analysis and assessment of advanced hydrogen …
TRANSCRIPT
ANALYSIS AND ASSESSMENT OF ADVANCED HYDROGEN
LIQUEFACTION SYSTEMS
by
Anwar Hammad
A Thesis Submitted in Partial Fulfillment
of the Requirement for Doctor of Philosophy Degree
in
Mechanical Engineering
Faculty of Engineering and Applied Science
University of Ontario Institute of Technology
Oshawa, Ontario, Canada
September 2019
© Anwar Hammad, 2019
i
THESIS EXAMINATION INFORMATION
Submitted by: Anwar Hammad
Ph.D. in Mechanical Engineering
Thesis title: ANALYSIS AND ASSESSMENT OF ADVANCED HYDROGEN LIQUEFACTION
SYSTEMS
An oral defense of this thesis took place on August 8, 2019 in front of the following examining committee:
Examining Committee:
Chair of Examining Committee Dr. Sayyed Hosseini
Research Supervisor Dr. Ibrahim Dincer
Examining Committee Member Dr. Martin Agelin-Chaab
Examining Committee Member
University Examiner
Dr. Haoxiang Lang
Dr. Mustafa El-Gindy
Eternal Examiner Dr. Ziad Sagir
The above committee determined that the thesis is acceptable in form and content and that a satisfactory knowledge of the field covered by the thesis was demonstrated by the candidate during an oral examination. A signed copy of the Certificate of Approval is available from the School of Graduate and Postdoctoral Studies.
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ABSTRACT
As global warming and energy crisis issues continue to increase, it becomes critical to
investigate new sources of clean and affordable energy. Liquid hydrogen, is an
attractive energy alternative as the byproduct of hydrogen combustion is non-pollution
and useful water vapor. High hydrogen liquefaction work represents the most important
obstacle to achieving feasibility in the hydrogen economy.
In this thesis, a hydrogen liquefaction system is analyzed by using a hydrogen
liquefaction method both with and without catalyst infused heat exchangers. The goal
is to assess, modify and improve the proposed systems with the ultimate goal of
achieving sustainable and environment friendly hydrogen production.
The main objective of this work is to present detailed thermodynamic,
environmental, and economic analyses of the proposed multi-generation energy
systems. The study shows that when compared to the primary (main system), significant
improvements in energy and exergy efficiencies can be made by modifying the system
by employing vortex tubes, Organic Rankine Cycle (ORC), and the aid of a catalyst. In
fact, at 25oC the overall exergy efficiency of a configuration employing ORC is 42%
as opposed to 12% for the main system. This system also has the highest energy
efficiency of 76% as oppose to 10% for the main base system.
KEYWORDS: Hydrogen Liquefaction, Exergy Analysis, Energy Analysis, System
Optimization
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STATEMENT OF CONTRIBUTIONS
I hereby certify that I am the sole author of this thesis and that no part of this thesis has been published or submitted for publication. I have used standard referencing practices to acknowledge ideas, research techniques, or other materials that belong to others. Furthermore, I hereby certify that I am the sole source of the creative works and/or inventive knowledge described in this thesis.
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AUTHORS DECLARATION
I hereby declare that this thesis consists of original work of which I have authorized.
This is a true copy of the thesis, including any required final revisions, as accepted by
my examiners. I authorize the Ontario Tech University to lend this thesis to other
institutions or individuals for the purpose of scholarly research. I further authorize
University of Ontario Institute of Technology to reproduce this thesis by photocopying
or by other means, in total or in part, at the request of other institutions or individuals
for the purpose of scholarly research. I under-stand that my thesis will be made
electronically available to the public.
Student name and signature
v
ACKNOWLEDGMENTS
I would like to express my most profound gratitude and appreciation to my supervisor,
Professor Ibrahim Dincer, for giving me this unique opportunity to work with him. His
immense knowledge, constant guidance, intellect, commitment, and passion as a
scientist encouraged me to be more dedicated throughout my research journey. Dr.
Dincer has unfailingly provided me with his expert guidance, which has significantly
helped me overcome the challenges I faced during my Ph.D. research. Without Dr.
Dincer’s patience, it would not have been easy to complete my research.
I am also grateful to my colleagues and friends in Professor Dincer`s research
group who have consistently given me both support and motivation.
Special thanks and love go to my beautiful wife, friends and family for their
support and encouragement to continue and finish my studies. They always been my
primary motivation during my stay in Canada. I am sincerely grateful for their sacrifice
and blessings.
Last but not the least; I would like to express my most profound love and thanks
to my mother for her endless support, determination, inspiration and reassurance
throughout my life. Her prayers and good wishes have always been with me. I will
always be grateful to her.
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TABLE OF CONTENTS ABSTRACT i
AUTHORS DECLARATION iii
ACKNOWLEDGMENTS v
NOMENCLATURE ix
LIST OF TABLES xi
LIST OF FIGURES xii
INTRODUCTION 1 1.1 Overview and Outlook for Hydrogen 1 1.2 Hydrogen Production and Storage 4 1.3 Hydrogen Liquefaction 5 1.4 Improvements in Hydrogen Liquefaction 7 1.5 Motivation and Novelties of the Thesis 8 1.6 Thesis Objectives 8 1.7 Thesis Outline 9
LITERATURE REVIEW 11 2.1 Ideal Work of Hydrogen Liquefaction 11 2.2 Hydrogen Liquefaction Plants 14 2.3 Previous and Current Plants 15 2.4 Advanced Liquefaction Techniques 17 2.5 Existing Large-Scale Plants 18 2.6 Lab Scale Hydrogen Liquefaction Methods 19 2.7 Future Developments and Presentation of New Conceptual Designs 21 2.8 Orthohydrogen and Parahydrogen 24 2.9 Catalyst Conversion of Ortho- to Parahydrogen 28 2.10 Organic Rankine Cycle (ORC) 29 2.11 Vortex Tube 29 2.12 Closing Remarks 30
SYSTEM DESCRIPTION 31 3.1 Description of System 1 33
3.1.1 System 1A: Reference system without a catalyst 33 3.1.2 System 1B: Reference system with a catalyst 34
3.2 Description of System 2 38
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3.2.1 System 2A: System with ORC and without a catalyst 38 3.2.2 System 2B: System with ORC and with a catalyst 39
3.3 Description of System 3 41 3.3.1 System 3A: System with Vortex Tubes and without a catalyst 42 3.3.2 System 3B: System with Vortex Tubes and with a catalyst 42
SYSTEM ANALYSIS, MODELLING AND SIMULATION 45 4.1 Basic Thermodynamic Concepts 45 4.2 Conservation of Mass Principle 46 4.3 Conservation of Energy Principle 46 4.4 Entropy Balance and Entropy Generation 46 4.5 Exergy Analysis 47 4.6 Components used in the systems 49 4.7 Energy and Exergy Efficiencies 51 4.8 Sustainability Assessment 53 4.9 Exergoeconomic Assessment 54 4.10 Environmental Impact Assessment 56 4.11 Optimization Study 57
RESULTS AND DISCUSSION 60 5.1 Base System Results 60 5.2 Systems 1A and 1B Results 73
5.2.1 Pre-cooling phase at systems S1A and S1B 77 5.2.2 Liquefaction Phase at systems S1A and S1B 83
5.3 Systems 2A and 2B Organic Rankine Cycles 92 5.3.1 Pre-cooling phase at systems S2A and S2B 96 5.3.2 Liquefaction Phase at systems S1A and S1B 101
5.4 Systems 3A and 3B – Vortex tubes 111 5.4.1 Pre-cooling phase at systems S3A and S3B 112 5.4.2 Liquefaction Phase at systems S3A and S3B 118
5.5 Property set 129 5.6 Comparative analysis results 131 5.7 Optimization results 133
5.7.1 Objective function 133 5.7.2 Design conditions 133 5.7.3 Variables 134 5.7.4 Constraints 134
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5.8 Optimum case 134 5.9 Simulation comparison 135
CONCLUSIONS AND RECOMMENDATIONS 138 6.1 Conclusions 138 6.2 Recommendations 139
REFERENCES 140
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NOMENCLATURE
c Unit cost of exergy, $/kJ E Energy, kJ ex Specific exergy, kJ/kg Ex Exergy rate, kW 𝐸��& Exergy destruction rate, kW h Specific enthalpy, kJ/kg K Equilibrium constant m Mass, kg �� Mass flow rate, kg/s 𝑛 Number of moles �� Molar flow rate, mole/s P Pressure, kPa �� Heat flow rate, kW s Specific entropy, kJ/kg K ��+,- Entropy generation rate, kW/K T Temperature, °C or K t Time, s u Specific internal energy, kJ/kg U Internal energy, kJ V Volume, m3 𝜐 Specific volume, m3/kg Y Yield, % �� Capital cost, $ Greek Letters 𝜌 Density, kg/m3 𝜂 Energy efficiency 𝜓 Exergy efficiency Subscripts 0 Ambient conditions cv Control volume cold Cold ex Exergy hot Hot in Inlet out Exit consumption Consumption loss loss Q Thermal energy s Surface w Work
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Acronyms CHP Combined heat and power CPV Concentrated photovoltaic COP Coefficient of Performance EX Expansion valve HE Heat Exchanger EX Heat Exchanger HX Heat Exchanger H2FEED Hydrogen Feed LH2 Liquified Hydrogen LN2 Liquid Nitrogen LNG Liquefied Natural gas ORC Organic Rankine Cycle SI Sustainability Index TPD Ton per Day VT Vortex Tube
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LIST OF TABLES
Table 1.1 Exergy efficiencies of different renewable energy hydrogen production methods
before and after liquefaction. ..................................................................................................... 5
Table 2.1 Details of Commercial Hydrogen Liquefaction Plants ............................................ 20
Table 2.2 Efficiencies of some conceptual plants .................................................................... 25
Table 4.1. Energy Balance for System Components ............................................................... 50
Table 4.2 Base system components exergy equations ............................................................ 55
Table 4.3. Properties of Hydrogen, Nitrogen and Carbon dioxide .......................................... 59
Table 5.1 Constraints of Selected Variables .......................................................................... 134
Table 5.2 Simulation and initial experimental data of the proposed system ......................... 137
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LIST OF FIGURES
Figure 1.1 Total Primary Energy Consumption – World (data from [1]) .................................. 1
Figure 1.2 World primary energy consumption (data from [3]) ................................................ 2
Figure 1.3 Generalized process flow for syngas production and industrial hydrogen (adapted
from [8]). .................................................................................................................................... 4
Figure 1.4 Graphical overview of hydrogen storage technologies (adapted from [9]) .............. 5
Figure 1.5 Schematic of the Vortex Tube, showing the inlet (𝑉𝑖𝑛) and cold (𝑉𝑐𝑜𝑙𝑑) and hot (𝑉ℎ𝑜𝑡)
outlets. Adapted from [13] ......................................................................................................... 7
Figure 2.1 Linde-Hampson liquefaction cycle schematic representation (adapted from [29]).
.................................................................................................................................................. 13
Figure 2.2 Schematic representation of the Claude cycle (adapted from [30]). ...................... 14
Figure 2.3 Flow chart of the hydrogen liquefier system [25]. ................................................. 16
Figure 2.4 Praxair hydrogen liquefaction process flow diagram (left) and improved hydrogen
liquefaction process flow diagram (right), adapted from [38]). ............................................... 17
Figure 2.5 Basic scheme of hydrogen liquefaction process based on renewable energy (adapted
from [52]). ................................................................................................................................ 23
Figure 2.6 Equilibrium composition as a function of temperature (adapted from [54]) .......... 26
Figure 2.7 Spin isomers of molecular hydrogen (adapted from [57]). .................................... 26
Figure 3.1 Advanced hydrogen liquefaction systems considered for analysis. ....................... 31
Figure 3.2 The main systems schematic diagram .................................................................... 35
Figure 3.3 Schematic diagram for the reference system .......................................................... 36
Figure 3.4 Schematic diagram for the main system with reactor ............................................ 37
Figure 3.5 Organic Rankine Cycles ......................................................................................... 38
Figure 3.6 Schematic diagram for the system with ORCs ....................................................... 40
Figure 3.7 Schematic diagram for the system ORCs and reactor ............................................ 41
Figure 3.8 Added VTs .............................................................................................................. 42
Figure 3.9 Schematic diagram for the system with VTs and no reactor .................................. 43
Figure 3.10 Schematic diagram for the system with VTs and reactor ..................................... 44
Figure 5.1 Exergy and energy efficiencies for each component .............................................. 61
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Figure 5.2 Effect of pre-cooling Hydrogen on Compressors work losses and yield. The graphs
illustrate a) work loss of compressors C1, C2, and C3 and, b) daily hydrogen yield against
hydrogen Feed temperature ...................................................................................................... 62
Figure 5.3. Effect of pre-cooling hydrogen on overall energy and exergy efficiencies .......... 63
Figure 5.4. Effect of hydrogen mass flow rate change on work and yield. The graphs show a)
work (kW) of (a)Turbo expander (TE1), Turbo expander (TE2), (b)Adsorber (A0) and; (c)
Liquid hydrogen generation per day against hydrogen Feed Mass Flow rate (kg/h) .............. 64
Figure 5.5 Effect of hydrogen mass flow rate change on the overall energy and exergy
efficiencies ............................................................................................................................... 65
Figure 5.6 Effect of changing turbo expander TE1 pressure on main components work ....... 65
Figure 5.7 Effect of changing turbo expander TE1 pressure on the overall energy and exergy
efficiencies ............................................................................................................................... 66
Figure 5.8 Effect of changing flash drums pressure on the overall yield ................................ 67
Figure 5.9 Effect of flash drum pressure rate change on overall energy and exergy .............. 67
Figure 5.10 Effect of hydrogen feed pressure change on the compressors ............................. 68
Figure 5.11 Effect of hydrogen feed pressure change on the overall energy and exergy
efficiencies ............................................................................................................................... 68
Figure 5.12 Effect of the Nitrogen gas mass flow rate change on the overall energy and exergy
efficiencies ............................................................................................................................... 69
Figure 5.13 Heat Exchanger HX1 Heat composite curves ...................................................... 70
Figure 5.14 Heat Exchanger HX2 Heat composite curves ...................................................... 71
Figure 5.15 Heat Exchanger HX3 Heat composite curves ...................................................... 72
Figure 5.16 Energy efficiencies for main components of System 1A and 1B ......................... 74
Figure 5.17. Effect of hydrogen feed pressure on overall efficiencies for System 1A ............ 75
Figure 5.18 Effect of Compressor C1 pressure on overall efficiencies for System 1A ........... 75
Figure 5.19 Effect of Compressor C2 pressure overall efficiencies for System 1A ................ 75
Figure 5.20 Effect of hydrogen H2 feed pressure on overall efficiencies for System 1B ........ 76
Figure 5.21 Effects of Compressor C2 pressure on overall energy and exergy efficiencies for
System 1B ................................................................................................................................ 76
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Figure 5.22 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream N2LIQ inlet for System 1A ............................................................................. 77
Figure 5.23 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 28 inlet for System 1A ..................................................................................... 78
Figure 5.24 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 9 inlet for System 1A ....................................................................................... 78
Figure 5.25 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 36 inlet for System 1A ..................................................................................... 79
Figure 5.26 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 44 inlet for System 1A ..................................................................................... 79
Figure 5.27 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream R inlet for System 1A ...................................................................................... 80
Figure 5.28 Heat Load, Exergy flow vs Temperature for Precooling Phase heat exchanger HX1
at stream N2LIQ inlet for System 1B ...................................................................................... 80
Figure 5.29 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 28 inlet for System 1B ..................................................................................... 81
Figure 5.30 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 9 inlet for System 1B ....................................................................................... 81
Figure 5.31 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 36 inlet for System 1B ..................................................................................... 82
Figure 5.32 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 44 inlet for System 1B ..................................................................................... 82
Figure 5.33 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX2 at stream 49 inlet for System 1A ..................................................................................... 83
Figure 5.34 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX2 at stream 13 inlet for System 1A ..................................................................................... 84
Figure 5.35 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream S10 inlet for System 1A ................................................................................... 84
Figure 5.36 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 14 inlet for System 1A ..................................................................................... 85
Figure 5.37 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 22 inlet for System 1A ..................................................................................... 86
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Figure 5.38 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 32b inlet for System 1A ................................................................................... 86
Figure 5.39 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream S1 inlet for System 1A ..................................................................................... 87
Figure 5.40 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 17 inlet for System 1A ..................................................................................... 87
Figure 5.41 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream S10 inlet for System 1B ................................................................................... 88
Figure 5.42 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 14 inlet for System 1B ..................................................................................... 88
Figure 5.43 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 13 inlet for System 1B ..................................................................................... 89
Figure 5.44 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 49 inlet for System 1B ..................................................................................... 90
Figure 5.45 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 22 inlet for System 1B ..................................................................................... 90
Figure 5.46 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 17 inlet for System 1B ..................................................................................... 91
Figure 5.47 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream S1 inlet for System 1B ..................................................................................... 91
Figure 5.48 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 32b inlet for System 1B ................................................................................... 92
Figure 5.49 Systems (a) 2A and (b) 2B Exergy and Energy efficiencies for each component 93
Figure 5.50. Effect of pre-cooling hydrogen feed pressure variations on overall energy and
exergy efficiencies for systems (a) S2A and (b) S2B .............................................................. 94
Figure 5.51 Effect of Compressor 1 (C1) pressure variations on overall energy and exergy
efficiencies for systems (a) S2A and (b) S2B .......................................................................... 95
Figure 5.52 Effect of Compressor 5 (C5) pressure variations on overall energy and exergy
efficiencies for systems (a) S2A and (b) S2B .......................................................................... 96
Figure 5.53 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream N2LIQ inlet for System 2A ............................................................................. 97
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Figure 5.54 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 28 inlet for System 2A ..................................................................................... 97
Figure 5.55 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 9 inlet for System 2A ....................................................................................... 98
Figure 5.56 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 36 inlet for System 2A ..................................................................................... 98
Figure 5.57 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 44 inlet for System 2A ..................................................................................... 99
Figure 5.58 Heat Load, Exergy flow vs Temperature for Precooling Phase heat exchanger HX1
at stream N2LIQ inlet for System 2B ...................................................................................... 99
Figure 5.59 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 28 inlet for System 2B ................................................................................... 100
Figure 5.60 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 9 inlet for System 2B ..................................................................................... 100
Figure 5.61 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 36 inlet for System 2B ................................................................................... 101
Figure 5.62 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 44 inlet for System 2B ................................................................................... 102
Figure 5.63 Heat Load, Exergy flow and Temperature for liquefaction Phase heat exchanger
HX2 at stream 49 inlet for System 2A ................................................................................... 102
Figure 5.64 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 13 inlet for System 2A ................................................................................... 103
Figure 5.65 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 26 inlet for System 2A ................................................................................... 103
Figure 5.66 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 14 inlet for System 2A ................................................................................... 104
Figure 5.67 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 22 inlet for System 2A ................................................................................... 105
Figure 5.68 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 32b inlet for System 2A ................................................................................. 105
Figure 5.69 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream S1 inlet for System 2A ................................................................................... 106
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Figure 5.70 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 17 inlet for System 2A ................................................................................... 106
Figure 5.71 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 49 inlet for System 2B ................................................................................... 107
Figure 5.72 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 26 inlet for System 2B ................................................................................... 108
Figure 5.73 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 14 inlet for System 2B ................................................................................... 108
Figure 5.74 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 13 inlet for System 2B ................................................................................... 109
Figure 5.75 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 22 inlet for System 2B ................................................................................... 109
Figure 5.76 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 17 inlet for System 2B ................................................................................... 110
Figure 5.77 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream S1 inlet for System 2B ................................................................................... 110
Figure 5.78 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 32b inlet for System 2B ................................................................................. 111
Figure 5.79 Systems 3A and 3B Exergy and Energy efficiencies for each component ........ 112
Figure 5.80 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream N2LIQ inlet for System 3A ........................................................................... 113
Figure 5.81 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 28 inlet for System 3A ................................................................................... 114
Figure 5.82 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 9 inlet for System 3A ..................................................................................... 114
Figure 5.83 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 36 inlet for System 3A ................................................................................... 115
Figure 5.84 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 44 inlet for System 3A ................................................................................... 115
Figure 5.85 Heat Load, Exergy flow vs Temperature for Precooling Phase heat exchanger HX1
at stream N2LIQ inlet for System 3B .................................................................................... 116
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Figure 5.86 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 28 inlet for System 3B ................................................................................... 116
Figure 5.87 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 9 inlet for System 3B ..................................................................................... 117
Figure 5.88 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 36 inlet for System 3B ................................................................................... 117
Figure 5.89 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger
HX1 at stream 44 inlet for System 3B ................................................................................... 118
Figure 5.90 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 49 inlet for System 3A ................................................................................... 119
Figure 5.91 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 13 inlet for System 3A ................................................................................... 119
Figure 5.92 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream S10 inlet for System 3A ................................................................................. 120
Figure 5.93 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 14 inlet for System 3A ................................................................................... 121
Figure 5.94 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 22 inlet for System 3A ................................................................................... 122
Figure 5.95 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 32b inlet for System 3A ................................................................................. 122
Figure 5.96 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream S1 inlet for System 3A ................................................................................... 123
Figure 5.97 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 17 inlet for System 3A ................................................................................... 123
Figure 5.98 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 49 inlet for System 3B ................................................................................... 124
Figure 5.99 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream S10 inlet for System 3B ................................................................................. 125
Figure 5.100 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 14 inlet for System 3B ................................................................................... 126
Figure 5.101 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX2 at stream 13 inlet for System 3B ................................................................................... 126
xix
Figure 5.102 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 22 inlet for System 3B ................................................................................... 127
Figure 5.103 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 17 inlet for System 3B ................................................................................... 127
Figure 5.104 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream S1 inlet for System 3B ................................................................................... 128
Figure 5.105 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger
HX3 at stream 32b inlet for System 3B ................................................................................. 128
Figure 5.106 T-xy plot for temperature versus liquid composition for isobaric data ........... 129
Figure 5.107 T-x plot for temperature versus liquid composition for isobaric data .............. 130
Figure 5.108 K-values for Vapor-liquid vs fraction of Para-hydrogen and Ortho Hydrogen 130
Figure 5.109 y-x diagram for vapor vs liquid composition for the para-hydrogen ............... 131
Figure 5.110 Activity coefficients vs mole fraction for Para-hydrogen and orthohydrogen . 131
Figure 5.111 Exergy efficiency for the proposed hydrogen liquefaction systems at 0°C, 10°C,
25°C, and 45°C. ..................................................................................................................... 132
Figure 5.112 Energy efficiency for the hydrogen liquefaction systems. ............................... 132
Figure 5.113 Work done for liquefaction per unit mass (kJ/kg). ........................................... 133
Figure 5.114 Overall exergy efficiency ................................................................................. 135
Figure 5.115 Optimized system ............................................................................................. 136
1
INTRODUCTION
1.1 Overview and Outlook for Hydrogen
Over the last few decades, global energy consumption has grown continuously and this
trend is expected to continue as presented in Figure 1.1 [1]. This significant growth has
created considerable interest in sustainable energy, including hydel, wind, solar, and
geothermal resources. Over the past century, factors such as population growth,
increasing water demand, industrial development, and the bulk production of agriculture
products have also motivated the research and development of substitutes for fossil fuels.
Figure 1.1 Total Primary Energy Consumption – World (data from [1])
The share of sustainable energy in the global market is around 24% as of 2017
and advancement is dependent on technological developments, society, world politics,
and the environment. Over 75% of the total energy consumption comes from fossil
fuels, according to the World Bank data. Additionally, carbon-based fuels are very
harmful to the environment, causing 87% of carbon emissions [2]. Hence, a substitute
for fossil fuels is required since they are finite resources that may create and cause
future crises and instability. Alternative clean energy sources are needed to meet rising
energy demands in an environmentally friendly manner. As clean energy sources,
renewable energies can be considered as sustainable alternatives due to their significant
advantages over fossil fuels. Figure 1.2 shows the energy share between traditional and
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renewable sources. Hydrogen, which is considered as a sustainable energy source, can
be considered in direct competition with electricity as an energy carrier, with each using
a separate production and distribution system.
Figure 1.2 World primary energy consumption (data from [3])
As an energy carrier, hydrogen plays a crucial part in the reduction of
greenhouse gas emissions thereby curtailing the effects of climate change. It is therefore
often considered as the energy carrier of the future. Since it can primarily be produced
from water, hydrogen can provide a solution to issues of sustainability, greenhouse gas
and other pollutant emissions, and also offer energy security. A hydrogen economy that
is the same size as the U. S. would require approximately 150 million tons per year of
hydrogen for transportation, which would be equal to the consumption of two to five
billion tons of water, taking into account current hydrogen production efficiencies. This
consumption would be considerably less than the current consumption of water for
thermoelectric power generation in the U. S. in power plants, which is approximately
300 billion tons, while an additional US1.2 billion 1.2 billion is spent in the process of
gasoline production. Therefore, the most likely scenario is that the hydrogen economy
could significantly reduce water consumption in the process of energy generation [4].
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140000
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180000
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
Ener
gy (T
Wh)
Year
Coal Solar Crude oil Natural gas
Traditional biofuels Other renewables Hydropower Nuclear
3
Hydrogen energy content per weight is around 125 MJ kg-1, over two times
higher than any other fuel currently in use. Fossil fuels have a weight in the range of
20-50 MJ kg-1, with diesel fuel at the high end and natural gas at the low end, while
batteries have 0.1-0.5 MJ kg-1 [5]. This energy outcome makes hydrogen the most
effective energy carrier. As a result, considerable effort has been expended on
improving its production and storage. However, energy content per volume of hydrogen
is relatively low when not highly compressed or liquefied. Even then, it is significantly
lower than that of fossil fuels: 8 MJ L-1 for liquid hydrogen, the most efficient form of
hydrogen storage, in comparison to 32 MJ L-1 for gasoline. In spite of intense research
efforts that have been devoted to the development of more efficient means of storing
hydrogen, hydrogen liquefaction remains the most economical method of hydrogen
storage to date, regardless of its deficiencies. Therefore, when considering the
efficiency of hydrogen production, the cost of liquefaction should be considered.
The phrase “hydrogen economy”, which was first used by John Bockris in 1970,
refers to a proposed system of energy delivery utilizing hydrogen as an energy carrier.
This concept is meant to alleviate some of the adverse effects of hydrocarbon fuel
consumption as both the primary source of energy and as the main energy carrier. Since
burning fossil fuels leads to the emission of carbon dioxide and other pollutants that
have considerable adverse effects on the environment, a global hydrogen economy is
seen as an environmentally eco-friendlier alternative for delivering energy to end-users,
particularly in transport. A 2004 study found that "most of the hydrogen supply chain
pathways would release significantly less carbon dioxide into the atmosphere than
would gasoline used in hybrid electric vehicles" and that significant reduction in carbon
dioxide emissions could result from utilizing carbon capture or carbon sequestration
technology at the site of energy or hydrogen production [6].
If hydrogen is considered to be a renewable fuel for the future, with the
numerous challenges that come with its production, storage and use, the issue of
efficiency is considered as just one of the many factors that determine the viability and
usability of these systems. The investment of both financial and other resources in
durability, and the stability of operation along with safety are important parameters in
determining the viability of each proposed solution that might become a part of the
hydrogen cycle.
4
1.2 Hydrogen Production and Storage
Most of the hydrogen produced today is obtained from methane reforming, with carbon
dioxide as the side product of the reaction. A generalized process flow is shown in
Figure 1.3. Since this increase greenhouse gas emission, significant efforts have been
invested in the exploration of alternative methods of hydrogen production that rely on
renewable energy. The use of hydrogen produced in this alternative process could
contribute to a reduction in the level of greenhouse gases. There are several of these
methods, which differ significantly in their efficiencies [7].
Figure 1.3 Generalized process flow for syngas production and industrial hydrogen
(adapted from [8]).
Hydrogen storage includes a number of methods that can be broadly divided
into three groups: mechanical storage: storage in a solid material through physisorption:
and solid material storage through chemical bonding or chemisorption. Each of these
has advantages and disadvantages, and a graphical overview of their capabilities is
given in Figure 1.4.
In order to overcome storage issues, hydrogen has been liquefied at high cost
but room for improvement in the liquefaction process is still present. Table 1.1 shows
the efficiencies of different renewable energy sources used in hydrogen production
before and after liquefaction. The difference in efficiencies clearly show that
liquefaction has a great room of improvement through advanced research and
development studies.
Cool/Biomass
Natural gas/ fuel oil
Natural gas/ naphtha
Syngas(H2, CO, CO2, H2O)
O2 and/or Air Steam
(Water- gas shiftH2,O, H2, CO, CO2,
shift to H2 and CO2)
(H2 and CO2 separation)
PSA, physical absorption
H2 and CO2 to ammonia and F-T Processes
CO2:- Vented - To urea production - Enhanced oil recovery
Gasifier
Partial OXI (POX)
Reformer (SMR/ATR)
Shift Reactor Gas Clean up
O2 and/or Air Steam
Feedstock in:
5
Figure 1.4 Graphical overview of hydrogen storage technologies (adapted from [9])
Table 1.1 Exergy efficiencies of different renewable energy hydrogen production methods before and after liquefaction.
Method Exergy efficiency after electrolysis (%)
Exergy efficiency after liquefaction (%)
Photovoltaic solar 7.2 1.0 Photothermal solar 8.6 1.2 Wind power 30.8 4.1 Hydropower 41.6 5.6 Biomass Combustion 27.5 3.9 Biomass Gasification 40.6 5.4
Source: [7].
Mechanical storage methods, in both gas and liquid form, are the most common
hydrogen storage methods used today. Storage of liquid hydrogen creates less risk than
high-pressure gas storage and represents the dominant form of hydrogen storage for
large-scale production and transport.
1.3 Hydrogen Liquefaction
Liquefied hydrogen has thus far played a role mostly as fuel for space exploration and
other related applications, as well as in the semiconductor industry. However, with a
shift in the energy landscape towards clean energy, it is expected that liquid hydrogen
will be used as a future energy carrier, both for automotive transportation and long
6
distance and overseas transport. In order to achieve this, hydrogen liquefaction
technology requires significant improvements to reduce energy consumption from the
current levels of 12-15 kWh per kg of liquid hydrogen to about 6 kWh per kg. This can
only be achieved through a series of improvements in the different stages of hydrogen
liquefaction. Some of them are already available, such as: improved operation of the
recycled gas compression system that applies chillers; closed refrigeration loops for
pre-cooling; upgraded turbine designs; and adjusted concepts for the main refrigeration
loop. Adoption of these technologies could reduce energy consumption to 7.5 to 9 kWh
per kg of liquid hydrogen [34]. Further improvements could be gained by applying
more innovative process schemes, such as that described above, as well as improved
machinery and equipment.
Some of those improvements could include turboexpanders. They are
considered to be one of the most challenging aspects of hydrogen liquefaction [10],
where low molecular weight and size, as well as the high speed of sound of hydrogen,
require very high peripheral speeds. The material properties and the difficulty presented
in forming reliable seals, along with the propensity of hydrogen to embrittle materials,
restrict the speed as well as limit each stage to a low-pressure ratio. In addition,
hydrogen gas must be thoroughly purified to remove oxygen and other possible
contaminants that would freeze in the system, clogging the Joule-Thomson valve or
damaging an expander. The presence of frozen oxygen in the product tank also
represents a potential explosion hazard, therefore purification requirements for
hydrogen liquefiers can be as high as 1 ppm. A number of authors point out that due to
the long development history and long-term qualification procedures, improvements in
modern components can be difficult to achieve and implement, and their
implementation could most likely be achieved only on a long-term basis [11,12], due
to a relatively small number of plants in operation. However, if more hydrogen
liquefaction plants are set up due to increasing demand, it may become economically
feasible to develop new components specifically designed for liquefaction cycle flow
rates and pressure levels, providing an additional increase in the efficiency of the
liquefaction process.
Other techniques to improve the liquefaction process could include Vortex
tubes. The Vortex tube devices cool a fluid by separating the inlet flow into an inner
cold and outer hot stream within the tube, as described in Figure 1.6. To achieve higher
7
efficiency, the components in the streams have to compress. Vortex tubes can be
considered to be effective solutions for heat exchange applications. As shown in Figure
1.6, the fluid is tangentially injected at high pressure and expanded in the tube. The
fluid flows into the tube at a high velocity along the sidewall, and a cold stream is then
created through the expansion of the centre of the tube. Fluid with a higher temperature
than Vin ejects from the Vhot outlet and the flow that has not been ejected flows back
into the center of the tube, exiting through the Vcold outlet[13].
Figure 1.5 Schematic of the Vortex Tube, showing the inlet (𝑉𝑖𝑛) and cold (𝑉𝑐𝑜𝑙𝑑) and hot (𝑉ℎ𝑜𝑡) outlets. Adapted from [13]
1.4 Improvements in Hydrogen Liquefaction
Krasae-in et al. proposed a series of improvements to the overall design of the hydrogen
liquefaction process, primarily through the application of a multi-component refrigerant
refrigeration system [14,15]. The new system uses a mixture of 4% neon, 12% nitrogen,
26% methane, 30% ethane, and 28% butane as a coolant, resulting in lower power
consumption (for the production of 100 tons per day) on the pre-cooling, compared to
conventional refrigeration systems with 1.76 kWh per kg of hydrogen, compared to
4.86 kWh per kg for an actual hydrogen liquefaction plant in Ingolstadt (capacity 4.4
tons per day) [16]. This could reduce the specific energy consumption for liquefaction
of the overall cycle from 13.58 to 5.35 kWh per kg of hydrogen.
In the near future, the application of these improvements, the development of
several new improved technologies and reduced costs due to economies of scale could
make hydrogen liquefaction an economically viable solution for energy storage in a
renewable energy sourced hydrogen economy.
Vin
Vhot
Vcold
8
1.5 Motivation and Novelties of the Thesis
With the current rapid worldwide energy consumption, fossil-fuel reserves prove to be
in constant reduction [17]. In 2012, the total world energy consumption amounted to
580 kJ, which is expected to rise to 711 kJ by 2025 and 860 kJ in 2040 [1]. In 2016,
the U.S. Energy Information Administration reported that the global use of petroleum
and other liquid fossil fuels had risen from 67.2 million barrels per day in1990 to 90.3
million barrels per day in 2012, and would rise to 109.1 million barrels per day in 2030,
and even 120.9 million barrels per day by 2040.
Growing concerns over greenhouse gas emissions from the combustion of fossil
fuels, and an awareness of the need for a clean high-energy fuel, have prompted interest
in the production of hydrogen. Building a hydrogen-based economy for a sustainable
energy system is the long-term view of many [18]. The main goal of building a
hydrogen economy is to replace fossil-based energy sources with hydrogen. The
technology of producing, liquefying, and storing hydrogen is vital for its feasibility.
Therefore, it is extremely important to build a sophisticated and efficient system that is
a feasible replacement for another energy source.
Hydrogen energy systems represent a potential solution for these highly
important problems, where the requirement is to deliver high-efficiency output while
lowering the total emissions per energy used.
From the open literature, it can be seen that researchers and scientists have
attached considerable weight to hydrogen production systems. However, there has not
yet been sufficient research on hydrogen liquefaction systems, especially catalyst-based
models.
The systems presented in the thesis will contribute in the following: (i) new
advanced liquefaction system configurations, (ii) ull and comprehensive analysis for
systems and (iii) simulation of each system.
1.6 Thesis Objectives
The main objective of this work is to outline the novel advanced hydrogen liquefaction
systems. This will include energy, exergy and environmental analyses to compare the
studied systems. In more detail, these objectives can be listed as follows:
9
• To develop and design three advanced hydrogen liquefaction systems with
two configurations each based on a patented commercial system number
US8042357B2 [19] that was never studied and analyzed.
• To perform comparative analysis and simulate multiple advanced hydrogen
liquefaction systems using ASPEN plus.
• To build a novel configuration of an advance hydrogen liquefaction system,
involving the development of a complete thermodynamic model with full
exergy analysis of the proposed systems, including calculating exergy flow,
energy and exergy efficiency, exergy destruction ratios, and other related
thermodynamics measures.
• To have environmental impact assessment of each of the proposed systems
by studying the CO2 emission and sustainability indices.
• To conduct a parametric study to evaluate system performance utilising
parametric study on individual components and the effect of environmental
conditions on each system.
• To determine the optimum design parameters through an optimization
analysis of the proposed liquefaction system using Matlab.
1.7 Thesis Outline
This thesis consists of six main chapters. The first chapter includes an introduction and
background information regarding the hydrogen economy and hydrogen liquefaction
developments over time. Furthermore, the novelties of the proposed integrated systems,
together with the motivation and objectives of this thesis, are included. Chapter 2
provides a comprehensive literature review of different advanced liquefaction
techniques, ortho- parahydrogen and catalysts. Moreover, a literature review of the
different components that will be utilized in the proposed integrated systems, such as
Organic Rankine Cycles (ORC) and Vortex Tubes (VT), is incorporated. Chapter 3
explains in detail the proposed systems and their components. Chapter 4 contains the
general thermodynamic equations that are used to model the introduced integrated
systems along with detailed thermodynamic modelling for the main components in each
integrated system. An exergoeconomic analysis is the main part of the system along
with optimization. Chapter 5 shows the results of the systems, combined with a
comprehensive comparison. The results of the exergoeconomic analysis and an
10
optimization study for each system are also provided. Chapter 6 highlights conclusions
from the thesis together with recommendations for future studies.
11
LITERATURE REVIEW
The significance of hydrogen as a clean energy source in the present world has already
been explained in Chapter 1. This chapter provides a brief literature review of the
hydrogen liquefaction process which is essential for using hydrogen as a fuel. Hydrogen
was first liquefied by Sir James Dewar in 1898 [20]. Following this significant step,
several procedures were developed for hydrogen liquefaction, forming a broad range of
technological solutions from laboratory liquefaction apparatus to large-scale plants
[21]. In this present review, developments from 1898 to 2016 are presented.
Hydrogen has shown potential as an important energy carrier for use in
transportation vehicles of the future, leading to considerable hydrogen research activity.
The greatest challenge today is the relatively low efficiency of the currently used
liquefaction plant cycles. Several recent studies have explained methods and ways to
overcome efficiency issues, where some have proposed conceptual plants with
efficiencies that can be increased up to 40– 60% [20].
In this chapter, the literature available regarding the characteristics of hydrogen
are discussed first. The process of hydrogen liquefaction and its evolution over time is
then covered followed by a discussion on laboratory scale hydrogen liquefaction
processes. The chapter concludes by identifying the literature gaps and the necessity
for researching the various hydrogen liquefaction systems.
2.1 Ideal Work of Hydrogen Liquefaction
The first successful hydrogen liquefaction was achieved in 1898 by a small device made
and invented by Scottish scientist James Dewar [22,23]. Dewar’s process used a
combination of carbolic acid and liquid air to pre-cool compressed hydrogen gas at 180
bars and the Joule-Thompson effect for liquefaction [20]. The amount of work required
by a reversible cycle to bring hydrogen from the initial conditions, e.g. 300K, 100 kPa,
and 25% parahydrogen, to the final liquid state at 100 kPa and equilibrium
parahydrogen content is referred to as the ideal work of hydrogen liquefaction.
Most current hydrogen liquefier systems utilize steady flow processes,
including the pre-cooled Linde-Hampson cycle, the Claude cycle and the helium
hydrogen condensing cycle [24]. The choice of a particular thermodynamic cycle
12
depends on the projected size of the plant, the available level of technology, equipment
cost and, principally, cycle efficiency[25].
The most simplified hydrogen liquefaction cycle is the Linde-Hampson or
Joule-Thomson expansion cycle, as shown in Figure 2.1. The process consists of
compressing gas at ambient pressure, cooling it in a heat exchanger and then passing it
through a throttle valve producing liquid through isenthalpic Joule-Thomson
expansion. The liquid product is then collected and removed, while the cooled gas is
returned to the compressor through the heat exchanger. However, unlike most gasses,
hydrogen warms under expansion at room temperature, and therefore requires pre-
cooling to the temperature below the corresponding inversion temperature (which
depends on pressure), which is typically 78 K. This is usually accomplished using liquid
nitrogen as a coolant, where nitrogen gas can be recovered and then reused in a
continuous refrigeration loop[26,27].
Most large-scale hydrogen liquefaction processes are based on the Claude cycle,
as illustrated in Figure 2.2, where hydrogen is both the product and the working fluid
[16,21]. One or more heat exchangers reduce the temperature of the working fluid and
a Joule-Thomson valve brings the fluid into the two-phase regime when the saturated
liquid is removed from the cycle. The input of gas at the warm end maintains a constant
mass of hydrogen in the system. Modifications of the Claude cycle include the addition
of a second compressor, where the first compresses hydrogen from low to medium
pressures and the second compresses from medium to high pressures. In this case, the
expander operates between medium and low pressures, providing additional cooling to
the high-pressure gas through its exhaust. Variations of this system are often used in
large-scale hydrogen liquefaction plants [23,28], combined with nitrogen pre-cooling,
multiple ortho-para conversion catalysts and, typically, two or three expanders.
One cycle that can be considered as a combination of a helium refrigerator
(Claude cycle) and a hydrogen liquefier (pre-cooled Linde-Hampson) cycle is the
helium hydrogen condensing cycle that utilizes helium as a primary refrigeration
working fluid. The main advantage of this cycle is its safety features, where hydrogen
compression is relatively limited and only increases to a high enough pressure to
overcome the pressure drop in the heat exchangers. Considering this pressure change,
and with the helium gas temperature at below 20 K, complete liquefaction of hydrogen
13
can be achieved after the expansion by using the correct ratio of flow rates of helium
and hydrogen. However, a return hydrogen stream can be constructed by the heat of
conversion of ortho- to parahydrogen in the liquid hydrogen receiver.
Figure 2.1 Linde-Hampson liquefaction cycle schematic representation (adapted from
[29]).
A comparison of these three hydrogen liquefaction cycles [24] shows that while
the Linde-Hampson and Claude cycle have a liquid hydrogen yield of 12-20%, helium
hydrogen condensing achieves a 100% yield for normal hydrogen and a 54% yield for
parahydrogen production. However, energy-wise, the Claude cycle is the most efficient
14
with an energy cost of 100-140 MJ kg-1, followed by helium hydrogen condensing with
120-200 MJ kg-1, and Linde-Hampson with a considerably higher 260-285 MJ kg-1 [26].
The liquefaction processes covered in this section are employed in liquefaction plants
to produce hydrogen. The next sections describe the transition of hydrogen liquefaction
from small scale plants over time starting from plants using Dewar’s process to more
recent plants employing advanced hydrogen liquefaction systems.
Figure 2.2 Schematic representation of the Claude cycle (adapted from [30]).
2.2 Hydrogen Liquefaction Plants
Since Dewar’s first successful hydrogen liquefaction in the late 1800s [23], more
efficient systems, such as Claude, pre-cooled Claude and the helium-refrigerated
system, were developed in the early 1900s [31]. Construction of the first large hydrogen
plants in the United States took place in 1957, to satisfy the growing needs of the
aerospace and petrochemical industries. These plants utilized the modified pre-cooled
Claude cycle, using liquid nitrogen as a pre-coolant, cooling input hydrogen to 80 K,
which was then further cooled down to 20 K using the hydrogen refrigeration system.
These plants had an energy efficiency of less than 20%, with a focus on reliability and
safety rather than maximum efficiency. Since then, there has been little improvement
15
in this regard; most large-scale hydrogen plants in operation today use similar cycles as
these first types with energy efficiencies of up to 40% [32]. A study of the efficiency
of hydrogen liquefaction plants by Linde Kryotechnik and the Nippon Sanso
Corporation [33] showed that about one-third of the exergy is lost in the liquefier
system, with an additional 27% lost before the cold end of the liquefier. Only 39.7%
exergy is left as product flow.
A typical liquefier system is shown in Figure 2.3, which includes a hydrogen
feed stream entering the cold box and featured a continuous conversion of ortho- to
parahydrogen during cooling using a catalyst placed directly in the heat exchangers.
Hydrogen pressure between the seventh and eighth heat exchangers (HE 7 and HE 8)
is reduced to tank pressure with an ejector, which functions similar to a water jet blast,
removing displaced or flash gas from the tank to be re-liquefied in HE 8.
Refrigeration of hydrogen gas down to about 80 K is achieved utilizing a
nitrogen pre-cooler by running liquid nitrogen through a phase separator and flooding
HE 2 with liquid, which cools the hydrogen stream down to approximately 81 K. This
refrigeration process creates evaporated nitrogen, which is further used in HE 1 to cool
the feed stream, while warming up to ambient temperature. Refrigeration from 80 K to
about 30 K is accomplished through expansion of high-pressure hydrogen gas (2 MPa)
in three expanders placed in a series. The Joule-Thomson cycle is applied in HE 7 and
8 to cool hydrogen from 30 K to liquefaction. The system has a throttle valve at the
bottom, where the high-pressure gas is throttled to low pressure, reducing its
temperature in the process. This is the lowest temperature point of the system. After
removal of the liquid, the warmed-up gas is again compressed, to medium pressure, and
inserted into the stream of the return gas from the expanders.
2.3 Previous and Current Plants
As the demand for liquid hydrogen grew, the liquefaction plants also changed to
accommodate the demand. Economies of scale mean that centralized hydrogen
production is more cost effective and energy efficient than distributed production. In
fact, hydrogen liquefaction plants tend to be more efficient with an increase in size [34]
as well as limited by financial rather than technical constraints. The capital costs
accounts for approximately 63% of the total lifetime cost of a hydrogen liquefaction
plant.
16
The main operating cost of hydrogen liquefaction is the cost associated with
input power of 12-15 kWh/kg, accounting for about 32% of the total lifetime cost of a
hydrogen liquefaction plant [35,36]. The capacities of hydrogen liquefaction plants
vary from 5 tons per day for the Air Products plant in Sacramento to 66 tons per day
for the Air Products plant in New Orleans. An economic analysis of three hydrogen
liquefaction systems [37] illustrates that, while power consumption costs remain
relatively constant, fixed charges, as well as operation and maintenance costs, rapidly
decrease with increased production rates. The cost of production decreased to about
0.7$ and 0.8$ per kg H2 for a production rate of 29,700 kg per hour for an optimized
large-scale hydrogen liquefier and a two-stage Claude hydrogen liquefier, respectively.
Figure 2.3 Flow chart of the hydrogen liquefier system [25].
The currently used hydrogen liquefaction process is highly integrated with air
separation and typically uses liquid nitrogen as a coolant. In 2011, Praxair introduced
improvements to its existing hydrogen liquefaction process, as shown two
corresponding flow diagrams in Figure 2.6. The improved process reduces the overall
power consumption by 2.4% and the liquid nitrogen requirement by 11%. The cooling
load is moved from the second to the first heat exchanger, with the result that external
refrigeration is increased by 17% and the recycle flow is reduced [38]. In addition, it
was reported that the novel ortho-para conversion process was able to achieve an overall
17
process improvement of around 8%. However, all of these improvements failed to
deliver the desired 20% improvement in overall process performance. Therefore,
researchers have worked on new advanced methods with the motivation to improve
hydrogen liquefaction efficiency. These are discussed in the next section.
Figure 2.4 Praxair hydrogen liquefaction process flow diagram (left) and improved hydrogen liquefaction process flow diagram (right), adapted from [38]).
2.4 Advanced Liquefaction Techniques
The hydrogen liquefaction process has undergone numerous refinements since the first
successful hydrogen liquefaction. However, the fundamental elements of the early
liquefaction process have not disappeared from currently employed cycles. These
basics include [12]:
• Using Joule-Thomson expansion, where the pressure of the compressed
hydrogen is reduced using a nozzle or valve, which represents an adiabatic
process leading to a reduction in gas temperature. That is, at a hydrogen
inversion temperature of 204 K, this temperature reduction can be achieved only
with gas that has been pre-cooled in the process to a temperature below this
inversion point. Thus, hydrogen liquefaction requires more than just this
refrigeration technique.
18
• Using an external auxiliary refrigerating fluid in the hydrogen liquefaction
process in order to cool hydrogen below inversion point. Typical refrigeration
fluid is liquid nitrogen, where the nitrogen liquefaction may, in turn, use its
auxiliary refrigerating fluid, typically a halogenated hydrocarbon. In addition to
liquid nitrogen, helium is also used for some small-scale liquefiers as the
auxiliary refrigeration liquid in order to achieve hydrogen liquefaction via
Joule-Thomson expansion.
• Using an expansion engine for compressed hydrogen. In addition to expansion
by a Joule-Thomson valve, the compressed hydrogen can also expand in an
expansion engine. The developed work is then excluded from the system using
the engine shaft and can be recovered for additional external use.
The first reported hydrogen liquefaction cycles include the pre-cooled Linde-
Hampson cycle, invented in 1895, and the Claude cycle that was invented in 1903 [20].
Since many of the advancements were based on trying to achieve better efficiency, most
of the related literature focuses on optimization of the liquefaction process,
improvements to the process equipment, and the improvement of the ortho-para
conversion to reduce the amount of power consumed by liquefaction.
A patent by Schwartz et al. [39] identified and quantified some of the ways to
reduce the cost of the liquefaction process, which would, in turn, significantly reduce
the cost of hydrogen distribution. The aim of this research was to achieve a reduction
of 20% in power consumption, followed by a further reduction in capital cost. When
the targeted efficiency improvement was not achieved, Praxair Inc. stopped the project
before the potential savings in capital costs were addressed.
Other projects introduced by NASA were for small and medium-scale hydrogen
liquefaction processes. The technology included domestically produced wet cryogenic
turboexpanders [40]. The few large-scale hydrogen liquefaction plants in operation use
variations of the cycles described above.
2.5 Existing Large-Scale Plants
The most common cycles now used for hydrogen liquefaction are the helium Brayton
cycle and (pre-cooled) Claude cycle [11]. The Brayton cycle achieves refrigeration
capacity solely with expansion turbines, while the Claude cycle uses recycle
compressors with lower power consumption, and optimizing the refrigeration loop with
19
expansion by turbines and finally via a Joule-Thomson valve [11,41]. The energy
consumptions of the Brayton and Claude cycles are 12.3 to 13.4 kWh per kg H2 and
10.8 to 12.7 kWh per kg H2, respectively [11].
The first liquefaction plants were constructed in the late 1950s to support
NASA's programs [20,42]. Today, the greatest use of liquid hydrogen continues to be
in space programs as rocket fuel. In the future, if production efficiency is increased, it
might be used as a fuel for vehicles. Table 2.1 exhibits some hydrogen liquefaction
plants that exist globally. The largest producers are Praxair, Air Products, Air Liquide
and Linde [11].
Praxair Inc. currently has five fully operational hydrogen liquefaction plants in
the United States, with different production capacities ranging from 6 to 35 tons per
day (TPD) of liquid hydrogen. Production is based on a modified pre-cooled Claude
cycle, where the usual rates of power consumption are in the range of 12.5−15
kWh/kgLH2 [11]. Air Products has six hydrogen liquefaction plants: five in the US and
one in the Netherlands. Four out of five of the US plants have a producing capacity of
about 30 TPD, while the others (one in the US and one in the Netherlands) have a 5
TPD capacity [11]. Air Liquide has a plant in each of France and Canada, both with
production capacities of around 10 TPD. The two plants utilize the Claude cycle, where
hydrogen was used as the cycle fluid [11]. Linde is another large-scale producer whose
production is also based on the pre-cooled Claude cycle [11,42]. The plant is located in
Ingolstadt, near Munich. The liquefier has a capacity of 4.4 TPD [32]. In addition to
these large-scale operations, various experimental laboratory scale hydrogen
liquefaction processes are also in existence. It is hoped that over time, some of these
processes will develop sufficiently to be economically viable in an industrial setting.
2.6 Lab Scale Hydrogen Liquefaction Methods
Most of the recent designs on a small and laboratory scale are based on the
magnetocaloric effect. This kind of liquefier, together with an appropriate cyclic
thermodynamic process, can use isentropic demagnetization of a ferromagnetic
material near its Curie point temperature as a refrigeration procedure [42].
Magnetocaloric refrigerators were investigated and constructed for a temperature range
of about 1 K to 20 K.
20
Table 2.1 Details of Commercial Hydrogen Liquefaction Plants
Country Location Operated by Capacity Tons Per Day
Canada
Ontario Air Products 30.0
Quebec
Air Liquide 10.0 Air Liquide 12.0 BOC by Linde 15.0 BOC by Linde 14.0
French Guyana Kourou Air Liquide 5.0
USA
Missouri Air Products 3.0
Florida Air Products 3.2 Air Products 27.0
Mississippi Air Products 32.7 California Union Carbide/Linde Division 54.0 Louisiana Air Products 34.0 New York Praxair 18.0 California Air Products 6.0 New York Praxair 18.0 Mississippi Air Products 30.0 McIntosh Praxair 24.0 Indiana Praxair 30.0
France Lille Air Liquide 10.0
Germany Linde 4.4 Linde 5.0
Netherlands Air Products 5.0 China CALT 0.6
India ISRO 0.3 Asiatic Oxygen 1.2 Andhra Sugars 1.2
Japan
Iwatani 0.6 MHI 0.7 Tashiro 1.4 Pacific Hydrogen 1.4 Japan Liquid Hydrogen 2.2 Japan Liquid Hydrogen 0.3 Air Products 0.3 Iwatani (Hydro Edge) 11.3 Iwatani, built by Linde 10.0
Source: [20]
They can be integrated into thermodynamic processes in a manner similar to
isentropic expansion in expanders with pure gas processes. To accomplish a continuous
cooling process, an appropriate ferromagnetic material, whose Curie point temperature
is in the range of the cooling temperature, has to be cyclically magnetized and
21
demagnetized [42]. Pressure and specific volume of the gas process correspond to the
magnetic field intensity and the magnetization, respectively. Thus, from the
corresponding gas processes, the magnetic Carnot, Brayton, Ericson and Stirling cycles
can be derived [42]. Many of these types of systems and magnetic material have been
investigated, and it is demonstrated that there is a possibility for these systems to be
improved and combined with existing hydrogen liquefaction cycles [43–47].
In research by Kamiya et al. [45], a newly designed magnetic refrigerator is run
by the Carnot cycle and liquefies pre-cooled 20.28 K gas hydrogen, absorbing the latent
heat. This Carnot liquefaction system consists of magnetic materials, a superconducting
magnet and a heat switch. Its capacity is 12 kg LH2/day at 1.25 Hz.
Two-stage active magnetic regenerative refrigerator systems, which have been
investigated and presented, can be improved and used in the future for the liquefaction
of hydrogen [47].
2.7 Future Developments and Presentation of New Conceptual Designs
In research by Ohlig and Decker [11], some future developments with 40-60%
efficiency and about 7.5 kWh/kg/ LH2 energy consumption are recognized as follows:
1. One of the key areas of exergy loss is the recycle compressor of the refrigeration
loop, including its interstage and after-coolers. Improvements could be made by
shifting to the more efficient turbo-compressors with a higher number of stages
and frequent intercooling, hence bringing compression closer to the isothermal
optimum.
2. Precooling to 77 K (usually about 80 K) is achieved by use of liquid nitrogen
which is then released to the atmosphere. Energy for nitrogen generation from
the air is lost and sensible heat only partially used. A closed loop consisting of
a nitrogen re-liquefier increases investment costs by 20 to 30% but results in
significant energy savings of 10%.
3. Shifting towards systems including energy recovery solutions regarding
expansion turbines even though, due to complexity respective, solutions will
need to undergo extensive qualification procedures prior to approval as the
standard for industrial applications.
4. The optimization of the refrigeration loop and use of unproven concepts in this
field such as new refrigeration media.
22
In a study of Kuendig et. al. proposed pre-cooling by LNG instead of liquid
nitrogen. The authors suggested that efficiency would be improved from 10 to 4 kW
h/kg LH2, when compared to currently used liquefaction processes, such as the plant at
Leuna, which uses liquid nitrogen to precool, but with the compression at ambient
temperature. However, this method is only applicable to hydrogen production from
LNG, which would necessitate the location of the plant near a seaport [20].
Another conceptual design is the WE-NET project, which is a 300 TPD large-
scale process delivering LH2 at 1.06 bar from a feed stream of equal pressure [20]. The
plant is based on a pre-cooled Claude cycle and is similar to the plant in Ingolstadt but
with some modifications that lead to an increased efficiency of 46.2% and a specific
liquefaction power calculated to 8.5 kWh/kgLH2 [48].
Quack [49] produced a study in which the design is based on modern helium
liquefiers that are built with up to ten expansion turbines placed strategically in a cycle
to obtain optimal overall efficiency. Efficiencies obtainable by this concept are up to
60% and specific energy consumption is 5-7 kWh/kg LH2.
Another study by Kuz’menko et al. [50] proposed a helium refrigeration cycle
which showed higher efficiency than the Ingolstadt plant. Valenti and Macchi [51]
found an innovative, efficient and large hydrogen liquefier. It is a large-scale plant since
the production rate is 10 kg/s of L H2. The system utilizes four cascaded helium Joule–
Brayton cycles and has a reported efficiency of 47.73% [51].
Research conducted by Ratlamwala et al. [52] reports on hydrogen liquefaction
based on renewable energy (solar photovoltaic/thermal) based on the Linde-Hampson
23
cycle. It is schematically shown in
Figure 2.5. The efficiencies of some of these prototype plants, and some already
in existence, are compared in Table 2.2.
From the open literature, it can be seen that there are three fundamental ways to improve liquefaction efficiency. The first is to improve the ortho-para hydrogen conversion, thereby improving the energy carrying capacity of the final product. The second approach would be to improve the overall energy efficiency of the process through cogeneration or energy recovery. Finally, improving the overall cooling capacity of the system can improve the energy conversion efficiency. In this study, three methods (catalytic ortho-para conversion, integrating Organic Rankine Cycle, and introducing vortex tubes) are investigated to increase the energy conversion efficiency of hydrogen liquefaction. An introduction to there three inventions are provided below.
Figure 2.5 Basic scheme of hydrogen liquefaction process based on renewable energy (adapted from [52]).
24
2.8 Orthohydrogen and Parahydrogen
An understanding of the physical characteristics of hydrogen is essential to develop an
efficient hydrogen liquefaction method. This section explains how the isomeric forms
of hydrogen are relevant in its use as a fuel and how these states are affected by the
liquefaction process. Molecular hydrogen occurs in two isomeric forms depending on
the alignment of its two proton spins namely orthohydrogen which has parallel spin
alignment, and parahydrogen which has antiparallel spin alignment. Since these two
differ in the nuclear spin state, rather than in chemical structure, they are also referred
to as spin isomers. The existence of different hydrogen forms of molecular hydrogen
was first proposed in 1927 by Heisenberg and Hund [53]. The two forms are para- and
orthohydrogen [53]. However, Harteck and Bonhoeffer first synthesized pure
parahydrogen in the following year. Parahydrogen represents a lower energy state than
orthohydrogen although, due to thermal excitation, at room temperature and pressure,
hydrogen consists of around 75% ortho- and 25% parahydrogen [53]. However, at low
temperatures, in the hydrogen liquefaction process, there is a spontaneous increase in
parahydrogen content, accompanied by a release of energy of around 1091 J mol-1,
which is higher than the heat of vaporization of hydrogen (904 J mol-1) [53].
25
Table 2.2 Efficiencies of some conceptual plan
Source: [20]
The two states have different energy levels so that the content of each species
at equilibrium is temperature dependent. Moreover, the energy balance of this reaction
has important implications for hydrogen storage because spontaneous conversion inside
a hydrogen tank can cause significant hydrogen vaporization. At hydrogen's boiling
point of 20K, as shown in Figure 2.6, the equilibrium is shifted almost completely
towards parahydrogen [54].
26
Figure 2.6 Equilibrium composition as a function of temperature (adapted from [54])
In the ortho-state, electron spins are in the opposite direction to the ortho-state,
and in the same direction in the para-state when the temperature falls, as shown in
Figure 2.2. A number of catalysts can be used to accelerate the conversion from ortho-
state hydrogen to parahydrogen [55,56], allowing conversion of all the liquid hydrogen
to parahydrogen prior to storage. One of the challenges of this reaction is that it is
typically performed at low temperature, usually at 77 K, during hydrogen liquefaction,
which additionally decreases the reaction rate.
Figure 2.7 Spin isomers of molecular hydrogen (adapted from [57]).
It was shown as far back as 1949 that the activation of hydrogen gas by
transition metals is connected with the presence of unpaired d-electrons or holes in the
electronic d-band of the metal [58]. Therefore, the key component of any catalyst for
this reaction is a paramagnetic metal ion, typically a transition metal or a lanthanide.
Para Hydrogen Ortho Hydrogen
Proton
Electron
27
Most of the widely used catalysts are made of various metal oxides such as
ferric, chromic, cerium, neodymium, manganese (both supported [12] and
unsupported), activated carbon, different metal compounds (such as uranium and
nickel), rare-earth metals and various organo-metallic compounds. The catalytic
activity of these materials is directly dependent on the specific surface area of the
catalyst [59]. A recent investigation of interconversion kinetics using paramagnetic
complexions as catalysts revealed a direct correlation between the rate constants and
the concentration of the catalyst, and that second-order rate constants are related to the
magnetic moment of solvated metals and, in most cases, to the size of the ligand in the
complex. While the dependence on magnetic moment can be explained using Wigner's
theory [60], the size of the ligand has a greater effect on the second-order rate constants
than previously expected [56]. Furthermore, recent research has demonstrated the
ability of C60 fullerene to act as a catalyst for ortho-para conversion in liquid oxygen at
77 K [61]. After the removal of oxygen, enriched parahydrogen adsorbed at C60
fullerene is stable for many days (half-life of around 15 days in the absence of
paramagnetic catalyst), while the conversion rate back to orthohydrogen of this material
dissolved in organic solvent at room temperature was determined to be three orders of
magnitude lower than the conversion rate of parahydrogen dissolved in organic solvents
and not protected by a C60 fullerene shell [62–64].
Hydrogen with parahydrogen in excess of its natural 3:1 ratio is used to study
hydrogenation reactions because the resulting products exhibit hyperpolarized signals
in proton NMR spectra [65,66]. Parahydrogen and orthohydrogen also exhibit some
significant differences in properties, such as ideal gas specific heat. Parahydrogen
exhibits as much as 50% higher ideal gas isobaric specific heat than orthohydrogen in
the 65-320K region, leading to a significant difference in behaviour between it and
ordinary hydrogen, which is 75% orthohydrogen [67]. This leads to different equations
of state proposed for parahydrogen and normal hydrogen. Recently, Leachman et al.
formulated new fundamental equations of state for parahydrogen, normal hydrogen,
and orthohydrogen, with upper limitations of pressure and temperature of 2000 MPa
and 1000 K, respectively [68]. Based on this work, Lemmon et al. developed a truncated
virial equation for use in fuel consumption applications, using the normal hydrogen
equation of state, providing a correlation for density as a function of temperature and
pressure [69]. After understanding the behaviour of hydrogen at various states, the next
28
step is to study the literature available on hydrogen liquefaction. The various hydrogen
liquefaction process is described in the next section.
2.9 Catalyst Conversion of Ortho- to Parahydrogen
When hydrogen is condensed, the transition from ortho- to parahydrogen form is slow
at 1.14%/ hour [70]. For this reason, catalysts are used to speed up this transition. In
1933, a mechanism for this catalysis was proposed by Wigner [71], and it is still the
most widely used version even though other mechanisms have since been proposed
[70,71]. Wigner’s mechanism was refined by Kalckar and Teller [23]. In research
published in 1965 by Leffler [23] and Kasai et al. [72], two general mechanisms were
distinguished:
1. Magnetic: Wigner’s mechanism belongs to this group, which assumes that a
strongly inhomogeneous magnetic field decouples the proton spins and allows
the ortho-para transition to occur.
2. Dissociative: This mechanism assumes disassociation of hydrogen molecule
and rearrangement such that parahydrogen is obtained.
The most commonly used catalysts are Cr2O3, Fe-oxides and hydroxides [70]. In
[73], CrO3/SiO2-gel, chromic anhydride, nickel silica gel, FeNi alloys, activated
charcoal, Fe(OH)3, and Fe2O3 are reported as commonly used catalysts. In [73], the use
of LaFeO3 as a catalyst is proposed.
A study by Boeva et al. [74] reported the use of gold nanoparticles as a catalyst
at low temperatures. It seems that gold nanoparticles exhibit magnetic properties at low
temperatures.
Iron, Platinum and Nickel Cata1ysts were investigated for use in ortho-para
conversion of hydrogen in a study conducted by Emmett and Harknes [75], which also
examined the influence of temperature, pressure, time of contact and poisons.
The theory was also formulated as a case study rather than for general purposes.
One of them is published in an article by Ishii and Sugano [76], where the theoretical
framework for the conversion of ortho- to parahydrogen on magnetic surfaces was
presented. This involved two energy flow paths to the translational motion of the
hydrogen molecule and spin excitation of the substrate, which was then compared with
the experimental results of two catalysts: antiferromagnetic α-Cr2O3, and ferromagnetic
29
EuO. Three factors identified by the authors as important in ortho-para hydrogen
conversions in [62], namely Fermi contact interaction; the Steric effect; and Dynamical
quantum filtering have been investigated.
2.10 Organic Rankine Cycle (ORC)
The Organic Rankine Cycle (ORC) works with organic, high molecular mass working
fluid, which has the normal for having the fluid vapour stage change, or breaking point,
at a lower temperature than the water-steam stage change. The ORC is a potential
contender for integrated systems. A significant amount of research has been conducted
related to Rankine cycle performance in energy systems. Researchers have investigated
the ideal working liquid, and the ideal reinjection state of the liquid, the capacity of
cogeneration and the economic analysis of the systems.
Organic Rankine Cycles (ORCs) offer several advantages. A standout feature
amongst the most essential qualities of natural working liquids is their generally low
enthalpy drop through the turbine that causes a higher mass flow rate and reduces the
entire waste, and which therefore builds turbine adiabatic proficiency. Additionally,
superheated vapour at the turbine exit of an ORC cycle, stimulates avoiding dissolution,
permitting dependable activity and quick start-up [77,78].
2.11 Vortex Tube
Through broad research endeavours, it is recognized that there have been no recent
critical investigations into the conduct of a Vortex Tube (VT) using supercritical,
cryogenic hydrogen as its working liquid. Accordingly, there exists no approved
computational liquid element (CFD) model of a VT under these conditions. The earliest
study to report the phenomenon of energy separation in a VT was conducted by Ranque
[79]. Imperative exploratory examinations on VT parameters were led by researchers
such as Takahama [80], while hypothetical and expository depictions of the vitality
detachment, as well as temperature and speed profiles in a VT were provided by
Deissler and Perlmutter [81] and Ahlborn et al. [82]. Saidi and Yazdi [83] investigated
an exergy examination on a VT while Valipour [79] confirmed a trial model of a VT
icebox.
30
2.12 Closing Remarks
The review of existing literature shows that there is a need for moving away from fossil
fuels and hydrogen as a fuel has significant advantages. However, the efficiency of
hydrogen liquefaction plants is at present too low to make the process environmentally
and economically viable. Multiple attempts have been made by researchers to improve
the efficiency of hydrogen liquefaction systems. Some of these have resulted in large
scale hydrogen liquefaction plants while new methods and modifications are largely
available in laboratory settings. While there has been research on individual methods
for efficiency improvement, little data is available on comparing possible hydrogen
liquefaction systems. Such an approach would enable researchers to identify the system
that is likely to generate liquified hydrogen at the best possible efficiency. In this light,
the thesis investigates various hydrogen liquefaction systems with a focus on energy
and exergy efficiency. Such an investigation will help identify the system
configurations that are likely to have maximum energy and exergy efficiency. The
systems employing catalyst, ORC, and vortex tubes being simulated are described in
detail in the next chapter.
31
SYSTEM DESCRIPTION
Chapter 3 highlights the importance of understanding various hydrogen liquefaction
systems so that the process can be carried out at a greater efficiency. In that light, this
chapter examines various possible combinations and system configurations that may
generate better efficiencies. For this purpose, seven hydrogen liquefaction systems are
introduced, the first of which is based on a patent introduced by Schwartz et al. [84].
The remaining six are advanced hydrogen liquefaction systems that simulate large plant
size processes. The proposed integrated systems are described in detail in order to
demonstrate how they function. Assuming initial pressure to be atmospheric for the H2
feed and liquid H2 product as a saturated liquid, these systems were simulated in Aspen
Plus in two versions: the process is simulated once without the catalyst and again with
the catalyst. The energy systems for the production of liquid hydrogen, which are
discussed in this chapter, are modified from the basic patent in order to reach optimum
process. It is expected that these systems will meet the desired exergy objectives. Figure
3.1 illustrates the analyzed systems in this proposal.
Figure 3.1 Advanced hydrogen liquefaction systems considered for analysis.
These efficiency improvements provide a benefit and can lead to future
improvements in the design of a hydrogen liquefaction system. These systems increase
understanding of the intricacies of the process of hydrogen liquefaction using modelling
and a thorough examination of the ortho-para conversion process and their separation.
The process modelling techniques provide additional benefit to the public. The planning
and projections incorporate the effect of parahydrogen into process modelling software,
allowing more accurate modelling of the liquefaction processes compared to only using
Main Liquefaction
System
Advanced Liquefaction System (S1)
Without Catalyst (S1-
A)
With Catalyst (S1-B)
Advanced Liquefaction System with CO2 Orgainc Rankine Cycle (S2)
Without Catalyst (S2-
A)
With Catalyst (S2-B)
Advanced Liquefaction System with Vortex tubes (S3)
Without Catalyst (S2-
A)
With Catalyst (S2-B)
32
normal hydrogen. An additional simple method of reduction in liquefaction power
consumption has also been identified.
Figure 3.2 illustrates the main system simulated and analyzed to provide an
understanding of the actual patent and validate results against it. A brief description of
the system and hydrogen flow are provided below.
H2FEED (stream 1) enters mixer before travelling to compressor C1 where it is
compressed from atmospheric pressure to 3 bar. Stream 3 exits from compressor C1
and enters heat exchanger EX1 as a hot product. A stream of liquid CO2, serves as the
cold stream. The exchanger lowers the temperature of the stream and it exits at a
temperature of -20 oC as stream 4. Stream 4 further enters mixer M2. Stream 5 exits
M2 and enters compressor C2, where the gas is compressed to 20 bar. Stream 6 exits
C2 and enters heat exchanger EX2 as a hot product, where it is cooled until -23.15 oC.
Stream 7 exits EX2 and enters the first multi-heat exchanger HX1, as a hot feed.
The output stream is stream 8 with the temperature of -173.15oC, which enters splitter
D1. In D1 it is split on streams 9 and 15. Stream 9 enters mixer M3, from which exits
stream 10 and enter the second multi-heat exchanger HX2 as a hot feed. The output
stream is stream 11, which exists at -230 oC. Stream 11 further enters the third heat
exchanger HX3 as a hot feed. Stream 12 is an outlet stream on -253 oC that goes directly
to valve V1, where the pressure is decreased again to atmospheric. Exit of V1 is stream
13, which again enters HX3 and then exits it as a final product – stream H2LIQProduct,
a liquid H2 product at the atmospheric pressure and temperature of -253 oC.
Stream 15, after splitter D2, enters HX2 as a hot feed and exits at -230 oC to
enter D3 as stream 16. Stream 16 goes to splitter D3, where it is split on streams 17 and
18. Stream 17 enters turbo expansion compressor TE2, from which exits stream 29 as
a vapor on 20 bar. This stream goes directly to the HX3 as a hot feed, from where it
exits as stream 30 as a liquid on -253 oC. Stream 30 further enters mixer M5 and exits
as stream 31. Stream 30 enters now HX2 as a hot feed, from where it exits as stream 32
at -235 oC. Stream 32 is split on streams 33 and 39 in splitter D4. Stream 33 goes to the
absorption tank AD, where ortho H2 species are absorbed. They are routed as stream
34B and mixed in mixer M4. From M4 stream 26 enters HX1 as a hot feed. Output
33
stream 27 exits on -120 C and goes to mixer M1, where it is mixed together with
H2FEED.
Para species from AD tank are represented as stream 34 and it enters HX1 as a
cold stream. The output stream 35 at -23.15 oC enters compressors C3, where it is
compressed to 20 bar and exists as stream 36. Stream 36 is a hot feed for heat exchanger
EX3, where it is cooled to -20 oC and rerouted back to HX1 as stream 37 as a hot feed.
Stream 45 is a cooling medium in EX3, which is liquid CO2, which exits as stream 46.
Stream 37 is cooled up to -173.15 oC and exits as stream 38, which is mixed in mixer
M3 together with stream 9.
Stream 39 from splitter D4 enters HX1 as a cold stream and exits as stream 40
at 215.5 oC. It is then mixed with stream 4 in mixer M2. Stream 18 from splitter D3
enters HX3 as a hot feed. It is cooled until -253 oC and it exits as stream 23, which goes
to valve V2 to decrease the pressure to atmospheric and becomes a saturated vapor as
stream 20. It is then flashed in F1 and the vapor fraction, stream 21, is rerouted back to
HX1 as a cold feed. The output stream 24 exits as vapor at-230 oC and goes directly to
HX2 as a cold stream. It exits as stream 25 at 26 oC, and enters mixer M5 together with
stream 34B.
Stream 17 from splitter D2 goes to turbo expansion valve TE1, from where it
exits as stream 28 at 20 bar. Stream 28 then enters mixer M5, where it is mixed together
with stream 30. Liquid product of flash separation F1 is stream 22 and it enters HX3 as
a cold feed. The output stream enters again F1. The inlet to flash separator F2 is a stream
of N2 Liq, from where the vapor fraction is rerouted to HX2 as a cold feed. The output
stream enters F2 again at 26 oC. The liquid part after F1 enters HX1 as a cold feed, and
exits HX1 as a stream of N2Gas at 215.5 oC. The additional cold feed to HX1 is stream
N2LIQ, which exits as N2GAS.
3.1 Description of System 1
Two hydrogen liquefaction systems are proposed in this section. The base system, which
is considered as a reference (base), is shown in Figure 3.3 while Figure 3.4 displays the
first developed integrated system with added catalysts.
3.1.1 System 1A: Reference system without a catalyst
34
This system is based on the layout of the original system described in the previous
section. Figure 3.3 shows the modified base system. The base system that is analyzed,
this system has a hydrogen feed stream and a liquefier. Ortho-species of hydrogen the
hydrogen feed stream are converted to the para-species in higher and lower temperature
converters. An adsorption unit, between the higher and lower temperature catalytic
converters, adsorbs a portion of the ortho content of the feed stream. The adsorbed
portion is desorbed during regeneration of an adsorbent in the bed of the adsorption
unit. It is then re-circulated in the higher temperature catalytic converter to reduce the
degree to which the ortho-species are converted to the para-species in the lower
temperature catalytic converter and at lower temperatures.
The process starts with feeding hydrogen, containing both the ortho- and para-
species of hydrogen, to the system from outside and recycling it from inside the system.
The proportion is about 75% ortho-species and 25% para-species, which are the
approximate values at atmospheric condition.
3.1.2 System 1B: Reference system with a catalyst
In this simulation, three catalysts were added to the heat exchangers to speed up the
conversion and reach 90% para-conversion. The system layout otherwise remains the
same as that of the reference system except for the addition of catalysts. With the
addition, ortho-para conversion consumes a significant amount of refrigeration energy
because it requires cooling at low temperatures. Further improvements in ortho-para
conversion can lead to a significant reduction in power requirement. Figure 3.4 shows
the configuration with the added catalysts. The colored exchangers are two exchanger
with catalysts. The catalysts are expected to help in speeding up the liquefaction
process but on the other hand it is also expected that efficiency will decrease due to the
high-energy requirement to run and operate the catalyst. Changes made on this
configuration are significant for utilizing the developed system imitating the
commercial system for further improvements are needed to increase the overall
efficiency of the system and could lead to less CO¬2 emissions. This is configuration
is aimed to lead to improvements to: reach simplicity, system integration, high thermal
efficiency and quick thermal response, wide turn-down window, low emissions, and
better fuel flexibility in the same design.
35
Figure 3.2 The main systems schematic diagram
Legend:HX : Heat ExchangerAO : Adsorption Unit B : BoilerC : CompressorD : SplitM : Mixer/DividerTE : Turbo ExpanderF : FlashV : Valve
36
Figure 3.3 Schematic diagram for the reference system
37
Figure 3.4 Schematic diagram for the main system with reactor
38
3.2 Description of System 2
In a further improvement to the efficiency of the system, three Organic Rankine
Cycles (ORCs) are added to the compressors. The ORC represents the most
commonly used low heat source temperature-based system.
3.2.1 System 2A: System with ORC and without a catalyst
System 2A explores the use of ORC in hydrogen liquefaction. While the basic system
remains the same, an ORC is integrated to improve compressor work gain. An ORC
system consists of the same components as a conventional steam power plant. However,
the working fluid is an organic component, which exhibits a lower boiling temperature
than water, allowing a reduction in the evaporating temperature. The selection of this
CO2 fluid in an ORC is mainly due to the nature of use in the cycle and, most
importantly, the maximum temperature of the cycle. Figure 3.6 shows a schematic
representation of the proposed integrated system, where an ORC provides the necessary
electricity.
The additions of the ORCs to the compressor gained work by a total of about
140 kJ in which the total system efficiency can be improved. Figure 3.5 illustrate the
ORCs that are added to the system.
(a)
(b)
(c)
Figure 3.5 Organic Rankine Cycles
For each ORC, the work outcome could make a significant positive impact on
the system’s overall efficiency and can be optimized for future improvements. In a
further improvement to the efficiency of the system, three ORCs are added to the
compressors. The ORC, which represents the most commonly used low heat source
temperature-based system, consists of the same components as a conventional steam
power plant. However, the working fluid is an organic component that exhibits a lower
M1
H2FEED
C1
2
EX1
3
M2
C2
4
9
HX1
46
M3
S4
14
17
V-1
HX2
N2LIQ
1
D2
D3
D519
20
C4
C5
30
32
21
31HX3
22
49
F1
V-2
23
24
S1
25
33
D1
M412 13
B627
51
28
26
AO
D4
34
44
35
PROD
N2GAS
29
43
EX2
S9
33
S3
36
E-51
E-52
S3
S6
E-53
S11
S5
46B
8
E-57
E-58
E-59
11
10
E-56
S8
S7
S12
Ex3
B13
E-61
41
40
B11
S13
S15
P-91
39
P-93
M1
H2FEED
C1
2
EX1
3
M2
C2
4
9
HX1
46
M3
S4
14
17
V-1
HX2
N2LIQ
1
D2
D3
D519
20
C4
C5
30
32
21
31HX3
22
49
F1
V-2
23
24
S1
25
33
D1
M412 13
B627
51
28
26
AO
D4
34
44
35
PROD
N2GAS
29
43
EX2
S9
33
S3
36
E-51
E-52
S3
S6
E-53
S11
S5
46B
8
E-57
E-58
E-59
11
10
E-56
S8
S7
S12
Ex3
B13
E-61
41
40
B11
S13
S15
P-91
39
P-93
M1
H2FEED
C1
2
EX1
3
M2
C2
4
9
HX1
46
M3
S4
14
17
V-1
HX2
N2LIQ
1
D2
D3
D519
20
C4
C5
30
32
21
31HX3
22
49
F1
V-2
23
24
S1
25
33
D1
M412 13
B627
51
28
26
AO
D4
34
44
35
PROD
N2GAS
29
43
EX2
S9
33
S3
36
E-51
E-52
S3
S6
E-53
S11
S5
46B
8
E-57
E-58
E-59
11
10
E-56
S8
S7
S12
Ex3
B13
E-61
41
40
B11
S13
S15
P-91
39
P-93
39
boiling temperature than water, allowing a reduction in the evaporating temperature.
The selection of this CO2 fluid in an ORC is mainly due to the nature of use in the cycle
and, most importantly, the maximum temperature of the cycle.
For each ORC, the work outcome could make a significant positive impact on
the system’s overall efficiency and can be optimized for future improvements. The use
of ORCs allows or a reduction in CO2 usage in exchangers EX1-3 from 350kg/sec to
5kg/sec.
3.2.2 System 2B: System with ORC and with a catalyst
For a catalyst-infused advanced hydrogen system, three ORCs have also been added.
The importance of system relay on the catalyst beds in the heat exchangers that have
been added to speed up the process of liquefaction. The objective of the ORCs is to
successfully utilize the outlet temperature. This also limits the emissions that will be
reflected in the environmental analysis by the reduction of CO2 emissions per MW of
energy. The colored exchangers are two exchangers with catalysts. The catalysts are
expected to help in speeding up the liquefaction process but on the other hand it is also
expected that efficiency will decrease due to the high-energy requirement to run and
operate the catalyst but with ORC its can be said that ORCs will compensate on the
high emissions. Changes made on this configuration are significant for utilizing the
developed system imitating the commercial system for further improvements are
needed to increase the overall efficiency of the system and could lead to less CO2
emissions. This is configuration is aimed to lead to improvements to: reach simplicity,
system integration, high thermal efficiency and quick thermal response, wide turn-
down window, low emissions, and better fuel flexibility in the same design.
This has shown interesting results and was analyzed thoroughly in the next
chapter to maximize the efficiency with keeping in mind other factors within the limits
and the boundaries of the optimization models.
40
Figure 3.6 Schematic diagram for the system with ORCs
41
Figure 3.7 Schematic diagram for the system ORCs and reactor
3.3 Description of System 3
In a further improvement to the efficiency of the system, Vortex tubes before the
exchanger inlet. Two configurations are explained further.
42
3.3.1 System 3A: System with Vortex Tubes and without a catalyst
In this system, Vortex Tubes (VTs) were added using a splitter and a turbo expander to
simulate the splitting and cooling effects of the VTs. Apart from the addition of VTs,
the system layout corresponds to that of the base system. The split was simulated to be
at 50% and the turbo expander outlet pressure was simulated to be 15 bars. Adding a
VT in this manner resulted in additional cooling capacity in the system. There is an
optimization opportunity to split more of the stream through to the VT, and to further
reduce the outlet pressure on the expander to increase cooling capacity. Figure 3.9
shows the VTs added to the main system.
Figure 3.8 Added VTs
3.3.2 System 3B: System with Vortex Tubes and with a catalyst
In this system, Vortex Tubes (VTs) were added using a splitter and turbo expander to
simulate the splitting and cooling effects of the VT. The split was simulated to be at
50% and the turbo expander outlet pressure was simulated to be 15 bars. Adding a VT
in this manner resulted in additional cooling capacity in the system. There is an
optimization opportunity to split more of the stream through to the VT, and to further
reduce the outlet pressure on the expander to increase cooling capacity. The use of the
reactors puts additional energy requirement on the system. This energy requirement is
reduced by using the VTs.
The next step of this study is to investigate the energy and exergy efficiencies
of the six proposed systems. Systems 1A and 1B are the base configurations with and
without catalyst addition. Systems 2A and 2B are modifications of the same systems
with the addition on ORC while systems 3A and 3B include vortex tubes.
It is hoped that this analysis of varying configurations will yield insights
regarding methods to improve efficiency of hydrogen liquefaction. The next chapter
therefore presents energy and exergy analysis of each configuration for comparison.
M1
H2FEED
C1
2
EX1
3
M2
C2
4
9HX1
46
M3
S4
14
17
V-1
HX2
N2LIQ
1
D2
D3
D519
20
C4
C5
30
32
21
31HX3
22
49
F1
V-2
23
24
S1
25
33
D1
M412
13
27
51
28
26
AO
D4
34
44
35
PROD
N2GAS
29
43
EX2
S9
33
S3
36
VT1
VT2
P-78
text 51 44
78 32
43
Figure 3.9 Schematic diagram for the system with VTs and no reactor
44
Figure 3.10 Schematic diagram for the system with VTs and reactor
45
SYSTEM ANALYSIS, MODELLING AND
SIMULATION
The thermodynamics analyses of the proposed systems will be based on energetic and
exergetic methods. Exergoeconomic concepts will be utilized to examine the created
frameworks economically. The performances of the proposed systems will be assessed
by deciding the energy and exergy efficiencies for the presented frameworks. In this
chapter, basic equations of energy and exergy will be presented. The investigation of
the principle controlling choices will be portrayed.
4.1 Basic Thermodynamic Concepts
In thermodynamic analyses, overall mass, energy, entropy and exergy balance
equations are written for the fuel that will be blended with ammonia. In order to
understand the combustion process, how the generator operates and its performance, a
comprehensive thermodynamic analysis is carried out regarding energy and exergy
studies to evaluate efficiencies of the system. The general assumptions taken in to
account for the thermodynamic analysis and calculations of the system can listed as:
• The reference temperature is selected as T0 = 25 oC (outside temperature during the
experimental studies) and reference pressure P0 = 101.325 kPa.
• The variations in the kinetic and the potential energies and exergies are ignored.
• The ideal gas laws apply for the gases operating in the system.
• Air used in the system is an ideal gas with constant specific heat.
• The relative humidity of the inlet air and hydrogen is taken as 90%.
• No pressure drops in the system
• No heat losses in pipes nor other equipment (perfectly insulated)
• The kinetic and potential energy changes are negligible.
• The liquefaction capacity is 36000 kg/day.
• Compressors are assumed to be isentropic.
Thermodynamic systems are, in principle, open or closed in nature [85]. Open
systems interact with the environment, exchanging heat, mass and work, while closed
systems include only exchange heat and work. When the system is defined as positive,
the Mass flow and the transfer of heat goes into, and the transfer of work goes out of
46
the system. During the hydrogen liquefaction process, there is a constant flow of
hydrogen mass in and out of the system; a hydrogen liquefaction system would
represent an open system.
4.2 Conservation of Mass Principle
For a non-steady flow process, occurring during a time interval from t1 to t2, the balance
of mass for this process can be written as follows:
∑ 𝑚; −∑ 𝑚> = 𝑚@ −𝑚A>; (4.1)
where mi and mo denote the mass entering the system through the input and exiting the
system through the output in time (t2-t1), respectively. In a more generalized form, for
an infinitesimally period dt, the conservation of mass in the system can be described as:
&BC&D
= ∑ ��; −; ∑ ��>> (4.2)
where ṁ represents mass flow (dm/dt), with i and o denoting the input and output of
the system, respectively; mv denotes the mass inside the system volume V.
4.3 Conservation of Energy Principle
The balance of energy for a non-steady flow process can be written as:
∑ (𝑒 + 𝑃𝑣);𝑚;; −∑ (𝑒 + 𝑃𝑣)>𝑚> +∑ (𝑄K)A,@ −(𝑊)A,@K> = 𝐸@ −𝐸A (4.3)
where e, P, v and m represent specific energy, pressure, specific volume and mass, i
and o denote input and output of the system, (Qr)1,2 denotes the heat transferred into the
system volume across the region r, (W)1,2 denotes the work transferred out of the system
and E1 and E2 represent the energy of the system at time t1 and t2, respectively. For a
hydrogen liquefier, which is a steady state system, where the energy of the system is a
constant and infinitesimally small period of time, this equation can be rewritten as:
��N + ��N + ∑ ��;ℎ; = ��P + ��P + ∑ ��> ℎ> (4.4)
where �� and �� represent the heat transfer and work rate exchanged between the system
and its environment, and ṁ and h represent mass flow rate and the specific enthalpy of
the streams.
4.4 Entropy Balance and Entropy Generation
Entropy generation occurring during the processes these systems undergo can be
described using the following equation:
47
&QRC&D
= ∑ ��;-𝑠;- − ∑ ��>TD𝑠>TD + ∑URCV+ ��+,- (4.5)
where s denotes specific entropy and ��+,- represents the rate of entropy generation.
4.5 Exergy Analysis
Exergy analysis is a technique of thermodynamic analysis formed based on the Second
Law of Thermodynamics. It gives alternative means of assessing and comparing
different systems and processes in a meaningful way, yielding efficiencies that
represent a factual representation of how close the performance of a given system
comes to an ideal form, and allows us to identify the causes and locations of
thermodynamic loses more effectively than with the energy analysis [85]. In this regard,
exergy analysis can help to improve and optimize system designs. Exergy
corresponding to a particular quantity of energy represents a quantitative assessment of
its usefulness, recognizing that, while energy cannot be created or destroyed, its quality
and value can be degraded. For an energy storage system, which liquid hydrogen
essentially is, exergy analysis determines the maximum potential of the incoming
energy. Liquid hydrogen is preserved and recovered only if the process of storing
energy is fully reversible. Since this can never occur under realistic working conditions,
where processes are always irreversible, the process of energy storage represents a
source of loss in the system’s potential for exergy recovery. This means that exergy
analysis quantitatively specifies more practical boundaries by providing the
information on losses as indicated by lost exergy.
For a steady state system, the exergy balance equation considering the system
components can be constructed in the following general form [86]:
&WXY&D
= ∑ ��𝑥U − ∑ ��𝑥Z + ∑ ��𝑥[\>Z; − ∑ ��𝑥[\>Z> − 𝐸��& (4.6)
where ��𝑥U denotes the rate of exergy transfer with the heat energy exchange across the
system volume. ��𝑥Z denotes the rate of exergy transfer by the boundary or work
applied on or done by the system. The term ��𝑥[\>Z represents the exergy transfer rate
with flow transfer through the system. The exergy destruction, which describes the
system irreversibility, is shown in the equation as 𝐸��&.
The exergy transfer due to the exchange of heat with the environment can be
formulated as follows:
48
Ex]^ = QN `1 −bcbd^e (4.7)
where To is the environmental temperature and TS is the temperature of the source (for
heat penetration process) or sink (for a heat loss process).
The exergy transfer rate associated with work, taking into consideration the
potential change of the volume of the system as a result of this work, can be written as
follows:
��𝑥Z = ��f + 𝑃g&hC&D
&BC&D
= ∑ ��; − ∑ ��> (4.8)
where m and �� denote mass and mass flow rate, respectively. The subscripts v, i and o
indicate the control volume and the inlet and exit of the control volume, respectively.
where P0 is the pressure of the system in a dead state.
Exergy of a flowing stream of gas or liquid can be represented as a sum of the
different exergies (chemical, physical, kinetic and/or potential) of the flow:
𝐸𝑥i>D = 𝐸i>D (4.9)
𝐸𝑥j;- = 𝐸j;- (4.10)
𝐸𝑥kl,B = 𝐸x = ∑ m𝜇; − 𝜇,op𝑁;; (4.11)
𝐸𝑥[\>Z = (𝐻; − 𝐻g) −𝑇g(𝑆; − 𝑆g) (4.12)
where Epot and Ekin are the potentials and kinetic energy, respectively, μ is the chemical
potential (i denotes environmental state, eq denotes equilibrium state), T0 is the
temperature, Ni is the component mole fraction and H and S represent the enthalpy and
the entropy of the system, respectively. The kinetic and potential terms of the flow
exergy can be disregarded as negligible since the changes in velocities and elevation
across the system components are too small compared to the values of other two terms.
Therefore, the flow exergy term can be written like this:
𝑒𝑥; = ℎ; − ℎg − 𝑇g(𝑠; − 𝑠g) + 𝑒𝑥 (4.13)
where h and s denote the enthalpy and the entropy, respectively, while ex-denotes
chemical exergy term.
The exergy destruction rate can be calculated according to the following:
49
𝐸��&t = 𝑇g��+,-,; (4.14)
whereSvwx,Ndenotes the rate of entropy generation for the system component i, which
can be correlated to the entropy balance equation for a steady state operation of each
system component:
��+,-,; = ∑ ��> 𝑠> − ∑ ��;𝑠; −∑(UV) (4.15)
For each individual system component, the corresponding exergy balance
equations and exergy efficiency, including those of individual subsystems. The exergy
efficiency for a particular process can be expressed through the ratio of exergy output
produced by the system to the total exergy input.
From the above, the exergy balance equation can be reformulated as follows:
&WXC&D
= ��𝑥f = ∑ y1 − VcVz �� − ��f + 𝑃g
&hC&D+ ∑ ��;𝑒𝑥; − ∑ ��>𝑒𝑥> − 𝑇g��+,-(4.16)
Exergy destruction in each component can be determined to utilize the exergy
balance on the system components at a steady state, as follows:
𝐸��&; = 𝐸𝑥U{ − 𝐸��|t + ∑ ��;𝑒𝑥; − ∑ ��> 𝑒𝑥> (4.17)
where Ex}N denotes the rate of exergy destruction occurring at the system component i,
Ex~^ and Ex]^ represent the exergy rates corresponding to work and heat transfer,
respectively, across the system limitations, while the exergy rates carried in and out of
the system with the flow are represented by exN, exP.
4.6 Components used in the systems
System components that are utilized in the different systems can be analyzed
energetically and exergetically in Table 4.1. Some components included in the table are
pressure regulator, expander, and compressor, for each component mass, energy,
entropy and exergy balance equations are listed.
50
Table 4.1. Energy Balance for System Components
Component Component Name
Balance Equation
Mixer Energy Balance ��AℎA + ��@�ℎ@� = ��@ℎ@ Entropy Balance ��A𝑠A + ��@�𝑠@� + ��+,-,BA = ��@𝑠@ Exergy Balance ��A𝑒𝑥A + ��@�𝑒𝑥@� = ��@𝑒𝑥@ + 𝐸��&,BA
Compressor Energy Balance ��@ℎ@ + ��kA = ���ℎ� Entropy Balance ��@ℎ@ + ��+,-,kA = ���ℎ� Exergy Balance ��@𝑒𝑥@ + ��kA = ���𝑒𝑥� + 𝐸��&,kA
Cooler, Heat Exchanger (Without heat exchanger efficiency)
Energy Balance ���ℎ� + ���ℎ� = ���ℎ� + ���ℎ� Entropy Balance ���𝑠� + ���𝑠� + ��+,-,,XA = ���𝑠� + ���𝑠� Exergy Balance ���𝑒𝑥� + ���𝑒𝑥� = ���𝑒𝑥� + ���𝑒𝑥� + 𝐸��&,,XA
Cooler, Heat Exchanger (With heat exchanger efficiency)
Energy Balance ���ℎ� + ���ℎ� = ���ℎ� + ���ℎ� + ��\>��,,XA Entropy Balance ���𝑠� + ���𝑠� + ��+,-,,XA = ���𝑠� + ���𝑠� +U����,���𝑻𝒃𝒐𝒖𝒏𝒅𝒓𝒚
𝑻𝒃𝒐𝒖𝒏𝒅𝒓𝒚𝒐𝒓𝑻𝒔𝒖𝒓𝒇𝒂𝒄𝒆𝒐𝒓𝑻𝟎𝒐𝒓𝑻𝒂𝒎𝒃𝒊𝒆𝒏𝒕 Exergy Balance ���𝑒𝑥� + ���𝑒𝑥� = ���𝑒𝑥� + ���𝑒𝑥� +𝐸��&,,XA + 𝐸��U����,��� 𝐸��U����,��� = y1 − Vc
V���z ��\>��,,XA𝑎𝑛𝑑𝐸��&,,XA = 𝑇g��+,-,,XA
𝐹𝑜𝑟𝑇,XA𝑚𝑎𝑘𝑒𝑎𝑛𝑎𝑠𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛𝑑𝑒𝑝𝑒𝑛𝑑𝑖𝑛𝑔 𝑜𝑛𝑠𝑡𝑎𝑡𝑒𝑝𝑜𝑖𝑛𝑡𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒𝑠.
Valve Energy Balance ��@�ℎ@� = ��@�ℎ@� Entropy Balance ��@�𝑠@� + ��+,-,f@ = ��@�ℎ@� Exergy Balance ��@�𝑒𝑥@� = ��@�𝑒𝑥@� + 𝐸��&,f@
M
2
1
3
C1
2
HX4
5
6
1
2
3
HX4
5
6
1
2
3
V
12
51
Flash Energy Balance ��@�ℎ@� + ���@ℎ�@ = ��@�ℎ@� + ���@ℎ�@ Entropy Balance ��@�𝑠@� + ���@𝑠�@ + ��+,-,[A = ��@�𝑠@� + ���@𝑠�@ Exergy Balance ��@�𝑒𝑥@� + ���@𝑒𝑥�@ = ��@�𝑒𝑥@� + ���@𝑒𝑥�@ +𝐸��&,[A
Vortex Tube Energy Balance ��@ℎ@ + ��kA = ���ℎ� Entropy Balance ��@ℎ@ + ��+,-,kA = ���ℎ� Exergy Balance ��@𝑒𝑥@ + ��kA = ���𝑒𝑥� + 𝐸��&,kA
4.7 Energy and Exergy Efficiencies
The exergy efficiency for the liquefaction process of the system is expressed as follows:
Ψ = WX¦t§|¨�©ª«����«
(4.18)
For each component exergy and exergy efficiency, in Table 4.2, Exergy
destruction and exergy efficiency are presented.
Efficiency can be defined as "the ability to produce the desired effect without
waste of, or with minimum use of, energy, time, resources, etc.,”. It is typically used in
the context of effectiveness which something is produced from something else, or how
close to the ideal situation the system is in performing a given task [85]. In engineering
systems, efficiency is usually expressed through nondimensional ratios of quantities,
like the energy in systems for transformation of energy. Energy efficiency, formulated
in this manner, is based on the First Law of Thermodynamics, where it can be stated
that the maximum efficiency is achieved if the input of energy equals the recoverable
output of energy. However, efficiency determined in this form does not mean its a true
measure of mimicking the ideal situation. A more meaningful efficiency can be
determined using exergy: through a formulation that the maximum efficiency can only
be achieved through a fully reversible process. This can be quantified through entropy,
where maximum efficiency can be achieved in through conservation of entropy, and,
correspondingly, the magnitude of the creation of entropy can be utilized as the measure
F
2
1
3
VT
12
52
of the degree to which the process is irreversible. However, using the entropy ratios
would not provide the measure of how close the process is to the ideal.
If maximum efficiency is defined to have been attained when, at the end of a
process, the sum of all energy in the process has the ability to perform work equal to
the sum before the process occurred, then exergy, as the measure of the ability to
perform work, has to be conserved in a process with maximum efficiency [67]. This
approach provides a measure of an approach of the system to the ideal. Another
advantage of exergy efficiency is that the values between 0 and 100% are always
obtained, unlike for energy consideration, where factors like coefficient of performance
can sometimes have values greater 100%. The energy (η) and the exergy (ψ)
efficiencies can be written as:
𝜂 = W�Wt
(4.19)
𝜓 = WX�WXt
= 1 − ∑WX����WXt
(4.20)
where i and o denote input and output of the system, and E and Ex denote energy and
exergy of the system, respectively. Exergy destruction ratio, or depletion factor Dp, can
be defined as the ratio of exergy destruction rate to the input exergy rate:
𝐷i =WX
WXt (4.21)
There are two other, commonly used exergy-based equations for the efficiency
of a steady-state device:
𝑅𝑎𝑡𝑖𝑜𝑛𝑎𝑙𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = 1 −WX,K+±k>-�TBiD;>-V>D²\,X,K+±;-iTD
(4.22)
𝑇𝑎𝑠𝑘𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦 = Vl,>K,D;k²\B;-;BTB,X,K+±;-iTD³,oT;K,&´kDT²\,X,K+±;-iTD
(4.23)
Exergy efficiencies weigh individual energy flows in the system, taking into
consideration their respective exergy contents, and separate inefficiencies into two
groups according to the cause: those originating from effluent losses and those
originating from irreversibilities in the system. This makes the information about
inefficiencies more useful, because they provide a quantitative measure of the system’s
potential for improved efficiency.
The yield calculation is based on the fraction of liquefaction:
53
𝑌 = B¶
B= l�·l¸
l�·l¶ (4.24)
where 𝑚[ is the mass flow rate of the fraction of liquefaction. The total system
efficiency is determined by:
𝜂@-&(%) = ml¶·l�p·Vc.(�¶·��¸)
Zº�º.B¶. 100 (4.25)
The energy per unit mass liquefied in the system is calculated by:
𝑤; = |B¶
= |B½
(4.26)
For the cooling cycle, the Coefficient of performance (COP) is determined by:
𝐶𝑂𝑃 = ml¶·l�pZº�ºÀ�
(4.27)
4.8 Sustainability Assessment
Exergy analysis, as a whole, evaluates the quality of the underlying thermodynamic
processes and serves as a potential tool for achieving maximum sustainability.
However, when considering a particular process in the real world, there are two main
aspects to be considered: its cost or efficiency and its environmental impact. Increase
in exergy efficiency of a particular system serves both to minimize the destructed
exergy (and reduce waste) and decrease the environmental impact of the process,
making exergoeconomic and exergoenvironmental analysis invaluable tools for
achieving sustainable development.
Another parameter is the sustainability index SI [87]:
𝑆𝐼 = A[ (4.28)
which represents a measure of the effectiveness of the process: higher sustainability
index means that less exergy is destroyed during the process as a portion of the input
exergy. This also means that the process with a higher sustainability index would have
a lower environmental impact.
54
4.9 Exergoeconomic Assessment
The exergoeconomic analysis represents a combination of exergy analysis and
economic analysis of a process. The goal is to help achieve maximum exergy system
optimization by determining the trade-off between the input cost (like the cost of fuel),
and capital and production cost. It uses a combination of thermodynamic and economic
principles to describe the system at the individual component level, providing
information for design improvement and cost-effective operation. The exergy part of
the model considers the exergy efficiencies at each point of the system, while the
economic part of the model takes into account the costs associated with the capital,
operation and maintenance of the system to determine the flow of costs in the system
and provide optimization of both specific variables of a particular component and the
system as a whole.
Exergy balance equation can be rewritten to include the cost associated with
each term of exergy flow:
𝐸��|t𝐶| = 𝐸𝑥U{ 𝐶V − 𝐸��&;𝐶Q + m∑ ��;𝑒𝑥; − ∑ ��> 𝑒𝑥>p𝐶g +𝑍j (4.29)
where CW is the unit cost of the rate of work production, while CT, CS, and C0 represent
unit costs corresponding to the thermal exergy flow, exergy rate of destruction and
chemical exergy flow, respectively. The term 𝑍j includes all financial costs correlated
with ownership, operation and maintenance of any particular system component, and
can be defined as [62]:
𝑍j = ÂÃÄÃ
��gg×ÆÇ (4.30)
where ϕk is maintenance factor (typically equal to 1.06), ��j is the unit cost rate and Nh
is the annual number of operational hours of the system component.
The cost balance equation can also be written in terms of unit cost rates for each
individual exergy flow component:
𝐶| = 𝐶V −𝐶Q +𝐶g + ��j (4.31)
where different �� parameters represent the corresponding unit cost rates: �� = 𝐶𝐸𝑥.
Application of exergy cost balance equation for every system component yields a set
of non-linear equations which can be solved for �� or C.
55
Table 4.2 Base system components exergy equations
Component Exergy Destruction Rate Exergy Efficiency Cooler 1 Exd,EX1 = m3ex3 + m5ex5
− m4ex4− m6ex6
ηex,EX1 =(m4ex4 − m6ex6)(m3ex3 − m5ex5)
Cooler 2 Exd,EX2 = m8ex8 + m10ex10− m9ex9− m11ex11
ηex,EX2 =(m9ex9 − m11ex11)(m8ex8 − m10ex10)
Cooler 3 Exd,EX3 = m38ex38 + m40ex40− m39ex39− m41ex41
ηex,EX3 =(m39ex39 − m41ex41)(m38ex38 − m40ex40)
Expansion Valve 1 Exd,V1 = m18ex18 − m16ex16 ηex,V1 =
(m16ex16)(m18ex18)
Expansion Valve 2 Exd,V2 = m23ex23 − m24ex24 ηex,V2 =
(m24ex24)(m23ex23)
Compressor 1 Exd,C1 = m3ex3 + Win,C1− m2ex2
ηex,C1 =(m3ex3 − m2ex2)
Win,C1
Compressor 2 Exd,C2 = m8ex8 + Win,C2− m7ex7
ηex,C2 =(m8ex8 − m7ex7)
Win,C2
Compressor 4 Exd,C3 = m38ex38 + Win,C3− m37ex37
ηex,C3 =(m38ex38 − m37ex37)
Win,C3
Turbo Expander 1 Exd,T1 = m31ex31 − m30ex30− Wout,T1 ηex,T1 =
Wout,GT1
(m31ex31 − m30ex30)
Turbo Expander 2 Exd,T2 = m32ex32 − m21ex21− Wout,T2 ηex,T2 =
Wout,GT2
(m32ex32 − m21ex21)=
Flash Drum 1 Exd,F1 = ms1exs1 + m25ex25−ms2exs2− m24ex24− Wout,F1
ηex,F1 =Wout,ST
(ms2exs2 − m24ex24)
Flash Drum 2 Exd,F2 = ms4exs4 + ms5exs5−ms3exs3− ms6exs6− Wout,F2
ηex,F2 =Wout,ST
(m56ex56 − m54ex54)
Heat Exchanger 1 Exd,HX1 = m1ex1 + m29ex29 +m37ex37 + m42ex42 + m46ex46+mN2gasexN2gas + ms5exs5 −m9ex9− m28ex28 −m36ex36 + m39ex39 −m44ex44− mN2liqexN2liq − ms7exs7
Heat Exchanger 2 𝐸��𝑑,𝐻𝑋2 = ��15𝑒𝑥15 + ��19𝑒𝑥19 +��27𝑒𝑥27 + ��34𝑒𝑥34
+��𝑠4𝑒𝑥𝑠4 −��6𝑒𝑥6 − ��13𝑒𝑥13 −��14𝑒𝑥14+ ��26𝑒𝑥26 −��49𝑒𝑥49
Heat Exchanger 3 Exd,HX3 = m18ex18 + m23ex23 +m26ex26 + m33ex33+mH2liqproexH2liqpro + ms2exs2 −m15ex15− m16ex16 −m22ex22 − m25ex25 −m32ex32−ms1exs1
56
The ratio of exergy loss to the capital cost of the system provides information
about the relative exergy loss (waste plus destruction) in the system compared to its
capital cost:
𝑅 = �� ��⁄ (4.32)
where �� represents the overall financial cost of the system and �� is the total exergy loss
in the system defined as:
�� = 𝐸��& + 𝐸��\>�� (4.33)
where 𝐸��\>�� represents the sum of exergy losses due to heat exergy transfer and flow
exergy leaving the system.
Finally, exergoeconomic factor for the component k can be defined as the ratio
of the capital cost of the system to the sum of the capital cost and exergy loss of the
system [88]:
𝑓,,j = ëÃ
ëÃìÄí,ÃîÃ= ëÃ
ëÃìÄí,Ã(WXìWX����) (4.34)
where CF,k is the unit cost of the exergy of the fuel expended by the component k. A
higher value of the,k means that the process is more efficient because the cost of lost
exergy is smaller compared to the capital cost of the system.
4.10 Environmental Impact Assessment
Environmental problems and issues have become a major factor in the adoption of new
technologies and construction of new production facilities due to increased public
awareness over the past few decades. These include an increasing number of pollutants
and ecosystem deterioration factors affecting the environment at the local, regional and
global level. Since these problems are often complex and in a state of constant flux,
where industrial and technological development often creates new environmental
problems, requiring analysis of environmental impact before their introduction.
Since hydrogen liquefaction is a thermal process, involving no exhaust gas or
another pollutant, the most important environmental factor stems from its energy
consumption. Since a large amount of energy is obtained by burning fossil fuels, it has
to be taken into consideration that any waste in the system would produce additional
greenhouse gas emissions. The amount of greenhouse gas emission from any thermal
process can be calculated using thermodynamic analysis and then compared to the
57
similar systems to evaluate its environmental impact relative to the technology currently
in use. The environmental impact of a process can be correlated directly with the rate
of exergy consumption or destruction through environmental impact factor f [87]:
𝑓 = ïªð
(4.35)
which represents the efficiency of exergy depletion, i.e. what portion of exergy flow in
the system is destroyed.
4.11 Optimization Study
Cost accounting is concerned with calculating the real cost of production, providing a
rational economic basis for pricing, providing means for allocation and control of
expenditure and providing information for making an evaluation of operating decisions.
Since cost can be defined, in the broadest of terms, as the number of resources necessary
to obtain a functional product, that amount of resources can be expressed through
exergy, allowing us to exercise thermoeconomic optimization of the system, with the
goal of minimizing the cost associated with exergy flow. In order to do this, it is
necessary to choose the proper equipment, in type and size, and the best configuration
of the system, along with optimal operating pressure and temperature ranges for the
process. In calculating the approximate average cost of thermal processes, it is
recommended to use the lowest possible aggregation level, since the sophistication of
the formulation of the cost balances has a considerable effect on the results of any
thermoeconomic analysis. This level is typically represented by the individual system
components, even in cases where all of the data is not available, because it is generally
preferable to make appropriate assumptions about the exergy cost of each individual
component, rather than to consider only a group of components as a whole [85].
The first step in the definition of the optimization problem is to clearly define
the system boundaries in such a way to include all the important components and
system parameters. Secondly, the selection of criteria that will form the basis of the
system's evaluation and optimization represents the key element of any optimization
[89]. The optimization criteria can be divided into three groups:
- Economic (capital investment, cost of operation and maintenance, net profit)
- Technological (efficiency, production time and rate, fuel consumption)
- Environmental (rate of pollutant emission)
58
Optimized system design is characterized by a minimum and maximum value
for each of the selected criteria [87]. A third essential element in system optimization
is the selection of design variables which characterize the available design options,
taking care to include all the possible factors which affect the efficiency and cost
effectiveness of the system. These can be independent or design variables, dependent
variables and parameters.
A mathematical model for solving the optimization problem consists of an
objective function that is to be minimized, a set of equality constraints and a set of
inequality constraints. The objective function, in this case, can be the maximization of
exergy efficiency or minimization of exergy loss or destruction or minimization of the
product cost. Equality constraints stem from appropriate thermodynamic and economic
models with properly defined system boundaries. Inequality constraints usually specify
the allowed operating ranges, minimum and maximum performance required and limits
on the available resources. Thermoeconomic optimization methods typically use a
primary optimization performance measure: minimizing the total cost of the system
product, although multicriteria optimization and environmental factors may also be
considered. The differences between the systems include is the amount of para species
in the liquid product. Without a catalyst, there were no improvements in the transition
of ortho to para species. The catalyst system provided up to 90% of parahydrogen
species in total, which is good as para- hydrogen provides stability of liquid hydrogen.
On the other hand, the total liquefaction yield (total liquid product per total hydrogen
input) is the same for both systems, due to the fact the amount of liquid product was the
same in both simulations. It is assumed that is because less energy was needed in the
main heat exchangers. The catalyst system provided up to 90% of parahydrogen species
in total, which is good as para- hydrogen provides stability of liquid hydrogen.
Simulation outcome and parameter changes have been utilized for the parametric study.
For Ortho-Para reactor conversion of the Hydrogen in the catalyst-based
systems The o-p conversion reaction is set as the following for the reactors:
𝐻𝑦𝑑𝑟𝑜𝑔𝑒𝑛 ⟶ 𝑝 − 𝐻𝑦𝑑𝑟𝑜𝑔𝑒𝑛 + ℎ𝑒𝑎𝑡 (4.36)
Conversion percentage for the equilibrium concentration of ortho and para hydrogen
is temperature dependent the and follows:
𝐶𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛(%) = 𝐶g + 𝐶A ∗ 𝑇 + 𝐶@ ∗ 𝑇@ (4.37)
59
where C0, C1, and C2 denote conversion confidents and T denotes the temperature in
kelvin. Yet, the coefficients are adjusted based on the composition of the outgoing
streams of the conversion reactors meets the experimental data. Properties of Hydrogen,
Nitrogen and Carbon dioxide used in the simulation shown in the Table 4.3.
Table 4.3. Properties of Hydrogen, Nitrogen and Carbon dioxide
Parameters Units Component
H2 LH2 N2 CO2
API Gravity 340 1860.95 340 340
Freeze Point C -259.2 -259.347 -210.001 -56.57
Std. Enthalpy cal/mol -57757.7 -57772 0 0
Heat of Fusion cal/mol 27.9689 28.0644 171.969 2154.15
Molecular Weight 2.01588 2.01588 28.0135 44.0098
Pitzer Acentric Factor -0.215993
-0.220755 0.0377215 0.223621
Critical Pressure bar 13.13 12.928 34 73.83
Some of Entropies cal/mol-K
31.2124 45.765 50.3696
Specific Gravity 0.3 0.0710179 0.3 0.3
Boiling Point C -252.76 -252.882 -195.806 -78.45
Critical Temp. C -239.96 -240.174 -146.95 31.06
Triple Point Temp C -259.2 -259.347 -210.001 -56.57
Liquid Molar Vol. cc/mol 28.5681 28.4572 34.6723 35.0189
Critical Vol. cc/mol 64.147 64.144 89.21 94
Standard Liquid Molar Vol. cc/mol 53.5578 28.4572 53.5578 53.5578
Critical Compressory Factor 0.305 0.302 0.289 0.274
60
RESULTS AND DISCUSSION
This chapter discusses the performance details of the proposed systems. This includes the
base system, and results obtained from systems 1A and 1B, 2A and 2B, and 3A and 3B.
The results of the exergoeconomic analysis and optimization study are also included.
Finally, comparative analysis between the introduced integrated system is carried out. The
analyzed and assessed systems are:
• Base Liquefaction System
• Advanced Liquefaction System without Catalyst (S1A)
• Advanced Liquefaction System with Catalyst (S1B)
• Advanced Liquefaction System without Catalyst with ORGs (S2A)
• Advanced Liquefaction System with Catalyst with ORGs (S2B)
• Advanced Liquefaction System without Catalyst with VTs (S3A)
• Advanced Liquefaction System with Catalyst with VTs (S3B)
5.1 Base System Results
The reference system presented was simulated based on a patent assigned to a
commercial entity in [19]. A comprehensive energy and exergy analyses was carried
out to examine the system performance.
Figure 5.1 illustrates the energy and exergy efficiencies of individual system
components. Equipment variables changed to test system outcomes as they are changed.
The Cooler (EX1), with 23% efficiency is the least efficient equipment.
Expansion Valve (V2) has the highest exergy efficiency and Expansion Valve
(V1) and Compressor (C2) are the second and third highest efficient equipment among
other units. Noticeably, Compressors and Valves are working with efficiency higher
than 80% and heat exchangers and expanders are working with lower exergy efficiency
than other equipment. The lower exergy efficiency of individual equipment affects the
overall system exergy efficiency. Different catalyst types and performance
enhancement of the heat exchangers could create a dramatic improvement to the
process efficiency.
61
Figure 5.1 Exergy and energy efficiencies for each component
Figure 5.2 demonstrates the effect of feeding the system with precooled
hydrogen starting from negative 50 °C on Compressors work losses. Compressor (C1)
is affected to most with loss of 20% as temperature increases while the rest of the
compressors are not affected by the feed temperature change
Figure 5.3 illustrates the effect of the pre-cooled hydrogen on the overall energy
and exergy efficiencies. The energy efficiency increases as feed temperature rises but
the overall exergy efficiency does not change on a degree of significance. This is due
to the change on only mainly one compressor C1. Kanoglu et al. in [90] researched
precooling using geothermal energy and showed improvement by decreasing the work
consumption by
Figure 5.4 displays the effect of hydrogen mass flow rate change on the work
of the turbo expanders, the adsorption unit and the overall yield. It can be noticed that
the work of turbo expanders TE1, TE2 and the overall yield are increased in a linear
manner while the work of the adsorption unit declined. In case of both turbo expanders,
the work increases from zero to roughly 3500 kw as the hydrogen mass flow rate
increases from 0 to 7000kg/h
24
49
34
9299
69
82 81
53
65
46
6253
47
26
80 80 80
100100 99
88 90
68 6876
66
99 99 99
0
20
40
60
80
100
Cooler (EX1)
Cooler (EX2)
Cooler (EX3)
Expansio
n Valve (V
1)
Expansio
n Valve (V
2)
Compressor (
C1)
Compressor (
C2)
Compressor (
C3)
Turbo Expander (T
E1)
Turbo Expander (T
E2)
Flash
Drum (F1)
Adsorp
tion Unit
(AO)
Heat Exch
anger (HX1)
Heat Exch
anger (HX2)
Heat Exch
anger (HX3)
Effic
ienc
y (%
)
System Components
Exergy Efficiency
62
(a)
(b)
Figure 5.2 Effect of pre-cooling Hydrogen on Compressors work losses and yield. The graphs illustrate a) work loss of compressors C1, C2, and C3 and, b) daily
hydrogen yield against hydrogen Feed temperature
0
100
200
300
400
500
600
-60 -40 -20 0 20 40 60
Wor
k Lo
ss (k
W)
H2 Feed temperature (oC)
Compressor C1
Compressor C2
Compressor C3
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
-50 -40 -30 -20 -10 0 10 20 30 40 50
Yiel
d (T
ons/
Day)
H2 Feed Temperature (oC)
63
Figure 5.3. Effect of pre-cooling hydrogen on overall energy and exergy efficiencies
(a)
10
11
12
13
14
15
16
17
18
19
20
-60 -40 -20 0 20 40 60
Effic
ienc
y (%
)
Hydrogen Feed Tempreture (oC)
Exergy efficiency
Energy Effeciency
0
500
1000
1500
2000
2500
3000
3500
4000
0 1000 2000 3000 4000 5000 6000 7000
Wor
k (k
W)
H2 Mass flow rate (kg/h)
Turbo Expander TE1
Turbo Expander TE2
64
(b)
(c)
Figure 5.4. Effect of hydrogen mass flow rate change on work and yield. The graphs show a) work (kW) of (a)Turbo expander (TE1), Turbo expander (TE2), (b)Adsorber (A0) and; (c) Liquid hydrogen generation per day against hydrogen Feed Mass Flow
rate (kg/h)
The effect of hydrogen mass flow rate change on the overall energy and exergy
efficiencies is shown in Figure 5.5. The exergy efficiency plummets after 10% increase
and energy efficiency declines gradually by a small percentage. An optimum mass flow
rate can therefore give a higher yield without compromising on the exergy efficiency.
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
0 1000 2000 3000 4000 5000 6000 7000
Work of Adsorber A0
0
20
40
60
80
100
120
140
160
180
200
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500
Liqu
id H
2yi
eld
(ton
es/d
ay)
H2 Feed Mass Flow Rate (kg/h)
65
Figure 5.5 Effect of hydrogen mass flow rate change on the overall energy and exergy efficiencies
Figure 5.6 illustrates how changing turbo expander TE1 pressure is affecting
main components work. It can clearly be seen from figure 5.6 that the performance of
key components are not affected by variations in pressure of TE1. The graphs clearly
show that the pressure of turbo expander TE1 does not affect the work rate of the other
main components in the system.
Figure 5.6 Effect of changing turbo expander TE1 pressure on main components work
15.46
0.32
0
2
4
6
8
10
12
14
16
18
0 2000 4000 6000 8000
Effic
ienc
y (%
)
Hydrogen Feed Mass Flow Rate, kg/h
Exergy Effeciency
Energy Effeciency
-10
190
390
590
790
990
0 50 100 150 200
Wor
k Ra
te (k
W)
Turbo Expander TE1 Pressure, bar
Turbo Expander TE1 Compressor C1 Compressor C2
Compressor C3 Adsorption Unit A0 Turbo Expander TE2
66
Figure 5.7 illustrates the effect of changing turbo expander TE1 pressure on the
overall energy and exergy efficiencies. It is seen that TE1 has a notable impact on the
energy efficiency and a slight impact on the exergy efficiency. Both values decrease
with increase in TE1 pressure. In this system design, the turbo expander TE1 works
independently where is failure does not affect the overall system.
Figure 5.7 Effect of changing turbo expander TE1 pressure on the overall energy and exergy efficiencies
Figure 5.8 demonstrates the effect of changing pressure of flash drums on the
overall yield. Though it shows volatility in the production per day, it’s not significant
for a system of its scale and has minimal effect on the outcome. However, the yield
values show no discernible trend or consistency.
The effect of pressure of flash drums on energy and exergy are also negligible
as shown in Figure 5.9. In fact, the effective change in overall energy and exergy is
close to zero. This shows that the system startup period has a noticeable effect on the
10
11
12
13
14
15
16
0 50 100 150 200
Effic
ienc
y (%
)
TE1 Pressure (bar)
Exergy Effeciency Energy Effeciency
67
flash drum. It requires about 400 iterations in the simulation model to reach the steady
state for having the system to give required results.
Figure 5.8 Effect of changing flash drums pressure on the overall yield
Figure 5.9 Effect of flash drum pressure rate change on overall energy and exergy
10
11
12
13
14
15
16
0 1 2 3 4 5 6 7
Effic
ienc
y (%
)
Flash Drum Pressure (bar)
Exergy Effeciency
Energy Effeciency
68
Figure 5.10 Effect of hydrogen feed pressure change on the compressors
Changing the feed pressure does not help the overall system form compressors
perspective. In addition, both the exergy and energy efficiencies are unaffected by
variations in feed pressure. Therefore, the only effect of hydrogen feed pressure
increase is an increase in the work of compressor C1 as shown in Figure 5.10.
Figure 5.11 Effect of hydrogen feed pressure change on the overall energy and exergy efficiencies
236.28
236.3
236.32
236.34
236.36
236.38
236.4
0
100
200
300
400
500
600
700
800
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Wor
k (k
W)
Wor
k Ra
te (k
W)
H2 Feed Pressure (bar)
Work of Compressor C2 Work of Compressor C3 Work of compressor C1
10
11
12
13
14
15
16
17
18
19
20
0 5 10 15 20 25
Effic
ienc
y (%
)
Hydrogen Feed Pressure (bar)
Exergy Effeciency
Energy Effeciency
69
Figure 5.11 depicts the effect of the hydrogen feed pressure change on energy
and exergy are also negligible. In Figure 5.12, the Nitrogen gas mass flow rate change
is changing on the overall energy and exergy efficiencies. The change in energy
efficiency is noticeable but there is no significant change in exergy efficiency. The feed
can be utilized to cool the hydrogen feed. Other gases may also be utilized for the
cooling process that can replace nitrogen.
Figure 5.12 Effect of the Nitrogen gas mass flow rate change on the overall energy and exergy efficiencies
Temperature approach of the heat exchangers has a considerable effect on the
liquefaction plants. The more the pinch temperatures, the more the operating costs or
energy costs of the process. Figure 5.13, 5.14, and 5.15 show that hot composite curves
and heat flow through the main heat exchangers HX1 to HX2, and HX3 respectively.
10
11
12
13
14
15
16
0 10000 20000 30000 40000
Effic
ienc
y (%
)
Nitrogen Gas Mass Flow Rate (kg/h)
Exergy efficiency
Energy efficiency
70
Figure 5.13 Heat Exchanger HX1 Heat composite curves
-100
-50
0
50
100
150
-1.2E+08 -80000000 -40000000 0
Tem
pera
ture
(o C)
Heat Duty (kJ/h)(a)
-200
-150
-100
-50
0
-5E+08 -3E+08 -1E+08
Tem
pera
ture
(o C)
Heat Duty (kJ/h)(b)
-200
-150
-100
-50
0
-12000000 -8000000 -4000000 0
Tem
pera
ture
(o C)
Heat Duty (kJ/h)(c)
-250
-200
-150
-100
-50
0
1000000 6000000 11000000 16000000
Tem
pera
ture
(o C)
Heat Duty (kJ/h)(d)
-235
-185
-135
0 2000000 4000000 6000000 8000000 10000000 12000000 14000000
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(e)
71
Figure 5.14 Heat Exchanger HX2 Heat composite curves
Heat exchangers HX1, HX2, and HX3 Heat duties are the very sensitive to the temperatures and
each inlet is duty increase with the temperature increase. It can be said that in extreme cooling process there
is need to optimize the process with consideration of the inlet stream intake in the efficiency.
-230
-220
-210
-200
-190
-180
-1.35E+08 -8.50E+07 -3.50E+07 1.50E+07
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(a)
-230
-220
-210
-200
-190
-180
-170
-3.60E+07 -2.60E+07 -1.60E+07 -6.00E+06
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(b)
200
250
300
350
400
450
0.00E+00 1.00E+08 2.00E+08
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(c)
-235
-225
-215
-205
-195
-2.00E+07 -1.50E+07 -1.00E+07 -5.00E+06 0.00E+00
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(d)
72
Figure 5.15 Heat Exchanger HX3 Heat composite curves
-254
-249
-244
-239
-234
-1.00E+08 -6.00E+07 -2.00E+07
Tem
pera
ture
(o C)
Heat Duty (kJ/h)(a)
-253
-252.9
-252.8
-252.7
-252.6
-1.80E+06 -1.30E+06 -8.00E+05 -3.00E+05 2.00E+05
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(b)
-254
-249
-244
-239
-234
-2.00E+07 -1.50E+07 -1.00E+07 -5.00E+06 0.00E+00
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(c)
-248
-244
-240
-236
-232
-228
0.00E+00 2.00E+06 4.00E+06 6.00E+06 8.00E+06
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(e)
-255
-235
-215
-195
-175
-1.20E+07 -7.00E+06 -2.00E+06 3.00E+06
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(d)
-250
-150
-50
50
150
250
350
-3.00E+07 2.00E+07 7.00E+07 1.20E+08
Tem
pera
ture
(o C)
Heat Duty (kJ/h)
(f)
73
5.2 Systems 1A and 1B Results
With results in the base system, it was clear that some of the system components need
to be changed. The performance aspects of the base system are investigated by a
comprehensive study covering energy and exergy analyses. Various operating conditions,
reference state parameters, and system parameters are altered to examine each parameter’s
effects on the performance of system 1A. From this analysis, it is evident that some aspects
had to be modified and changed to produce a working functional system illustrates the
energy and exergy efficiencies of the equipment in the system individually. Equipment
variables changed to test system outcomes as they are changed.
The results show that the cooler (EX1) has the lowest efficiency (23%).
Expansion Valve (V2) has the highest exergy efficiency and Expansion Valve (V1) and
Compressor (C2) are the second and third highest efficient equipment among other
units. In fact, Compressors and Valves are working with efficiency higher than 80%,
while, heat exchangers and expanders are working with lower exergy efficiency than
other equipment. The lower exergy efficiency of almost all equipment affects the
overall system exergy efficiency. Different catalyst types and performance
enhancement of the heat exchangers could create a dramatic improvement to the
process efficiency. Figure 5.16 shows the exergy and energy efficiency of systems 1A
and 1B.
Figure 5.17 shows the effect of hydrogen feed pressure on energy and exergy
efficiency of system 1A. It can be seen that the energy efficiency does vary with
variation in hydrogen feed pressure. In fact, the energy efficiency increased 12.4% as
the hydrogen feed pressure increased from 1 bar to 5 bars. The exergy efficiency also
increases from 17.42% to 18.04% for the same change.
74
(a)
(b) Figure 5.16 Energy efficiencies for main components of System 1A and 1B
Figure 5.18 shows that changes in compressor (C1) pressure does have an effect
on both energy and exergy efficiency of system 1A. The energy efficiency increases
from 41.83% to 67.59% as the compressor pressure increased from 2 bar to 4 bar. For
the same change in compressor pressure, the exergy efficiency increases moderately
from 17.59% to 16.78%.
24
49
34
92 99
69
82 81
53
65
23
62
31
63
41
99 99
8389
98
64
85
28
74
0102030405060708090
100
Heat Ex
chan
ger (H
X1)
Heat Ex
chan
ger (H
X2)
Heat Ex
chan
ger (H
X3)
Expan
sion Valv
e (V1)
Expan
sion Valv
e (V2)
Compressor (C
1)
Compressor (C
2)
Compressor (C
3)
Compressor (C
3)
Compressor (C
3)
Flash
Drum (F1)
Adsorptio
n Unit (AO)
Effic
ienc
y (%
)
Exergy Efficiency
Energy Efficiency
77
18
42
99
60
23
77 78 7867
23
62
23
55
78
30
99 99 9987
30
74
0102030405060708090
100
Heat Ex
chan
ger (H
X1)
Heat Ex
chan
ger (H
X2)
Heat Ex
chan
ger (H
X3)
Expan
sion Valv
e (V1)
Expan
sion Valv
e (V2)
Compressor (C
1)
Compressor (C
2)
Compressor (C
3)
Compressor (C
3)
Compressor (C
3)
Flash
Drum (F1)
Adsorptio
n Unit (AO)
Effic
ienc
y (%
)
Exergy Efficiency
Energy Efficiency
75
Figure 5.17. Effect of hydrogen feed pressure on overall efficiencies for System 1A
Figure 5.18 Effect of Compressor C1 pressure on overall efficiencies for System 1A
Figure 5.19 Effect of Compressor C2 pressure overall efficiencies for System 1A
10
15
20
25
30
35
0 5 10 15 20 25
Effic
ienc
y (%
)
H2 Feed Pressure (bar)
Exergy Efficiency
Energy Efficiency
0
10
20
30
40
50
60
70
80
1.5 2 2.5 3 3.5 4 4.5
Effic
ienc
y (%
)
Compressor C1 Pressure (bar)
Energy Efficiency
Exergy Efficiency
7
9
11
13
15
17
19
1 2 3 4 5 6
Effic
ienc
y (%
)
Compressor C2 Pressure (bar)
Energy Efficiency
Exergy Efficiency
76
Figure 5.19 shows how the effect of variations in compressor C2 on the energy
and exergy efficiency of system 1A. The variation in exergy efficiency is negligible.
However, the energy efficiency initially drops and then rises. As the compressor (C2)
rises from 2 bar to 4 bar, the energy efficiency decreases from 15.85% to 11.14%.
However, as the compressor (C2) pressure rises from 4 bar to 5 bar, the energy
efficiency rises from 11.14% to 13%.
Figure 5.20 shows that changes in H2 feed pressure on overall energy and exergy
efficiency of system 1B. It can be seen that the variation in efficiency caused by feed
pressure variation is minimal in both cases. Both efficiencies do rise, but by less than
0.5% in both cases.
Figure 5.20 Effect of hydrogen H2 feed pressure on overall efficiencies for System 1B
Figure 5.21 Effects of Compressor C2 pressure on overall energy and exergy efficiencies for System 1B
17
18
19
20
21
22
23
0 5 10 15 20 25
Effic
ienc
y (%
)
Pressure (bar)
Exergy Efficiency
Energy Efficiency
1517192123252729313335
2 2.5 3 3.5 4
Effic
ienc
y (%
)
Pressure (bar)
Exergy Efficiency
Energy Efficiency
77
Figure 5.21 clearly shows that variations in compressor (C2) pressure has no
effect on either energy or exergy efficiency of system 1B.
5.2.1 Pre-cooling phase at systems S1A and S1B
The precooling phase in the liquefaction cycle helps cool the hydrogen gas for faster
liquefaction. An analysis has been conducted to understand the precooling phase of the
heat exchangers. Figure 5.22 shows the Heat Load, Exergy flow and Temperature
against specific exergy flow for Precooling Phase heat exchanger HX1 at the liquid
nitrogen inlet. The graphs indicate that as specific exergy decreases, total exergy
decreases while heat load increases beyond a certain value. While temperature initially
rises with the decrease in specific exergy, it soon stabilises.
Figure 5.23 shows the heat load, exergy flow and temperature for Precooling
Phase heat exchanger HX1 at stream 9 inlet and depicts a different view from the
Nitrogen inlet but in decreasing exergy flow. At inlet 9 and inlet 28, the exergy flow
increases slightly in the heat exchanger as a sign of need of optimization as shown in
Figure 5.23 and Figure 5.24.
Figure 5.22 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream N2LIQ inlet for System 1A
-195.805
-195.8
-195.795
-195.79
-195.785
-195.78
-195.775
0
200
400
600
800
1000
1200
760.2 760.1 709.6 658.9 608.3 557.7 507.1 456.5 405.9 355.3 304.7 254.1 203.5
Tem
pera
ture
(˚C
)
Hea
t Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
78
The precooling phase shows good indicator that it can effect the changes on the
overall system and possibly on the efficiency as can bee seen at the end of the chapter
with the analysis.
Figure 5.23 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 28 inlet for System 1A
Figure 5.24 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 9 inlet for System 1A
At inlet 36 and 44 the exergy flow drops at an almost constant rate as shown in
Figure 5.25 and Figure 5.26. It can be seen that in both cases, as the specific exergy
-140
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-80
-60
-40
-20
0
-20
-15
-10
-5
0
5
10
15
20
25
30
401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
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0
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0
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400
3903.8 3822.1 3756.5 3709.2 3682.8 3680.5 3706.6 3766.2 3866.3 4016.2 4228.7 4522.1
Tem
pera
ture
(o C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy (kj/kg)
Total ExergyHeat LoadTemperature
79
flow rises, heat load increases and exergy decreases. Additionally, higher specific
energy flows correspond to higher inlet temperatures in both cases.
Figure 5.25 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 36 inlet for System 1A
Figure 5.26 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 44 inlet for System 1A
Figure 5.27 shows the Heat Load, exergy flow, and temperature for heat
exchanger HX1 at stream (R) inlet for system 1A. It can be seen that both the heat load
and exergy plots diverge with increase in specific energy flow. The total exergy flow
increases while heat load increases with increase in specific exergy flow. Lower values
of specific exergy flow also correspond to lower temperatures. In addition, Figure 5.28
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-120
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-80
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-20
0
0
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40
50
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401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
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-20
0
0
5
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25
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45
401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
80
shows the heat load, exergy flow, and temperature for heat exchanger (HX1) at stream
N2LIQ inlet for system 1B and signals that as specific exergy goes down also the total
exergy goes down with slight change in temperature value.
Figure 5.27 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream R inlet for System 1A
Figure 5.28 Heat Load, Exergy flow vs Temperature for Precooling Phase heat
exchanger HX1 at stream N2LIQ inlet for System 1B
Figure 5.29 shows the changes in heat load, Exergy flow and Temperature for
Precooling Phase heat exchanger HX1 at stream 28 inlet. It is seen that with increase in
specific exergy flow rate, the total exergy flow increases while heat load decreases.
Additionally, lower temperatures correspond to higher values of specific exergy flow.
-200-180-160-140-120-100-80-60-40-200
-20
0
20
40
60
80
100
401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
-195.805
-195.8
-195.795
-195.79
-195.785
-195.78
-195.775
0
200
400
600
800
1000
1200
760.2760.1709.2658.3607.3556.4505.5454.5403.6352.6301.7250.8199.8
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
81
Figure 5.29 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 28 inlet for System 1B
Figure 5.30 illustrates the variation of heat load, exergy flow, and temperature
for heat exchanger HX1 in the precooling phase at stream inlet 9. The total exergy flow
has a nonlinear relationship with specific exergy flow in this case. The trend is that as
specific exergy flow increases, the total exergy flow also increases, but the two have a
non-linear relationship.
Figure 5.30 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 9 inlet for System 1B
Figure 5.31 shows the variation of temperature, exergy flow and heat load for
precooling heat exchanger HX1 at stream 36 inlet. With increase in specific exergy
flow, it is seen that total exergy flow rate decreases and heat load increases. Similarly,
lower inlet temperatures correspond to lower values of specific exergy flow. In this
-140
-120
-100
-80
-60
-40
-20
0
-20-15-10
-505
1015202530
396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
-150
-100
-50
0
50
100
150
-400
-300
-200
-100
0
100
200
300
400
3042.8 2961.9 2897.0 2850.3 2824.5 2822.6 2849.0 2908.7 3008.6 3158.0 3369.6 3661.8
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
82
case, it can be noted that the relationship between specific exergy flow and all three
other variables is linear.
Figure 5.31 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 36 inlet for System 1B
Figure 5.32 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 44 inlet for System 1B
Figure 5.32 shows the variation of temperature, exergy flow and heat load for
precooling heat exchanger HX1 at stream 44 inlet. With the increase in specific exergy
-180
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-165
-160
-155
-150
-145
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-135
0
10
20
30
40
50
60
396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
-180
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-170
-165
-160
-155
-150
-145
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-135
0
5
10
15
20
25
30
35
40
396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
83
flow, it is seen that total exergy flow rate decreases and heat load increases. Similarly,
lower inlet temperatures correspond to lower values of specific exergy flow.
5.2.2 Liquefaction Phase at systems S1A and S1B
This section deals with the variation of Heat Load, exergy, and temperature for the
liquefaction phase of systems S1A and S1B. Figure 5.33 shows heat load, Exergy flow
vs Temperature for Precooling Phase heat exchanger HX2 at stream 49 inlet for System
1A. It can be seen that total exergy flow increases with increase in specific exergy flow
while heat load decreases. The variation of heat load is linear, while total exergy flow
has a nonlinear relationship. Higher specific exergy flows can be achieved at lower
temperatures at the actual liquidation phase.
Figure 5.33 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX2 at stream 49 inlet for System 1A
Figure 5.34 illustrates the variation of heat load, exergy flow and temperature
for Precooling Phase heat exchanger HX2 at stream 13 inlet for System 1A. Much like
in Figure 5.33, it can be seen that total exergy flow increases with increase in specific
exergy flow while heat load decreases. The variation of heat load is linear, while total
exergy flow has a nonlinear relationship. Higher specific exergy flows are achieved at
lower temperatures. The variation of these parameters for Precooling Phase heat
exchanger HX2 at stream inlet S10 is shown in Figure 5.35. The trends are similar to
those at stream inlet 13 and stream inlet 9. However, the nonlinearity in total exergy
flow is more noticeable at stream inlet S10.
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-100
-50
0
50
100
150
200
401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
84
Figure 5.34 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX2 at stream 13 inlet for System 1A
Figure 5.35 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream S10 inlet for System 1A
Figure 5.36 illustrates the variation of heat load, exergy flow and temperature
for Liquefaction Phase heat exchanger HX2 at stream 14 inlet for System 1A. It can be
seen that the heat exchanger HX1, the specific exergy flow is lower at lower inlet
temperatures. Additionally, the heat load increases linearly with increase in specific
exergy flow, while total exergy flow has a nonlinear, but decreasing relationship with
specific exergy flow.
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-100
0
100
200
300
400
500
401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
-250
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-150
-100
-50
0
0
20
40
60
80
100
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140
401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
85
The variation brought about by pre cooling in heat exchanges HX3 can be seen
in Figure 5.37, where the heat load, exergy flow and temperature for Liquefaction Phase
heat exchanger HX3 at stream 22 inlet for System 1A is illustrated. The key difference
is that total exergy flow now increases with increase in specific exergy flow while
overall heat load decreases. Specific exergy flow is higher at lower temperatures,
though the variation of temperature vs. specific exergy is minor.
Figure 5.38 and 5.40 illustrate variations of temperature, total exergy flow,
and heat load for liquefaction phase heat exchanger HX3 at stream inlets 32b and S1
respectively. At stream inlet 32b, the variation of total exergy flow offers a unique
trend. It is seen that as specific exergy flow increases, the total exergy flow increases
initially before dropping sharply at a point. The specific exergy values are higher at
lower temperatures. The heat load decreases slightly with increase in specific exergy.
With variations at S1, it is seen in Figure 5.39 that the heat load and total exergy flow
remains constant with variations in specific exergy. However, the temperature and
specific exergy flow have a slightly nonlinear relationship with specific exergy being
lower at lower temperatures.
Figure 5.36 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 14 inlet for System 1A
Figure 5.40 shows the variation of heat load, exergy and temperature for HX3
at stream inlet 17 for system 1A. It can be seen that heat load increases with increase in
specific exergy flow while total exergy flow decreases. At lower temperatures, the
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
0
20
40
60
80
100
120
140
401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
86
specific exergy flow is higher and vice versa. The next set of graphs illustrate the
variation of exergy, heat load and temperature for system 1B.
Figure 5.37 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 22 inlet for System 1A
Figure 5.38 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 32b inlet for System 1A
Figures 5.41, 5.42, and 5.43 illustrate the variations in temperature, heat load,
and exergy for system 1B for stream inlets S10, 14, and 13. Figure 5.41 and Figure 5.42
show similar trends. The total exergy decreases with increase in specific exergy flow
while the heat load increases. Specific exergy flow is lower at lower temperatures.
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401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
-235
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401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
87
Figure 5.39 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream S1 inlet for System 1A
Figure 5.40 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 17 inlet for System 1A
-245.04
-245.03
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-244.99
-244.98
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401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
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pera
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(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
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401.4 431.1 462.2 494.8 528.9 564.6 601.9 641.0 681.9 724.6 769.4 816.1
Tem
pera
ture
(˚C
)
Hea
t Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
88
Figure 5.41 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream S10 inlet for System 1B
Figure 5.42 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 14 inlet for System 1B
However, the variation of heat load is more noticeable in inlet 14, and the
variation of total exergy flow exhibits a nonlinear pattern in this case (Figure 5.42). In
figure 5.43 (corresponding to inlet 13) the trends are different. There is a slight increase
in total exergy flow and a slight increase in heat load as the specific exergy flow
increases. The value of specific exergy flow is higher at lower temperatures.
-203
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396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
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(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
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0
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396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
89
Figure 5.43 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 13 inlet for System 1B
In the next set of figures, the variations in exergy, temperature, and heat load
for heat exchanger HX3 in system 1B is illustrated. Figure 5.44 and Figure 5.45 show
the variations related to stream inlets 49 and 22 respectively. Both exhibit similar
trends, with lower temperatures corresponding to higher values of specific exergy flow.
The heat load in both cases decrease with increase in specific exergy flow while total
exergy flow increases.
Figure 5.46 corresponds to variations in heat load, exergy and temperature for
HX3 at stream inlet 17. The value of specific exergy flow is lower at lower values of
temperature. The heat load increases with increase in specific exergy flow. However,
the variation in total exergy flow shows an interesting trend. Initially, with increase in
specific exergy flow the total exergy flow increases. However, at a certain point, the
trend changes, and the total; exergy flow decreases in a nonlinear manner with increase
in specific exergy flow.
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-50
0
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300
396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
90
Figure 5.44 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 49 inlet for System 1B
Figure 5.45 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 22 inlet for System 1B
Figure 5.47 ilustrates the variations corresponding to stream inlet S1. It can be
seen that the total exergy flow and heat load are unvaried in this case with increase in
specific exergy flow. The temperature and specific exergy flow have a slightly
nonlinear relationship, with lower values of specific exergy flow corresponding to
lower temperatures.
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0
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396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
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396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2Te
mpe
ratu
re (˚
C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
91
Figure 5.46 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 17 inlet for System 1B
Figure 5.47 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream S1 inlet for System 1B
-250
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396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
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(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
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-245.01
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396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
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(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
92
Figure 5.48 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 32b inlet for System 1B
The final figure in this series, Figure 5.48 shows the variation in heat load, Exergy
flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 32b inlet
for System 1B. Specific exergy flow is lower at higher temperatures in this case. It is
seen that the heat load decreases with increase in specific exergy flow. The variation of
total exergy flow initially increases with increase in specific exergy flow up to a point,
but then decreases linearly.
5.3 Systems 2A and 2B Organic Rankine Cycles
Figure 5.49 illustrates the energy and exergy efficiencies of the equipment in the system
individually. Equipment variables changed to test system outcomes as they are
changed. The least efficiency is at 23 % for the Cooler (EX1). Expansion Valve (V2)
has the highest exergy efficiency and Expansion Valve (V1) and Compressor (C2) are
the second and third highest efficient equipment among other units. Noticeably,
Compressors and Valves are working with efficiency higher than 80%. While, heat
exchangers and expanders, are working with lower exergy efficiency than other
equipment. The lower exergy efficiency of almost all of some equipment affects the
overall system exergy efficiency. Different catalyst types and performance
enhancement of the heat exchangers could create a dramatic improvement to the
process efficiency.
-235
-230
-225
-220
-215
-210
-205
-10
0
10
20
30
40
50
60
70
396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
93
Figure 5.49 Systems (a) 2A and (b) 2B Exergy and Energy efficiencies for each component
Figure 5.50 illustrates the effect of the hydrogen gas feed pressure on the overall
energy and exergy efficiencies. The energy efficiency increases as feed temperature
rises but the overall exergy efficiency does not change on a degree of significance. This
is due to the change on only mainly one compressor C. Kanoglu et al. in [90] researched
precooling using geothermal energy and showed improvement by decreasing the work
consumption by 25%.
44
8
4732
11
33 35
9
77
9
77 76 77 75
37
57
10
61
38
13
40 46
12 12
92 99 92 90
44
0102030405060708090
100
Heat Ex
chan
ger (H
X1)
Heat Ex
chan
ger (H
X2)
Heat Ex
chan
ger (H
X3)
Cooler EX1
Cooler EX2
Cooler EX3
Cooler EX4
Cooler EX5
Cooler EX6
Compressor (C
1)
Compressor (C
2)
Compressor (C
3)
Compressor (C
4)
Compressor (C
5)
Flash
Drum (F1)
Effic
ienc
y (%
)
Exergy Efficiency
Energy Efficiency
(a)
25
7 5
32 30
86
1832
9
77 76 77 78 78
4030
8 6
42 39
23
42
11
92 91 92 94
48
0102030405060708090
100
Heat Ex
chan
ger (H
X1)
Heat Ex
chan
ger (H
X2)
Heat Ex
chan
ger (H
X3)
Cooler EX1
Cooler EX2
Cooler EX3
Cooler EX4
Cooler EX5
Cooler EX6
Compressor (C
1)
Compressor (C
2)
Compressor (C
3)
Compressor (C
4)
Compressor (C
5)
Flash
Drum (F1)
Effic
ienc
y (%
)
Exergy Efficiency
Energy Efficiency
(b)
94
(a)
(b)
Figure 5.50. Effect of pre-cooling hydrogen feed pressure variations on overall energy and exergy efficiencies for systems (a) S2A and (b) S2B
The effect of Compressor (C1) pressure change on the overall energy and
exergy efficiencies is shown in Figure 5.51 and it shows the exergy efficiency plummets
at after the 10% increase and the energy efficiency declines by a small percentage. An
optimum mass flow rate can make give and a higher yield without compromising on
the exergy efficiency.
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
1.0 3.9 6.7 9.6 12.4 15.3 18.1
Perc
enta
ge
H2 Feed Pressure (bar)
Exergy Efficiency
Energy Efficiency
13.00%
13.50%
14.00%
14.50%
15.00%
15.50%
1.0 3.9 6.7 9.6 12.4 15.3 18.1
Perc
enta
ge
H2 Feed Pressure (bar)
Exergy Efficiency
Energy Efficiency
95
(a)
(b)
Figure 5.51 Effect of Compressor 1 (C1) pressure variations on overall energy and exergy efficiencies for systems (a) S2A and (b) S2B
Figure 5.52 illustrates the effect of changing Compressor 5 (C5) pressure on the
overall energy and exergy efficiencies that C5 has an impact on the energy efficiency
and very slight impact on the exergy efficiency.
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
80.00%
1.50 2.00 2.50 3.00 3.50 4.00 4.50
Effic
ienc
y
Compressor 1 (C1) pressure (bar)
Exergy Efficiency
Energy Efficiency
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
70.00%
1.50 2.00 2.50 3.00 3.50 4.00 4.50
Effic
ienc
y
Compressor 1 (C1) pressure (bar)
Exergy Efficiency
Energy Efficiency
96
(a)
(b) Figure 5.52 Effect of Compressor 5 (C5) pressure variations on overall energy and
exergy efficiencies for systems (a) S2A and (b) S2B 5.3.1 Pre-cooling phase at systems S2A and S2B
The precooling phase in the liquefaction cycle helps cool the hydrogen gas for faster
liquefaction. An analysis has been conducted to understand the precooling phase of the
heat exchangers. Figure 5.53 shows the Heat Load, Exergy flow and Temperature
against specific exergy flow for Precooling Phase heat exchanger HX1 at the liquid
nitrogen inlet. The graphs indicate that as specific exergy decreases, total exergy
decreases while heat load increases beyond a certain value. While temperature initially
rises with the decrease in specific exergy, it soon stabilises.
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
2.00 2.22 2.44 2.67 2.89 3.11 3.33 3.56 3.78 4.00 20.00
Effic
ienc
y
Compressor 5 (C5) pressure (bar)
Exergy Efficiency
Energy Efficiency
0.00%
5.00%
10.00%
15.00%
20.00%
25.00%
2.00 2.22 2.44 2.67 2.89 3.11 3.33 3.56 3.78 4.00 20.00
Effic
ienc
y
Compressor 5 (C5) pressure (bar)
Exergy Efficiency
Energy Efficiency
97
Figure 5.55 shows the heat load, exergy flow and temperature for Precooling
Phase heat exchanger HX1 at stream 9 inlet and depicts a different view from the
Nitrogen inlet but in decreasing exergy flow. At inlet 9 and inlet 28, the exergy flow
increases as can be seen in Figure 5.54 and Figure 5.55.
Figure 5.53 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream N2LIQ inlet for System 2A
Figure 5.54 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 28 inlet for System 2A
At inlet 36 and 44 the exergy flow drops at an almost constant rate as shown in
Figure 5.56 and Figure 5.57. It can be seen that in both cases, as the specific exergy
flow rises, heat load increases and exergy decreases. Additionally, higher specific
energy flows correspond to higher inlet temperatures in both cases.
-195.805
-195.8
-195.795
-195.79
-195.785
-195.78
-195.775
-10000
0
10000
20000
30000
40000
50000
60000
70000
80000
181.6 181.6 181.6 181.6 181.6 181.6 181.6 181.5 181.5 181.5 181.5 181.5 45.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFL
Heat duty
Temperature
-160
-140
-120
-100
-80
-60
-40
-20
0
-60
-40
-20
0
20
40
60
80
100
120
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5 Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat duty
98
Figure 5.55 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 9 inlet for System 2A
Figure 5.56 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 36 inlet for System 2A
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-600
-400
-200
0
200
400
600
800
1000
891.4 901.9 915.9 934.0 956.8 984.9 1019.3 1061.1 1111.6 1172.8 1247.4 1339.3
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-250
-200
-150
-100
-50
0
0
20
40
60
80
100
120
140
160
180
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
99
Figure 5.57 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 44 inlet for System 2A
Figure 5.58 Heat Load, Exergy flow vs Temperature for Precooling Phase heat
exchanger HX1 at stream N2LIQ inlet for System 2B
Figure 5.59 shows the changes in heat load, Exergy flow and Temperature for
Precooling Phase heat exchanger HX1 at stream 28 inlet. It is seen that with increase in
specific exergy flow rate, the total exergy flow increases while heat load decreases.
Additionally, lower temperatures correspond to higher values of specific exergy flow.
-250
-200
-150
-100
-50
0
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-1.96E+02
-1.96E+02
-1.96E+02
-1.96E+02
-1.96E+02
-1.96E+02
-1.96E+02
-10000
0
10000
20000
30000
40000
50000
60000
70000
80000
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
100
Figure 5.59 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 28 inlet for System 2B
Figure 5.60 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 9 inlet for System 2B
Figure 5.60 illustrates the variation of heat load, exergy flow, and temperature
for heat exchanger HX1 in the precooling phase at stream inlet 9. The total exergy flow
has a nonlinear relationship with specific exergy flow in this case. The trend is that as
specific exergy flow increases, the total exergy flow also increases, but the two have a
non-linear relationship.
-1.60E+02
-1.40E+02
-1.20E+02
-1.00E+02
-8.00E+01
-6.00E+01
-4.00E+01
-2.00E+01
0.00E+00
-30
-20
-10
0
10
20
30
40
50
60
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-300
-200
-100
0
100
200
300
400
500
600
700
3732.7 3775.8 3833.7 3908.4 4002.1 4117.8 4259.0 4430.3 4637.5 4888.7 5195.0 5572.4
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyVapor fraction
101
Figure 5.61 shows the variation of temperature, exergy flow and heat load for
precooling heat exchanger HX1 at stream 36 inlet. With increase in specific exergy
flow, it is seen that total exergy flow rate decreases and heat load increases. Similarly,
lower inlet temperatures correspond to lower values of specific exergy flow. In this
case, it can be noted that the relationship between specific exergy flow and all three
other variables is linear.
Figure 5.61 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 36 inlet for System 2B
Figure 5.62 shows the variation of temperature, exergy flow and heat load for
precooling heat exchanger HX1 at stream 44 inlet. With increase in specific exergy
flow, it is seen that total exergy flow rate decreases and heat load increases. Similarly,
lower inlet temperatures correspond to lower values of specific exergy flow.
5.3.2 Liquefaction Phase at systems S1A and S1B
This section deals with the variation of Heat Load, exergy, and temperature for the
liquefaction phase of systems S2A and S2B. Figure 5.63 shows heat load, Exergy flow
vs Temperature for liquefaction Phase heat exchanger HX2 at stream 49 inlet for
System 2A. It can be seen that total exergy flow increases with increase in specific
exergy flow while heat load decreases. The variation of heat load is linear, while total
exergy flow has a nonlinear relationship. Higher specific exergy flows are achieved at
lower temperatures.
-2.50E+02
-2.00E+02
-1.50E+02
-1.00E+02
-5.00E+01
0.00E+00
0
50
100
150
200
250
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
102
Figure 5.62 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 44 inlet for System 2B
Figure 5.63 Heat Load, Exergy flow and Temperature for liquefaction Phase heat
exchanger HX2 at stream 49 inlet for System 2A
Figure 5.64 illustrates the variation of heat load, exergy flow and temperature
for Liquefaction Phase heat exchanger HX2 at stream 13 inlet for System 2A. Much
like in Figure 5.63 it can be seen that total exergy flow increases with increase in
specific exergy flow while heat load decreases. The variation of heat load is linear,
while total exergy flow has a nonlinear relationship. Higher specific exergy flows are
-2.50E+02
-2.00E+02
-1.50E+02
-1.00E+02
-5.00E+01
0.00E+00
0
20
40
60
80
100
120
140
160
180
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-250
-200
-150
-100
-50
0
50
-600
-400
-200
0
200
400
600
800
1000
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5 Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
103
achieved at lower temperatures. The variation of these parameters for Liquefaction
Phase heat exchanger HX2 at stream inlet 26 is shown in Figure 5.65. The trends are
similar to those at stream inlet 13 and stream inlet 9. However, the nonlinearity in total
exergy flow is more noticeable at stream inlet 26.
Figure 5.64 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 13 inlet for System 2A
Figure 5.65 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 26 inlet for System 2A
-250
-200
-150
-100
-50
0
-200
0
200
400
600
800
1000
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-300
-250
-200
-150
-100
-50
0
-800
-600
-400
-200
0
200
400
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
104
Figure 5.65 illustrates the variation of heat load, exergy flow and temperature
for Liquefaction Phase heat exchanger HX2 at stream 14 inlet for System 2A. It can be
seen that as with HX1, the specific exergy flow is lower at lower inlet temperatures.
Additionally, the heat load increases linearly with increase in specific exergy flow,
while total exergy flow has a nonlinear, but decreasing relationship with specific exergy
flow.
The variation brought about by pre cooling in heat exchanges HX2 can be seen
in Figure 5.67, where the heat load, exergy flow and temperature for Liquefaction Phase
heat exchanger HX2 at stream 22 inlet for System 2A is illustrated. The key difference
is that total exergy flow now increases with increase in specific exergy flow while
overall heat load decreases. Specific exergy flow is higher at lower temperatures,
though the variation of temperature vs. specific exergy is minor.
Figure 5.66 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 14 inlet for System 2A
Figure 5.68 Figure 5.69 illustrate variations of temperature, total exergy flow,
and heat load for liquefaction phase heat exchanger HX3 at stream inlets 32b and S1
respectively. At stream inlet 32b, the variation of total exergy flow offers a unique
trend. It is seen that as specific exergy flow increases, the total exergy flow increases
initially before dropping sharply at a point. The specific exergy values are higher at
lower temperatures. The heat load decreases slightly with increase in specific exergy.
-300
-200
-100
0
100
200
300
400
500
600
700
800
0
100
200
300
400
500
600
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
105
With variations at S1, it is seen in Figure 5.69 that the heat load and total exergy flow
remains constant with variations in specific exergy. However, the temperature and
specific exergy flow have a slightly nonlinear relationship with specific exergy being
lower at lower temperatures.
Figure 5.67 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 22 inlet for System 2A
Figure 5.68 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 32b inlet for System 2A
-2.42E+02
-2.40E+02
-2.38E+02
-2.36E+02
-2.34E+02
-2.32E+02
-2.30E+02
-2.28E+02
-2.26E+02
-2.24E+02
-800
-700
-600
-500
-400
-300
-200
-100
0
100
200
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-235
-230
-225
-220
-215
-210
-205
-50
0
50
100
150
200
250
300
350
400
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
106
Figure 5.69 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream S1 inlet for System 2A
Figure 5.70 shows the variation of heat load, exergy and temperature for HX3
at stream inlet 17 for system 2A. It can be seen that heat load increases with increase in
specific exergy flow while total exergy flow decreases. At lower temperatures, the
specific exergy flow is higher and vice versa. The next set of graphs illustrate the
variation of exergy, heat load and temperature for system 2A.
Figure 5.70 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 17 inlet for System 2A
-3.00E+02
-2.50E+02
-2.00E+02
-1.50E+02
-1.00E+02
-5.00E+01
0.00E+00
-20000
0
20000
40000
60000
80000
100000
120000
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
0.00E+00
1.00E+02
2.00E+02
3.00E+02
4.00E+02
5.00E+02
6.00E+02
7.00E+02
8.00E+02
-500
-400
-300
-200
-100
0
100
200
300
400
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
107
Figure 5.71 shows heat load, Exergy flow vs Temperature for Liquefaction
Phase heat exchanger HX2 at stream 49 inlet for System 2B. It can be seen that total
exergy flow increases with increase in specific exergy flow while heat load decreases.
The variation of heat load is linear, while total exergy flow has a nonlinear relationship.
Higher specific exergy flows are achieved at lower temperatures.
Figure 5.71 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 49 inlet for System 2B
Figure 5.72 and Figure 5.73 and Figure 5.74 illustrate the variations in
temperature, heat load, and exergy for system 2B for stream inlets 26, 14, and 13. Figure
5.73 Figure 5.74 show similar trends. The total exergy decreases with increase in
specific exergy flow while the heat load increases. Specific exergy flow is lower at
lower temperatures. However, the variation of heat load is more noticeable in inlet 14,
and the variation of total exergy flow exhibits a nonlinear pattern in this case. In figure
Figure 5.74 (corresponding to inlet 13) the trends are different. There is a slight increase
in total exergy flow and a slight increase in heat load as the specific exergy flow
increases. The value of specific exergy flow is higher at lower temperatures.
-2.50E+02
-2.00E+02
-1.50E+02
-1.00E+02
-5.00E+01
0.00E+00
-300
-200
-100
0
100
200
300
400
500
600
700
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
108
Figure 5.72 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream 26 inlet for System 2B
Figure 5.73 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream 14 inlet for System 2B
The next set of figures illustrate the variations in exergy, temperature, and heat
load for heat exchanger HX3 in system 2B. Figure 5.75 and Figure 5.76 show the
variations related to stream inlets 22 and 17 respectively. Both exhibit similar trends,
with lower temperatures corresponding to higher values of specific exergy flow. The
heat load in both cases decrease with increase in specific exergy flow while total exergy
flow increases.
-203
-202.5
-202
-201.5
-201
-200.5
-200
-199.5
-199
-198.5
0
20
40
60
80
100
120
140
396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
0
20
40
60
80
100
120
396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
109
Figure 5.74 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream 13 inlet for System 2B
Figure 5.75 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 22 inlet for System 2B
Figure 5.77 corresponds to variations in heat load, exergy and temperature for
HX3 at stream inlet 17. The value of specific exergy flow is lower at lower values of
temperature. The heat load increases with increase in specific exergy flow. However,
the variation in total exergy flow shows an interesting trend. Initially, with increase in
specific exergy flow the total exergy flow increases. However, at a certain point, the
-131
-130
-129
-128
-127
-126
-125
-124
-123
-122
-50
0
50
100
150
200
250
300
396.7 427.2 459.1 492.6 527.7 564.5 603.1 643.5 685.9 730.2 776.6 825.2
Tem
pera
ture
(˚C)
Heat
Loa
d an
d Ex
ergy
Flo
w (k
W)
Specific Exergy Flow (kJ/kg)
Total ExergyHeat LoadTemperature
-2.42E+02
-2.40E+02
-2.38E+02
-2.36E+02
-2.34E+02
-2.32E+02
-2.30E+02
-2.28E+02
-2.26E+02
-2.24E+02
-350
-300
-250
-200
-150
-100
-50
0
50
100
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
110
trend changes, and the total; exergy flow decreases in a nonlinear manner with increase
in specific exergy flow.
Figure 5.76 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 17 inlet for System 2B
Figure 5.77 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream S1 inlet for System 2B
Figure 5.77 ilustrates the variations corresponding to stream inlet S1. It can be
seen that the total exergy flow and heat load are unvaried in this case with increase in
specific exergy flow. The temperature and specific exergy flow have a slightly
-2.50E+02
-2.00E+02
-1.50E+02
-1.00E+02
-5.00E+01
0.00E+00
0
50
100
150
200
250
300
350
400
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-3.00E+02
-2.50E+02
-2.00E+02
-1.50E+02
-1.00E+02
-5.00E+01
0.00E+00
-1500
-1000
-500
0
500
1000
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
111
nonlinear relationship, with lower values of specific exergy flow corresponding to
lower temperatures.
The final figure in this series, Figure 5.78 shows the variation in heat load, Exergy
flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 32b inlet
for System 1B. Specific exergy flow is lower at higher temperatures in this case. It is
seen that the heat load decreases with increase in specific exergy flow. The variation of
total exergy flow initially increases with increase in specific exergy flow up to a point,
but then decreases linearly.
Figure 5.78 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 32b inlet for System 2B
5.4 Systems 3A and 3B – Vortex tubes
Figure 5.79 illustrates the energy and exergy efficiencies of the equipment in the system
individually. Equipment variables changed to test system outcomes as they are
changed. The least efficiency is at 23 % for the Cooler (EX1). Expansion Valve (V2)
has the highest exergy efficiency and Expansion Valve (V1) and Compressor (C2) are
the second and third highest efficient equipment among other units.
Noticeably, Compressors and Valves are working with efficiency higher than
80% while, heat exchangers and expanders, are working with lower exergy efficiency
than other equipment. The lower exergy efficiency of almost all of some equipment
affects the overall system exergy efficiency. Different catalyst types and performance
-2.35E+02
-2.30E+02
-2.25E+02
-2.20E+02
-2.15E+02
-2.10E+02
-2.05E+02
-50
0
50
100
150
200
250
300
134.9 144.4 154.3 164.6 175.5 186.9 198.8 211.3 224.4 238.1 252.5 267.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
112
enhancement of the heat exchangers could create a dramatic improvement to the
process efficiency.
Figure 5.79 Systems 3A and 3B Exergy and Energy efficiencies for each component
Noticeably, Compressors and Valves are working with efficiency higher than
80% while, heat exchangers and expanders, are working with lower exergy efficiency
than other equipment. The lower exergy efficiency of almost all of some equipment
affects the overall system exergy efficiency. Different catalyst types and performance
enhancement of the heat exchangers could create a dramatic improvement to the
process efficiency.
5.4.1 Pre-cooling phase at systems S3A and S3B
The precooling phase in the liquefaction cycle helps cool the hydrogen gas for faster
liquefaction. An analysis has been conducted to understand the precooling phase of the
heat exchangers. shows the Heat Load, Exergy flow and Temperature against specific
7
84 81
23
2.2 4
35
9
77 7… 78 78
78
97 95
23
3 5
36
9
85 8095 93
36
0102030405060708090
100
Heat Ex
chan
ger (H
X1)
Heat Ex
chan
ger (H
X2)
Heat Ex
chan
ger (H
X3)
Expan
sion Valv
e (V1)
Expan
sion Valv
e (V2)
Expan
sion Valv
e (V3)
Vortex T
ube (VT1
)
Vortex T
ube (VT2
)
Compressor (C
1)
Compressor (C
2)
Compressor (C
3)
Flash
Drum (F1)
Adsorptio
n Unit (AO)
Effic
ienc
y (%
)
Exergy Efficiency
Energy Efficiency
93
16
81
2331
3
35
9
7786 90 96
15
96
19
94
2434
3
39
9
85 100 99 99
44
0102030405060708090
100
Heat Ex
chan
ger (H
X1)
Heat Ex
chan
ger (H
X2)
Heat Ex
chan
ger (H
X3)
Expan
sion Valv
e (V1)
Expan
sion Valv
e (V2)
Expan
sion Valv
e (V3)
Vortex T
ube (VT1
)
Vortex T
ube (VT2
)
Compressor (C
1)
Compressor (C
2)
Compressor (C
3)
Flash
Drum (F1)
Adsorptio
n Unit (AO)
Effic
ienc
y (%
)
Exergy Efficiency
Energy Efficiency
113
exergy flow for Precooling Phase heat exchanger HX1 at the liquid nitrogen inlet. The
graphs indicate that as specific exergy decreases, total exergy decreases while heat load
increases beyond a certain value. While temperature initially rises with the decrease in
specific exergy, it soon stabilises.
Figure 5.80 shows the heat load, exergy flow and temperature for Precooling
Phase heat exchanger HX1 at stream 9 inlet and depicts a different view from the
Nitrogen inlet but in decreasing exergy flow. At inlet 9 and inlet 28, the exergy flow
increases as can be seen in Figure 5.81 and Figure 5.82
Figure 5.80 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream N2LIQ inlet for System 3A
At inlet 36 and 44 the exergy flow drops at an almost constant rate as shown in
Figure 5.83 and Figure 5.84. It can be seen that in both cases, as the specific exergy
flow rises, heat load increases and exergy decreases. Additionally, higher specific
energy flows correspond to higher inlet temperatures in both cases
-250
-200
-150
-100
-50
0
0
200
400
600
800
1000
1200
760.2 760.1 687.9 615.6 543.4 471.1 398.8 326.6 254.3 190.4 182.2 125.0 84.8 56.1
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
114
Figure 5.81 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 28 inlet for System 3A
Figure 5.82 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 9 inlet for System 3A
.
-130
-125
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-115
-110
-105
-100
-20
-10
0
10
20
30
40
50
60
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-130
-125
-120
-115
-110
-105
-100
-20
-10
0
10
20
30
40
50
60
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
115
Figure 5.83 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 36 inlet for System 3A
Figure 5.84 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 44 inlet for System 3A
-152
-150
-148
-146
-144
-142
-140
-10
0
10
20
30
40
50
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-152
-150
-148
-146
-144
-142
-140
-5
0
5
10
15
20
25
30
35
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
116
Figure 5.85 Heat Load, Exergy flow vs Temperature for Precooling Phase heat
exchanger HX1 at stream N2LIQ inlet for System 3B
Figure 5.86 Heat Load, Exergy flow and Temperature for Precooling Phase heat exchanger HX1 at stream 28 inlet for System 3B
Figure 5.85 shows the changes in heat load, Exergy flow and Temperature for
Precooling Phase heat exchanger HX1 at stream N2LIQ inlet. It is seen that with
increase in specific exergy flow rate, the total exergy flow increases while heat load
decreases. Additionally, lower temperatures correspond to higher values of specific
exergy flow.
-195.805
-195.8
-195.795
-195.79
-195.785
-195.78
-195.775
0
200
400
600
800
1000
1200
760.2 760.1 711.5 662.8 614.1 565.5 516.8 468.1 419.5 370.8 322.1 273.4 224.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-195.805
-195.8
-195.795
-195.79
-195.785
-195.78
-195.775
0
200
400
600
800
1000
1200
760.2 760.1 711.5 662.8 614.1 565.5 516.8 468.1 419.5 370.8 322.1 273.4
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
117
Figure 5.87 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 9 inlet for System 3B
Figure 5.88 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 36 inlet for System 3B
Figure 5.85 illustrates the variation of heat load, exergy flow, and temperature
for heat exchanger HX1 in the precooling phase at stream inlet 9. The total exergy flow
has a nonlinear relationship with specific exergy flow in this case. The trend is that as
-195.805
-195.8
-195.795
-195.79
-195.785
-195.78
-195.775
0
200
400
600
800
1000
1200
760.2 760.1 711.5 662.8 614.1 565.5 516.8 468.1 419.5 370.8 322.1 273.4
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-195.805
-195.8
-195.795
-195.79
-195.785
-195.78
-195.775
0
200
400
600
800
1000
1200
760.2 760.1 711.5 662.8 614.1 565.5 516.8 468.1 419.5 370.8 322.1 273.4
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
118
specific exergy flow increases, the total exergy flow also increases, but the two have a
non-linear relationship.
Figure 5.88 shows the variation of temperature, exergy flow and heat load for
precooling heat exchanger HX1 at stream 36 inlet. With increase in specific exergy
flow, it is seen that total exergy flow rate decreases and heat load increases. Similarly,
lower inlet temperatures correspond to lower values of specific exergy flow. In this
case, it can be noted that the relationship between specific exergy flow and all three
other variables is linear.
Figure 5.89 Heat Load, Exergy flow and Temperature for Precooling Phase heat
exchanger HX1 at stream 44 inlet for System 3B
Figure 5.89 shows the variation of temperature, exergy flow and heat load for
precooling heat exchanger HX1 at stream 44 inlet. With increase in specific exergy
flow, it is seen that total exergy flow rate decreases and heat load increases. Similarly,
lower inlet temperatures correspond to lower values of specific exergy flow.
5.4.2 Liquefaction Phase at systems S3A and S3B
This section deals with the variation of Heat Load, exergy, and temperature for
the liquefaction phase of systems S3A and S3B. Figure 5.90 shows heat load, Exergy
flow vs Temperature for Liquefaction Phase heat exchanger HX2 at stream 49 inlet for
System 3A. It can be seen that total exergy flow increases with increase in specific
exergy flow while heat load decreases. The variation of heat load is linear, while total
-180
-175
-170
-165
-160
-155
-150
-145
-140
-135
0
5
10
15
20
25
30
35
40
760.2 760.1 711.5 662.8 614.1 565.5 516.8 468.1 419.5 370.8 322.1 273.4
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
119
exergy flow has a nonlinear relationship. Higher specific exergy flows are achieved at
lower temperatures.
Figure 5.90 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream 49 inlet for System 3A
Figure 5.91 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream 13 inlet for System 3A
Figure 5.91 illustrates the variation of heat load, exergy flow and temperature
for Liquefaction Phase heat exchanger HX2 at stream 13 inlet for System 3A. Much
like in Figure 5.92 it can be seen that total exergy flow increases with increase in
-160
-140
-120
-100
-80
-60
-40
-20
0
-40
-20
0
20
40
60
80
100
120
140
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-132
-130
-128
-126
-124
-122
-120
-118
-50
0
50
100
150
200
250
300
350
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
120
specific exergy flow while heat load decreases. The variation of heat load is linear,
while total exergy flow has a nonlinear relationship. Higher specific exergy flows are
achieved at lower temperatures. The variation of these parameters for Liquefaction
Phase heat exchanger HX2 at stream inlet S10 is shown in Figure 5.93 the trends are
similar to those at stream inlet 13 and stream inlet 9. However, the nonlinearity in total
exergy flow is more noticeable at stream inlet 26.
Figure 5.92 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream S10 inlet for System 3A
Figure 5.93 illustrates the variation of heat load, exergy flow and temperature
for Liquefaction Phase heat exchanger HX2 at stream 14 inlet for System 2A. It can be
seen that as with HX1, the specific exergy flow is lower at lower inlet temperatures.
Additionally, the heat load increases linearly with increase in specific exergy flow,
while total exergy flow has a nonlinear, but decreasing relationship with specific exergy
flow.
The variation brought about by pre cooling in heat exchanges HX2 can be seen
in Figure 5.94 where the heat load, exergy flow and temperature for Liquefaction Phase
heat exchanger HX2 at stream 22 inlet for System 3A is illustrated. The key difference
is that total exergy flow now increases with increase in specific exergy flow while
overall heat load decreases. Specific exergy flow is higher at lower temperatures,
though the variation of temperature vs. specific exergy is minor.
-203
-202.5
-202
-201.5
-201
-200.5
-200
-199.5
-199
-198.5
0
20
40
60
80
100
120
140
160
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
121
Figure 5.95 and Figure 5.96 illustrate variations of temperature, total exergy
flow, and heat load for liquefaction phase heat exchanger HX3 at stream inlets 32b and
S1 respectively. At stream inlet 32b, the variation of total exergy flow offers a unique
trend. It is seen that as specific exergy flow increases, the total exergy flow increases
initially before dropping sharply at a point. The specific exergy values are higher at
lower temperatures. The heat load decreases slightly with increase in specific exergy.
With variations at S1, it is seen in Figure 5.96 that the heat load and total exergy flow
remains constant with variations in specific exergy. However, the temperature and
specific exergy flow have a slightly nonlinear relationship with specific exergy being
lower at lower temperatures.
Figure 5.93 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 14 inlet for System 3A
Figure 5.97 shows the variation of heat load, exergy and temperature for HX3
at stream inlet 17 for system 3A. It can be seen that heat load increases with increase in
specific exergy flow while total exergy flow decreases. At lower temperatures, the
specific exergy flow is higher and vice versa. The next set of graphs illustrate the
variation of exergy, heat load and temperature for system 3A.
-180
-160
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-120
-100
-80
-60
-40
-20
0
0
20
40
60
80
100
120
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
122
Figure 5.94 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 22 inlet for System 3A
Figure 5.95 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 32b inlet for System 3A
-160
-140
-120
-100
-80
-60
-40
-20
0
-40
-20
0
20
40
60
80
100
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140
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-235
-230
-225
-220
-215
-210
-205
-10
0
10
20
30
40
50
60
70
80
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
123
Figure 5.96 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream S1 inlet for System 3A
Figure 5.97 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream 17 inlet for System 3A
-245.005
-245
-244.995
-244.99
-244.985
-244.98
-244.975
-244.97
-244.965
-244.96
-244.955
-20
0
20
40
60
80
100
120
140
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-160
-140
-120
-100
-80
-60
-40
-20
0
-40
-20
0
20
40
60
80
100
120
140
643.7 659.6 675.7 692.2 709.0 726.0 743.4 761.0 779.0 797.3 815.9 834.8
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
124
Figure 5.98 shows heat load, Exergy flow vs Temperature for Liquefaction
Phase heat exchanger HX2 at stream 49 inlet for System 3B. It can be seen that total
exergy flow increases with increase in specific exergy flow while heat load decreases.
The variation of heat load is linear, while total exergy flow has a nonlinear relationship.
Higher specific exergy flows are achieved at lower temperatures.
Figure 5.98 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat exchanger HX2 at stream 49 inlet for System 3B
Figure 5.99, Figure 5.100 and Figure 5.101 illustrate the variations in
temperature, heat load, and exergy for system 3B for stream inlets S10, 14, and 13.
Figure 5.100 show similar trends. The total exergy decreases with increase in specific
exergy flow while the heat load increases. Specific exergy flow is lower at lower
temperatures. However, the variation of heat load is more noticeable in inlet 14, and
the variation of total exergy flow exhibits a nonlinear pattern in this case. In figure
Figure 5.101 (corresponding to inlet 13) the trends are different. There is a slight
increase in total exergy flow and a slight increase in heat load as the specific exergy
flow increases. The value of specific exergy flow is higher at lower temperatures.
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
-40
-20
0
20
40
60
80
100
120
140
160
3381.5 3457.7 3541.2 3632.7 3732.8 3842.5 3962.6 4094.3 4238.8 4397.7 4572.6 4765.7
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
125
Figure 5.99 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream S10 inlet for System 3B
The next set of figures illustrate the variations in exergy, temperature, and heat
load for heat exchanger HX3 in system 3B. Figure 5.102 and Figure 5.103 show the
variations related to stream inlets 22 and 17 respectively. Both exhibit similar trends,
with lower temperatures corresponding to higher values of specific exergy flow. The
heat load in both cases decrease with increase in specific exergy flow while total exergy
flow increases.
The value of specific exergy flow is lower at lower values of temperature in
inlet 17 in Figure 5.103. The heat duty increases with increase very slightly in specific
exergy flow. However, the variation in total exergy flow shows a moderate trend.
However, the total exergy flow is remaining steady with decrease in specific exergy
flow.
-203
-202.5
-202
-201.5
-201
-200.5
-200
-199.5
-199
-198.5
0
20
40
60
80
100
120
140
2858.3 2848.0 2837.7 2827.4 2817.2 2807.1 2797.0 2786.9 2776.9 2766.9 2756.9 2747.0
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFL
Heat duty
Temperature
126
Figure 5.100 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream 14 inlet for System 3B
Figure 5.101 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX2 at stream 13 inlet for System 3B
Figure 5.104 ilustrates the variations corresponding to stream inlet S1. It can be
seen that the total exergy flow and heat load are unvaried in this case with increase in
specific exergy flow. The temperature and specific exergy flow have a slightly
nonlinear relationship, with lower values of specific exergy flow corresponding to
lower temperatures.
-203
-202.5
-202
-201.5
-201
-200.5
-200
-199.5
-199
-198.5
0
20
40
60
80
100
120
140
2858.3 2848.0 2837.7 2827.4 2817.2 2807.1 2797.0 2786.9 2776.9 2766.9 2756.9 2747.0
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-131
-130
-129
-128
-127
-126
-125
-124
-123
-122
-50
0
50
100
150
200
250
300
3661.8 3668.5 3675.2 3681.9 3688.6 3695.4 3702.3 3709.2 3716.1 3723.1 3730.1 3737.2
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
127
Figure 5.102 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 22 inlet for System 3B
Figure 5.103 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 17 inlet for System 3B
The final figure in this series, Figure 5.105 shows the variation in heat load,
Exergy flow and Temperature for Liquefaction Phase heat exchanger HX3 at stream
32b inlet for System 3B. Specific exergy flow is lower at higher temperatures in this
case meaning that more power is needed to cool and liquefy. It is seen that the heat load
decreases with increase in specific exergy flow. The variation of total exergy flow
increases with increase in specific exergy flow.
-250
-200
-150
-100
-50
0
-100
-50
0
50
100
150
200
250
300
3737.2 3842.2 3956.8 4081.9 4218.6 4368.2 4532.1 4712.3 4910.7 5129.8 5372.8 5643.2
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-250
-200
-150
-100
-50
0
-1600
-1400
-1200
-1000
-800
-600
-400
-200
0
200
-1388
0.1
-1387
6.6
-1387
3.1
-1386
9.5
-1386
6.0
-1386
2.5
-1385
9.0
-1385
5.5
-1385
2.0
-1384
8.5
-1384
5.0
-1384
1.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
128
Figure 5.104 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream S1 inlet for System 3B
Figure 5.105 Heat Load, Exergy flow and Temperature for Liquefaction Phase heat
exchanger HX3 at stream 32b inlet for System 3B
-245.012
-245.01
-245.008
-245.006
-245.004
-245.002
-245
-244.998
-244.996
-244.994
-1600
-1400
-1200
-1000
-800
-600
-400
-200
0
200
-1388
0.1
-1387
6.6
-1387
3.1
-1386
9.5
-1386
6.0
-1386
2.5
-1385
9.0
-1385
5.5
-1385
2.0
-1384
8.5
-1384
5.0
-1384
1.5
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFLHeat dutyTemperature
-250
-200
-150
-100
-50
0
-100
-50
0
50
100
150
200
250
300
3737.2 3842.2 3956.8 4081.9 4218.6 4368.2 4532.1 4712.3 4910.7 5129.8 5372.8 5643.2
Tem
pera
ture
(˚C
)
Hea
t Dut
y an
d Ex
ergy
Flo
w (k
W)
Spesific Exergy Flow (kJ/kg)
TOTAL EXERGYFL
Heat duty
Temperature
129
5.5 Property set
In the property set in Figure 5.106 the T-xy diagram (vapor mole fraction versus liquid
mole fraction) for vapor-liquid equilibrium (VLE) is shown for a Hydrogen mixture
being fed to the cycle and it shows that hr composition changes at temperatures between
-253 °C and -245 °C at the liquid phase. Figure 5.107 shows the T-x diagram. K-values
for Vapor-liquid and fraction of Para-hydrogen and Ortho Hydrogen is shown in Figure
5.108 and Figure 5.109 illustrates the y-x diagram for vapor vs liquid composition for
the para-hydrogen
Figure 5.106 T-xy plot for temperature versus liquid composition for isobaric data
Figure 5.110 shows the activity coefficients vs mole fraction for Para-hydrogen
and orthohydrogen and illustrates the point of where ortho and para hydrogen have
similar molar fraction with Activity Coefficient ~=1.2.
Liquid/vapor mole fraction, HYDRO-01
Tem
pera
ture
, C
0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 0.875 0.900 0.925 0.950 0.975 1.000-253.0
-252.5
-252.0
-251.5
-251.0
-250.5
-250.0
-249.5
-249.0
-248.5
-248.0
-247.5
-247.0
-246.5
-246.0
-245.5
-245.0x 1.0133 bary 1.0133 bar
130
Figure 5.107 T-x plot for temperature versus liquid composition for isobaric data
Figure 5.108 K-values for Vapor-liquid vs fraction of Para-hydrogen and Ortho
Hydrogen
T-x diagram for HYDRO-01/HYDRO-02
Liquid mole fraction, HYDRO-01
Tem
pera
ture
, C
0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 0.875 0.900 0.925 0.950 0.975 1.000-253.0
-252.5
-252.0
-251.5
-251.0
-250.5
-250.0
-249.5
-249.0
-248.5
-248.0
-247.5
-247.0
-246.5
-246.0
-245.5
-245.0
1.0133 bar
K-values for HYDRO-01/HYDRO-02
Liquid/vapor mole fraction, HYDRO-01
K-va
lues
0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 0.875 0.900 0.925 0.950 0.975 1.0000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Liquid1 HYDRO-01 1.0133 barLiquid1 HYDRO-02 1.0133 bar
131
Figure 5.109 y-x diagram for vapor vs liquid composition for the para-hydrogen
Figure 5.110 Activity coefficients vs mole fraction for Para-hydrogen and orthohydrogen
5.6 Comparative analysis results
The exergy efficiencies of the systems are looking quite low when T0 is changed. The
systems were simulated at 0°C, 10°C, 25°C, and 45°C. System S2A shows the highest
exergy efficiency at 40% as shown in Figure 5.111.
y-x diagram for HYDRO-01/HYDRO-02
Liquid/vapor mole fraction, HYDRO-01
Vapo
r mol
e fra
ctio
n, H
YDRO
-01
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.000.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.0133 bar
Activity coefficients for HYDRO-01/HYDRO-02
Liquid/vapor mole fraction, HYDRO-01
Activ
ity c
oeffi
cient
s
0.000 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250 0.275 0.300 0.325 0.350 0.375 0.400 0.425 0.450 0.475 0.500 0.525 0.550 0.575 0.600 0.625 0.650 0.675 0.700 0.725 0.750 0.775 0.800 0.825 0.850 0.875 0.900 0.925 0.950 0.975 1.0001.00
1.05
1.10
1.15
1.20
1.25
1.30
1.35
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
1.95
2.00
2.05
2.10
2.15
2.20
2.25
2.30
2.35
2.40
Liquid1 HYDRO-01 1.0133 barLiquid1 HYDRO-02 1.0133 bar
132
Figure 5.111 Exergy efficiency for the proposed hydrogen liquefaction systems at 0°C, 10°C, 25°C, and 45°C.
Figure 5.112 shows system S2A shows the highest energy efficiency at 76%
followed by S3B then S3A. Energy efficiency with Vortex tubes is creating better
efficiencies overall. Similarly, work done per unit liquefaction is highest for S3B and
lowest for S3A when compared to peer systems as shown in Figure 5.113.
Figure 5.112 Energy efficiency for the hydrogen liquefaction systems.
12.0
11.9 16
.2
42.0
6.0
21.0
15.4
13.4 17
.7 19.3 24
.0
10.6
17.5
16.8
11.9 17
.4
18.9 22
.4
10.4 17
.2
16.5
9.4
8.0
5.0
12.0
4.0 6.
7
2.3
0
10
20
30
40
50
Main S1A S1B S2A S2B S3A S3B
Exer
gy E
ffici
ency
(%)
Exergy effeciency (0°C) Exergy effeciency (10°C)
Exergy effeciency (25°C) Exergy effeciency (45°C)
10 12
32
76
7
54
74
0
10
20
30
40
50
60
70
80
Main S1-A S1-B S2-A S2-B S3-A S3-B
Ener
gy E
ffici
ency
(%)
133
Figure 5.113 Work done for liquefaction per unit mass (kJ/kg).
5.7 Optimization results
The optimization model is solved using genetic algorithm utilizing MATLAB
software through the internal software’s calculator and an optimizer.
5.7.1 Objective function
The objective function for optimal operation is simpler than for optimal design,
discussed by Jensen [91], because the investment costs, the capital costs, and others are
not considered. The simplified cost function to reduce total compressor consumption to
be minimized then becomes:
min ��� + 𝐶
max 𝐸𝑥
subject to ��[,,& = 𝑔𝑖𝑣𝑒𝑛 = 3.628𝑘𝑔/ℎ𝑟
𝑐 ≤ 0
Here, ��� is the sum of all compressor powers (kW). 𝑐 ≤ 0represents the
mathematical formulation of the operational constraints and the model equations. And
feed ��[,,& is maintained at the nominal feed rate.
5.7.2 Design conditions
Feed hydrogen gas stream: normal hydrogen gas enters with P = 1 bar and T = 25 oC
after gas.
15941.215017.9
12688.1
3403.9
1657.8
17216.1
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
14000.0
16000.0
18000.0
20000.0
S1A S1B S2A S2B S3A S3B
Wor
k (k
w)
134
Nominal flow rate is 3.628 TPD = 3628 kg/24 hours = 151.2 kg/hour =
0.042 kg/s.
Product: 95% para-liquid hydrogen is at P = 1.3 bar and T = −253 oC equivalent
to the product at Ingolstadt plant by Bracha et al [32].
Pressure: Pressure drops inside HX1, HX2, and HX3 are assumed to be zero
because the information about design criteria of all heat exchangers is assumed to be
insignificant.
5.7.3 Variables
The number of manipulated variables are based on the number of main components
seen showing changes in the system overall through the sensitivity analyses for each
system. Hydrogen compressor powers, Expansions valves, Flash power are mainly
manipulated. Additional equipment based on the proposed systems were manipulated
to calculation ran to get results.
5.7.4 Constraints
The exergy efficiencies for each system are optimized to be the maximum possible value.
The total cost rates obtained from each system is optimized to have the lowest value to
reduce the cost of the system. Eventually, for each system, the exergy efficiency equation
and the total cost rate equation are combined in a function, at which the exergy efficiency
is divided by the total cost rate and the function is set to be maximized. The constraints of
some selected variables are shown in Table. 5.1, at which the upper and lower bounds are
set based on the available date from previous studies.
Table 5.1 Constraints of Selected Variables Variable Lower Upper Unit
Ambient temperature, 𝑇0 -10 50 °C
Compressor Pressure -10 100 °C
LN Pressure 400 1500 Bar
Heat exchanger minimum approach temperature
5 10 °C
5.8 Optimum case
After running the optimization for the all the systems, results show that slight changes
to model do make significant improvement but integrating the systems make a great
improvement. In the case of system 2B along with 3B, it can be found that adding
135
Vortex tubes with ORCs increased the exergy efficiency to 27% with the configuration
shown in Figure 5.115 with at work done per liquefaction mass at 3476.3(kJ/kg).
Figure 5.114 Overall exergy efficiency
5.9 Simulation comparison
For validating the liquefaction system Krasae-In et. Al. [15] experiment data is
compared with the simulation model assumption and setup. Table 5.2 indicates that the
simulation and experimentally measured power consumptions of the two compressors
were equal because the simulation data was calculated using the experimental data. The
compressor power was calculated from the flow rate, inlet and outlet pressures, and
temperatures. According to the law of conservation of energy, the calculated brake
horsepower was the same as the measured one. The hydrogen gas flow rate from the
measurement was only 0.6 kg/h instead of the initially designed 2.0 kg/h.
The main conclusion is that the compressor power and liquefier efficiency were
the same as the simulation data. Although the test rig was capable of cooling hydrogen
gas using the refrigeration system, it was only able to reach a temperature of −158 °C
instead of the designed value of −230 °C.
As the proposed plans are larger that the one in the experiment by [15], the
proposed simulation data are at 1:3 ratio with ±10%change.
17 19
22
10
17 16
27
0%
5%
10%
15%
20%
25%
30%
S1A S1B S2A S2B S3A S3B OPTIMUM
Effic
ienc
y
136
Figure 5.115 Optimized system
137
Table 5.2 Simulation and initial experimental data of the proposed system Parameter Simulation data Experimental data
H2 mass flow 3.628 kg/h 0.6 kg/h
H2 compressor power 0.4 kW 0.067 kW
Isentropic efficiency of compressor 80% 80%
Actual work 6.8 kW 1.127 kW
138
CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusions
This report gives a brief view of the prime movers of the hydrogen economy, hydrogen
liquefaction and the advanced liquefaction systems. In the literature review, a fair
number of research papers discussing liquid hydrogen production and basic liquefaction
processes and cycles have been discussed. The objective of this work is to propose
different hydrogen liquefaction systems based on an existing one created by Praxair
Inc. A comprehensive thermodynamic is then performed. Energy and exergy
efficiencies will be analyzed to assess and make further improvements. System
feasibility will be assessed with Economic and environmental evaluation. Operating
conditions will be optimized to generate the best scenarios with and will be validated.
The results for the analyzed cases show that there is room for improvement of
on advanced liquefaction system and novel work can be produced that can overcome
efficiency challenges.
• The overall energy and exergy efficiencies of systems 1A and 1B are found to
be 16.69 % and 18.87 % respectively.
• The work done per liquefaction mass of systems 1A and 1B are found to be
15941.20 kJ/kg and 15017.91 kJ/kg respectively.
• The overall energy and exergy efficiencies of systems 2A and 2B are found to
be 22.55 % and 10.44 % respectively.
• The work done per liquefaction mass of systems 2A and 2B are found to be
12688.07 kJ/kg and 3403.89 kJ/kg respectively.
• The overall energy and exergy efficiencies of systems 3A and 3B are found to
be 17.55 % and 16.44 % respectively.
• The work done per liquefaction mass of systems 3A and 3B are found to be
1657.76 kJ/kg and 17216.12 kJ/kg respectively.
• The overall energy and exergy efficiencies of the optimized system is found to
be 26.89 % and the work done per liquefaction mass is 3476.34 kJ/kg.
139
• The power obtained and supplied from and to the optimized system was
sufficient to provide nearly half of the power required for compressor work.
6.2 Recommendations
Regarding the future development of study liquefaction cycles of increasing
complexity. An attempt was made create VTs, TEs and ORCs. The expander added
extra variables that made the cycle much more complex. The recommendations for the
future studies are as follows; the use of chemical reactions in the simulations may create
more realistic conclusions.
• Base system simulation cycle indicated that the TEs may improve the cycles
efficiency. However, modified cycle designs could yield more positive results.
Testing TEs in different parts may achieve interesting results.
• In view of this study, it can be inferred that more cryogenic experiments should
be conducted using a VT. Experimentation has the possibility to determine the
expected range of effectiveness values for a cryogenic VT, under any condition.
• Optimization of the proposed large-scale plant explained is simplified and it is
preliminary. More information is still required for more complicated work. It is
a must that there is a study about computer simulation work deep inside about
optimization of the new more efficient cycle.
• For the proposed systems, performing dynamic modeling should be done to
evaluate their actual performances and the released emissions during the
different phases of the driving cycles.
• Considering the hydrogen compression method to compress liquid hydrogen is
required to allow for storing hydrogen onboard at very high pressures.
• Conducting life cycle assessments for the proposed systems to confirm that the
operational emissions produced by the proposed systems are feasible for
hydrogen and sustainable fuel source.
140
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