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International Journal of Greenhouse GasControl 19 (2013) 312

Contents lists available at ScienceDirect

InternationalJournal ofGreenhouse Gas Control

j ournal homepage: www.elsevier .com/ locate / i jggc

Analysis and predictive correlation ofmass transfer coefficient KGavofblended MDEA-MEA for use in post-combustion CO2capture

Abdulaziz Naamia,b, Teerawat Semaa, Mohamed Edalib, Zhiwu Lianga,,Raphael Idema,b, Paitoon Tontiwachwuthikula,b

aJoint International Center for CO2 Capture andStorage (iCCS), College of Chemistry andChemical Engineering, Hunan University, Changsha 410082, PR

Chinab International Test Centre for CO2 Capture (ITC), Faculty of Engineering andApplied Science, University of Regina, Regina,Saskatchewan S4S0A2, Canada

a r t i c l e i n f o

Article history:

Received 22March2013

Received in revised form 7 August 2013

Accepted12 August 2013

Available online 18 September 2013

Keywords:

CO2 absorption

Packed column

Structuredpacking

Mass transfer coefficient

Cyclic capacity

a b s t r a c t

The mass transfer performance ofthe absorption ofCO2 in aqueous blended MDEA-MEA solutions was

evaluatedexperimentally in a lab-scaleabsorber packedwith high efficiencyDX structured packing over

MDEA-MEA concentrations of 27/3, 25/5, and 23/7%wt under atmospheric pressure using a premixed

feed gas containing 15% CO2 balanced with N2. The absorption performance was presented in terms

of overall mass transfer coefficient (KGav) and CO2 concentration profile. The results showed that themass transfer performance increased as ratio ofMEA in the blended solution, temperature, and liquid

flow rate increased but decreased as CO2 loading increased. In addition, it was found that the cyclic

capacity and relative solvent regeneration ability decreased as the ratio ofMEA in the blended solution

increased. Based onmass transfer performance, cyclic capacity, and relative solvent regeneration ability,

23/7%wtMDEA-MEAwas found to be the most effective blend ratio among the three ratios investigated

in the present work. Also, the correlation for predicting KGavfor CO2 absorption into aqueous blendedMDEA-MEAwas successfully developedwith an AAD of21.8%.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

One of the options for reducing carbon dioxide (CO2) emis-

sions is absorption of CO2 from gas streams using reactive amine

solvents, which has recently been considered as one of the

most mature and reliable CO2 reduction technologies (Kohl and

Nielsen, 1997; Rao and Rubin, 2002; Liang et al., 2011). The

use of an effective solvent is considered to be one of the key

parameters of this technology. Charkravarty et al. (1985) firstly

introduced blended solvent systems by mixing primary (or sec-

ondary) amines with tertiary amines in order to capitalize on

the advantages of each amine and counter the disadvantages

of one amine with another amine. Presently, several blendedconventional amines have been introduced for absorbing CO2suchasblendedmonoethanolamine(MEA)-methyldiethanolamine

(MDEA), diethanolamine (DEA)-MEA, triethanolamine (TEA)-MEA,

MEA-2-amino-2-methyl-1-propanol (AMP), AMP-MDEA, AMP-

piperazine (PZ), and MDEA-PZ (Horng and Li, 2002; Mandal et al.,

2003; Aroonwilas and Veawab, 2004; Ramachandran et al., 2006;

Setameteekul et al., 2008; Huang et al., 2011; Samanta and

Corresponding author. Tel.: +8613618481627; fax: +8673188573033.

E-mail address: [email protected](Z. Liang).

Bandyopadhyay, 2011). Blended MDEA-MEA solutions have been

widely investigated for several years. This is because, firstly, both

the single amine solvents ofMEA andMDEAare mature since they

have been used for decades in fossil fuel-fired power generation,

natural gasprocessing, and chemical production industries. Exam-

ples of thecommercial facilities that have employed these solvents

are the Warrior Run Power Plant in Maryland, USA, Shady Point

PowerPlant inOklahoma,USA, PlatteRiverPowerAuthorityin Col-

orado, USA, Searles Valley Minerals Soda Ash Plant in California,

USA, Schwarze Pumpe Pilot Plant in Germany, Salah Natural Gas

Production Facility in Algeria, Sumitomo Chemicals Plant in Japan,

andProsint Methanol Production Unit in Brazil (Barchas andDavis,

1992; Dooley et al., 2009; Eswaran et al., 2010). Secondly, blendedMDEA-MEA canbe easily used in commercial absorbers since both

MEA and MDEA have generally been used at the industrial scale.

Blended MDEA-MEA has been commercially used in large-scale

processes at Yokosuka Power Plant in Japan and, Tokyo Electric

Power Corporation in Japan (Eswaran et al., 2010). This blended

amine system capitalizes on the advantages of high absorption

capacity, high solvent stability, low corrosiveness, and low energy

requirement for regenerationofMDEA andfast reaction kinetics of

MEA.

Even though blended MDEA-MEA was introduced in 1985,

most of the investigations have focusing on the reaction kinetics

1750-5836/$ seefrontmatter 2013 Elsevier Ltd. All rightsreserved.

http://dx.doi.org/10.1016/j.ijggc.2013.08.008
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4 A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312

Nomenclature

AAD absolute average deviation

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

Cs concentration for amine in the solution, kmol/m3

CO2 carbon dioxide

DEA diethanolamine

GI inert gas flow rate, kmol/m2 h

kL liquid phase mass transfer coefficient in the case ofchemical reaction,m/h

KG overall interfacial areagasphasemasstransfercoef-

ficient, kmol/m2 hkPa

KGav overall volumetric mass transfer coefficients,kmol/m3 hkPa

L1 liquid flow rate, m3/m2 h

MDEA methyldiethanolamine

MEA monoethanolamine

N2 nitrogen

P total pressure of the system, kPa

PZ piperazine

T absolute temperature, K

TEA triethanolamine

xCO2 mole fraction of CO2yA

mole fraction of solute A at interface

yA,G mol fraction of solute A at bulk gas phase

YA,G mole ratio of soluteA in gas phase

Z height of the absorption column, m

Greek letters

CO2 loading,mol CO2/mol amine

(Critchfield and Rochelle, 1987; Versteeg et al., 1990; Glasscock

etal., 1991;Rangwalaet al., 1992;Hagewiescheetal., 1995;Mandal

etal.,2001; Liao andLi,2002;Ramachandranet al., 2006;Edaliet al.,

2009), solubility (Shen and Li, 1992; Li and Shen, 1993; Dawodu

and Meisen, 1994; Li and Mather, 1994; Kaewsichan et al., 2001;Mamun et al., 2005), physiochemical and thermodynamics prop-

erties (Austgen et al., 1991; Li and Shen, 1992; Jou et al., 1994;

Li and Lai, 1995; Hsu and Li, 1997; Weiland et al., 1998; Bensetiti

et al., 1999; Mandal et al., 2005; Vrachnos et al., 2006), and sol-

vent degradation (Lawal et al., 2005; Lawal and Idem, 2005, 2006;

Dawodu and Meisen, 2009). Only a few studies have investigated

the mass transfer of CO2 absorption into blends of MDEA-MEA in

packed columns.

Aroonwilas andVeawab (2004)investigatedmass transfer per-

formance forCO2 absorption intoblendedMDEA-MEA,MDEA-DEA,

and AMP-MEA in structured DX packing using 10% CO2 balanced

with nitrogen (N2) as feed gas. The mass transfer performance

was reported in terms of the overall mass transfer coefficient,

removal efficiency, and relative columnheight requirement.How-ever, the experiments for the blended amines were conducted at

only 1/1 mole ratio (with total amine concentration of 3M). Itwas

found that the mass transfer performance of blended AMP-MEA

is higher than those of MDEA-MEA and MDEA-DEA, respectively.

Since the reaction kinetics of CO2 absorption can be ranked as:

AMP>DEA>MDEA, (i) themass transfer performanceofAMP-MEA

then exceeds that of MDEA-MEA and (ii) that of MDEA-MEA then

exceeds that of MDEA-DEA. However, reboiler heat duty should

also be taken into consideration since it contributes about 70% of

the operating cost of the CO2 capture process (Kohl and Nielsen,

1997). Sakwattanapong et al. (2005) investigated the reboiler heat

duty for CO2 capture with blends of AMP-MEA, MDEA-MEA, and

MDEA-DEA. They reported that the reboiler heat duty of MDEA-

DEAis lower than thoseofMDEA-MEAandAMP-MEA, respectively.

By taking into consideration both mass transfer performance and

heat duty requirement for solvent regeneration, blended MDEA-

MEA seems to be the best solvent combination among the three

because it provides good performance in both mass transfer and

reboiler heat duty requirement.

Later, Setameteekul et al. (2008) studied themass transfer per-

formance of blended MDEA-MEA at various MDEA-MEA blending

ratios of 1/3, 1/1, and 3/1 molar. They concluded that (i) the blend-

ing ratio of the blended MDEA-MEA has a significant effect on

themass transfer performance, (ii) themass transfer performance

increases asmolar ratio ofMEA increases, and (iii) theMDEA-MEA

of 1/3molar ratio provided the best mass transfer performance.

Additionally, the CO2 absorption not only depends on using

an effective solvent, but the contact between gas and liquid

solvent also plays an important role in the CO2 absorption per-

formance in a packed column. It has been over forty years since

the tray columns have been replaced by the more effective packed

columns (Aroonwilas and Tontiwachwuthikul, 1998; Aroonwilas

and Veawab, 2004). In the packed columns, the packing cre-

ates a gasliquid contact area by generating liquid droplets. The

most promising packing should provide large surface area per

volume ratio, low pressure drop across the column, and uniform

gasliquid distribution throughout the column. Aroonwilas and

Tontiwachwuthikul (1998)and deMontignyet al. (2001)compared

the CO2 absorption performance using randomly (IMTP#15, 0.63,

3 in. Pall Ring, 0.5in. Berl Saddles) and structured (Gempack 4A,

Sulzer EX) packings. They concluded that the structured pack-

ing provide higher mass transfer performance than the random

packing, which is in good agreement with the results observed by

Fernandes et al. (2009).

Even though CO2 absorption technology has been industrially

used for over half a century, themost promising solvents have still

to be discovered. Recently, a number of novel solvents (e.g., 2-N-

methylamino-2-methyl-1-propanol,2-N-ethylamino-2-methyl-1-

propanol, 2-(isopropylamino)ethanol, 2-(isobutylamino)ethanol,

2-(secondarybutyamino)ethanol, 2-(isopropyl)diethanolamine, 1-

methyl-2-piperidineethanol, 4-diethylamino-2-butanol, RITE-A

and RITE-B) has been introduced (Chowdhury et al., 2011; Gotoet al., 2011; Sema et al., 2013). The CO2absorption performance of

these novel amines was found to exceed the conventional MEA,

AMP, DEA, and MDEA. However, the major drawbacks of these

novel solvents are (i) expense, (ii) difficult synthesis, (iii) difficult

mass production, (iv) phase separation or turning into solidsunder

certain conditions, and (v) undiscovered disadvantages.

Another approach for improving the CO2 absorption perfor-

mance rather than working with novel solvents is to enhance and

optimize the performance of conventional solvents. In order to

achieve this, the conventional solvents have to be comprehen-

sively investigated. Even though blended MDEA-MEA has been

widely investigated, the predictive correlation for overall mass

transfer performance of CO2 absorption into aqueous solutions of

blended MDEA-MEA has not yet been established. In the presentwork, the blended MDEA-MEA ratio was selected at 27/3, 25/5,

and 23/7%wt (equivalent to MDEA-MEA molar ratio of 2.3/0.5,

2.1/0.8, and 1.95/1.16, respectively). These blend concentrations

were selected in order to capitalize on the great advantages of

both MDEA and MEA in that MDEA requires low energy for sol-

vent regeneration and has high absorption capacity while MEA

provides fast CO2 absorption rate. Since the heat requirement for

solvent regeneration contributes about 70% of the cost of captur-

ing CO2 (Kohl and Nielsen, 1997), MDEA was, therefore, selected

as the major component of the blended solutions in order tomin-

imize the heat requirement for solvent regeneration. The addition

ofMEA,which has fast reactionkinetics,can then enhance the CO2absorption rate of the blended solutions. However, the total con-

centration of the blended solution cannot be high because MDEA


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A. Naami et al. / International Journal of Greenhouse Gas Control 19 (2013) 312 5

itself is considered to be a slightly viscous solvent already. Thus,

the total concentration of the blended MDEA-MEA used in the

present work was then limited at 30% wt. In addition, the CO2absorption experiments were done over ranges of temperatures

(298, 303, and 318K), CO2loadings at the absorber top (0.05, 0.17,

and 0.25mol CO2/mol amine), and liquid flow rates (2.8, 3.8, and

5m3/m2 h). Theexperimentswere performedin a laboratory-scale

absorption column with DX structured packing (27.5mm ID from

Sulzer Chemtech Canada, Inc.) at atmospheric pressure. The over-

allvolumetricmass transfer coefficients(KGav) atvarious operating

conditions were then used to establish thepredictive correlation.

The data obtained from the present workwill be very crucial to

further pilot plant-scale test at ITCs technology pilot plant (diam-

eter of 12 incheswith capture capacity of 1 ton of CO2/day). Itwas

mentioned by Idem et al. (2006,2011a) that only one molar ratio

of 1/4 of blended MDEA-MEA was previously tested in the pilot

plant so the results from the present work will be very useful to

(i) determine the scope of the pilot plants operating conditions

with blendedMDEA-MEAand(ii) expandthe pilotplant-scale test-

ing to test various blended ratios other than 1/4 molar ratio of

blended MDEA-MEA. In addition, the results obtained from this

study can also lead to improvement of CO2 absorption perfor-

mance of blended MDEA-MEA by addition of a third component

into the system. Up to the present, several promising amines

(e.g., AMP, PZ, 1-methyl-2-piperidineethanol, 4-diethylamino-2-

butanol)have beenintroducedfor capturingCO2 (Chowdhuryet al.,

2011; Goto et al., 2011; Sema et al., 2013). These solvents can

potentially enhance the performance of the well-known blended

MDEA-MEA. In order to effectively achieve this goal, a complete

understanding of blended MDEA-MEA over the range of relevant

operatingconditionsis required.Additionally,the thirdcomponent

canalso be ionic liquids, physical solvents, or even solid/liquidcat-

alyst that canreducethe heat requirement forsolvent regeneration

andimprovethe performanceofCO2removal as suggestedby Idem

et al. (2011b) and Shannon andBara (2012).

2. Determination of theoverall mass transfer coefficient

It hasbeen widelyunderstood that oneof theprimary concerns

in removing CO2 using absorption processes is the concentration

of CO2 in the gas phase. Also, it is easier and more convenient

to measure the concentration of CO2 in gas phase than in liq-

uid phase. Thus, for the CO2 absorption process, the gas phase

mass transfer coefficient is generally accepted to be more suit-

able than the liquid phase coefficient. However, it is very difficult

to measure the gasliquid interfacial area in an absorption col-

umn. Therefore, the mass transfer coefficient in a packed column

is generally represented by overall volumetric gas phase mass

transfer coefficient (KGav; kmol/m3 hkPa) instead of that based

on interfacial area (KG; kmol/m2 hkPa) (Astarita et al., 1983; Kohl

and Nielsen, 1997; Aroonwilas and Tontiwachwuthikul, 1998;

deMontigny et al., 2001; Aroonwilas and Veawab, 2004; Fu et al.,

2012). The overall volumetric gas phase mass transfer coefficient

KGavcan be calculated as follow:

KGav=

GI

P(yA,G yA)

dYA,GdZ

(1)

whereGIis inert gasflow rate (kmol/m2 h), Pis totalpressureof the

system (kPa), Zis height of the absorption column (m),yA,Gis mol

fractionof soluteA atbulkgas phase,yAis mole fractionof soluteA

at interface,YA,Gis the concentration (in terms ofmole ratio) in gas

phaseofsoluteA,which isCO2 inthepresentwork,and(dYA,G/dZ) is

thesolutemoleratio concentrationgradient,whichcanbeobtained

by taking a slope ofYA,Gversus Zplot.

3. Experimental

3.1. Chemicals

MDEAand MEA were purchased from Fisher Scientific, Canada,

with purities of99%. The premixed 15% CO2 (balanced with N2)

wassuppliedby Praxair Inc.,Canada.Allmaterialsin thisstudywere

used as receivedwithout further purification.Aqueous solutionsof

blendedMDEA-MEA of desired concentrationswere prepared by a

known amount of deionizedwater andpredetermined amounts of

MDEA and MEA.

3.2. CO2absorption in packed column

ThemasstransferinapackedcolumnoccurswhenCO2 inthegas

phasetransfersacrossthegasliquidinterface intothe liquidphase.

In thepresent work, themass transfer performancewasevaluated

in termsof theoverall volumetricmass transfercoefficientKGavandthe CO2 concentration profile along the height of the column. The

KGavcan be calculated using Eq. (1), in which several parameterscan be obtained from the experiment. Generally, the experiments

were done at atmospheric pressureP. The inert gas flow rateGIcan

bedetermined from thegas flowratemeasurementusingelectron-ics Aalborg GFM-17mass flow meter (ranging from 5 to 50L/min

with a 0.15%/C accuracy). The mole fraction of CO2 in the gas

phase yA,G can be determined from an infrared CO2 gas analyzer

(model 301D, Nova Analytical System Inc., Hamilton, ON,Canada),

which is capable of measuring CO2 concentration up to 20% with

0.5% accuracy. Themeasurement of CO2concentration was done

along the height of the column through the sampling ports, which

are connected to the CO2gas analyzer. The mole fraction of CO2at

interface (yA) canbe calculated using theHenrys law relationship.

TheHenrys lawconstant of blendedMDEA-MEA canbe calculated

bythecorrelationestablishedbyWangetal.(1992). Themoleratios

ofCO2 inthegasphase(YA,G) atvariousheights of thecolumncanbe

obtained from the CO2analyze, and the, plotted against theheight

of the column to get dYA,G/dZ.The glass laboratory absorption column (diameter of

27.5102m and height of 2.15m) was packed with 37 ele-

ments of stainless steel Sulzer DX structured packing (with

900m2/m3 packing surface area). The schematic diagram of the

experimental setup of CO2 absorption in the packed column is

presented in Fig. 1. The operational procedure of the absorption

columncan be found in our previousworks (Fu et al., 2012; Naami

et al., 2012). Inorder to validate the absorption columnused in the

present work, 2M MEA solution was tested and compared with

the results from deMontigny (2004). It was found that the results

obtained in the present work are in good agreement with those

obtained in deMontigny (2004) as shown inFig. 2.

4. Results and discussion

Themass transfer performance of CO2 absorption into aqueous

solutions of blended MDEA-MEA was experimentally determined

in the laboratory-scale absorption column packed with DX struc-

turedpacking andreportedin termsofKGavandCO2concentrationprofile along the height of the column. The values ofKGavare pro-

portional to the mass transfer performance in that the higher the

KGav, the higher mass transfer performance. For the CO2 concen-tration profile, a lower CO2 concentration profile indicates a larger

amountof CO2 thathasbeenremovedfromthegasstreamresulting

in highermass transfer performance.

The experiments were done at various MDEA-MEA con-

centrations of 27/3, 25/5, and 23/7%wt (which are equivalent

to MDEA-MEA molar ratios of 2.3/0.5, 2.1/0.8, and 1.95/1.16,


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Fig. 1. Schematicdiagram of theexperimental setup of CO2 absorption in packed column.

respectively) over a range of temperatures (298, 303, and 318K),

CO2 loadings at the absorber top (0.05, 0.17, and 0.25molCO2/mol

amine), and liquid flow rates (2.8, 3.8, and 5m3/m2 h). A mass

balance error,which canbecalculated using Eq. (2), was taken into

consideration in order to confirm the validity of each absorption

experimental run. The mass balance error compares the amount

of CO2 removed from the gas phase (which can be measured via

an infrared CO2 gas analyzer) with that added into liquid phase

(which can be measured by titration with standard 1.0M HCl

until methyl orange end point as mentioned in Association of

Official Analytical Chemists (AOAC) methods by Horwitz (1975)).

Theoretically, these twovalues shouldbe equal.However, because

of (i) experimental errors from the CO2 gas analyzer and CO2

loading measurement apparatus and (ii) an elongated liquid trap

in the packed column, the average mass balance error obtained

from the present work was found to be 4%, which is considered

to be in an acceptable range of less than 10%. Therefore, it can

be inferred from this observation that the experimental results

obtained from the present work are correct and reliable.

Mass balance error=

absorbed CO2 removed CO2absorbed CO2

100% (2)The total experimental data points obtained from the present

work are 600 from30 experimental runs, which include 300 mea-

sured points of CO2 concentration and another 300 points of

temperature. From these data, the effects of MDEA-MEA blended


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Fig. 2. Validation of packed column using 2.0M MEAat temperatureof 294K, CO2loading at absorber topof 0.2molCO2/molamine,liquidflowrate of5 m

3/m2, inert

gas flow rate of 17.85kmol/m2 h, and YCO2 of 0.09.

ratio, temperature, CO2 loading at the absorber top, and liquid

flow rate onmass transfer performance (in terms ofKGavand CO2concentration profile along the height of the column) were then

examined.

4.1. Effect ofMDEA-MEA blended ratio onmass transfer

performance

In order to examine the effect of MDEA-MEA blended ratio

on the mass transfer performance, three blended ratios of 27/3,

25/5, and 23/7%wt were tested in the packed column. The results

showed that the mass transfer performance in terms ofKGavandCO2 concentration profile increased as the weight ratio of MEA in

theblended solutions increasedas presented in Figs. 3 and 4. It can

bereasoned thatMEAhasfaster reactionkinetics ofCO2absorption

thanMDEA (Kohl and Nielsen, 1997; Semaet al., 2012). Thehigherthe ratio of MEA in the blended solutions, the higher amounts of

the more reactive MEA molecules that can absorb CO2; thus, a

highermass transfer performance was observed. Therefore, it can

be seen from Figs. 3 and 4 that 23/7%wt MDEA-MEA (molar ratio

of 1.95/1.16) provides the best mass transfer performance among

the three investigated ratios. However, by increasing the ratio of

MEA, theratio ofMDEA in theblended solutionswouldbereduced,

Fig. 3. Effect of MDEA-MEA blended ratio on CO2 concentration profile at temper-

ature of 294K, CO2 loading at absorber topof 0.25mol CO2/mol amine, liquidflow

rate of 5m3

/m2

, and inert gas flow rate of 15.99kmol/m2

h.

Fig. 4. Effect of MDEA-MEA blended ratio on KGav at temperature of 294K, CO2loading at absorber top of 0.25mol CO2/mol amine, liquid flow rate of 5m

3/m2,

inert gas flowrate of 15.99kmol/m2 h, and YCO2 of 0.09.

which can lead to (i) a reduction of CO2 absorption capacity and

(ii) an increment of heat requirement of solvent regeneration. This

is because MDEA has higher CO2absorption capacity and requires

much lower heat forsolvent regeneration(Kohl andNielsen, 1997;

Sakwattanapong et al., 2005; Lianget al., 2011). Itwasdiscussedby

Kohl and Nielsen (1997) that the energy requirement for solvent

regeneration contributes about 70% of the cost of capturing CO2.

Therefore, in order to maintain low heat requirement for solvent

regeneration andhighabsorption capacity characteristicsofMDEA

in theblended solutions,much higherblended ratiosofMDEA over

MEAwere selected in this work.

4.2. Effect of temperature onmass transfer performance

In the present work, the liquid inlet temperature was varied

from294 to 318K. It was found that the CO2concentration profile

became lower as temperature increased as presented in Fig. 5. The

mass transfer performance in terms ofKGavwas also found to cor-respond well with that of the CO2concentrationprofile in that the

KGavincreased as temperature increasedas shown in Fig. 6. There-fore, it can be inferred from Figs. 5 and 6 that the mass transfer

performance of the blended MDEA-MEA increases as temperature

increases over a temperature range of 294318K. This is because

the reaction kinetics of blended MDEA-MEA increase as tempera-

ture increases as discussed in the works ofMandal et al. (2001),

Liao and Li (2002), and Edali et al. (2009).

Fig.5. Effect of temperatureon CO2 concentrationprofileof 23/7%wtMDEA-MEAat

CO2loading at absorbertopof 0.04mol CO2/mol amine, liquidflow rateof 5m3/m2,

and inert gas flowrate of 15.99kmol/m3

h.


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Fig. 6. Effect of temperature on KGav of blended MDEA-MEA at blended ratios of

27/3, 25/5, and 23/7%wt, CO2 loading at absorber top of 0.04mol CO2/mol amine,

liquidflow rateof 5m3/m2, inert gasflow rate of 15.99kmol/m2 h,andYCO2 of 0.09.

4.3. Effect of CO2loading on mass transfer performance

For the effectof CO2loadingat theabsorbertoponmass transferperformanceofblendedMDEA-MEA,it canbe found inFigs.7and8

that the mass transfer performance (in terms of CO2 concentra-

tion profileandKGav) is significantlyaffected byCO2loading at the

Fig.7. Effectof CO2loadingatabsorber topon CO2 concentrationprofileof 23/7%wt

MDEA-MEA at temperature of 294K, liquidflow rate of 5m3/m2, and inert gas flow

rate of 18.65kmol/m2 h.

Fig. 8. Effect of CO2 loading at absorber top on KGav of blended MDEA-MEA at

blended ratios of 27/3, 25/5, and 23/7%wt, temperature of 294K, liquid flow rate

of5m

3

/m

2

, inert gas flowrate of 18.65kmol/m

2

h, and YCO2 of 0.09.

Fig.9. Effectof liquidflowrateonCO2concentrationprofile of 23/7%wtMDEA-MEA

at temperatureof 294K, CO2loading at absorbertop of 0.2molCO2/molamine, and

inert gas flowrate of 18.65kmol/m2 h.

Fig. 10. Effect of liquid flow rate on KGavof blended MDEA-MEA at blended ratiosof27/3, 25/5,and 23/7%wt, at temperatureof 294K, CO2 loading at absorber topof

0.2mol CO2/mol amine, inert gasflow rate of 18.65kmol/m2 h, and YCO2 of 0.09.

absorber top in that the mass transfer performance decreased as

CO2 loading increased over a CO2 loading range of 0.050.25mol

CO2/mol amine. This is because the amounts of active free amines

(MEA and MDEA) decreases as CO2loading increases.

4.4. Effect of liquid flow rate onmass transfer performance

It can be seen from Fig. 9 that the CO2 concentration profile

became lower as liquid flow rate increased over the range of

2.85.0m3/m2 h. This observation correspondswell with themasstransfer performance in terms ofKGav. As presented in Fig. 10,the KGav increased as liquid flow rate increased. Regard to the

results observed in the present work (from Figs. 9 and 10), it

can be inferred that the liquid flow rate has a very significant

effect on the mass transfer performance for CO2 absorption into

aqueous solutionsof blendedMDEA-MEA in that themass transfer

performance increases as liquidflow rate increases over the liquid

flow rate range of 2.85.0m3/m2 h. Thus, it can be reasoned that

increase of liquid flow rate results in an increase of liquid side

mass transfer coefficient (kL), which significantly increases the

KGavin the case of liquid-phase controlled mass transfer (Astaritaet al., 1983). Additionally, by increasing the liquid flow rate, the

gasliquidsurfaceareaisgreatly increased,resultinginmoreliquid

spreading on the packing surface. The increase of liquid flow rate


7/10


not only increases the mass transfer performance in the packed

column, but also leads to higher circulation and regeneration

costs; thus, it might not improve the overall system efficiency. In

addition, the range of liquid flow rate used in the present work

(2.85.0m3/m2 h) is in the effective range of using DX structured

packing, which is 0.15.0m3/m2 h, as provided by the packing

supplier (Sulzer Chemtech Canada, Inc.). The use of a liquid flow

rate higher than 5.0m3/m2 h can lead to a decline ofmass transfer

performance.

4.5. Effect ofMDEA-MEA blending ratio on cyclic capacity

Astarita et al. (1983) and Maneeintr et al. (2009) described the

cyclic capacity concept in which cyclic capacity is defined as the

differenceof molesof CO2absorbedin thesolutionper unit volume

of solution in theabsorption step andthat in theregenerationstep,

which is, then, crucial to the CO2 absorption process. With the

same volume of solvent, the solvent with higher cyclic capacity

can carry a higher amount of absorbed CO2 than that with lower

cyclic capacity. Therefore, it can be inferred from this concept that

for a solvent that has high cyclic capacity, a low liquid circulation

rate can be applied, which results in (i) a lower operation cost

for pumping liquid and (ii) a smaller volume of solvent can beused in the packed column. In order to investigate the effect of

MDEA-MEA blended ratio on the cyclic capacity, three blended

ratios of 27/3, 25/5, and 23/7%wt were tested in the present work.

The experimental and calculation procedures for cyclic capacity

can be found in our previous work (Maneeintr et al., 2009). The

results showed that the cyclic capacity of the blended MDEA-MEA

increased as the ratio of MDEA in the blended solution increased

and can be ranked as: 27/3%wt MDEA-MEA> 25/5%wt MDEA-

MEA>23/7%wt MDEA-MEA, as presented in Fig. 11. Based on this

observation, this result occurred because MDEA has higher cyclic

capacity than MEA (Chowdhury et al., 2011); thus, by increasing

the ratio of MDEA (in another words, decreasing the ratio of MEA)

in theblended solution, the cyclic capacity of the blended solution

was then found to be increased.

Fig. 11. Effectof MDEA-MEA blended ratio on cyclic capacity.

4.6. Relative solvent regeneration ability

One of the key points for the CO2 absorption process usingchemical solvent is the ability of solvent to be generated. It has

beengenerallyacceptedthatgood/promisingsolventsfor thistech-

nology should be easy to regenerate; in other words, they should

require lowheatfor solvent regeneration.Thisis becausethemajor-

ity (about 70%) of the cost of CO2 capture comes from the solvent

regeneration unit.

In the present work, the relative solvent regeneration ability

of the blended MDEA-MEA was determined and compared with

those of 2M MDEA and 5M MEA. The experiment was conducted

at 80 C in a temperature controlled bath (Cole-Parmer; the tem-

perature can range from -20 to 200 C with 0.01 C accuracy).

3000ml of the solutions, which include 27/3%wt (or 2.3/0.5 molar

ratio) MDEA-MEA, 2M MDEA, and 5M MEA, at the same initial

CO2 loading of 0.5 were introduced into round-bottom flasks

Fig. 12. Experimental set up for determining relative solvent regenerationability.


8/10


Fig. 13. Relative solvent regeneration ability of 5M MEA, 2M MDEA, and2.3/0.5M

(27/3%wt) MDEA-MEA.

connected with condensers as shown in Fig. 12. Within the same

operating conditions, the CO2loadings of the three testedsolvents

were then measured as the experiment proceeded by titrating

with standard 1.0M HCl using methyl orange as an indicator.

The results obtained from this experiment suggest thesolvents

relative regeneration ability in that the CO2 loading of a solu-

tion that has high relative solvent regeneration ability (in other

words, one that is relatively easy to regenerate) would decrease

more rapidly than one that has lower relative solvent regeneration

ability. Itwas found that the CO2 loadings of the three testing solu-

tions decreased as the operational time increased as presented in

Fig. 13. Even though the results observed from Fig. 13 can gener-

ally beunderstood as showing that therelativeregenerationability

of blended MDEA-MEAwould fall in between those of MDEA and

MEA, the results obtained from the present work clearly show

how close the relative regeneration ability of 27/3%wt (or 2.3/0.5

molar ratio) blended MDEA-MEA is to that of 2M MDEA. This newobservation is considered to be very useful in order to (i) use the

blended MDEA-MEAmore effectively in the pilot plant-scale test-

ing and (ii) enhance the performance of the blended MDEA-MEA

by addition of the third component (e.g., amine or additive). In

addition, at the end of the experiment, the total amine concentra-

tions of the three testing solutions were determined (by titration

with standard 1.0M HCl (Horwitz, 1975)) and compared with the

initial concentrations. It was found that the total amine concen-

trations were very close to the initial concentrations. Thus, it can

be said that the experimental set up as shown in Fig. 12 is effec-

tive. More importantly, the results obtained from this study are

reliable.

In the present work, the mass transfer performance of CO2

absorption intoaqueoussolutionsof blendedMDEA-MEAwas com-prehensively investigated in a DX structured packed column at

various operating conditions of MDEA-MEA blend ratio, temper-

ature, CO2 loading at the absorber top, and liquid flow rate. The

mass transfer performancewasevaluated in termsofKGavandCO2concentration profile along the height of the column. It was found

that themass transfer performanceincreasedas theratioof MEAin

the blended solution, temperature, and liquid flow rate increased

butdecreased asCO2loading increased(aspresentedin Figs.310)

over the testing conditions. In addition, the mass transfer perfor-

mance (in terms of both KGav and CO2 concentration profile) of

blended MDEA-MEA increased as the ratio of MEA in the blended

solution increasedas shown inFigs. 3,4, 6,8 and 10. This is because

MEAhasfasterreactionkineticsofCO2 absorption thanMDEA(Kohl

andNielsen,1997;Semaetal., 2012). The higherthe ratio ofMEA in

theblended solutions, thehigher theamounts of themore reactive

MEA molecules that can absorb CO2; thus, a higher mass transfer

performance was observed. By comparing the three MDEA-MEA

blendratios(27/3, 25/5, and23/7%wtMDEA-MEA), itwasobserved

that the23/7%wtMDEA-MEA provided the best mass transfer per-

formance among the three as shown in Figs. 3, 4, 6, 8 and 10.

However, in order to effectively use blended MDEA-MEA, not only

mass transfer performance,but also thecycliccapacityandrelative

regeneration ability shouldbe taken into consideration.

In summary, by increasing theratio ofMEAin theblended solu-

tion, the mass transfer performance would be increased but the

cyclic capacity and the relative solvent regeneration ability would

be decreased. As shown in Figs. 3, 4, 6, 8 and10, it can be observed

that themass transfer of 23/7%wtMDEA-MEA ismuchhigher than

those of 25/5 and 27/3%wt MDEA-MEA, respectively. On the other

hand, the cyclic capacities of the three blend ratios are consider-

ably close to each other as presented in Fig. 11. Therefore, it can be

reported that the effect of ratio ofMEA in the blendedMDEA-MEA

solutions on mass transfer performance is more significant than

that on cyclic capacity. As a result, based on above discussion, it

is reasonable to conclude that the 23/7%wt MDEA-MEA provides

the best CO2 absorption performance among the three blend-

ing ratios over the operating conditions conducted in the present

work.

4.7. Empirical predictive correlation for KGavof blended

MDEA-MEA

Liang et al. (2011) mentioned that in order to effectively

design an absorption column, the KGav is required. However, itis an expensive and time consuming process to experimentally

determine the KGav in a packed column. Thus, the predictive cor-relation for KGav i s then found to be important since it can be

used to determine the KGav from operating conditions withoutexperimental work. Dey and Aroonwilas (2009) proposed a pre-

dictive correlation for blended AMP-MEA. The model was tested

with the experimental mass transfer data of blended AMP-MEA

in a laboratory-scale absorption column packed with DX struc-

tural packing. They concluded that the predictive results were

found to be in good agreement with the experimental results.

For the MDEA-MEA system, the predictive correlation can be seen

in Eq. (3).

KGav= K eA(MDEA/MEA)

eB eCxCO2 LD1 eECs eF/T (3)

where KGav is the overall volumetric mass transfer coefficient(kmol/m3 hkPa), (MDEA/MEA) is themolar ratioofMDEAandMEA,

xCO2 is the mole fraction of CO2, is theCO2loading (mol CO2/molamine), Csis theconcentration of amine in thesolution (kmol/m

3),

L1 is the liquid flow rate (m3/m2 h), Tis the absolute temperature

(K), and K, A, B, C, D, E, and Fare the coefficients for the respective

parameters modeled in theproposed equation.Theexperimental values ofKGavobtainedin thepresentwork at

various operating conditions of MDEA-MEA blending ratios (27/3,

25/5, and23/7%wt), temperatures (298, 303, and318K), CO2load-

ings at the absorber top (0.05, 0.17, and 0.25mol CO2/mol amine),

and liquid flow rates (2.8, 3.8, and 5m3/m2 h) were then used to

correlate the predictive correlation in Eq. (3) using the nonlinear

regression analysis package NLREG (with a minimum confidence

level of 97%). The coefficients (K, A, B, C, D, E, and F) obtained

from the NLREG program are presented in Table 1. By compar-

ing the predicted and experimental results in the parity chart,

as shown in Fig. 14, it can be seen that the predicted values of

KGav calculated from Eq. (3) are in fairly good agreement withthe experimental valueswith an absolute average deviation (AAD)

of 21.8%.


9/10


Table 1

Summary of parameters for predictive equationpresented in Eq. (3).

Concentrationof

blended

MDEA-MEA (%wt)

Confidential level A B C D E F K AAD

27/3 0.97 0.0032 4 10.325 0.8531 0.1438 595.211 0.9254 20.9%

25/5 0.97 0 1.1 14.3 0.559 0.3665 511.5 1 21.7%

23/7 0.99 0.04 3.65 11.4 0.91 0.2 443 1 22.8%

Fig. 14. Parity chart compares predicted and experimental values ofKGav for CO2absorption intoblendedMDEA-MEA solutions.

5. Conclusions

Themass transfer performance of CO2 absorption into aqueous

solutions of blended MDEA-MEA was experimentally determined

(in terms ofKGavand CO2 concentration profile) in a laboratory-scale absorption column packed with DX structured packing. The

experiments were conducted at various operating conditions ofMDEA-MEA blending ratios (27/3, 25/5, and 23/7%wt), temper-

atures (298, 303, and 318K), CO2 loadings at the absorber top

(0.05, 0.17, and 0.25mol CO2/mol amine), and liquid flow rates

(2.8, 3.8, and 5m3/m2 h). The results show that the mass transfer

performance increasesas ratio ofMEAintheblendedsolution, tem-

perature,and liquidflow rate increasebutdecreases asCO2loading

increaseswithin the range of conditions using in thepresent work.

On the other hand, the cyclic capacity and the relative solvent

regeneration ability decrease as the ratio of MEA in the blended

solution increases. However, after taking into consideration all

three parameters (i.e., mass transfer performance, cyclic capac-

ity, and relative solvent regeneration ability), it can be concluded

that 23/7%wtMDEA-MEAprovides thebest CO2absorption perfor-

mance among the three blend ratiosover the operating conditionsconducted in thepresent work.

Acknowledgments

The first author (A. Naami) would like to acknowledge and

appreciate the scholarship support from the Libyan Higher Edu-

cational studies through thecultural section of theLibyanEmbassy

inOttawa, Canada. Thefinancial support from theNationalNatural

Science Foundation of China (NSFC No. 21276068, 21250110514,

and 21376067), Ministry of Science and Technology of the Peo-

ples of Republic of China (MOST No. 2012BAC26B01), Ministry

of Education of the Peoples of Republic of China-Supported Pro-

gram for Innovative Research Team in University (No. IRT1238),

Shaanxi Yanchang Petroleum (Group) Co., LTD, Chinas State

Project985inHunanUniversityNovelTechnologyResearchand

Development for CO2 Capture as well as Hunan University to

the Joint International Center for CO2 Capture and Storage (iCCS)

is gratefully acknowledged. In addition, we would also like to

acknowledge the research supports over the past many years of

the Industrial Research Consortium Future Cap Phase II of the

International Test Center for CO2Capture (ITC) at theUniversity of

Regina. We would also like to acknowledge the research support

from the following organizations: Natural Sciences and Engineer-

ing Research Council of Canada (NSERC), Canada Foundation for

Innovation (CFI), Saskatchewan Ministry of Energy & Resources,

Western Economic Diversification, Saskatchewan Power Corpo-

ration, Alberta Energy Research Institute (AERI), and Research

Institute of Innovative and Technology for the Earth (RITE).

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