29

9.18

0.

72

61

Figure 4.1-5 shows the effect of the number of –OH groups on the initial CO2 absorption

rate of unhindered amines. By comparing the number of –OH groups as in 1°, 2° and 3° amines,

the initial CO2 absorption rate tend to decrease in the order: BUTYL (no –OH) >

butylethanolamine ((BEA), 1 –OH) > BDEA (2 –OH. This trend confirms the results obtained by

Singh who attributed this to the electron withdrawing effect of the -OH group which reduced the

electron density around the amino group and consequently reducing the absorption. It is interesting

to note that the reduction in the initial CO2 absorption rate from BUTYL to BEA was only

marginal. This may be due to the fact that the impact of the -OH group is countered by the presence

of the alkyl group. However, the presence of two -OH groups became sufficiently strong to cause

a drastic drop in the initial CO2 absorption rate for BDEA. From Figure 4.1-6 the impact of the

electron withdrawing effect of the -OH group on the initial CO2 absorption rates of hindered

secondary (tBEA) and tertiary (tBDEA) amines is clearly seen in the trend tBEA > tBDEA.

4.1.2 Viscosity of Dilute Single Solvent Systems and their Effects on Initial CO2 Absorption Rate

As is well known, mass transfer of CO2 in CO2 absorption in amines is influenced by the

viscosity of the medium, as the rate of chemical absorption is not only affected by the kinetics and

the structure of the amine, but also, by mass transfer. We decided to further explain the absorption

rates obtained for the studied alkanolamines on the basis of possible mass transfer limitations due

to viscosity. Since it is well established that mass transfer is influenced by the viscosity of the

medium, we decided to measure this property for all the alkanolamine solvent systems. The

viscosity results are given in Table 4.1-3. The slow CO2 absorption rate of sterically hindered

amines as compared with their straight chain analogues is caused primarily by the sterical

hindrance with possible modification by mass transfer limitation due to higher viscosity.

62

Figure 4.1-5: Effect of number of hydroxyl group substituents on the initial CO2 absorption rate of 1o, 2o and 3o unhindered amines

0

0.2

0.4

0.6

0.8

1

1.2

1.4

BUTYL(no -OH group) BEA(1 -OH group) BDEA(2 -OH groups)

Initi

al C

O2

Abs

orpt

ion

rate

,*10

-2m

ol/l.

min

63

Figure 4.1-6: Comparison of the initial CO2 absorption rates of 1o, 2o and 3o hindered amines

64

Table 4.1-3: Viscosity (mPa.s) data for primary, secondary and tertiary alkanolamines @ 2M and 40oC

AMINE Viscosity, mPa.s Viscosity, mPa.s

Primary Amines without CO2 with CO2

4-A-1-B 1.156 1.6

AMP 1.234 1.68

MEA 0.923 1.113

Secondary Amines without CO2 with CO2

BEA 1.702 2.047

tBEA 1.809 2.526

Tertiary Amines without CO2 with CO2

BDEA 2.092 2.879

tBDEA 2.358 3.063

65

4.1.3 Acid Dissociation Constant (pKa)

The dissociation constant (pka) shows the basic strength of the amine and their reactivity

towards CO2. Tables 4.1-1, 4.1-2 and 4.1-4 show the pKa data for all the amines studied. The

tables show the effect of the alkyl and -OH substitutions at the amine group on the pKa value. The

alkyl group has an electron donating effect which increases their basicity. Consequently,

alkylamines have higher pKa values than alkanolamines which suffer from an electron

withdrawing effect from their –OH groups. Also, the number of –OH groups as in BUTYL, BEA

and BDEA increases the electron withdrawing effect of the –OH group which lowers the basicity

and consequently, the pKa in that order as shown in Figure 4.1-7. A similar trend in pKa was

observed from 2o to 3o hindered amine (tBEA > tBDEA) as shown in Figure 4.1-8. BEA and MEA

each have one –OH group and should suffer the electron withdrawing effect of just one –OH group.

However, for BEA, the contribution of the electron donating effect from the alkyl group is higher

and hence appears to counter the effect of the –OH group. This can also be explained in terms of

the distance of the amino group from the –OH group. BEA is further away from the –OH group

than MEA, and therefore has a higher pKa. The pKa results for primary alkyl amines are shown in

Figure 4.1-9. From these results it can be seen that the pKa order was BUTYL > ISO > SEC. It

shows that the substitution of a methyl group at the -carbon atom next to the nitrogen group

weakens the N-H bond thereby making it a weaker base as reported by Chakraborty et al. (1988).

As such, we can see a significant reduction in the pKa value of SEC ( -position) than ISO ( -

position) when compared to that of BUTYL. The same effect is also seen in sterically hindered

primary and tertiary alkanolamines where 4-A-1-B > AMP and BDEA > tBDEA, respectively

(Figure 4.1-8). This is consistent with the higher reactivity towards CO2 by the higher basic

strength of the non-sterically hindered amines. The sterically hindered amines except tBEA have

lower pKa values, and consequently, relatively lower absorption rates than their analogue

unhindered straight chain amines. Interestingly for secondary amines, this trend is not seen as the

pKa value for tBEA > BEA.

66

Table 4.1-4: Comparison of experimental pKa values with literature values

AMINE pKa @ 25oC (this work)

pKa @ 25oC (literature)

Reference

MEA 9.33 9.45 Rayer et al., 2014

BUTYL 10.66 10.6 Perin, 1972

SEC 10.53 10.56 Perin, 1972

ISO 10.6 10.68 Perin, 1972

4-A-1-B 10.12 10.32 Perinu et al., 2014

AMP 9.78 9.78 Da Silva and Svendson, 2007

BEA 10.13 10 Chemicalize., 2010b

tBEA 10.44 10.29 Little et al., 1990

BDEA 8.89 8.9 Rayer et al., 2014

tBDEA 8.84 9.03 Rayer et al., 2014

67

Figure 4.1-7: Effect of number of hydroxyl group substituents on the pKa of 1o, 2o and 3o amines

8

8.5

9

9.5

10

10.5

11

MEA BUTYL(no -OHgroup)

BEA(1 -OHgroup)

BDEA(2 -OHgroups)

pKa

68

Figure 4.1-8: Comparison of the pKa of primary, secondary and tertiary alkanolamines

8

8.5

9

9.5

10

10.5

11

MEA 4-A-1-B AMP BEA tBEA BDEA tBDEA

pKa

69

Figure 4.1-9: Effect of side chain position on pKa of primary alkylamines

8.5

9

9.5

10

10.5

11

MEA BUTYL SEC( -carbon position)

ISO( -carbon position)

pKa

70

4.1.4 Equilibrium CO2 Solubility

The equilibrium CO2 solubility or CO2 absorption capacity results of the amines studied

are shown in Figures 4.1-10 and 4.1-11. The CO2 partial pressure-equilibrium loading relationships

are also shown in Figures 4.1-12 and 4.1-13. Figure 4.1-10 shows that the CO2 absorption

capacities for primary alkylamines increased in the order: BUTYL < ISO < SEC. The position of

the alkyl group strongly affects the solubility. The highest loading was observed in SEC due to

significant impact of steric hindrance at the -carbon position causing carbamate instability and

thus increasing bicarbonate formation due to carbamate hydrolysis resulting in higher loadings

(Satori and Savage, 1983). However, the level of sterical hindrance at the -position leads to only

marginal increase in the loading. For primary, secondary and tertiary alkanolamines, the same

trend followed and the order observed were: AMP > 4-A-1-B (for primary), tBEA > BEA (for

secondary) and tBDEA > BDEA (for tertiary) as shown in Figure 4.1-11. The same explanation

for SEC applies to this trend since all the substituent is located at the -carbon position.

Clearly, the level of sterical hindrance increases from primary hindered amine

(AMP) to secondary hindered amine which is reflected in their equilibrium CO2 solubility values

(tBEA > AMP). The number of hydroxyl group substituents increases from tBEA to tBDEA

reducing their reactivity and resulting in less absorption of CO2, and consequently tBEA > tBDEA.

4.1.5 CO2 Desorption Rate

Although sterical hindrance reduces the initial CO2 absorption rates, the literature appears

to suggest that CO2 desorption is favored significantly due to this effect. Due to carbamate

instability created by steric hindrance effect, the breakdown of this intermediates species to form

bicarbonate is enhanced. This leads to a higher concentration of bicarbonate in the system resulting

in higher and faster release of CO2 during desorption (Singh et al., 2008). This is confirmed from

the results obtained for primary and secondary amines. See Table 4.1-2 and Figure 4.1-14. For

secondary amines on the other hand, the reverse is seen, that is: BEA > tBEA.

71

Figure 4.1-10: Effect of side chain position on equilibrium CO2 solubility of primary alkylamines at 15% CO2 partial pressure @ 40oC

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

MEA BUTYL SEC( -carbon position)

ISO( -carbon position)

Equ

ilibr

ium

load

ing,

mol

CO

2/mol

am

ine

72

Figure 4.1-11: Comparison of the equilibrium CO2 solubility of primary, secondary and tertiary alkanolamines @ 40oC and 15% CO2 partial pressure

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Equ

ilibr

ium

load

ing,

mol

CO

2/mol

am

ine

73

Figure 4.1-12: Equilibrium solubility of butylamines at different partial pressures and temperatures (A: secButylamine (SEC); B: Butylamine (BUTYL); C: isoButylamine(ISO)).

74

Figure 4.1-13: Equilibrium solubility of alkanolamines at different partial pressures and temperatures (A:2M MEA; B: 5M MEA; C: 4-A-1-B; D: AMP; E: tBEA; F: BEA; G: BDEA; H: tBDEA)

020406080

100120140

0.38 0.58 0.78

CO

2pa

rtia

l pre

ssur

e,kP

a

loading,mol CO2/mol amine

T=40C

T=90C

T=50C

T=70C

75

Figure 4.1-14: Comparison of the initial CO2 desorption rates of primary, secondary and tertiary alkanolamines

0

2

4

6

8

10

12

14


Initi

al D

esor

ptio

n R

ate,

*10^

(-2)

mol

/l.m

in

76

Desorption is a complex reaction involving so many factors. As such, not only sterical

hindrance, but also, other factors like amine basicity contribute to the overall desorption

performance. In desorption, the following reaction mechanisms take place:

AmineH+ deprotonation:

BEAH+ + H2O BEA + H3O+ 4.1

Carbamate breakdown involves:

1. Proton transfer: BEA-COO- + H3O+ BEA-H+-COO-(zwitterion) + H2O 4.2

2. N-C bond breaking: BEA-H+-COO- (zwitterion) BEA + CO2 4.3

Reaction 9 is a critical reaction whose speed depends on the energy involved in transferring

the proton to water. The stronger the base the more the energy needed for the proton transfer.

Conversely, the weaker the base the easier it is for deprotonation to occur. This explains why BEA

being a relatively weaker base than tBEA (Fig. 4.1-9) provides faster desorption for CO2. Another

factor that may be considered in the desorption of CO2 from an amine is its propensity and quantity

of bicarbonate formation. In the case of the secondary amines studied, it is expected that tBEA, a

sterically hindered amine will produce more bicarbonate than BEA. However, the experimental

results on the initial rate of CO2 desorption of BEA as compared with tBEA show that the

contribution from lower basicity of BEA may be stronger than the contribution from sterical

hindrance of tBEA. The alkylamines studied in this work had boiling points in the range of 77-79 oC (for BUTYL), 67-69 oC (for ISO) and 63 oC (for SEC). As such, these amines were not tested

for this activity since regenerating low boiling point amines at temperatures higher than their

boiling points will result in highly significant amine vapor losses. By comparing hindered

alkanolamines in the primary, secondary and tertiary categories, it was found that desorption rates

decreased in the order AMP > tBEA > tBDEA (Figure 4.1-15). However, it is known that CO2

desorption does not only depend on the kinetics aspect, but also, on mass transfer as well, which

is related to viscosity. The results from Table 4.1-3 shows that there is an increase in the mass

transfer limitation by going from primary to tertiary hindered amines (tBDEA > tBEA > AMP),

resulting in a relatively slower desorption rate. This implies that there is increased difficulty for

77

desorbed CO2 to escape from the liquid phase into the gas phase. The CO2 desorption profile for

all alkanolamines studied are illustrated in Figure 4.1-16.

78

Figure 4.1-15: Comparison of the initial CO2 desorption rates of 1o, 2o and 3o hindered amines

79

Figure 4.1-16: CO2 Desorption Profile for Alkanolamines

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 20 40 60 80 100

load

ing,

mol

CO

2/mol

am

ine

Time,minMEA BDEA tBDEA BEAtBEA 4-A-1-B AMP

80

4.1.6 Heat Duty for Solvent Regeneration

The heat duty (Qreg) for the primary, secondary and tertiary alkanolamines studied are given

in Table 4.1-2 as well as in Figure 4.1-17. The figure shows that heat duty followed the order:

AMP < MEA < 4-A-1-B (for primary), BEA < tBEA (for secondary) and tBDEA < BDEA (for

tertiary). This can be explained on the basis that for the same amount of energy input, the amines

with the faster desorption rates release more CO2 within the same period of time thereby

consuming less energy per unit of CO2 desorbed. This behavior can be alluded to sterical hindrance

which destabilizes the carbamate, enhancing their hydrolysis into bicarbonate and facilitating easy

desorption thus requiring relatively less energy input than the analogue unhindered amines. For

unhindered primary, secondary and tertiary amines the order was 4-A-1-B > BDEA > BEA. For

hindered amines, the trend was tBDEA > tBEA > AMP (Figure 4.1-17). These latter trends did

not follow the normal order 1o > 2o > 3o for heat duty. The different variations on the structures of

these amines may account for their modifications in their CO2 capture performances.

4.1.7 Cyclic Capacity

The cyclic capacity of an amine represents the capacity to capture CO2 per mole of amine

per amine cycle through the capture unit. The cyclic capacity values for the alkanolamines studied

are given in Table 4.1-2 and Figure 4.1-18. It is seen that a higher cyclic capacity is seen for amines

with higher desorption rates, that is hindered amines > unhindered amines except for secondary

amines. The trend followed was in the order: AMP > 4-A-1-B (for 1o amines), BEA > tBEA (for

2o amines) and tBDEA > BDEA (for 3o amines). Unhindered amines followed the order: 4-A-1-B

< BDEA < BEA. For hindered amines, the trend was: tBDEA < tBEA < AMP

4.1.8 Heat of CO2 Absorption

4.1.8.1 Validation of the New Procedure for Determination of Heat of Absorption

The equilibrium solubility data obtained in this work for 5M MEA at different temperatures

were used to validate the developed procedure by comparing the results with that obtained in the

literature based on the calorimeter method.

81

Figure 4.1-17: Heat Duties of primary, secondary and tertiary alkanolamines

0

100

200

300

400

500

600


Hea

t Dut

y,kJ

/mol

CO

2de

sorb

ed

82

Figure 4.1-18: Cyclic Capacities of primary, secondary and tertiary alkanolamines

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Cyc

lic C

apac

ity,

mol

CO

2/Lso

ltn

83

In the literature, the method of using the exponential type of equation (and indirectly the

natural logarithmic function) or other functions to fit the CO2 partial pressure and loading

relationship has been reported (Xu et al., 2011; Arshad et al., 2014). Even though this type of

relationship has been used in the literature (Xu et al., 2011; Arshad et al., 2014) on the Gibbs

Helmhotz equation to estimate the heat of absorption for a specified loading, the focus on those

papers appeared to be on getting a fitting correlation (Xu et al., 2011; Arshad et al., 2014; Bougie

and Iliuta, 2012; Hwang et al., 2016; Park et al., 2002) rather than the heat of absorption

calculation. Consequently, other than by calorimetry, there are publications in the literature (Liu

et al, 2014; 2016; Singto et al. 2016; Liang et al., 2015; Zhang et al., 2016) which still use the trial

and error approach for the heat of absorption calculation. That is why we intentionally use the

simple but comprehensive “line of best fit” method which is not restrictive in terms of the partial

pressure - equilibrium loading relationship type or range. In our method, it is required that accurate

experimental data be obtained in order to produce a good line of best fit with a degree of correlation

> 93% as shown in Table 4.1-5 (which gives the lines of best fit equations of CO2 partial pressure-

equilibrium loading relationships (obtained from Figures 4.1-13) and their corresponding degrees

of correlation of the various amines studied at different temperatures). This contribution is

therefore not focused on introducing a new correlation, but has the important objective to highlight

that, if accurate experimental data are obtained, there is no need to use a trial and error approach

in coming up with a constant loading for partial pressures at different temperatures. The description

of the new procedure is illustrated in Figures 4.1-19 to 4.1-21.

The extracted lnPCO2 and 1/T at 0.45 CO2 loading are given in Table 4.1-6. The CO2 partial

pressures shown in Table 4.1-6, were obtained by substituting the selected CO2 loading values into

the equations at the respective temperatures to obtain the corresponding PCO2 values. A plot of ln

PCO2 versus 1/T was generated and used to obtain the heat of absorption. These values are shown

in Table 4.1-7 which compares very well with literature values (Bruder et al., 2011). As shown in

Table 4.1-7, the results obtained using the new procedure developed in this work is consistent with

those obtained using the calorimeter method thereby validating the new procedure for use with the

Gibbs-Helmhotz equation for estimating the heat of absorption.

84

Table 4.1-5: The equations of lines of best fit for CO2 partial pressure-equilibrium loading relationships and their corresponding degrees of correlation for various solvents at different temperatures:

A: Primary Amines

Solvent

Temperature, oC Equation of line of best fit Degree of Correlation, R2

5M MEA 40 y = 7E-07e31.686x 0.99

50 y = 4E-07e37.376x 0.98

60 y = 4E-06e34.108x 0.99

80 y = 0.009e19.315x 0.97

90 y = 0.212e14.636x 1.00

2M MEA 40 y = 0.0004e17.668x 0.95

60 y = 0.0001e22.503x 0.96

80 y = 0.0128e17.164x 0.97

90 y = 0.8297e9.2663x 0.97

2M 4-A-1-B 40 y = 0.0018e14.85x 1.00

50 y = 0.0019e15.802x 0.97

70 y = 2E-05e27.142x 1.00

90 y = 0.0022e20.676x 0.97

2M AMP 40 y = 0.0026e12.269x 0.95

60 y = 0.1781e8.2066x 0.94

90 y = 1.2158e8.7681x 0.93

Note: y is CO2 partial pressure, x is equilibrium loading

85

B: Secondary Amines

2M BEA 40 y = 0.0105e11.254x 0.97

50 y = 0.0349e10.873x 0.97

60 y = 0.0196e13.693x 1.00

90 y = 1.9836e10.696x 0.99

2M tBEA 40 y = 0.0042e10.644x 0.99

50 y = 0.0036e11.792x 0.98

60 y = 0.1059e8.2164x 0.99

80 y = 0.3446e9.3459x 1.00

90 y = 1.6911e8.0394x 0.99

C: Tertiary Amines

2M BDEA 40 y = 3.8083e4.2063x 0.99

50 y = 4.1457e6.4014x 0.96

60 y = 4.3859e9.1737x 0.97

90 y = 3.173e43.027x 0.99

2M tBDEA 40 y = 1.1539e5.7647x 1.00

50 y = 2.3585e6.687x 0.95

60 y = 4.1844e6.6683x 0.93

90 y = 2.5303e31.312x 0.97

86

Figure 4.1-19: Equilibrium Solubility data of 5M MEA at 313,323,333,353 and 363K

T=40C, y = 7E 07e31.686xR² = 0.9869

T=90C, y = 0.212e14.636xR² = 0.9995

y = 4E 07e37.376xR² = 0.9778

T=60C, y = 4E 06e34.108xR² = 0.9907

T=80C, y = 0.009e19.315xR² = 0.9686

0

20

40

60

80

100

120

0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

CO

2pa

rtia

l pre

ssur

e

CO2 loading, mol CO2/mol amine

87

Figure 4.1-20: Plot of lnPCO2 and 1/T at 0.40 CO2 loading

88

Figure 4.1-21: Plot of lnPCO2 and 1/T at 0.45 CO2 loading

89

Table 4.1-6: Partial pressure and corresponding temperatures obtained at selected loadings

Selected loading, molCO2/mol amine

CO2Partial pressure,kPa ln PCO2 T, K 1/T

0.45 8.06 2.09 323 0.0031

0.45 18.53 2.92 333 0.0030

0.45 53.58 3.98 353 0.0028

0.4 1.24 0.22 323 0.0031

0.4 3.37 1.21 333 0.0030

0.4 20.40 3.02 353 0.0028

90

Table 4.1-7: Comparison of heat of absorption of 5M MEA at specific loadings with literature

Specific loading, mol CO2/mol amine

Habs, kJ/mol CO2 absorbed

Source

0.4 88.34 This work

0.4 84.70 Bruder et al., 2011

0.45 58.863 This work

0.5 58.51 Bruder et al., 2011

91

4.1.8.2 Heat of Absorption for alkanolamines

The heat of CO2 absorption is the heat released during the reaction between amine and

CO2. The heat released during this reaction drops gradually as the reaction proceeds. However, at

equilibrium and after, the amount of heat drops drastically as the driving force is almost zero at

this point. The heat of absorption is a function of temperature and CO2 loading (Kim and Svendsen,

2007). Also, not only is the heat of absorption dependent on these two factors, but also, it is a

strong function of the CO2 partial pressure, and most importantly, the amine structure. To make a

fair comparison of the heat of absorption values of the amines studied, the loading selected was at

halfway the equilibrium solubility, which is an approximate average value of the heat of absorption

of each amine at 15% CO2 and at the absorption temperature of 40oC. In addition, this loading was

selected at the absorption temperature and the typical flue gas composition of CO2 in order to have

this under practical conditions used in industry. From the results (Fig. 4.1-22), it can be observed

that all the side chain (hindered) amines had higher heat of CO2 absorption values than the straight

chain (unhindered) amines. That is: AMP > 4-A-1-B for primary alkanolamines, tBEA > BEA

(secondary amines) and tBDEA > BDEA (tertiary amines). This might be attributed to the higher

amount of CO2 absorbed in hindered amines with a corresponding larger amount of heat produced

as compared to their straight chain analogues.

By comparing primary, secondary and tertiary hindered amines, the general trend observed

was: AMP > tBEA > tBDEA. The decrease from AMP to tBEA was however insignificant or only

marginal. The trend obtained can be explained by the fact that carbamate formation is seen in

primary and secondary amines. Tertiary amines on the other hand produce little or no carbamate

but bicarbonate. The release of energy during carbamate formation is higher than that in

bicarbonate formation hence tertiary amines tend to have a relatively lower heat of absorption than

primary and secondary. For straight chain amines, BEA > 4-A-1-B > BDEA. This does not follow

the regular trend that 1o > 2o. The reason might be due to the same reason suggested by Yang et al.

(2012) in their work that other conditions might also be contributing.

92

Figure 4.1-22: Heat of CO2 absorption values for 1o, 2o and 3o alkanolamines

0

10

20

30

40

50

60

70

80

90

100


Hea

t of A

bsor

ptio

n,

kJ/m

ol C

O2

abso

rbed

93

4.1.9 Correlations between Different Activities

4.1.9.1 Rate of CO2 Absorption versus Heat of CO2 Absorption

It has been generally accepted in the literature that the heat of CO2 absorption in

alkanolamines has a linear relationship with the CO2 absorption rate. However, no studies have

been reported to show if the structure of the amine has any effect on this relationship. In this work,

we tested this structure – linearity concept of CO2 absorption rate in alkanolamines versus heat of

absorption using MEA, 4-A-1-B, AMP, BEA, tBEA, BDEA, and tBDEA, which have widely

different structural characteristics. Consequently, a graph of the Initial Absorption Rate was

plotted against the Heat of Absorption. The results are shown in Figure 4.1-23. As can be seen

from the figure, it does not appear that there is a linear correlation between the heat of absorption

and the absorption rate. This implies that not all amines with high heat of reaction will necessarily

be fast in absorbing CO2. The absence of linearity can be explained on the basis of an energy

diagram shown in Figure 4.1-24.

As shown in the figure, the heat of reaction is defined as the energy difference between the

reactant ground state and the product ground state. As can be seen in the diagram, the heat of

reaction depends on the reactant and product state. This implies that the heat of reaction is a state

function and not a path function. For any reaction to take place reactants should be able to have an

energy greater than the activation energy in order to overcome the activation complex to form

products. This highly depends on the pathway implying that the higher the Ea assuming the

frequency factor is constant), the more difficult reactants will proceed to form products. It is

important to note that the contributing factors to the rate of reaction are the frequency factor and

the Ea (Equation 4.4).

- 4.4

where ko is the frequency factor, and Ea is the activation energy, kJ/mol, while a and b are

respectively the orders of reactions with respect to reactant A and B. When the rate of reaction is

altered by lowering the activation energy, Ea, by changing the pathway such as using a catalyst or

invoking the structure of the amine, the heat of reaction will still remain unaffected as shown in

Figures 4.1-24. This explains why the relationship between heat of absorption and reaction rate

may not be linear.

94

Figure 4.1-23: Absorption Rate versus Heat of absorption

0

0.2

0.4

0.6

0.8

1

1.2

1.4

40 50 60 70 80 90 100

InitialAb

sorptio

nRa

te,*10

2 mol/l.m

in

Heat of Absorption,kJ/mol CO2 absorbed

MEA

AMP

BEA

tBEA

BDEA

tBDEA

4 A 1 B

95

Figure 4.1-24: Reaction Progress with and without catalyst

Source: https://en.wikipedia.org/wiki/File:Activation_energy.svg

96

4.1.9.2 Heat Duty for Solvent Regeneration Versus Heat of CO2 Absorption

It has also been generally accepted in the literature that the heat of CO2 absorption in

alkanolamines has a linear relationship with the heat duty for solvent regeneration. Also, no studies

have been reported to show if the structure of the amine has any effect on this relationship. In this

work, we tested this structure – linearity concept for heat of CO2 absorption in alkanolamines

versus heat duty for solvent regeneration using MEA, 4-A-1-B, AMP, BEA, tBEA, BDEA, and

tBDEA, which have widely different structural characteristics. A graph of Heat duty versus heat

of absorption was also generated to determine if such a linear relationship existed for the amines

studied. The plot is shown in Figure 4.1-25. Based on the figure, there appears to be no linear

relationship between the heat of absorption and the heat duty. From the results, it appears that

some of the amines with higher heat of absorption such as AMP rather have a relatively low heat

duty. In particular, the hindered amines have a higher heat of absorption than the straight chain

amines but have lower heat duty than their analogue unhindered amines. It may not be very

accurate to anticipate that amines with higher heat of absorption will always have higher heat

duties as the different structural properties modify their CO2 performance behavior in different

regards. Oexmann and Kather (2010) stated that the focus on low heat of absorption solvents

without considering the process in its entirety is inadequate to determine the overall energy

performance of solvents. From their work, a process parameter specifically the desorber pressure

was found to affect the heat duty. Our work shows that, in addition to the process parameters, the

chemical structure of the amine affects the energy of regeneration.

97

Figure 4.1-25: Heat Duty for Solvent Regeneration versus Heat of CO2 Absorption

0

100

200

300

400

500

600

40 50 60 70 80 90 100

Heat

Duty,kJ/molCO

2de

sorbed

Heat of Absorption,kJ/mol CO2 absorbed

MEA

4 A 1 B

AMP

BEA

tBEA

BDEA

tBDEA

98

4.2 Effect of alkyl and alkanol chain length of alkanolamines

4.2.1 Acid Dissociation Constant (pKa)

The absorption of an acid gas such as CO2 in amines is dependent on the alkalinity of the

amine. This property for amines can be measured by the acid dissociation constant of the amine.

The pKa results for all the amines studied are given in Table 4.2-1, which gives a summary of all

the results obtained for all activities studied.

4.2.1.1 Primary Alkanolamines

The primary alkanolamines studied were: monoethanolamine (MEA) or 2-amino-1-ethanol

(2-A-1-E), 3-amino-1-propanol (3-A-1-P), 4-amino-1-butanol (4-A-1-B), and 5-amino-1-pentanol

(5-A-1-P). The effect of the alkanol chain length on their pKa values are illustrated in Figure 4.2-

1. In the figure, it is seen that pKa increases as the alkanol chain length increases. The literature

(Singh, 2011) shows that the introduction of an OH-, which is an electron withdrawing group, close

to the amino group reduces the alkalinity of the amine. Therefore, an increase in the alkanol chain

length increases the distance between the amino group and the OH- group, and consequently

reduces the influence of the OH- group, thereby resulting in an increase in pKa. From the results

we can see a significant increase of about 7.5% from MEA to 3-A-1-P. Further increase in alkanol

chain length resulted in only a marginal increase in the pka. This trend is consistent with the results

obtained by Singh (2011), whose work showed an increase of about 8% from MEA to 3-A-1-P but

a relatively lower increase after further addition of - CH2. The pKa results obtained in this work

are compared and validated with the literature data. This is shown in Table 4.2-2.

4.2.1.2 Secondary Alkanolamines

In the case of secondary amines, the alkanol chain length from the amino group to the OH-

group was fixed (i.e. to ethanol length), but instead, the alkyl chain length as a substitute to one of

the H atoms of the primary corresponding primary amine (MEA) was varied. The amines studied

were methylmonoethanolamine (MMEA), ethylmonoethanolamine (EMEA),

propylmonoethanolamine (PMEA) and butylmonoethanolamine (BEA). MEA was included here

for this study to illustrate the effect of retaining the H atom as compared with replacing it with an

alkyl group. Their pKa results are given in Figure 4.2-2.

99

Table 4.2-1: Effect of chain length on CO2 absorption–desorption performance of primary, secondary and tertiary amines

Amine Initial Absorption rate, *10-2 mol/l. min

Equilibrium loading, mol

CO2/mol amine

pKa

Heat duty,

kJ/mol CO2

Initial Desorption rate, *10-2 mol/l.min

Cyclic Capacity,

mol CO2/L.soltn

Primary Amines

MEA 1.12 0.55 9.33 391.21 4.7 0.33 3-A-1-P 1.14 0.58 10.03 652.02 2.82 0.26 4-A-1-B 1.12 0.63 10.12 537.63 3.42 0.36 5-A-1-P 1.17 0.63 10.25 464.31 3.96 0.36

Secondary Amines MMEA 1.24 0.61 9.98 322.01 5.71 0.42 EMEA 1.26 0.63 9.99 297.04 6.19 0.54 PMEA 1.18 0.63 9.94 265.71 6.92 0.54 BEA 1.2 0.65 10.13 200.29 9.18 0.72

Tertiary Amines MDEA 0.42 0.41 8.67 331.29 5.55 0.56 EDEA 0.66 0.50 8.87 220.10 8.32 0.72 BDEA 0.46 0.32 8.89 294.66 6.24 0.46

100

Table 4.2-2: Validation of pKa values

Amine pKa (this work) pKa (Singh, 2011) % Deviation

MEA 9.33 9.16 1.86

3-A-1-P 10.03 9.91 1.21

4-A-1-B 10.12 10.32 1.94

101

Figure 4.2-1: Effect of alkanol chain length of primary alkanolamines on their pKa values (2 is 2-Amino-1-ethanol (MEA); 3 is 3-Amino-1-propanol; 4 is 4-Amino-1-butanol)

9.2

9.4

9.6

9.8

10

10.2

10.4

2 3 4 5 6

pKa

No of C atoms in the primary amino alcohol chain length groups

102

Figure 4.2-2: Effect of alkyl chain length in secondary alkanolamines on their pKa values (0 is MEA; 1 is MMEA; 2 is EMEA; 3 is PMEA; 4 is BEA)

9.2

9.3

9.4

9.5

9.6

9.7

9.8

9.9

10

10.1

10.2

0 1 2 3 4 5

pKa

No of C atoms in the secondary alkyl substituent chain length groups

103

The figure shows that the replacement of one of the H’s in MEA with an alkyl group

resulted in a big increase in pKa from 9.33 in MEA to 9.98 in MMEA. The literature (Singto et

al., 2016) indicates that the substitution of an alkyl group in place of the H in the alkanolamine

implies the introduction of an electron donating group, and consequently an increase in the pKa.

We see a marginal increase, (though considered a significant trend) in pka with chain length. This

trend is consistent with the work of Singh (2011). They studied the effect of alkyl chain length in

primary alkyl amines. Their results showed only a marginal increase in the pKa. This marginal

increase, affirms the assertion that the alkyl group is known to have just a weak electron donating

(+I) effect.

4.2.1.3 Tertiary Alkanolamines

In the case of tertiary amines, the alkanol chain length from the amino group to the OH-

group was also fixed (i.e. to ethanol length), but instead, the alkyl chain length as a substitute to

one of the H atoms of the primary corresponding primary amine (MEA) was varied. In the tertiary

amines, we also substituted the remaining H (as in secondary amines) with an ethanol group. The

amines studied were: methyldiethanolamine (MDEA), ethyldiethanolamine (EDEA) and

butyldiethanolamine (BDEA). MEA and MMEA were included in this group for comparison

aimed at illustrating the effect of two ethanol groups versus one ethanol group (for the specific

situations in primary and secondary alkanolamines). Their pKa results are given in Figure 4.2-3.

In the figure, it is seen that the introduction of two OH- groups relative to both MEA and MMEA

(each with one OH- group) drastically reduces the pKa. On the other hand, the trend obtained

within the tertiary amines shows that, an increase in the chain length of the substituted alkyl group

results in an increase in pKa, though marginal. This can be attributed to the electron donation

characteristics of the substituted alkyl group which are enhanced with chain length (Singh, 2011).

4.2.2 Equilibrium CO2 Solubility

The equilibrium CO2 solubility gives an indication of the capacity of the amine to hold

CO2. A large equilibrium CO2 solubility is desirable for an amine used for post-combustion CO2

capture. In this work, equilibrium CO2 solubility was studied as a function of alkanol chain length

for primary alkanolamines, and alkyl chain length for secondary and tertiary alkanolamines.

104


The equilibrium CO2 solubility results for the primary alkanolamines as a function of the

alkanol chain length are shown in Figure 4.2-4. It is seen that equilibrium CO2 solubility increases

with alkanol chain length. The results for equilibrium CO2 solubility mirrors the results for pKa to

some extent, and the same reasons adduced for pKa are applicable to equilibrium CO2 solubility.

This is consistent with the literature (Tomizaki et al., 2008) which shows that, all things being

equal, equilibrium CO2 solubility is a good reflection of pKa, and consequently of the alkalinity.


The equilibrium CO2 solubility results for the secondary alkanolamines as a function of the

substituted alkyl chain length are shown in Figure 4.2-5. The equilibrium CO2 solubility result for

MEA is also included for comparison. It is seen in the figure that the equilibrium CO2 solubility

increases with the alkyl chain length. The figure also shows a larger equilibrium CO2 solubility

value for MMEA (with a substituted methyl group) relative to MEA (with no substituted alkyl

group). The results for equilibrium CO2 solubility mirrors the results for pKa and the same reasons

adduced for pKa are also applicable to equilibrium CO2 solubility. Again, this is consistent with

the work of Tomizaki et al. (2008) which shows that, all things being equal, equilibrium CO2

solubility is a good reflection of pKa, and consequently of the alkalinity.


The equilibrium CO2 solubility results for the tertiary alkanolamines studied as a function of the

alkyl chain length are shown in Figure 4.2-6. The equilibrium CO2 solubility results for MEA and

MMEA are also included for comparison. The figure shows a larger equilibrium CO2 solubility

for MMEA (with a substituted methyl group) relative to MEA (with no substituted alkyl group).

However, the equilibrium CO2 solubility results for the tertiary alkanolamines (two OH- groups)

are smaller relative to MEA and MMEA (with one OH- group).

105

Figure 4.2-3: Effect of alkyl chain length in tertiary alkanolamines on their pKa values (MEA and MMEA included for comparison)

8

8.5

9

9.5

10

10.5

pKa

106

Figure 4.2-4: Equilibrium CO2 loading for primary alkanolamines

0.5

0.52

0.54

0.56

0.58

0.6

0.62

0.64

2 3 4 5

CO

2E

quili

briu

m lo

adin

g,m

ol C

O2

/mol

am

ine


107

It is also seen in the figure that the equilibrium CO2 solubility increases with the alkyl chain length

from MDEA (0.41 mol CO2/mol amine) to EDEA (0.51 mol CO2/mol amine). Further increase in

the alkyl chain length to BDEA resulted in a decrease in equilibrium CO2 solubility (0.32 mol

CO2/mol amine). This anomaly can be attributed to the increased viscosity for BDEA relative to

MDEA and EDEA as shown in Table 4.2-3. The increased viscosity of BDEA creates a mass

transfer limitation for absorption of CO2 into the solvent resulting in reduced equilibrium CO2

solubility, thus masking the true effect of the inherent structural electron donating effect of the

longer alkyl group. Except for BDEA, these results for equilibrium CO2 solubility mirrors the

results for pKa and the same reasons adduced for pKa are applicable to equilibrium CO2 solubility.

This is consistent with the literature (Tomizaki et al., 2008) which shows that, all things being

equal, equilibrium CO2 solubility, is a good reflection of pKa, and consequently of the alkalinity.

The mass transfer limitation created by viscosity in the BDEA anomaly is consistent with the trend

obtained by Liu et al. (2017) whose work showed that mass transfer limitation sets in when

viscosity is increased thus resulting in a lower equilibrium loading.

4.2.3 Initial Rate of CO2 Absorption

The absorption profiles for the primary, secondary and tertiary alkanolamines studied in this

work are shown in Figures 4.2-7a, b and c, respectively. The initial absorption rate for each amine

was taken as the slope of the initial linear portion of the respective absorption profile of the amine.


The initial CO2 absorption rates for primary alkanolamines as a function of the alkanol chain

length are given in Figure 4.2-8. In the figure, it is seen that there is an overall increase in the initial

CO2 absorption rate from MEA to 5-A-1-P. However, the initial rate for 4-A-1-B decreased, which

is an anomaly. The increasing distance of the amino group from the -OH group reduces the

influence of the electron withdrawing effect of the -OH group thereby increasing the absorption

rate

108

Figure 4.2-5: Equilibrium CO2 loading for secondary alkanolamines and MEA

0.5

0.55

0.6

0.65

0.7

0 1 2 3 4 5

CO

2eq

uilib

rium

load

ing,

m

ol C

O2/m

ol a

min

e

No of C atoms in the secondary alkyl chain length groups

109

Figure 4.2-6: Equilibrium CO2 loading for tertiary alkanolamines, MEA and MMEA

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7E

quili

briu

m lo

adin

g,

mol

CO

2/mol

am

ine

110

Table 4.2-3: Viscosities of primary, secondary and tertiary alkanolamines at 40oC

Amine Viscosity at equilibrium

loading, mPa.s

Amine Viscosity at equilibrium

loading, mPa.s

Amine Viscosity at equilibrium loading, mPa.s

Primary Amines

Secondary Amines

Tertiary Amines

MEA 1.018

MMEA 1.212

MDEA 1.475

3-A-1-P 1.255

EMEA 1.492

EDEA 1.834

4-A-1-B 1.463

PMEA 1.842

BDEA 2.551

5-A-1-P 1.722

BEA 1.975

111


The initial CO2 absorption rates for secondary alkanolamines as a function of the alkyl chain

length are given in Figure 4.2-9. The initial CO2 absorption rate for MEA is also included in the

figure for comparison. In the figure, it is seen that there is an increase in the initial CO2 absorption

rate from MEA to MMEA to EMEA. However, any additional increase in alkanol chain length

beyond EMEA resulted in a decrease in initial CO2 absorption rate. The increase in initial CO2

absorption rate between MEA and EMEA can be attributed to the increased electron density

brought about by the reduction of the substitution of one of the H’s on the amino group of MEA

by an alkyl group. However, as in the case for primary alkanolamines, this comes at the expense

of increasing viscosity with alkyl chain length. The effect of viscosity (see Table 4.2-3) is clearly

manifested beyond EMEA where the beneficial effect of increase in electron density because of

alkyl chain length (which should result in increased initial CO2 absorption rate) is not able to match

the detrimental effect of mass transfer limitation created by high viscosity (which ultimately results

in reduced initial CO2 absorption rate.


The initial CO2 absorption rates for tertiary alkanolamines as a function of the alkyl chain

length are given in Figure 4.2-10. The initial CO2 absorption rates for MEA and MMEA are also

included in the figure for comparison. The figure shows an increase in the initial CO2 absorption

rate from MEA (primary alkanolamine) to MMEA (secondary alkanolamine) because of the

substitution of one H in the amino group with a methyl group. However, there is a sharp reduction

in the initial CO2 absorption rate, when the remaining H is substituted with an OH- group. This is

attributed to the electron withdrawing effect of the OH- group. Within the tertiary alkanolamines,

Figure 4.2-10 shows that there is an increase in the initial CO2 absorption rate from MDEA to

EDEA. However, any additional increase in alkanol chain length beyond EDEA (for example to

BDEA) resulted in a decrease in the initial CO2 absorption rate. The increase in initial CO2

absorption rate between MDEA and EDEA can be attributed to the increased electron density

brought about by the reduction caused by the increase in the chain length of the alkyl group.

However, as in the case for primary and secondary alkanolamines, this comes at the expense of

112

increasing viscosity with alkyl chain length. The effect of viscosity (see Table 4.2-3) is clearly

manifested beyond EDEA where the beneficial effect of increase in electron density

113

Figure 4.2-7a: Absorption profile for Primary amines

Figure 4.2-7b: Absorption profile for secondary amines

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 100 200 300 400

CO

2lo

adin

g,

mol

CO

2/mol

am

ine

Time, min

2-A-1-E(MEA)3-A-1-P

4-A-1-B

5-A-1-P

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 200 400 600

CO

2lo

adin

g,

mol

CO

2/mol

am

ine

Time, min

114

Figure 4.2-7c: Absorption profile for tertiary amines

0

0.1

0.2

0.3

0.4

0.5

0.6

0 200 400 600 800

CO

2lo

adin

g,

mol

CO

2/mol

am

ine

Time, min

MDEA

EDEA

BDEA

115

Fig 4.2-8: Effect of alkanol chain length on the initial absorption rate of primary alkanolamines

1.11

1.12

1.13

1.14

1.15

1.16

1.17

1.18

2 3 4 5 6

Initi

al a

bsor

ptio

n ra

te,

*10^

(-2)

mol

/l/m

in


116

Figure 4.2-9: Effect of alkanol chain length on the initial absorption rate of secondary alkanolamines compared with MEA

1.07

1.12

1.17

1.22

1.27

0 1 2 3 4 5

Initi

al a

bsor

ptio

n ra

te,

*10^

(-2)

mol

/l/m

in


117

Figure 4.2-10: Initial Absorption rate for tertiary amines

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Initi

al A

bsor

ptio

n ra

te,

*10-2

mol

/l.m

in

118

because of alkyl chain length (which should result in increased initial CO2 absorption rate) is not

able to match the detrimental effect of mass transfer limitation created by high viscosity (which

ultimately results in reduced initial CO2 absorption rate). The results for initial CO2 absorption rate

in tertiary alkanolamines as a function of alkyl chain length shows the inter-play of two opposing

effects: the beneficial effect of increasing the electron density on the amino group, and the

detrimental effect of the attendant increase in viscosity.

4.2.4. Initial CO2 Desorption Rate

The desorption profiles for primary, secondary and tertiary alkanolamines studied in this work are

shown in Figures 4.2-11a, b and c, respectively. As in the case for the initial absorption rate, the

initial CO2 desorption rate for each amine was taken as the slope of the initial linear portion of the

respective desorption profile of the amine.


The initial CO2 desorption rates for primary alkanolamines as a function of the alkanol chain

length are given in Figure 4.2-12. In the figure, it is seen that there is a decrease in the initial CO2

desorption rate from MEA to 3-A-1-P. However, any additional increase in alkanol chain length

beyond 3-A-1-P resulted in an increase in the initial CO2 desorption rate. The increase in the

desorption rate can be attributed to the beneficial effect of bulkiness created by the chain length

(which ultimately results in increased initial CO2 desorption rate by making the CO2 to be less

tightly held by the amino group).


The initial CO2 desorption rates for secondary alkanolamines as a function of the alkyl chain

length are given in Figure 4.2-13. The initial CO2 desorption rate for MEA is added for

comparison. The figure shows that there is an increase in the initial CO2 desorption rate as one H

in the amino group of MEA is replaced with an alkyl group (as in MMEA) as well as with an

increase in the alkyl chain length from MMEA to BEA. It has been observed in the literature (Singh

et al., 2009; Shi et al., 2014; Singto et al., 2016) that the replacement of the OH- group by an alkyl

group can lead to the formation of more HCO32- ions which is easier to breakdown into CO2

119

thereby resulting in higher CO2 desorption. It is also indicated that the bulkier the alkyl group, the

easier the possibility to form HCO32- ions, and consequently, the quicker and larger the amount of

CO2 desorbed. There is an opposing effect: viscosity, which increases mass transfer limitations for

CO2 desorption. The results appear to show that the beneficial effect of higher bulkiness due to

increase in alkyl chain length overcame any increase in mass transfer limitation for CO2 desorption

due to increased viscosity.


The initial CO2 desorption rates for tertiary alkanolamines as a function of the alkyl chain

length are given in Figure 4.2-14. The initial CO2 desorption rates for MEA and MMEA are added

for comparison. The figure shows that there is an increase in the initial CO2 desorption rate as one

H in the amino group of MEA is replaced with an alkyl group (as in MMEA). However, there is a

slight decrease in the initial CO2 desorption rate for MDEA when the remaining H in MMEA is

replaced with a methyl group.

Within the tertiary alkanolamines, there is an increase in initial CO2 desorption rate from

MDEA to EDEA. Beyond EDEA (example BDEA), the initial CO2 desorption rate decreased. As

in the case of secondary alkanolamines, the increase from MDEA to EDEA can be attributed to

the bulkiness which reduces how CO2 is tightly bound to the amino group by forming larger

amounts of HCO32- ions (Singh et al., 2009; Shi et al., 2014; Singto et al., 2016). Also, there is a

second but opposing effect: viscosity, which increases mass transfer limitations for CO2

desorption. The results appear to show that the beneficial effect of bulkiness due to increase in

chain length overcame any increase in mass transfer limitation for CO2 desorption due to increased

viscosity from MDEA to EDEA. However, it appears that for BDEA, bulkiness due to increase in

chain length was not able to overcome the increase in mass transfer limitation for CO2 desorption

due to increased viscosity. Hence, the decrease in the initial CO2 desorption rate for BDEA.

4.2.5 Cyclic Capacity

The cyclic capacity of an amine represents the capacity to capture CO2 per mole of amine

per amine cycle through the capture unit. The cyclic capacity values for all the alkanolamines

studied are given in Table 4.2-1.

120

Figure 4.2-11a: Desorption profile for primary alkanolamines

Figure 4.2-11b: Desorption Profile for Secondary Alkanolamines

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0 20 40 60 80

CO

2lo

adin

g,

mol

CO

2/mol

am

ine

Time, min

MEA4-A-1-B3-A-1-P5-A-1-P

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0 20 40 60 80

CO

2lo

adin

g,

mol

CO

2/mol

am

ine

Time, min

121

Figure 4.2-11c: Desorption profile for tertiary amines

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 20 40 60 80

CO

2lo

adin

g,

mol

CO

2/mol

am

ine

Time, min

MDEA

EDEA

BDEA

122

Figure 4.2-12: Effect of alkanol chain length on the initial desorption rate of primary alkanolamines

2

2.5

3

3.5

4

4.5

5

2 3 4 5 6

Initi

al d

esor

ptio

n ra

te,

*10-2

mol

/l/m

in


123

Figure 4.2-13: Effect of alkyl chain length on the initial desorption rate of secondary alkanolamines and MEA

4

5

6

7

8

9

0 1 2 3 4

Initi

al d

esor

ptio

n ra

te,

*10^

(-2)

mol

/l/m

in


124


The cyclic capacities for primary alkanolamines are given in Figure 4.2-15 as a function of

alkanol chain length. It is seen that a higher cyclic capacity is observed for amines with higher

desorption rates. These alkanolamines also desorb larger amounts of CO2 between the equilibrium

CO2 loading at 40 and 90oC. The results reflect the same trend as initial desorption rate.


The cyclic capacities for secondary alkanolamines are given in Figure 4.2-16 as a function of

alkyl chain length. The cyclic capacity for MEA is included for comparison. It is seen that a higher

cyclic capacity is observed for amines with higher desorption rates.

Thus, the results for cyclic capacity follow the same trend as the results for initial CO2 desorption

rate for secondary alkanolamines. These alkanolamines desorb larger amounts of CO2 and reach

equilibrium faster.


The cyclic capacities for tertiary alkanolamines are given in Figure 4.2-17 as a function of

alkyl chain length. The cyclic capacities for MEA and MMEA are included for comparison. As in

the case for primary and secondary alkanolamines, it is also seen that a higher cyclic capacity is

observed for amines with higher desorption rates. Thus, the results for cyclic capacity follow the

same trend as the results for initial CO2 desorption rate for tertiary alkanolamines. These

alkanolamines desorb larger amounts of CO2 and reach equilibrium faster.

4.2.6 Heat Duty for Regeneration for Primary, Secondary and Tertiary Alkanolamines

The heat duty for regeneration (Qreg) for the primary alkanolamines as a function of alkanol

chain length, as well as secondary and tertiary alkanolamines as a function of alkyl chain length

are given in Figures 4.2-18, 4.2-19 and 4.2-20, respectively. These figures show that the trend for

heat duty follows the inverse trend for both the initial CO2 desorption rate and cyclic capacity for

all cases. This is expected based on the definition of heat duty as the heat rate (which is constant

for all experiments) per the CO2 desorption rate.

125

Figure 4.2-14: Effect of alkyl chain length on the initial desorption rate of tertiary alkanolamines, MEA and MMEA

0123456789

Initi

al D

esor

ptio

n ra

te,

*10-2

mol

/l.m

in

126

Figure 4.2-15: Effect of alkanol chain length on the cyclic capacity of primary amines

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

2 3 4 5 6

Cyc

lic C

apac

itym

ol C

O2/L

.soltn


127

Figure 4.2-16: Effect of alkyl chain length on the cyclic capacity of secondary alkanolamines and MEA

0.28

0.33

0.38

0.43

0.48

0.53

0.58

0.63

0.68

0.73

0 1 2 3 4 5

Cyc

lic C

apac

ity,

mol

CO

2/L.so

ltn


128

Figure 4.2-17: Effect of alkyl chain length on the cyclic capacity of tertiary alkanolamines, MEA and MMEA

0

0.1

0.2

0.3

0.4

0.50.6

0.7

0.8C

yclic

Cap

acity

, m

ol C

O2/L

.soltn

129

Figure 4.2-18: Effect of alkanol chain length on the heat duty of primary alkanolamines

350

400

450

500

550

600

650

700

2 3 4 5

HE

AT D

UT

Y,kJ

/mol

CO

2de

sorb

ed


130

Figure 4.2-19: Effect of alkyl chain length on the heat duty of secondary alkanolamines

190

240

290

340

390

440

0 1 2 3 4 5

HE

AT D

UT

Y,kJ

/mol

CO

2de

sorb

ed


131

Figure 4.2-20: Effect of alkyl chain length on the heat duty of tertiary alkanolamines

050

100150200250300350400450

Hea

t Dut

y,

kJ/m

ol C

O2

deso

rbed

132

CHAPTER FIVE: DEVELOPMENT OF SELECTION CRITERIA USING THE

STRUCTURE AND ACTIVITY RELATIONSHIP STUDIES OBTAINED FROM THE

SCREENING ANALYSIS

5.1 Criteria for Amine Component Selection for Blended Amine Solvents

In the literature (Yang et el., 2012; Liu et al., 2016), different strategies have been

formulated to represent the criteria for selecting components to make an amine blend. These are

based mostly on selecting any two pairs of activities at a time and then deciding which of the pairs

can be used to select the best components in a solvent blend. This is not easily achievable since

the pairs of activities typically yield contrasting results. It is necessary that only the relevant

activities are used in the selection strategy in order not to introduce distortions in the selection.

The must-have activities are those that have specific related impacts on CO2 capture performance.

These are as follows: CO2 equilibrium solubility (i.e. amine capacity to hold CO2 – solvent flow

rate), initial CO2 absorption rate (which determines the size of absorber), initial CO2 desorption

rate (which affects the size of the desorber), CO2 cyclic capacity (which determines the maximum

amount of CO2 that can be produced per cycle – solvent circulation rate), and heat duty for solvent

regeneration (which affect the operating costs in terms of energy penalty).

5.1.1 Rate of CO2 Absorption Versus pKa

In developing the criteria for selecting the amines for blending, it was useful to determine

whether to use both activities (if both provide a unique contribution) and which activity to discard

(if both provide the same contribution). Two of the activities tested were CO2 absorption rate and

pKa. In this study, we determined if there was a linear relationship (in which case we would select

only one activity) or a non linear relationship (in which case we would select the two activities)

for all amines studied which have widely different structural characteristics. A graph of CO2

absorption rate versus pKa was plotted as shown in Figure 5.1. It can be seen from the figure that

a somewhat poor linear relationship exists between pKa and the initial absorption rate. Some

amines have a faster absorption rate than would have been predicted by their pKa. The poor co-

linearity suggests that there are some aspects of the absorption rate that are not accounted for by

the pKa alone.

133

Figure 5.1: CO2 Absorption Rate – pKa Relationship

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

8.5 9 9.5 10 10.5 11

Initi

al A

bsor

ptio

n ra

te,

*10-2

mol

/l.m

in

pKaMEA 3-A-1-P 4-A-1-B 5-A-1-P MMEA EMEA PMEA BMEA MDEA

EDEA BDEA AMP tBEA tBDEA BUTYL SEC ISO

R2 =0.6661

134

5.1.2 CO2 Equilibrium Solubility Versus pKa

The acid dissociation constant, pKa, is an important characteristic of the amine which

reflects the alkalinity of the amine. Its direct influence on amine performance is related to high

equilibrium CO2 solubility (Shi et al., 2014) as well as the difficulty for amine deprotonation for

high pKa (Tomizaki et al., 2008). If there is perfect co-linearity in the relationship between pKa

and equilibrium CO2 solubility, then it would be more useful to use equilibrium CO2 solubility,

which is a direct measure of amine capacity for CO2 absorption. We tested this co-relation using

all the amines under study. The results are shown in Figure 5.2, which shows a somewhat poor

linear relationship with R2 of 72%. This means that there are certain aspects of equilibrium CO2

solubility that are not accounted for by pKa alone. With the lack of acceptable co-linearity between

pKa and equilibrium CO2 solubility as well as between pKa and rate of CO2 absorption, we decided

to include pKa as one of the parameters in the selection strategy.

5.2 Absorption-Desorption Parameters

The selection criteria were developed in terms of absorption parameter and desorption

parameter. These activities were group into parameters that affect absorption (which constituted

absorption parameter) and those that affect desorption (which constituted absorption parameter).

The absorption parameter included equilibrium solubility, pKa and rate of CO2 absorption and was

defined as in Equation 5.1. On the other hand, the desorption parameter included, rate of CO2

desorption, cyclic capacity and heat duty, and was defined as shown in Equation 5.2. Each activity

is assumed to have the same weight in their contribution to either the absorption parameter or

desorption parameter. In each equation, the activities in the numerator represent those that need to

be maximized while the activities in the numerator represent those to be minimized. The heat of

absorption was not included as one of the activities to be included in the absorption parameter.

This is because the heat of absorption is not an activity that truly defines the performance of an

amine, specifically in terms of the rate of absorption and the heat duty, as clearly seen from Figures

4.1-23 and 4.1-25, respectively. Specifically therefore, the absorption parameter and desorption

parameter were calculated as follows:

5.1

135

Figure 5.2: pKa-Solubility Relationship

00.10.20.30.40.50.60.70.80.9

8.5 9 9.5 10 10.5 11

Equ

ilibr

ium

load

ing,

m

ol C

O2/m

ol a

min

e

pKa

MEA 3-A-1-P 4-A-1-B 5-A-1-P MMEA EMEA

BMEA MDEA EDEA BDEA AMP tBEA

tBDEA BUTYL SEC ISO PMEA

R2 =0.72

136

5.2

These parameters were calculated for all amines studied. A plot of the absorption parameter

versus desorption parameter is shown in Figure 5.3. The objective was to use the plot to determine

the amine components with the best absorption parameter and/or best desorption parameter. Based

on Figure 5.3, it can be observed that AMP had the highest desorption parameter and a relatively

good absorption parameter. On the other hand, BEA had the highest absorption parameter and a

relatively good desorption parameter. Essentially, the X- and Y-coordinates respectively allow the

evaluation of the desorption and absorption characteristics of the amines all at once thereby

reducing a multi-dimensional set of amine activities to a planer two-dimensional criterion, which

provide a good strategy for the amines to be selected to form an amine blend. Thus, having

critically examined the performance of these amines based on their absorption and desorption

parameters, BEA and AMP were selected as solvents to be used in the blend. In order to minimize

mass transfer limitations in the blends of the selected components, a measurement of the viscosity

of the blends was carried out. The results are given in Table 5.1. The blends that were formulated

comprised of BEA-AMP-MEA (5 M total concentration), BEA-AMP (5 M total concentration),

BEA-AMP (4 M total concentration), BEA (5 M concentration) and MEA (5 M). MEA was added

to the first blend to reduce the viscosity of the medium as compared to the second blend, while the

5 M MEA was used for comparison.

5.2.1 Viscosity of Concentrated Single and Blended Solvent systems

Table 5.1 shows the impact of higher concentrations on the solvent viscosities. There was

a significant increase in the viscosities of the amines at higher concentrations. For example, 5 M

MEA was higher than 2 M MEA. Also, by comparing the tri-solvent blend composed of 2M BEA-

2M AMP-1M MEA and the 5M bi-solvent blend composed of 2.5M BEA-2.5M BEA, the effect

on viscosity of MEA addition is clearly seen as it reduced the viscosity by about 30% (from 5.72

to 3.99 GJ/tonne). Also, working at a more dilute concentration (i.e. 4M) of the bi-solvent blend

reduced the viscosity by about 52% (from 5.724 to 2.738 GJ/tonne). It is important to note that

due to precipitation issues, AMP was maintained at a maximum of 2.5 M.

137

Figure 5.3: Absorption versus Desorption Parameter in Selection Criteria

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0.0000 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600

Abs

orpt

ion

Para

met

er,

*10-2

(mol

CO

2ab

sorb

ed)2 /(

mol

am

ine.

min

.Lso

ltn)

Desorption Parameter, *10-2 (mol CO2 desorbed)3/ (kJ.(Lsoltn)2.min

MEA

3 A 1 P

4 A 1 B

5 A 1 P

MMEA

EMEA

PMEA

BMEA

MDEA

EDEA

BDEA

AMP

tBEA

tBDEA

138

Table 5.1: Viscosities (mPa.s) of amine blends and single amines at higher concentrations

AMINE Viscosity Viscosity

without CO2

with CO2 @15% CO2 partial pressure

5M MEA 1.589 2.475

5M BEA 6.092 > 10

5M tri-solvent blend 3.976 10.4

5M bi-solvent blend 5.724 > 10

4M bi-solvent blend 2.738 8.081

139

An increase in viscosity was seen when the amine solutions became saturated with CO2. The

viscosity of loaded 5 M BEA and 5 M BEA-AMP bi-solvent was so high that it was outside the

range for the capillary used for viscosity measurement.

5.2.2 Evaluation of Absorption Parameter and Desorption Parameter for Blended Amines

The blended amines were tested for CO2 absorption rate, CO2 equilibrium solubility, CO2

desorption rate, cyclic capacity and heat duty. These activities were then used to compile the

absorption parameter and desorption parameter for each blend in order to determine the most

optimum blend.

5.2.2.1 CO2 Absorption Rate for Blended Amines

The CO2 absorption profiles for CO2 absorption in the 5 M MEA and BEA as well as the

blended amines are shown in Figure 5.4. The initial CO2 absorption rates calculated from the linear

section of these profiles are shown in Figure 5.5 and Table 5. 2. The single amines MEA and BEA

showed equal absorption rates. The trend followed the order MEA = BEA > 5M tri-solvent blend

= 4M bi-solvent blend > 5M bi-solvent blend (Figure 5.5 and Table 5. 2). MEA and BEA had the

same rates; however, at dilute concentrations 2M, BEA was faster than MEA (see Figure 4.1-4).

At higher concentrations, BEA viscosity increases thereby creating mass transfer limitations, and

thus, reducing the overall rate. For this reason, BEA does not exhibit better absorption performance

than MEA at high concentrations. By adding MEA to the blend (tri-solvent blend) the overall

absorption efficiency is increased due to a reduction in the viscosity. The 5M bi-solvent blend has

higher mass transfer limitations because of the increased viscosity (see Table 5.1). An

improvement in the rate is seen when the concentration of the bi-solvent blend was reduced to 4M.

The concentration was not further reduced because there might be a compromise of the capacity

although working at more dilute concentrations will reduce mass transfer limitations drastically.

140

Figure 5.4: Absorption profiles for blended amines

0

0.1

0.2

0.3

0.4

0.5

0.6

0 100 200 300 400 500 600 700 800 900

Loa

ding

, mol

CO

2/mol

am

ine

Time, min

5M blend( BEA/AMP/MEA, 2:2:1)

5M MEA

5M BEA

5M blend (BEA/AMP,2.5:2.5)

4M blend( BEA/AMP,2:2)

141

Figure 5.5: Initial Absorption Rates of Blended Amines

142

Table 5.2: CO2 activities for amine blends

AMINE

BLENDS

Heat duty,

kJ/mol CO2

desorbed

Initial CO2

Absorption

rate,

*10-2 mol/l.min

Initial CO2

Desorptionrate,

*10 2

mol/l.min

Cyclic

Capacity,

mol

CO2/l.soltn

Equilibrium

loading, mol

CO2 /mol

amine

5M MEA 222.87 1.15 8.25 0.65 0.51

5M tri-blend

(BEA/AMP/ME

A; 2:2:1)

144.21 1 12.75 1.15 0.49

5M bi-blend

(BEA/AMP;

2.5:2.5)

118.62 0.8 15.5 1.35 0.47

5M BEA 136.71 1.15 13.45 1.35 0.44

4M bi-blend

(BEA/AMP;

2:2)

110.23 1 16.68 1.52 0.55

143

5.2.2.2 CO2 Desorption Rate for blended amines

The CO2 desorption profiles for CO2 desorption in the 5 M MEA and BEA as well as the

blended amines are shown in Figure 5.6. The initial CO2 absorption rates calculated from the linear

section of these profiles are shown in Figure 5.7. The CO2 desorption rate for the amines followed

the order, 4M bi-solvent blend > 5M bi-solvent blend > 5M BEA > 5M tri-solvent blend > 5M

MEA. The trend is not surprising as this is expected. Recall from Figure 31 that MEA had the least

value of desorption parameter, thus its presence in the tri-solvent blend reduces the overall rate of

desorption. BEA had a relatively good desorption parameter, and even at high concentrations, it

exhibited fast desorption rate. The combination of BEA and AMP (5M bi-solvent blend)

demonstrates faster desorption rate than the single amine (5M BEA). This is not surprising as AMP

(having the highest desorption parameter) displays its contribution significantly and increases the

overall desorption efficiency. Surprisingly, the 4M blend exhibited the fastest rate. This is because

by lowering the concentration of the bi-solvent blend from 5M to 4M the mass transfer limitation

was reduced significantly thus allowing for faster desorption.

5.2.2.3 Cyclic Capacity for blended amines

The cyclic capacities for the 5 M MEA and BEA as well as the blended amines are shown

in Figure 5.8. The cyclic capacities followed the same trend as the desorption rates. 4M bi-solvent

blend > 5M bi-solvent blend = 5M BEA > 5M tri-solvent blend > 5M MEA. A similar explanation

for the CO2 desorption rate can be applied to the cyclic capacities. The concentration of each

individual component of the blend, in other words, the mixing ratio determines the overall

performance of the blend. Each component displays its characteristics to different extents

depending on how they are mixed. Adding MEA to the blend reduced the cyclic capacity of the

tri-solvent blend. Lowering the bi-solvent blend concentration (from 5M to 4M) increased the

cyclic capacity by about 11% due to a significant reduction in the mass transfer limitation.

144

Figure 5.6: Desorption Profile for blended amines

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 20 40 60 80 100 120

Loa

ding

, mol

CO

2/mol

am

ine

Time, min

5M blend( BEA/AMP/MEA, 2:2:1) 5M MEA5M BEA 5M blend (BEA/AMP,2.5:2.5)4M blend( BEA/AMP,2:2)

145

Figure 5.7: Initial Desorption Rates of Blended Amines

146

Figure 5.8: Cyclic capacities of Blended Amines

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Cyc

lic C

apac

ity,

mol

CO

2/L.so

ltn

147

5.2.2.4 Equilibrium CO2 Solubility for blended amines

The equilibrium solubilities of CO2 in the 5 M MEA and BEA as well as the blended amines

are shown in Figure 5.9. The solubilities of the amines followed the order: 4M blend > MEA > 5M

tri-solvent blend > 5M bi-solvent blend > 5M BEA. It is seen from Table 5.1 that the viscosities

of these solvent systems increased in the order: MEA < 4M bi-solvent blend < 5M tri-solvent blend

< 5M bi-solvent blend < 5M BEA. The mass transfer limitation increases as the viscosity increases.

This explains why the solubilities decreased in the order 5M tri-solvent blend > 5M bi-solvent

blend > 5M BEA. Although MEA is much less viscous than 4M bi-solvent blend, the blend showed

a higher solubility than MEA. At dilute concentrations (2M) BEA and AMP had higher loadings

than MEA (See Figure 4.1-11). As such a blend of these two exhibited higher absorption capacities

irrespective of the mass transfer limitation; however, there is a compromise on their approach to

reaching equilibrium as they took a relatively longer time to attain this.

5.2.3 Determination of Optimum Amine Solvent Blend Using an Absorption Parameter-

Desorption Parameter Diagram

The optimum amine solvent blend was determined by plotting the absorption parameter

versus the desorption parameter of the blends (5 M tri-solvent blend, 5 M bi-solvent blend, 4 M

bi-solvent blend), higher concentration amines (5 M MEA and 5 M BEA) in an absorption

parameter-desorption parameter diagram. This is shown in Figures 5.10a and 5.10b. In Figure

5.10a, it is important to note that the absorption parameter was calculated by using the equilibrium

loading in mole CO2/mol amine. This plot is used to illustrate the efficiency of the blend on a per

mole basis in terms of absorption parameter. The absorption parameter for Figure 5.10b, on the

other hand was calculated using the equilibrium in mole CO2/Lsoltn. That is, the loading in mol

CO2/mol amine was converted to the actual capacity by multiplying it by the amine concentration.

This plot is used to illustrate the total carrying capacity of the blend in terms of absorption

parameter. The absorption parameter trend in Figure 5.10a was in the order 5M MEA > 4M bi-

solvent blend > 5M BEA > 5M tri-solvent blend > 5M bi-solvent blend showing that 5M MEA

had the highest absorption parameter where as 5M bi-solvent blend had the least absorption

parameter. From Figure 5.5, BEA and AMP both have a relatively higher absorption parameter

than MEA.

148

Figure 5.9: Equilibrium loadings of Blended Amines

149

Figure 5.10a: Absorption versus Desorption Parameter for all amines (using loading in mol CO2/mol amine)

Figure 5.10b: Absorption versus Desorption Parameter for all amines using loading in (mol CO2/L.soltn)

0

0.5

1

1.5

2

2.5

3

3.5

0 0.05 0.1 0.15 0.2 0.25

AA

bsor

ptio

n pa

ram

eter

,*1

0-2(m

ol C

O2

abso

rbed

)2 /(.m

in.(L

soltn

)2 )

Desorption Parameter,*10-2 (mol CO2 desorbed)3/ (kJ.(Lsoltn)2.min

5M MEA5M tri blend5M biblend5M BEA4M biblend

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.05 0.1 0.15 0.2 0.25AA

bsor

ptio

n pa

ram

eter

,*1

0-2(m

ol C

O2

abso

rbed

)2 /(m

ol

amin

e.m

in.L

soltn

)

Desorption Parameter,*10-2 (mol CO2 desorbed)3/ (kJ.(Lsoltn)2.min

5M MEA5M tri blend5M biblend5M BEA4M biblend

150

It is therefore anticipated that a combination of the two will show tremendous absorption

performance. However, this was not so. Due to mass transfer limitations, the performance was

drastically affected. It should be recalled from Table 5.1 that the 5M bi-solvent blend is about four

times as viscous as 5M MEA. This result shows the impact of mass transfer limitations. When the

concentration of this blend was stepped down to 4M, a tremendous enhancement of the absorption

performance is seen. This is because now that the mass transfer limitation is very small, the

performance is now controlled by the inherent structural absorption abilities of the components,

which are being clearly displayed. However, 5M MEA was slightly higher than the 4M bi-solvent

blend because this blend is still more viscous than 5M MEA (Table 5.1). MEA and BEA had the

same initial absorption rate (Figure 5.5), but the solubility of MEA was higher than BEA at 5M.

This is because as CO2 gets absorbed, the viscosity of BEA increases significantly thereby limiting

further absorption of CO2 and consequently reducing the CO2 capacity than would have been at

dilute concentration. Thus, the overall absorption performance is lower than that of MEA at the

higher concentration of 5M. The tri-solvent blend performed better than the bi-solvent blend at 5M

due to a significantly lower mass transfer limitation. BEA performed better than both the tri-

solvent blend and bi-solvent blend at 5M irrespective of the relatively higher mass transfer

limitation of the former (see Table 5.1). This shows that BEA has attractive absorption potential

and as such can be a promising solvent to add in a blend. Figure 5.10b on the other hand shows a

slightly different trend when the absorption capacity is used: 5MEA > 5M BEA > 5M tri-blend >

4M bi-blend > 5M bi- blend. Due to the lower amine concentration of the 4M bi-blend, the

absorption parameter became lower than it was in the per mole basis. However, this should not be

seen as a drawback since in a typical CO2 capture process setting, a lower amine concentration can

be overcome by a corresponding increase in the solvent circulation rate.

The desorption parameter followed the trend 5M MEA < 5M tri-solvent blend < 5M BEA

< 5M bi-solvent blend < 4M bi-solvent blend. MEA had the least desorption parameter, hence by

adding it in a blend it tends to drag the parameter down irrespective of the fact that the other two

components have very good desorption parameter. The performance of the 5M bi-solvent blend is

not surprising due to the combination of the two components that exhibit good desorption abilities.

The 4M bi-solvent blend performed better than the 5M bi-solvent blend because of a significant

reduction in the viscosity, thus allowing for faster desorption. From the results, the 4M bi-solvent

151

blend was the most optimum blend since it displayed the best desorption performance and a very

good absorption parameter as shown in Figure 5.10a.

152

CHAPTER SIX: PILOT PLANT VALIDATION

This chapter presents the validation of the optimum blend, 4M Bi-blend (BEA-AMP) in a

bench scale pilot plant. The performance of the blend was measured in terms of the absorber

efficiency, heat duty and cyclic capacity. Earlier work done by Decardi-Nelson (2016), Osei

(2016), Akachuku (2016), Srisang et al. (2017b) and Decardi-Nelson et al. (2017) shows a higher

absorber efficiency and cyclic capacity as well as a lower heat duty for 7 M MEA-MDEA system

when compare to the conventional 5M MEA system. Therefore, the current validation of the novel

solvent blend is done as a comparison with the benchmark 7M MEA-MDEA system. Accordingly,

this work compares the performance of the novel 4M BEA-AMP bi-blend system with the

benchmark 7 M MEA-MDEA blend system in terms absorber efficiency, heat duty and cyclic

capacity as well as concentration profiles and temperature profiles in the absorber. The role of

solid acid catalyst in aiding CO2 desorption and in further enhancing the performance of both

systems are compared and discussed.

6.1 Role of Catalyst

Without catalyst, the desorption process occurs according to the two main reactions shown

below:

For carbamate forming amines like MEA and BEA, desorption reactions are shown:

Step 1: Carbamate breakdown

MEA/BEACOO- + H3O+ MEA/BEA + CO2 + H2O 6.1

Step 2: Amine Deprotonation

MEA/BEAH+ + H2O MEA/BEA + H3O+ 6.2

The breakdown of carbamate in step one depends on the thermal heat supplied and the availability

of protons. The ease of the deprotonation step depends on the amine basicity. The deprotonation

step has been seen to have a high energy barrier due to high amine basicity (Xie et al., 2010).

With the addition of HZSM-5, a proton donor, the rate of desorption is further enhanced as now

the carbamate breakdown step will not have to wait for the deprotonation step but use the protons

readily made available by the catalyst.

For sterically hindered amine like AMP the reaction mechanism is as shown below.

153

Step 1: Carbamate hydrolysis

AMPCOO- + H2O AMP + HCO3- 6.3

Step 2: bicarbonate breakdown

AMPH+ + H2O AMP + H3O+ 6.4

HCO3- + H3O+ CO2 + 2H2O 6.5

HCO3- CO2 + OH- 6.6

H3O+ + OH- 2H2O 6.7

The breakdown of bicarbonate also depends on the availability of protons in the system. The

addition of a proton donor aids in the desorption because the bicarbonate need not wait for the

deprotonation step (equation 6.4).

For bicarbonate forming amines like MDEA, the reaction is as follows:

MDEA + CO2 + H2O MDEAH+ + HCO3- 6.8

MDEAH+ + H2O MDEA + H3O + 6.9

HCO3- + H3O+ CO2 + 2H2O 6.10

The breakdown of bicarbonate here still depends on the protons made available in the

system (Equation 6.9). The catalyst will aid in this step as it will make protons readily available

for bicarbonate breakdown into CO2. All the mechanisms described for the CO2 desorption process

hinges mainly on the availability of protons. This emphasises the need to utilise the proton donating

ability of the Bronsted acid catalyst, HZSM-5.

6.2 Absorber Efficiency

The absorber efficiency basically is a representation of the efficiency of the absorption

process. An amine with a high absorber efficiency is desired as this will impact significantly the

overall capture process. A high absorber efficiency will translate to a lower capital cost of the

absorber and all the associated peripheral units (rich amine pumps, heat exchangers, amine piping

equipment) attached to the absorber. Figures 6.1 shows the absorber efficiency for the two blend

154

systems. We can see that the efficiency of the BEA-AMP blend is almost twice that of the 7M

MEA-MDEA system for both catalytic and non-catalytic runs even though the total number of

moles of amine per litre of solution for MEA-MDEA blend is much higher (7 moles) than that for

BEA-AMP (4 moles). This excellent performance of BEA-AMP bi-blend can be explained using

the previous structural studies and selection chart that was developed and reported earlier in

Chapters 4 and 5. From the selection charts the location of these individual solvents (see Figure

5.3 showed that BEA and AMP had a relatively better absorption-desorption performance than

both MEA and MDEA due to their structural uniqueness as already discussed in Chapter 4.

The effect of catalyst is clearly seen in the performance of the two different systems in

Figures 6.2. A catalyst weight of 150g of HZSM-5 with Si/Al ratio of 19 was used in the catalytic

runs. This selection of catalyst weight was in tune with previous studies done by Osei et al. (2017)

and Akachuku (2016) which showed an optimum catalyst weight of 150g in terms of desorption

kinetics and overall mass transfer in desorption. As shown in Figures 6.2 an increase of about 19%

in absorber efficiency is seen for both BEA-AMP and 7M MEA-MDEA blend systems. HZSM-5,

a Bronsted acid catalyst, works by making protons available during the rate determining step

(carbamate breakdown) in the reaction. As is well known, the desorption process involving

carbamate breakdown to form free amines requires a proton. In the non-catalytic process, the

protons are made available by the deprotonation of the protonated amine to form free amine. The

ease of this step however depends on the amine basicity. Now, in the catalytic process when a

proton donor catalyst is employed, the rate of desorption is further enhanced as now the carbamate

breakdown step will not have to wait for the deprotonation step, but uses the protons readily made

available by the catalyst. The bicarbonate breakdown also involves acceptance of protons to form

carbonate which easily breaks down to release CO2 and water. The protons made available in the

system by the catalyst helps significantly in this step. In the catalytic aided process the amine

leaves the desorber leaner and the overall CO2 produced is increased, thus translating to a higher

absorber efficiency.

155

Figure 6.1 : Absorber Efficiency for catalytic and non-catalytic runs for solvent blends

Figure 6.2: Effect of catalyst on the absorber efficiency of blended systems

0

10

20

30

40

50

60A

bsor

ber

Eff

icie

cny,

%

Solvent system

MEA-MDEA

BEA-AMP

No catalystwith catalyst

0

10

20

30

40

50

60

MEA-MDEA BEA-AMP

Abs

orbe

r E

ffic

ienc

y, %

Efect of Catalyst

156

6.3 CO2 concentration and temperature profiles

6.3.1 CO2 concentration profile

The CO2 concentration profiles for both solvent blends are shown in Figures 6.3a and

6.3b. From the results, we can see that for the same section along the column height, the BEA

AMP has a relatively lower CO2 concentration than the MEA-MDEA. This means that the rate of

CO2 depletion in the gas phase is faster for the BEA-AMP system than the MEA-MDEA system.

The implication of this is that the rate of CO2 absorption into the liquid phase is faster for the BEA-

AMP than the MEA-MDEA system. The effect of catalyst is clearly seen in Figures 6.4a and 6.4b.

The results shown that the addition of the catalyst in the desorber makes the lean amine leaner;

hence this is able to absorb more CO2 than when there is no catalyst. As such we see a faster

depletion of CO2 along the column for the catalytic process than in the non-catalytic (blank) run.

6.3.2 Temperature profile

Figures 6.5a and 6.5b show the non-catalytic and catalytic temperature profiles of the

blends. From both figures, we see the temperature bulge occurring at the column mid section.

Figure 6.5a shows that the BEA-AMP temperature bulge is only marginally higher than the MEA-

MDEA system. The catalytic temperature profile, however shows a significance difference in the

temperature bulges as BEA-AMP has a significantly larger bulge than the MEA-MDEA. Figures

6.6a and 6.6b show the effect of catalyst on the temperature bulge of the BEA-AMP system. A

significant increase in the bulge is seen signifying the role of the catalyst in improving the

absorption process, hence an increase in the heat release due to a faster reaction. The MEA-MDEA

system shows a similar increase in the bulge, though only marginally.

6.4 Cyclic Capacity

Figure 6.7 shows the cyclic capacity of the non-catalytic and catalytic runs for both

solvent systems. The BEA-AMP shows a significantly higher cyclic capacity than the MEA-

MDEA system. The performance of the novel blend can be attributed to the unique structural

properties; the bicarbonate forming ability and the electron donating ability of the BEA-AMP

blend enable it to desorb a larger amount of CO2 than the MEA-MDEA.

157

Figure 6.3a: Non-catalytic CO2 concentration profiles for BEA-AMP and MEA-MDEA blends

Figure 6.3b: Catalytic CO2 concentration profiles for BEA-AMP and MEA-MDEA blends

0

5

10

15

20

25

30

35

40

45

8 10 12 14 16

Col

umn

heig

ht, i

nche

s

CO2 concentration , %

Non-catalytic run

MEA-MDEA(without catalyst)

BEA-AMP(withoutcatalyst)

0

5

10

15

20

25

30

35

40

45

7 9 11 13 15 17

Col

umn

heig

ht, i

nche

s


Catalytic Run

BEA AMP(with 150gHZSM 5)

MEAMDEA(with150g catalyst)

158

Figure 6.4a: Effect of catalyst on the CO2 concentration profile of MEA-MDEA system

Figure 6.4b: Effect of catalyst on the CO2 concentration profile of BEA-AMP system

0

5

10

15

20

25

30

35

40

45

7 9 11 13 15 17

Col

umn

heig

ht, i

nche

s


Effect of catalyst

BEA-AMP(with 150gHZSM-5)

BEA-AMP(withoutcatalyst)

0

5

10

15

20

25

30

35

40

45

12 13 14 15 16

Col

umn

heig

ht, i

nche

s


Effect of catalyst

MEA-MDEA( with 150gHZSM-5)


159

Figure 6.5a: Non-catalytic temperature profile of BEA-AMP and MEA-MDEA blends

Figure 6.5b: Catalytic temperature profile of BEA-AMP and MEA-MDEA blends

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Col

umn

heig

ht, i

nche

s

Absorber Temperature, oC

Non-catalytic run

BEA-AMP (without catalyst)


0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Col

umn

heig

ht, i

nche

s


Catalytic run

MEA-MDEA( with 150g HZSM-5)

BEA-AMP (with 150g HZSM-5)

160

Figure 6.6a: Effect of catalyst on the temperature profile of BEA-AMP system

Figure 6.6b: Effect of catalyst on the temperature profile of BEA-AMP system

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40

Col

umn

heig

ht, i

nche

s


Effect of catalyst

MEA-MDEA( with 150gHZSM-5)


0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50

Col

umn

heig

ht, i

nche

s


Effect of catalyst

BEA-AMP (with 150g HZSM-5)

BEA-AMP (without catalyst)

161

Figure 6.7: Cyclic capacities for catalytic and non-catalytic runs for solvent blends

Figure 6.8 : Effect of catalyst on the cyclic capacities of solvent blends

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8C

yclic

Cap

acity

,m

ol C

O2/L

. sol

tn

Solvent system

MEA-MDEA

BEA-AMP

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

MEA-MDEA BEA-AMP

Cyc

lic c

apac

ity,

mol

CO

2/L.so

ltn

Efect of Catalyst

162

Figures 6.8 show the effect of catalyst on the cyclic capacity. As already explained, the

role of the catalyst is to enhance CO2 desorption by making protons readily available in the system

such that the species that depend on the deprotonation steps will no longer have to wait for the

protons released from that step but rather use the proton provided by the catalyst.

6.5 Heat Duty

The heat duty results for both catalytic and non-catalytic runs for the blends are shown in

Figures 6.9. From the results, we see a significantly lower heat duty for the novel BEA-AMP blend

than the MEA-MDEA blend, showing the economical feasibility of employing such an energy

efficient solvent system in the CO2 capture process. The tremendous desorption behaviour of this

novel blend is not surprising due to the attractive structural characteristics of the individual

components which have been discussed earlier in Chapter 4. The role of catalyst in lowering the

heat duty is clearly seen in Figures 6.10. It is important to note that both systems were subjected

to the same desorption temperature of approximately 87oC, which is lower than the typical

desorption temperature of approximately 120oC.

The intent behind lowering the desorption temperature is to be able to see the effect of the

catalyst in improving the desorption process. Unlike the conventional process where steam is

supplied by the reboiler from the desorber column to strip CO2 from the rich amine stream, this

process employs the use of a hot water heater to heat up the rich amine stream. The preheated rich

amine enters the heater, and is heated to the desorption temperature of 87oC before entering the

catalyst bed in the desorber. Thus, the thermal desorption occurs in the hot water heater in the

catalyst aided process configuration, whereas the catalytic desorption occurs on the catalyst bed.

Figure 6.10 has clearly shown the impact of proton donating property of the catalyst which is very

essential in the desorption process. A reduction of about 21% in the heat duty of the catalytic

process is seen in the BEA-AMP blend whereas a reduction of 23% is seen in the MEA-MDEA

blend.

163

Figure 6.9: Heat duties for catalytic and non-catalytic runs for solvent blends

Figure 6.10: Effect of catalyst on the heat duties of solvent blends

0

2

4

6

8

10

12

14

16H

eat D

uty,

GJ/

tonn

e C

O2

Solvent system

MEA-MDEA

BEA-AMP

without catalyst

with catalyst

0

2

4

6

8

10

12

14

16

MEA-MDEA BEA-AMP

Hea

t Dut

y,

GJ/

tonn

e C

O2

Effect of catalyst

164

6.5.1 Calculation of the heat duty terms

The heat duty comprises of three terms: sensible heat, heat of vaporisation and heat of

desorption. The calculation procedure of each terms is outlined as follows: The sensible heat for

both gas and liquid phases were calculated and summed to give the total sensible heat. The heat of

vaporisation was estimated from steam tables while the heat of desorption was calculated using

the total heat duty and the other known terms (sensible heat and heat of vaporisation).

The sensible heat calculation is shown using the illustration given below in Figure 6.11.

The gas phase CO2 in the stream entering the heater, mg1 is obtained from the CO2 desorbed during

the preheating process in the lean-rich heat exchanger using the equation;

mg1 = - 1 6.11

The gas phase CO2 in the stream leaving the heater, mg2 is obtained from the thermal

desorption that takes place in the heater. The equation is shown:

mg2 = 1- 2 6.12

The sensible heat is given as mgaverage *Cpg (T2-T1), where Cpg is the specific heat capacity of CO2

at average T and mgaverage is the average mass of CO2 gas in streams 1 and 2, mg1 and mg2

The liquid phase CO2 in the stream entering the heater (stream 1) is given as mCO2, l. (which is

obtained from the loading 1). Having known the mass of the liquid solution (without CO2), the

total liquid solution in stream 1, ml1 is given as:

mCO2, l + mamine solution = ml1 6.13

The liquid phase in the stream leaving the heater (stream 2) is given as mCO2,2 (which is obtained

from the loading 2). Having known the mass of the liquid solution (without CO2), the total liquid

solution in stream 2, ml2 is given as:

mCO2, 2 + mamine solution = ml2 6.14

The sensible heat for liquid phase is given as mlaverage Cpl (T2-T1), where Cpl is the specific capacity of the liquid solution and mlaverage is the average mass of liquid solutions, ml1 and ml2.

165

The total sensible heat is then given as the sum of the liquid and gas phase sensible heats:

Figure 6.11: Schematic Illustration for Calculation of Sensible Heat

Desorber

Lean rich heat exchanger

166

mgaverage *Cpg (T2-T1) + mlaverage Cpl (T2-T1) 6.15

The heat of vaporisation Hvap is estimated from steam tables at the average temperature (average

of T1 and T2).

The heat of desorption is therefore calculated based on the known terms according to the equation:

Heat duty – (Hsens + Hvap) = Hdes 6.16

6.5.2 Comparison of the Heat Duty Terms for Catalytic and Non-Catalytic CO2 Desorption for

BEA-AMP and MEA-MDEA Solvent Blends

The results for the heat duty terms for catalytic and non-catalytic CO2 desorption for BEA-

AMP and MEA-MDEA solvent blends are reported in Table 6.1. From Table 6.1, we can see that

the MEA-MDEA system has a bigger sensible heat than the BEA-AMP system. This is due to the

fact that the former has a higher mass per litre of solution than the latter. The heat of vaporisation

for both systems are similar since the rate of vaporisation of water (using the Antoine equation) is

the same at a fixed temperature. Although the concentrations of the systems vary, implying that

the 4M BEA-AMP bi-blend has a higher water concentration than the 7M MEA-MDEA blend, the

amount of water evaporated per mol of CO2 produced appears to be the same.

In contrast, the table shows that the heat of desorption for the novel BEA-AMP blend is

about half less that of the MEA-MDEA blend. This is a confirmation and validation of the

attractive desorption characteristics of this blend. The lower Hdes for the BEA-AMP blend shows

that the energy required to break the bonded CO2-amine species and reverse the reaction to release

CO2 is very low relative to the MEA-MDEA system. This also confirms that the carbamate formed

by the novel BEA-AMP blend (consisting of a sterically hindered amine) is highly unstable, and

hence breaks down to form bicarbonate more readily. The energy barrier for the bicarbonate

breakdown to release CO2 is low, and hence, a lower Hdes for the novel blend. It should however

be noted that the MEA-MDEA system also has some limited amounts of bicarbonate ions due to

the presence of MDEA (which is a bicarbonate forming amine).

167

Table 6.1: A summary of the sensible heat, heat of vaporisation and heat of desorption of the blends

Non-catalytic Solvent System Hsens, GJ/tonne Hvap, GJ/tonne Hdes, GJ/tonne MEA-MDEA 7.43 0.016 6.3 BEA-AMP 5.25 0.015 3.45

Catalytic

MEA-MDEA 6.94 0.016 2.98 BEA-AMP 5.46 0.013 1.44

168

However, due to the presence of MEA, the carbamate species are also present in the

blended system. Since the carbamate species are highly thermally stable, the energy barrier for

breakdown to release CO2 is much higher. Based on the calculations, the change in sensible heat

for each blend from a non-catalytic run to a catalytic run appears to be very negligible. This is

reasonable since the catalyst is not expected to contribute much in this regard except for the ones

caused due to changes in the concentrations of the various species in the system. The table also

shows that the heat of evaporation is negligible for both catalytic and non-catalytic processes.

Thus, the role of catalyst is clearly manifested mainly in the Hdes for both systems. We see a

reduction of about 50% of the Hdes of the non-catalytic process as compared to when a catalyst is

employed in the system. For BEA-AMP, the Hdes reduced from 3.45 to 1.44 GJ/tonne while for

the MEA-MDEA blend, Hdes reduced from 6.3 to 2.98 GJ/tonne. Theoretically, the heat of CO2

desorption for the same solvent system is the same whether it is a catalytic process or a non-

catalytic process. However, what is reported here is the apparent heat of desorption which reflects

how much external energy is actually required to make up for the theoretical heat of desorption

required to break the amine-CO2 bonded species. Therefore, the results show that part of the energy

needed for CO2 desorption is contributed by the catalyst in proton donation thereby reducing the

external energy required for CO2 desorption from the amine solvent.

6.6 Analysis of amine cost

Sections 6.4 and 6.5 have been able to prove the excellent energy efficiency of the novel

solvent blend; the implication is that the BEA-AMP system will require a much lower cost of

energy than the MEA-MDEA system (based on the limited study in this aspect without taking into

account the solvent blend stability which will translate to additional energy of reclaiming). We

have decided to do a rough estimate of the solvent cost as this will also impact the operational cost

associated with the capture process. It is also important to note that the solvent stability will

indirectly affect the cost of solvent makeup; as such, a higher solvent cost could also imply a higher

cost of solvent makeup for a highly unstable solvent. Further studies have to be done in order to

determine the stability of the novel solvent. As such, this cost estimate is a one-time cost at the

plant start up. Considering the energy efficiency of the blend, it would imply that the cost of

recycling will be relatively low due to easy regeneration. Hence the cost of solvent makeup will

169

be relatively low (without taking the solvent blend stability into account which would translate to

additional energy for reclaiming).

BEA cost = $114.00/2L

AMP cost = $48.65/1L

MEA cost = $ 115.50/2.5L

MDEA cost = $ 98.50/2L

Amine flow rate for all systems 60ml/min

BEA system

Volumetric flow rate = =

Cost of BEA used = = $0.899/min

AMP system


Cost of AMP used = = $0.548/min

Total cost of blend = $0.899/min + $0.548/min = $1.447/min

Amount of CO2 removed = = 2.007

Cost of amine/g CO2 removed = = $721/kgCO2 removed

MEA system


Cost of MEA used = = $0.838/min

170

MDEA system


Cost of AMP used = = $0.677/min

Total cost of blend = $0.838/min + $0.677/min = $1.515/min



MEA system


Cost of MEA used = = $0.838/min



Table 6.2: Summary of amine cost/kg CO2 removed

System Cost of amine/kg CO2

5M MEA $794

4M BEA-AMP $721

7M MEA-MDEA $1171

171

By comparing the three systems, the calculations show that the novel blend not only perform

extremely well in terms of the absorption and desorption parameters but also, its cost/kg CO2

removed is better than MEA and much better than MEA-MDEA. This supports the point in

making BEA-AMP bi-blend a potential solvent for industrial application.

172

CHAPTER SEVEN: CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE

WORK

This chapter summarizes and presents the conclusions that have been derived from the

current research work and also presents recommendations for future work that could contribute to

the expansion of knowledge as well as the applicability of the work in a commercial setting.

7.1 Conclusions

7.1.1 The steric hindrance present in branched alkanolamines resulted in much faster desorption

rate, higher solubility, higher CO2 cyclic capacity and lower heat duty for solvent regeneration,

but just a slight a decrease in absorption rate when compared with their straight chain analogues.

7.1.2 A new procedure has been developed to determine the heat of CO2 absorption in amines

using the Gibbs-Helmholtz equation. This procedure does not require a trial and error

approach to obtain a coincident CO2 loading at a minimum of three temperatures which are

required to determine the corresponding CO2 partial pressure.

7.1.3 It has also been shown that the heat duty may not necessarily be proportional to the heat of

absorption. Any relationship between the heat duty and heat of absorption is modified by

the structure of the alkanolamines.

7.1.4 The longer alkanol chain length in primary alkanolamines and the longer alkyl chain length

in secondary and tertiary alkanolamines introduced higher electron density on the amino

group (either by reducing the influence of the OH- group on the amino group or directly

introducing more electrons to the amino group) which resulted in higher pKa, and

consequently, higher equilibrium solubility. However, in the case of the initial CO2

absorption rate, the positive effect of increased electron density with both alkanol and alkyl

chain lengths was countered by higher mass transfer limitation created by increasing

viscosity of the longer chain alkanolamines. This led to the appearance of a maximum point

for the initial CO2 absorption rate for secondary and tertiary alkanolamines.

7.1.5 The longer alkanol chain length in primary alkanolamines and the longer alkyl chain length

in secondary and tertiary alkanolamines led to more bicarbonate ions being produced when

CO2 is absorbed which resulted in faster CO2 desorption rate, higher CO2 cyclic capacity

and lower heat duty for solvent regeneration. In some cases of the longest chain

173

alkanolamines, the higher viscosity created the detrimental effect of mass transfer

limitations thereby adversely modifying the beneficial effects of chain length.

7.1.6 Rational criteria have been developed in terms of absorption parameter and desorption

parameter and used for selection of amine components to form an amine blend. Based on

this criterion, BEA and AMP were selected as components for formulation of a solvent

blend system.

7.1.7 The 4 M bi-solvent blend consisting of equimolar concentrations of BEA and AMP was

found to be the optimum blend because it displayed the best desorption performance and a

very good absorption parameter, with the minimum mass transfer limitations amongst the

single amines or amine blends made up to practical concentrations.

7.1.8 The performance of the 4M bi-blend was validated in a bench scale pilot plant and

compared to the 7M MEA-MDEA blend. The results showed tremendous performance of

the novel blend in terms of absorber efficiency, cyclic capacity and heat duty.

7.1.9 The absorber efficiency of the BEA-AMP blend was about twice that of the MEA-MDEA

blend for both non-catalytic and catalytic runs even though the molarity of MEA-MDEA

blend was much higher than that for the BEA-AMP blend.

7.1.10 The heat duty for the novel blend was almost half of that of the MEA-MDEA blend for

both catalytic and non-catalytic runs reflecting the trend in absorber efficiency even though

the molarity of 7M MEA-MDEA blend was much higher than that for the 4M BEA-AMP

bi-blend thereby showing the superiority of the BEA-AMP bi-blend solvent over 7M

MEA-MDEA solvent blend.

7.1.11 The role of catalyst in improving the CO2 desorption process via proton donation was

clearly seen when HZSM5, a Bronsted acid catalyst, was employed in the system. A

reduction of about 21% in the heat duty is seen in the BEA-AMP blend whereas a reduction

of 23% is seen in the MEA-MDEA blend when catalyst was used. The role of catalyst is

clearly manifested mainly in the reduction of the heat of CO2 desorption (Hdes) for both

systems. Sensible heat and heat of vaporization components are not affected by the

introduction of a catalyst.

7.1.12 The heat of desorption results reported are the apparent heats of desorption which reflects

how much external energy is actually required to make up for the theoretical heat of

174

desorption required to break the amine-CO2 bonded species. Therefore, the results show

that part of the energy needed for CO2 desorption is contributed by the catalyst in proton

donation thereby reducing the external energy required for CO2 desorption from the amine

solvent.

7.1.13 The use of catalyst led to an increase of about 19% in absorber efficiency for both BEA-

AMP and 7M MEA-MDEA blend systems.

7.1.14 The catalyst led to a 40% increase in the cyclic capacity of the 7 M MEA-MDEA blend

while an increase of about 19% in the cyclic capacity of the novel 4M BEA-AMP blend

was seen. This is because the catalyst worked harder for the less efficient solvent system

(7 M MEA-MDEA blend) than the more efficient solvent system (4M BEA-AMP bi-

blend).

7.1.15 The pilot plant test validated the selection of BEA and AMP as the components for

formulation of a good solvent blend. It also validated the criterion established in terms of

absorption parameter and desorption parameter for the selection. The novel 4M BEA-AMP

bi-blend proved to show very attractive CO2 capture performance, and as such is a good

potential solvent for post combustion CO2 capture.

7.2 Recommendations

Recommendations for future work and to expand the knowledge gained from the current

work are made in four main areas. These are: (i) expanding the amine structure – activity

relationships as a criterion for amine component selection, (ii) catalyst development/improvement

for CO2 absorption and CO2 desorption in relation to selected solvent blend, (iii) confirmation or

elimination of untested assumptions and (iv) further viscosity studies and its impact on CO2

capture performance

7.2.1 Expanding the Amine Structure – Activity Relationships as a Criterion for Selection

This research work has demonstrated the essence of solvent component selection based on the

fundamental understanding between the amine structure and activity relationships. The most

relevant CO2 capture activities that directly impact the capture process have been identified and

used to develop a selection strategy to identify potential solvents. In order to expand this selection

175

strategy to account for the typical operational problems encountered in industrial CO2 capture

processes, the following investigation is recommended for future research:

The effect of the amine structure on degradation, corrosion, foaming and emissions should be

studied so as to establish structure-amine stability trends that will aid in the selection of amines.

This will modify the selection strategy from a planar two-dimensional criterion into a three-

dimensional selection chart, with three axes namely: absorption parameter, desorption

parameter and solvent stability parameter.

7.2.2 Catalyst Development/Improvement for CO2 Absorption and CO2 Desorption in Relation to

Selected Solvent Blend

Having employed the use of catalyst in the validation process to enhance the desorption

characteristics, the following recommendations are made:

The major problem associated with the operation of heterogeneous catalytic system is the loss

of activity of catalyst which is known to increase overtime. In this circumstance, more research

work needs to be done to investigate the stability characteristics of the catalyst.

Investigations should also be done to develop what can be considered as the most optimum

catalyst for the desorption process.

All the aspects studied for the CO2 desorption process are also necessary for the CO2 absorption

process. Consequently, work should be initiated which should also focus on developing a

suitable catalyst for the absorber. This work should also look at type of catalyst, catalyst

stability in the CO2 capture environment, and the most optimum absorption catalyst.

7.2.3 Confirmation or Elimination of Untested Assumptions

In the literature, studies on the heat of absorption of amines have led researchers to

select amines with low heat of absorption based on the assumption that the heat of absorption is

the same as the heat of desorption, and as such, amines with low heat of absorption will have lower

heat duties. It is recommended that studies should be done to determine whether this assumption

is actually true and if the amine structure, the conditions under which the heat of absorption and

heat of desorption are determined, and the practical application of the different heat components

in a commercial CO2 capture plant will influence amine selection based on this criterion.

176

7.2.4 Viscosity Studies in relation to CO2 capture performance

We have shown in the current work that viscosity has a big negative influence which seem to

against the positive contribution that the chemical structure of the amines would provide. However,

we did not determine the viscosity limit where this negative influence completely wipes out the

positive influence of the chemical structure. Further studies should be focused on determining the

viscosity limit after which mass transfer limitation sets in and which limit we should avoid. This

will help in the blending formulation of amines as information in this area will help researchers

select the maximum practical concentration when mixing amines to form a blend.

177

REFERENCES

1. Abu-Zahra, M. R., Schneiders, L. H., Niederer, J. P., Feron, P. H., & Versteeg, G. F. (2007).

CO2 capture from power plants: Part I. A parametric study of the technical performance

based on monoethanolamine. International Journal of Greenhouse Gas Control, 1(1), 37-

46.

2. Arshad, M. W., Svendsen, H. F., Fosbøl, P. L., von Solms, N., & Thomsen, K. (2014).

Equilibrium total pressure and CO2 solubility in binary and ternary aqueous solutions of 2-

(diethylamino) ethanol (DEEA) and 3-(methylamino) propylamine (MAPA). Journal of

Chemical & Engineering Data, 59(3), 764-774.

3. Akachuku, A. (2016). Kinetic study of the catalytic desorption of carbon dioxide (CO2)

from CO2-loaded monoethanolamine (MEA) and blended monoethanol-methyl

diethanolamine (MEA-MDEA) during post combustion CO2 capture from industrial flue

gases, MASc Thesis, University of Regina, Regina.

4. Billingham, M., Lee, C., Smith, L., Haines, M., James, S., Goh, B., Peralta-Solorio, D.

(2011). Corrosion and materials selection issues in carbon capture plants. Energy Procedia,

4, 2020-2027.

5. Blauwhoff, P., Versteeg, G., & Van Swaaij, W. (1983). A study on the reaction between

CO2 and alkanolamines in aqueous solutions. Chemical Engineering Science, 38(9), 1411-

1429.

6. Bonenfant, D., Mimeault, M., & Hausler, R. (2003). Determination of the structural features

of distinct amines important for the absorption of CO2 and regeneration in aqueous solution.

Industrial & Engineering Chemistry Research, 42(14), 3179-3184.

7. Bougie, F., & Iliuta, M. C. (2012). Sterically hindered amine-based absorbents for the

removal of CO2 from gas streams. Journal of Chemical & Engineering Data, 57(3), 635-

669.

8. Bruder, P., Grimstvedt, A., Mejdell, T., & Svendsen, H. F. (2011). CO2 capture into aqueous

solutions of piperazine activated 2-amino-2-methyl-1-propanol. Chemical Engineering

Science, 66(23), 6193-6198.

178

9. Brúder, P., Lauritsen, K. G., Mejdell, T., & Svendsen, H. F. (2012). CO2 capture into

aqueous solutions of 3-methylaminopropylamine activated dimethyl-monoethanolamine.

Chemical Engineering Science, 75, 28-37.

10. Caplow, M. (1968). Kinetics of carbamate formation and breakdown. Journal of the

American Chemical Society, 90(24), 6795-6803.

11. Chakraborty, A., Astarita, G., & Bischoff, K. (1986). CO2 absorption in aqueous solutions

of hindered amines. Chemical Engineering Science, 41(4), 997-1003.

12. Chakravarti, S., Gupta, A., & Hunek, B. (2001, May). Advanced technology for the capture

of carbon dioxide from flue gases. In First National Conference on Carbon Sequestration,

Washington, DC, vol 2001, pp 15-17.

13. Chakravarty, T., Phukan, U., & Weilund, R. (1985). Reaction of acid gases with mixtures

of amines. Chem. Eng. Prog.,(United States), 81(4).

14. Chowdhury, F. A., Yamada, H., Higashii, T., Goto, K., & Onoda, M. (2013). CO2 capture

by tertiary amine absorbents: A performance comparison study. Industrial & Engineering

Chemistry Research, 52(24), 8323-8331.

15. Danckwerts, P. V., & Lannus, A. (1970). Gas�liquid reactions. Journal of the

Electrochemical Society, 117(10), 369C-370C.

16. Decardi-Nelson, B., Akachuku, A., Osei, P., Srisang, W., Pouryousefi, F., & Idem, R.

(2016). A flexible and robust model for low temperature catalytic desorption of CO2 from

CO2-loaded amines over solid acid catalysts. Chemical Engineering Science. In Press,

http://dx.doi.org/10.1016/j.ces.2016.12.068.

17. Decardi-Nelson, B. (2016). Modeling, simulation and experimental validation of a new

rigorous desorber model for low temperature catalytic desorption of CO2 from CO2-loaded

amine solvents over solid acid catalysts, MASc Thesis, University of Regina, Regina.

18. Decardi-Nelson, B., Akachuku, A., Osei, P., Srisang, W., Pouryousefi, F., & Idem, R.

(2017). Catalyst performance and experimental validation of a rigorous desorber model for

low temperature catalyst-aided desorption of CO2 in single and blended amine solutions

179

(Ms. Ref. No.: JECE-D-17-00660), Journal of Environmental Chemical Engineering.

Submitted.

19. Donaldsen, T., & Nguyen, Y. (1980). Carbon dioxide reaction kinetics and transport in

aqueous amine membranes. Industrial & Engineering Chemistry. Fundamentals, 19(3),

260-266.

20. Edali, M., Aboudheir, A., & Idem, R. (2009). Kinetics of carbon dioxide absorption into

mixed aqueous solutions of MDEA and MEA using a laminar jet apparatus and a

numerically solved 2D absorption rate/kinetics model. International Journal of Greenhouse

Gas Control, 3(5), 550-560.

21. El Hadri, N., Quang, D. V., Goetheer, E. L., & Zahra, M. R. A. (2017). Aqueous amine

solution characterization for post-combustion CO2 capture process. Applied Energy, 185,

1433-1449.

22. Feron, P., Hooper, B., Attalla, M., Dave, N., Kentish, S., Stevens, G., . . . Webley, P. (2009).

Research opportunities in post combustion CO2 capture. Scoping Study for Australian

National Low Emission Coal R&D Program (ANLEC R&D), Australia.

23. Figueroa, J. D., Fout, T., Plasynski, S., McIlvried, H., & Srivastava, R. D. (2008). Advances

in CO2 capture technology—the US department of energy's carbon sequestration program.

International Journal of Greenhouse Gas Control, 2(1), 9-20.

24. Glasscock, D. A., Critchfield, J. E., & Rochelle, G. T. (1991). CO2 absorption/desorption

in mixtures of methyldiethanolamine with monoethanolamine or diethanolamine. Chemical

Engineering Science, 46(11), 2829-2845.

25. Henni, A., Li, J., & Tontiwachwuthikul, P. (2008). Reaction kinetics of CO2 in aqueous 1-

amino-2-propanol, 3-amino-1-propanol, and dimethylmonoethanolamine solutions in the

temperature range of 298 313 K using the stopped-flow technique. Industrial &

Engineering Chemistry Research, 47(7), 2213-2220.

26. Hook, R. J. (1997). An investigation of some sterically hindered amines as potential carbon

dioxide scrubbing compounds. Industrial & Engineering Chemistry Research, 36(5), 1779-

1790.

180

27. Horwitz, W. (1975). Association of official analytical chemists (AOAC) methods. George

Banta Company, Menasha, WI

28. Hwang, S. J., Kim, H., & Lee, K. S. (2016). Prediction of VLE for aqueous blended amines

using VLE models of single amines. International Journal of Greenhouse Gas Control, 49,

250-258.

29. Idem, R., Shi, H., Gelowitz, D., & Tontiwachwuthikul, P. (2011). Catalytic method and

apparatus for separating a gas component from an incoming gas stream. WO Patent,

12013821.

30. Idem, R., Edali, M., & Aboudheir, A. (2009). Kinetics, modeling, and simulation of the

experimental kinetics data of carbon dioxide absorption into mixed aqueous solutions of

MDEA and PZ using laminar jet apparatus with a numerically solved absorption-

rate/kinetic model. Energy Procedia, 1(1), 1343-1350.

31. Idem, R., Shi, H., Gelowitz, D., & Tontiwachwuthikul, P. (2017). U.S. Patent No.

9,586,175. Washington, DC: U.S. Patent and Trademark Office.

32. Idem, R., Supap, T., Shi, H., Gelowitz, D., Ball, M., Campbell, C., & Tontiwachwuthikul,

P. (2015). Practical experience in post-combustion CO2 capture using reactive solvents in

large pilot and demonstration plants. International Journal of Greenhouse Gas Control, 40,

6-25.

33. Idem, R., Wilson, M., Tontiwachwuthikul, P., Chakma, A., Veawab, A., Aroonwilas, A., &

Gelowitz, D. (2006). Pilot plant studies of the CO2 capture performance of aqueous MEA

and mixed MEA/MDEA solvents at the University of Regina CO2 capture technology

development plant and the boundary dam CO2 capture demonstration plant. Industrial &


34. Intergovernmental Panel on Climate Change (IPCC), (2005). Carbon dioxide capture and

storage. Cambridge University Press.

35. Intergovernmental Panel on Climate Change (IPCC), 2001c, Climate Change 2001:

Mitigation. A Contribution of Working Group III to the Third Assessment Report of the

Intergovernmental Panel on Climate Change [Metz, B., O. Davidson, R. Swart, Pan J.

181

(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY,

USA, pp 752.

36. International Energy Agency (IEA), (2004). Energy balances of non-OECD countries,

2001–2002. OECD/IEA, Paris.

37. International Energy Agency (IEA), (2008b). World energy outlook, OECD/IEA, Paris

38. International Energy Agency (IEA), (2009). Technical report: Safety in Carbon Dioxide

Capture, Transport and Storage

39. Kadiwala, S., Rayer, A. V., & Henni, A. (2012). Kinetics of carbon dioxide (CO2) with

ethylenediamine, 3-amino-1-propanol in methanol and ethanol, and with 1-dimethylamino-

2-propanol and 3-dimethylamino-1-propanol in water using stopped-flow technique.

Chemical Engineering Journal, 179, 262-271.

40. Kim, G. H., Park, S. Y., You, J. K., Hong, W. H., Kim, J., & Kim, J. (2014). CO2 absorption

kinetics in a CO2-free and partially loaded aqueous ammonia solution. Chemical

Engineering Journal, 250, 83-90.

41. Kim, I., & Svendsen, H. F. (2007). Heat of absorption of carbon dioxide (CO2) in

monoethanolamine (MEA) and 2-(aminoethyl) ethanolamine (AEEA) solutions. Industrial

& Engineering Chemistry Research, 46(17), 5803-5809.

42. Kim, Y. E., Moon, S. J., Yoon, Y. I., Jeong, S. K., Park, K. T., Bae, S. T., & Nam, S. C.

(2014). Heat of absorption and absorption capacity of CO2 in aqueous solutions of amine

containing multiple amino groups. Separation and Purification Technology, 122, 112-118.

43. Kin-ya, T., Shimizu, S., Onoda, M., & Fujioka, Y. (2008). An acid dissociation constant

(pKa)-based screening of chemical absorbents that preferably capture and release

pressurized carbon dioxide for greenhouse gas control. Chemistry Letters, 37(5), 516-517.

44. Kittel, J., & Gonzalez, S. (2014). Corrosion in CO2 post-combustion capture with

Alkanolamines–A review. Oil & Gas Science and Technology–Revue d’IFP Energies

Nouvelles, 69(5), 915-929.

182

45. Kittel, J., Idem, R., Gelowitz, D., Tontiwachwuthikul, P., Parrain, G., & Bonneau, A.

(2009). Corrosion in MEA units for CO2 capture: Pilot plant studies. Energy Procedia, 1(1),

791-797.

46. Kladkaew, N., Idem, R., Tontiwachwuthikul, P., & Saiwan, C. (2009). Corrosion behavior

of carbon steel in the monoethanolamine H2O CO2 O2 SO2 system: Products, reaction

pathways, and kinetics. Industrial & Engineering Chemistry Research, 48(23), 10169-

10179.

47. Knudsen, J. N., Jensen, J. N., Vilhelmsen, P., & Biede, O. (2009). Experience with CO2

capture from coal flue gas in pilot-scale: Testing of different amine solvents. Energy

Procedia, 1(1), 783-790.

48. Kohl, A. L., & Nielsen, R. (1997). Gas purification. Gulf Professional Publishing.

49. Lepaumier, H., Martin, S., Picq, D., Delfort, B., & Carrette, P. (2010). New amines for CO2

capture. III. Effect of alkyl chain length between amine functions on polyamines

degradation. Industrial & Engineering Chemistry Research, 49(10), 4553-4560.

50. Lepaumier, H., Picq, D., & Carrette, P. (2009). New amines for CO2 capture. I. mechanisms

of amine degradation in the presence of CO2. Industrial & Engineering Chemistry Research,

48(20), 9061-9067.

51. Lepaumier, H., Picq, D., & Carrette, P. (2009). New amines for CO2 capture. II. Oxidative

degradation mechanisms. Industrial & Engineering Chemistry Research, 48(20), 9068-

9075.

52. Liang, Y., Liu, H., Rongwong, W., Liang, Z., Idem, R., & Tontiwachwuthikul, P. (2015).

Solubility, absorption heat and mass transfer studies of CO2 absorption into aqueous

solution of 1-dimethylamino-2-propanol. Fuel, 144, 121-129.

53. Liang, Z., Sanpasertparnich, T., Sema, T., Jiang, Z., Gelowitz, D., Idem, R. and

Tontiwachwuthikul, P, Design, modeling and simulation of post combustion CO2

capturesystems with reactive solvents in Recent Progress and New Developments in Post

Combustion Carbon-Capture Technology with Reactive Solvents; Tontiwachwuthikul, P.

183

and Idem, R. (Editors), Future Science, United Kingdom, ISBN: 978-1-909453-35-7, 78-

100 (2013).

54. Liu, H., Chen, G., & Liang, Z. (2016). Toward rational selection of amine solutions for PCC

applications: CO2 absorption kinetics and absorption heat in tertiary aqueous solutions.

International Journal of Greenhouse Gas Control, 50, 206-217.

55. Liu, H., Liang, Y., Liang, Z., Liu, S., Fu, K., Sema, T., & Rongwong, W. (2014). Solubility,

kinetics, absorption heat and mass transfer studies of CO2 absorption into aqueous solution

of 1-dimethylamino-2-propanol. Energy Procedia, 63, 659-664.

56. Liu, H., Liang, Z., Sema, T., Rongwong, W., Li, C., Na, Y., Idem, R., Tontiwachwuthikul,

P. (2014). Kinetics of CO2 absorption into a novel 1�diethylamino�2�propanol solvent

using stopped�flow technique. AIChE Journal, 60(10), 3502-3510.

57. Liu, X., Chen, J., Luo, X., Wang, M., & Meng, H. (2015). Study on heat integration of

supercritical coal-fired power plant with post-combustion CO2 capture process through

process simulation. Fuel, 158, 625-633.

58. Liu, H., Gao, H., Idem, R., Tontiwachwuthikul, P., & Liang, Z. (2017). Analysis of CO2

solubility and absorption heat into 1-dimethylamino-2-propanol solution. Chemical

Engineering Science. http://dx.doi.org/10.1016/j.ces.2017.02.032.

59. Maneeintr, K., Idem, R. O., Tontiwachwuthikul, P., & Wee, A. G. (2009). Synthesis,

solubilities, and cyclic capacities of amino alcohols for CO2 capture from flue gas streams.

Energy Procedia, 1(1), 1327-1334.

60. Maneeintr, K., Idem, R. O., Tontiwachwuthikul, P., & Wee, A. G. (2010). Comparative

mass transfer performance studies of CO2 absorption into aqueous solutions of DEAB and

MEA. Industrial & Engineering Chemistry Research, 49(6), 2857-2863.

61. Mangalapally, H. P., Notz, R., Hoch, S., Asprion, N., Sieder, G., Garcia, H., & Hasse, H.

(2009). Pilot plant experimental studies of post combustion CO2 capture by reactive

absorption with MEA and new solvents. Energy Procedia, 1(1), 963-970.

62. Martin, S., Lepaumier, H., Picq, D., Kittel, J., de Bruin, T., Faraj, A., & Carrette, P. (2012).

New amines for CO2 capture. IV. Degradation, corrosion, and quantitative structure

184

property relationship model. Industrial & Engineering Chemistry Research, 51(18), 6283-

6289.

63. McCann, N., Phan, D., Fernandes, D., & Maeder, M. (2011). A systematic investigation of

carbamate stability constants by ¹H NMR| NOVA. The university of newcastle's digital

repository.

64. Nainar, M., & Veawab, A. (2009). Corrosion in CO2 capture process using blended

monoethanolamine and piperazine. Industrial & Engineering Chemistry Research, 48(20),

9299-9306.

65. Nwaoha, C., Saiwan, C., Tontiwachwuthikul, P., Supap, T., Rongwong, W., Idem, R., AL-

Marri, MJ., Benamor, A. (2016). Carbon dioxide (CO2) capture: Absorption-desorption

capabilities of 2-amino-2-methyl-1-propanol (AMP), piperazine (PZ) and

monoethanolamine (MEA) tri-solvent blends. Journal of Natural Gas Science and

Engineering, 33, 742-750.

66. Nwaoha, C., Saiwan, C., Tontiwachwuthikul, P., Supap, T., Rongwong, W., Idem, R., AL-

Marri, MJ., Benamor, A. (2016). Carbon dioxide (CO2) capture: Absorption-desorption

capabilities of 2-amino-2-methyl-1-propanol (AMP), piperazine (PZ) and

monoethanolamine (MEA) tri-solvent blends. Journal of Natural Gas Science and

Engineering, 33, 742-750.

67. Oexmann, J., & Kather, A. (2010). Minimising the regeneration heat duty of post-

combustion CO2 capture by wet chemical absorption: The misguided focus on low heat of

absorption solvents. International Journal of Greenhouse Gas Control, 4(1), 36-43.

68. Osei, P. A., Akachuku, A., Decardi-Nelson, B., Srisang, W., Pouryousefi, F.,

Tontiwachwuthikul, P., & Idem, R. (2017). Mass transfer studies on catalyst-aided CO2

desorption from CO2-loaded amine solution in a post-combustion CO2 capture plant.

Chemical Engineering Science. In Press, http://dx.doi.org/10.1016/j.ces.2017.02.004.

69. Osei, P. A. (2016). Mass transfer studies on catalyst-aided CO2 desorption in a post

combustion CO2 capture plant, MASc Thesis, University of Regina, Regina.

185

70. Park, J., Yoon, S. J., & Lee, H. (2003). Effect of steric hindrance on carbon dioxide

absorption into new amine solutions: Thermodynamic and spectroscopic verification

through solubility and NMR analysis. Environmental Science & Technology, 37(8), 1670-

1675.

71. Park, S. H., Lee, K. B., Hyun, J. C., & Kim, S. H. (2002). Correlation and prediction of the

solubility of carbon dioxide in aqueous alkanolamine and mixed alkanolamine solutions.


72. Peeters, A., Faaij, A., & Turkenburg, W. (2007). Techno-economic analysis of natural gas

combined cycles with post-combustion CO2 absorption, including a detailed evaluation of

the development potential. International Journal of Greenhouse Gas Control, 1(4), 396-

417.

73. Pouryousefi, F., Idem, R., Supap, T. & Tontiwachwuthikul, P. (2016). Artificial neural

networks (ANN) for accurate prediction of physical properties of aqueous quaternary

systems of carbon dioxide (CO2)-loaded 4-(diethylamino)-2-butanol (DEAB) and

methyldiethanolamine (MDEA) blended with monoethanolamine (MEA), Industrial &

Engineering Chemistry Research, 55 (44), 11614–11621.

74. Puxty, G., Rowland, R., Allport, A., Yang, Q., Bown, M., Burns, R., Maeder, M., Attalla,

M. (2009). Carbon dioxide postcombustion capture: A novel screening study of the carbon

dioxide absorption performance of 76 amines. Environmental Science & Technology,

43(16), 6427-6433.

75. Rayer, A. V., Sumon, K. Z., Jaffari, L., & Henni, A. (2014). Dissociation constants (p K a)

of tertiary and cyclic amines: Structural and temperature dependences. Journal of Chemical

& Engineering Data, 59(11), 3805-3813.

76. Rinker, E. B., Sami, S. A., & Sandall, O. C. (1995). Kinetics and modelling of carbon

dioxide absorption into aqueous solutions of N-methyldiethanolamine. Chemical

Engineering Science, 50(5), 755-768.

77. Rochelle, G. T. (2009). Amine scrubbing for CO2 capture. Science (New York, N.Y.),

325(5948), 1652-1654. doi:10.1126/science.1176731 [doi]

186

78. Sakwattanapong, R., Aroonwilas, A., & Veawab, A. (2005). Behavior of reboiler heat duty

for CO2 capture plants using regenerable single and blended alkanolamines. Industrial &


79. Sartori, G., & Savage, D. W. (1983). Sterically hindered amines for carbon dioxide removal

from gases. Industrial & Engineering Chemistry Fundamentals, 22(2), 239-249.

80. Sexton, A. J., & Rochelle, G. T. (2009). Catalysts and inhibitors for MEA oxidation. Energy

Procedia, 1(1), 1179-1185.

81. Shi, H., Naami, A., Idem, R., & Tontiwachwuthikul, P. (2014). Catalytic and non catalytic

solvent regeneration during absorption-based CO2 capture with single and blended reactive

amine solvents. International Journal of Greenhouse Gas Control, 26, 39-50.

82. Shi, H., Sema, T., Naami, A., Liang, Z., Idem, R., & Tontiwachwuthikul, P. (2012). 13C

NMR spectroscopy of a novel amine species in the DEAB– CO2–H2O system: VLE model.


83. Singh, P. (2011). Amine based solvent for CO2 absorption" from molecular structure to

process" University of Twente.

84. Singh, P., Brilman, D. W., & Groeneveld, M. J. (2009). Solubility of CO2 in aqueous

solution of newly developed absorbents. Energy Procedia, 1(1), 1257-1264.

85. Singh, P., Brilman, D. W., & Groeneveld, M. J. (2011). Evaluation of CO2 solubility in

potential aqueous amine-based solvents at low CO2 partial pressure. International Journal

of Greenhouse Gas Control, 5(1), 61-68.

86. Singh, P., Niederer, J. P., & Versteeg, G. F. (2007). Structure and activity relationships for

amine based CO2 absorbents—I. International Journal of Greenhouse Gas Control, 1(1),

5-10.

87. Singh, P., Niederer, J. P., & Versteeg, G. F. (2009). Structure and activity relationships for

amine-based CO2 absorbents-II. Chemical Engineering Research and Design, 87(2), 135-

144.

187

88. Singh, P., Van Swaaij, W., & Brilman, D. W. (2013). Energy efficient solvents for CO2

absorption from flue gas: Vapor liquid equilibrium and pilot plant study. Energy Procedia,

37, 2021-2046.

89. Singh, P., & Versteeg, G. F. (2008). Structure and activity relationships for CO2

regeneration from aqueous amine-based absorbents. Process Safety and Environmental

Protection, 86(5), 347-359.

90. Singto, S., Supap, T., Idem, R., Tontiwachwuthikul, P., Tantayanon, S., Al-Marri, M. J., &

Benamor, A. (2016). Synthesis of new amines for enhanced carbon dioxide (CO2) capture

performance: The effect of chemical structure on equilibrium solubility, cyclic capacity,

kinetics of absorption and regeneration, and heats of absorption and regeneration.

Separation and Purification Technology, 167, 97-107.

91. Srinivasan, S., Veawab, A., & Aroonwilas, A. (2013). Low toxic corrosion inhibitors for

amine-based CO2 capture process. Energy Procedia, 37, 890-895.

92. Srisang, W., Pouryousefi, F., Osei, P. A., Decardi-Nelson, B., Akachuku, A.,

Tontiwachwuthikul, P., & Idem, R. (2017a). Evaluation of the heat duty of catalyst-aided

amine-based post combustion CO2 capture. Chemical Engineering Science. In Press,

http://dx.doi.org/10.1016/j.ces.2017.01.049.

93. Srisang, W., Pouryousefi, F., Osei, P.A., Decardi-Nelson, B., Akachuku, A.,

Tontiwachwuthikul, P. & Idem, R. (2017b). CO2 capture efficiency and heat duty of solid

acid catalyst-aided CO2 desorption using blends of primary-tertiary amines (Ref:

JGGC_2017_174), International Journal of Greenhouse Gas Control. Submitted.

94. Supap, T., Idem, R., Tontiwachwuthikul, P., & Saiwan, C. (2009). Kinetics of sulfur

dioxide-and oxygen-induced degradation of aqueous monoethanolamine solution during

CO2 absorption from power plant flue gas streams. International Journal of Greenhouse

Gas Control, 3(2), 133-142.

95. Tippayawong, N., & Thanompongchart, P. (2010). Biogas quality upgrade by simultaneous

removal of CO2 and H2S in a packed column reactor. Energy, 35(12), 4531-4535.

188

96. Vaidya, P. D., & Kenig, E. Y. (2009). A study on CO2 absorption kinetics by aqueous

solutions of N, N�Diethylethanolamine and N�Ethylethanolamine. Chemical Engineering

& Technology, 32(4), 556-563.

97. Wilson, M., Tontiwachwuthikul, P., Chakma, A., Idem, R., Veawab A., Aroonwilas, A.,

Gelowitz, D., Barrie, J., Mariz, C. (2004). Test results from a CO2 extraction pilot plant at

boundary dam coal-fired power station. Energy, 29(9), 1259-1267.

98. Xu, Q., & Rochelle, G. (2011). Total pressure and CO2 solubility at high temperature in

aqueous amines. Energy Procedia, 4, 117-124.

99. Yang, Q., James, S., Ballard, M., & Bown, M. (2012). Influence of amine chemical

structures to amine capacities in CO2 capture. Recent advances in post-combustion CO2

capture chemistry (pp. 29-42) ACS Publications.

100.Zhang, R., Liang, Z., Liu, H., Rongwong, W., Luo, X., Idem, R., & Yang, Q. (2016). Study

of formation of bicarbonate ions in CO2-loaded aqueous single 1DMA2P and MDEA

tertiary amines and blended MEA–1DMA2P and MEA–MDEA amines for low heat of

regeneration. Industrial & Engineering Chemistry Research, 55(12), 3710-3717.

101.Zhao, B., Sun, Y., Yuan, Y., Gao, J., Wang, S., Zhuo, Y., & Chen, C. (2011). Study on

corrosion in CO2 chemical absorption process using amine solution. Energy Procedia, 4,

93-100.

189

APPENDICES

APPENDIX A: Safety Precautions taken during experiments

Protective clothing was worn during all experiments. The following personal precautions were

taken:

Protective goggles were worn to protect the eye from injury.

The amine worked with were corrosive as such, laboratory gloves were worn to protect the

hands during amine handling.

Protective boots were also worn to protect feet from any injury due to broken glass, spilled

chemicals or falling items in the lab.

Safety precautions taken during equipment handling:

The water level in the water bath was regularly checked when working at temperatures

around 90C. This was to ensure that water level did not drop below the minimum point.

This was done to avoid possible fire hazard should overheating and boiling dry occur.

It was ensured that hands were dry before plugging into mains power outlet socket and

switching on/operating controls.

Other precautions taken:

Mineral oil was used as heating medium during the absorption and desorption experiments.

The flash point of mineral oil was checked in order not to work at a temperature above the

flash point to avoid a possible fire hazard.

The MSDS of all amines used was read and the necessary safety precautions taken.

Appendix A1: Solution Concentration determination

The concentration of the prepared amine solution had to be checked and confirmed using titration

analysis. A known sample volume of 2 mL was pipetted, delivered into an Erlenmeyer flask and

about 10 mL of s water is added and three drops of methyl orange indicator added. The sample

was titrated till end point (turned from yellow to pink). The concentration was derived form the

formula: C1V1=C2V2

190

Using 2M MEA as an example:

= solution concentration (mol/L) (unknown, to be determined)

= solution sample volume (ml) – 2ml

=HCl concentration (mol/l) (1mol/L)

= HCl volume from titration (ml) – 4ml

C1 = = (1 / 4 2 = 2.0mol/L

Appendix A2: CO2 loading Determination

The CO2 loading was determined using the titration technique. As already described in the

experimental section, the sample volume was taken and used for titration analysis and the

equation below was used to calculate the loading:

Taking 5M MEA as an example:

= = 0.51mol CO2/mol amine

Appendix A3: Initial absorption rate determination

The initial absorption rate was determined from the loading verses time profile of the amines. The

slope of the straight line region of the profile was taken and converted to rate of CO2 absorption

in mol CO2/(l. soltn*time). A typical example of the calculation is shown using MEA as an

example. The slope shown in the figure gives the absorption rate as:

Ac

191

Slope = 0.055

Initial absorption rate = 0.055 *2 = 1.11*10-2

Figure A1: Initial Absorption Rate profile

Appendix A4: Initial Desorption Rate

The initial desorption rate was also determined in the same manner as the initial absorption rate.

The slope of the desorption profile was used to calculate the initial desorption rate as shown:

y = 0.0055xR² = 0.9943

00.050.10.150.20.250.30.350.40.45

0 10 20 30 40 50 60 70 80CO2loading,molCO

2/molam

ine

Time, min

MEA linear absorption profile

192

Figure A2: Initial Desorption Rate Profile

Slope =-0.0235

Initial desorption rate = 0.0235 *2 = 4.7*10-2

Appendix A4: Cyclic Capacity Determination

The cyclic capacity was calculated using the difference between the rich and lean amine

loadings. This is shown below using 2M MEA as an example:

Rich loading = 0.55 mol CO2/mol amine

Lean loading = 0.385 mol CO2/ mol amine

Cyclic capacity = (0.55-0.385) * 2 = 0.33

Appendix A5: Calculation of heat transfer rate, q

The rate of heat transfer was calculated based on Fourier’s law of heat conduction:

q =

To find the area A, the area of the sphere was found as 4 R2

Knowing the volume of the flask = 560ml,

193

The radius of the spherical flask is derived from the equation V=

Therefore R = 51*10-3 ml

A = 4* (51*10-3)2 = 0.03269m2

Thermal conductivity, k of pyrex glass = 1.14 W/mK

The temperature difference, dT = 95.5-93 = 2.5K

Wall thickness of the glass, dx was measured and recorded as 4mm.

Thus, heat transfer rate, q = = 23.29J/s

Appendix A6: Heat duty determination

The heat duty was obtained from, Q= , therefore the heat duty for MEA for example

is shown below:

Q = = 391.21kJ/mol CO2 desorbed

Appendix A7: pKa determination

The pka of all amines was determined as described in chapter three.a volume of 100ml of 0.05M

of amine solution was titrated stepwise until endpoint was achieved. The ph was measured upon

addition of every 0.5ml of 1M HCl. The calculation procedure is as shown in the table:

The n MEA = C*V=0.05M *100*10-3ml = 0.005mol

At the beginning the HCl volume is zero and the pH measured as 11.1 as shown in the first row

of the table. Upon addition of 0.5ml HCl the ph is measured and recorded as 10.47.

The concentration of H+ is given as antilog of the ph: 10-10.47 = 3.388exp-11

nHCl = CHCl*VHCl = 1M *0.5*10-3ml = 0.0005mol

194

Vt, total volume of solution after HCl addition = 100+0.5 =100.5*10-3 l= 0.1005 l

Concentration of protonated MEA, MEAH+ is calculated using:

= 0.004975

The concentration of free MEA, [MEA] is calculated using:

[MEA] = = 0.004478

The dissociation constant, Kamine is calculated using:

= = 3.0495*10-10

Hence, pKa is determined using:

pKa = -log(Ka) = -log (3.0495*10-10) = 9.52

The same calculation is repeated for subsequent additions of HCL and the corresponding pKa is

obtained. The average of all the pka values is then calculated and taken as the pKa of the amine.

Thus, pKa of MEA was found to be 9.33.

195

Tabl

e A

1: p

Ka

calc

ulat

ed d

ata

molofM

EAHC

lvol

pH[H+

]nH

ClVt

Vt/l

[MEA

H+]

[MEA

]lnK

Kapka

0.005

011.

17.9

4328E

120

1000.1

7.9432

8E12

0.05

0.005

0.510.

473.3

8844E

110.0

005100

.50.1

0050.0

049751

240.0

44776

21.910

83.0

496E1

09.5

15757

0.005

110.

196.4

5654E

110.0

01101

0.101

0.0099

0099

0.0396

0422.

0772.5

8262E

109.5

8794

0.005

1.59.9

41.1

4815E

100.0

015101

.50.1

0150.0

147783

250.0

34483

22.040

42.6

7903E

109.5

72023

0.005

29.6

62.1

8776E

100.0

02102

0.102

0.0196

07843

0.0294

1221.

8375

3.2816

4E10

9.4839

090.0

052.5

9.44

3.6307

8E10

0.0025

102.5

0.1025

0.0243

90244

0.0243

921.

7364

3.6307

8E10

9.44

0.005

39.2

35.8

8844E

100.0

03103

0.103

0.0291

26213

0.0194

1721.

6583

3.9256

2E10

9.4060

910.0

053.5

8.91.2

5893E

090.0

035103

.50.1

0350.0

338164

240.0

14493

21.340

35.3

954E1

09.2

67977

0.005

48.2

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196

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197

Appendix B2: Packed Column Experimental data

Table B1-a: Experimental data for typical run (150g catalyst, 60ml/min amine floe rate BEA-AMP system)

Variables Value Units

Inlet Gas Flow rate Reading: 15.0 slpm

Meter Temperature: 23.4 oC

Meter Pressure: 15.8 psia

Off-gas flow rate reading 14.2 slpm

Inlet CO2 composition 15%

Outlet CO2 composition 7.30%

CO2 inlet flow rate 0.2459 kg/hr

CO2 outlet flow rate 0.1112 kg/hr

Rich amine concentration 4.1 mol/l

Rich loading 0.49 mol/mol

Lean amine concentration 4.2 mol/l

Lean loading 0.30 mol/mol

Hot water flow rate 0.386 kg/min

Hot water inlet temperature 96.93 oC

Hot water outlet temperature 87.43 oC

198

Table B1-b: Temperature and concentration profiles of BEA-AMP system (150g catalyst, 60 mL/min)

Absorber Stripper

Height, m Temperature, oC Concentration, % Temperature, oC

Gas outlet 36.377 7.3 83.856

42 31.592 7.3

36 32.927 8.6 86.130

30 43.555 11.3 86.154

24 44.361 12.9 85.529

18 42.105 13.5 85.132

12 39.935 14.1 84.877

6 33.397 14.7 84.371

0 22.267 15.2 75.299

199

Appendix B3: CO2 absorbed calculation from the gas side.

The absorbed CO2 was calculated based on the inlet and outlet CO2 gas flow rates. Thus, taking

the BEA-AMP non-catalytic system:

CO2 inlet flow rate = 0.2443 kg/hr

CO2 outlet flow rate = 0.1344 kg/hr

Therefore, CO2 absorbed = 0.1099 kg/hr

Appendix B4: loading CO2 production

The loading CO2 production was obtained from the CO2 remaining in the liquid phase. The CO2

produced was obtained from the difference in the rich and lean loadings. The calculation is as

shown:

Rich loading= 0.49mol CO2/ mol amine

Rich amine concentration =4.1 mol /L

Lean loading= 0.33 mol CO2/mol amine

Lean amine concentration= 4.1mol/L

CO2 produced = (0.49-0.33) mol CO2/mol amine * 4.1 mol amine/l = 0.656mol CO2/L

Amine flow rate = 60 mL/min

Converting CO2 produced to mass flow rate:

0.656 = 0.1039kg/hr

Loading CO2 production = 0.1039kg/hr

Appendix B5: Mass Balance Error

After every experimental run followed a mass balance error calculation to validate the run. The

mass balance error was determined by comparing the CO2 produced from the liquid side to the

one produced from the gas side. That is the amount of CO2 produced from the liquid side should

200

be the same as the one absorbed from the gas phase. The calculation for BEA-AMP blank run is

as follows:

Mass balance error =

Therefore, mass balance error for BEA-AMP non-catalytic run =

Appendix B6: Absorber Efficiency calculation

The absorber efficiency was calculated using the CO2 composition, absorbed CO2 form gas phase and loading production. The calculation is shown using BEA-AMP non-catalytic run as example:

Absorber efficiency based on CO2 composition:

Efficiency=

The absorber efficiency therefore is

Absorber efficiency based on loading production: * 100

= * 100 = 42.54%

Absorber Efficiency based on CO2 absorbed from gas phase: * 100

Therefore, the absorber efficiency based on gas phase = * 100% = 44.99%

CO2 inlet gas

CO2 outlet gasCO2 inlet liquid

CO2 outlet gas

201

Appendix B7: Reboiler Duty

The reboiler duty was calculated from the sensible heat of the heating medim (hot water). The

calculationis as follows using BEA-AMP catalytic sytme as an example:

Reboiler duty = mCpdT

Mass flow rate of water = 0.386kg/min

Cp of water at average T = 4.21kJ/kgoC

hot water outlet temperature =87.43 oC

hot water inlet temperature = 96.93oC

Reboiler duty = �

= 924.92kJ/hr

Appendix B8: Heat Duty

The heat duty was calculated by finding the ratio of the reboiler duty to the overall amount of CO2

produced based on loading CO2 production and absorber CO2 from the gas side. Heat duty is

calculated using BEA-AMP catalytic run as an example:

Heat duty =

Heat duty from amount of CO2 absorbed = = 6.86 GJ/tonne

Heat duty from loading CO2 production = = 6.96 GJ/tonne

Appendix B9: Sensible Heat

The sensible heat was calculated by taking an energy balance on the hot water unit. The sensible

heat was calculated for both gas and liquid phases and summed up to obtain the total sensible

heat. The calculation is shown using the illustration in Figure B2:

202

The gas phase CO2 in the stream (stream 1) entering the heater, mg1 is obtained from the

CO2 desorbed during the preheating process in the lean-rich heat exchanger using the equation;

mg1= - 1

Using BEA-AMP as an example:

mg1= (0.49-0.48) mol CO2/mol amine = 0.01 mol CO2/mol amine

mg1 =

The gas phase CO2 in the stream leaving the heater, mg2 is obtained from the thermal

desorption that takes place in the heater. The equation is shown:

mg2 = 1- 2

(0.48-035) mol CO2/mol amine = 0.13 mol CO2/mol amine

mg1 =

The sensible heat is given as mgaverage *Cpg (T2-T1)

= 87.32oC-47.51oC) = = 0.0096kJ/g CO2

The liquid phase CO2 in the stream entering the heater (stream 1) is given as mCO2, l. (which is

obtained from the loading 1 ). Having known the mass of the liquid solution (without CO2), the

total liquid solution in stream 1, ml1 is given as:

mCO2, l + mamine solution = ml1

mCO2, l =

mamine =412.66g/lsotn m water = 549.29g/lsoltn

mamine solution = 549.29g/lsoltn + 412.66g/lsotn

Therefore, ml1 = 84.50g/l.soltn + 549.29g/lsoltn + 412.66g/lsotn = 1046.45g/l.soltn

203

The liquid phase in the stream leaving the heater (stream 2) is given as mCO2,2 (which is obtained

from the loading 2). Having known the mass of the liquid solution (without CO2), the total liquid

solution in stream 2, ml2 is given as:

mCO2, 2 + mamine solution= ml2

or ml2 = ml1-mg2 = (1046.45 -22.89) g/l.soltn = 1023.56g/l.soltn

The sensible heat for liquid phase is given as mlaverage Cpl (T2-T1), where Cpl is the specific capacity

of the liquid solution and mlaverage is the average mass of liquid solutions, ml1 and ml2.

= 87.32oC-47.51oC) = = 5.45 kJ/gCO2

The total sensible heat is then given as the sum of the liquid and gas phase sensible heats:

=5.45 kJ/gCO2 + 0.0096kJ/gCO2 = 5.46kg/gCO2 / 5.46 GJ/tonne CO2

204

Figure B2: Schematic Illustration for Calculation of Sensible Heat

Desorber

= 87.32oC

= 0.35mol/mol amine

= 0.49mol/mol amine

= 47.51 oC

= 0.48mol/mol amine

205

Appendix B10: Heat of vaporisation

The heat of vaporisatioin calculated from steam tables at the average T of (T1 and T2)

T = 64.40oC

Hvap at T = 42.28kJ/mol H2O

Vapor pressure of water at T = 24.34kPa

Assuming ideal gas conditions,

PV=nRT to find the number of moles of water at T

= = 8.32*10-3 mol H2O/l * X H2O in amine soltuion

= 7.84*10-3 mol H2O/l * 0.84mol/mol *42.28kJ/mol H2O = 0.278kJ/lsoltn

Therefore, Hvap = = 0.013 kJ/g CO2 or 0.013 GJ/tonne CO2

Appendix B11: Heat of desorption

Having obtained the sensible heat and heat of vaporisation, the heat of desorption was then

calculated based on the known terms according to the equation:

Heat duty – (Hsens + Hvap) = Hdes

Therefore, the Hdes = 6.91GJ/tonne – (5.46GJ/tonne+0.013GJ/tonne) = 1.44GJ/tonne

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