Experimental and theoretical studies on adsorbed natural gas storage system using activated carbons

EXPERIMENTAL AND THEORETICAL STUDIES
ON ADSORBED NATURAL GAS STORAGE
SYSTEM USING ACTIVATED CARBONS
KAZI AFZALUR RAHMAN
NATIONAL UNIVERSITY OF SINGAPORE
2011
EXPERIMENTAL AND THEORETICAL STUDIES
ON ADSORBED NATURAL GAS STORAGE
SYSTEM USING ACTIVATED CARBONS
KAZI AFZALUR RAHMAN
(B.Sc. in Mechanical Engineering, BUET)
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2011
Acknowledgements
i
Acknowledgements
First of all I show my heartiest gratitude to the most merciful ALLAH who has given
me the strength and ability to write this thesis; without His help it would have been
impossible to end my project and write this doctoral thesis.
Then, I would like to express my deepest sense of gratitude and indebtedness to
my supervisor, Professor NG Kim Choon, for giving me the opportunity to work on
this project under his guidance. In particular, suggestions and recommendations of my
supervisor during the course of this research work have been invaluable. I would like
to thank National University of Singapore for the financial support throughout my
candidature.
I would like to express my special thanks and gratitude to Dr. Loh Wai Soong
for his continuous assistance rendered in many aspects. I am also indebted to Dr.
Bidyut Baran Saha (Professor, Kyushu University), Dr. Anutosh Chakraborty

(Assistant Professor, Nanyang Technological University), and Dr. Yanagi Hideharu
(Former Senior Research Fellow, NUS) for providing me opportunities to discuss the
technical results during their stay and research visits at NUS.
I wish to extend my appreciation to all the technical staffs of EBTS group,
particularly Mr. Sacadevan Raghavan and Mrs. Hung-Ang Yan Leng for their warm
relationship and kind cooperation.
I acknowledge my thanks to all the members of Prof. Ng’s research team: Dr.
M Kumja, Dr. Kyaw Thu, Dr. Mark Aaron Chan, Dr. He Jing Ming, Dr. Jayaprakash
Saththasivam, Mr. Aung Myat, Ms. Filian Arbiyani, Mr. Muhammad Wakil Shahzad,
Acknowledgements
ii
Mr. Li Ang, Mr. Azhar Bin Ismail for their kind cooperation and insightful
suggestions, which have been greatly helpful for the advancement of my research.
Last but not least, I record my heartfelt thanks to my respected parents, my
beloved wife, my daughter, and all other family members for their understanding,
kindness and encouragement during my study.
Kazi Afzalur Rahman,
(30 June, 2011)
List of Publications
iii
List of Publications
Patent
1. NG Kim Choon, LOH Wai Soong, Kazi Afzalur RAHMAN, Bidyut Baran
SAHA, Method and System for Storing Natural Gas, US Provisional Patent, PCT
Application No.: PCT/SG2011/000217, 17 June, 2011.
Journals Papers
1. K.A. Rahman, A. Chakraborty, B.B. Saha, K.C. Ng, On thermodynamics of
methane + carbonaceous materials adsorption, International Journal of Heat and
Mass Transfer, Article in press, DOI:10.1016/j.ijheatmasstransfer.2011.10.056,
2011.

2. K.A. Rahman, W.S. Loh, A. Chakraborty, B.B. Saha, W.G. Chun, K.C. Ng,
Thermal enhancement of charge and discharge cycles for adsorbed natural gas
storage, Applied Thermal Engineering, Vol. 31, No. 10, pp. 1630-1639, 2011.
3. A. Martin, W.S. Loh, K.A. Rahman, K. Thu, B. Surayawan, M.I. Alhamid,
Nasruddin, K.C. Ng, Adsorption isotherms of CH
4
on activated carbon from
Indonesian low grade coal, Journal of Chemical Engineering and Data, Vol. 56,
No. 3, pp. 361-367, 2011.
4. K.A. Rahman, W.S. Loh, H. Yanagi, A. Chakraborty, B.B. Saha, W.G. Chun,
K.C. Ng, Experimental adsorption isotherm of methane onto activated carbon at
sub- and supercritical temperatures, Journal of Chemical Engineering and Data,
Vol. 55, No. 11, pp. 4961-4967, 2010.
5. W.S. Loh, K.A. Rahman, A. Chakraborty, B.B. Saha, Y.S. Choo, B.C. Khoo,
K.C. Ng, Improved isotherm data for adsorption of methane on activated carbons,
Journal of Chemical Engineering and Data, Vol. 55, No. 8, pp. 2840-2847, 2010.
6. W.S. Loh, K.A. Rahman, K.C. Ng, B.B. Saha, A. Chakraborty, Parametric
studies of charging and discharging in adsorbed natural gas vessel using activated
carbon, Modern Physics Letters B, Vol. 24, No. 13, pp. 1421-1424, 2010.
List of Publications
iv
7. W.S. Loh, M. Kumja, K.A. Rahman, K.C. Ng, B.B. Saha, S. Koyama, I.I. El-
Sharkawy, Adsorption parameter and heat of adsorption of activated carbon/HFC-
134a pair, Heat Transfer Engineering, Vol. 31, No. 11, pp. 910-916, 2010.
8. K. Habib, B.B. Saha, K.A. Rahman, A. Chakraborty, S. Koyama, K.C. Ng,
Experimental study on adsorption kinetics of activated carbon/R134a and
activated carbon/R507A pairs, International Journal of Refrigeration, Vol. 33, No.
4, pp. 706-713, 2010.
9. W.S. Loh, K.A. Rahman, A. Chakraborty, B.B. Saha, K.C. Ng, W.G. Chun,
Evaluation and simulation of a waste heat driven pressurized solid-sorption chiller,

(Submitted to Transactions of JSRAE).
Conference papers
1. K.A. Rahman, W.S. Loh, K.C. Ng, W.G. Chun, Experimental investigations of
adsorbed natural gas storage system with enhanced thermal management, The
Forth International Symposium on Physics of Fluids, Lijiang, China, June 13-16,
2011.
2. K.A. Rahman, W.S. Loh, K.C. Ng, I. Alhamid, W.G. Chun, Adsorption isotherm
of methane/Maxsorb III pair for a wide range of temperature, Proceedings of the
Innovative Materials for Processes in Energy Systems (IMPRES2010), Published
by Research Publishing, ISBN: 978-981-08-7614-2, doi:10.3850/978-981-08-
7614-2_IMPRES058, 2010, pp. 313-317.
3. K.A. Rahman, W.S. Loh, K.C. Ng, Thermodynamic property evaluation and
adsorption characteristics of methane/Maxsorb III pair, Proceedings of the 3
rd
International Meeting of Advances in Thermofluids, Singapore, November 30,
2010, pp. 257-264.
4. K.A. Rahman, W.S. Loh, A. Chakraborty, B.B. Saha, W.G. Chun, K.C. Ng,
Theoretical modeling and simulation for adsorbed natural gas storage system
using activated carbon, Proceedings of the 9
th
International Conference on
Sustainable Energy Technologies, Shanghai, China, August 24-27, 2010, Paper
No. CO-32.
List of Publications
v
5. W.S. Loh, K.A. Rahman, B.B. Saha, A. Chakraborty, K.C. Ng, W.G. Chun,
Sorption rate and isotherms of methane on pitch-based activated carbon using
volumetric method, Proceedings of the 5
th
Asian Conference on Refrigeration and

Air-conditioning (ACRA), Tokyo, Japan, June 7-9, 2010, Paper No. 051.
6. K.A. Rahman, W.S. Loh, A. Chakraborty, B.B. Saha, K.C. Ng, Adsorption
Thermodynamics of natural gas storage onto pitch-based activated carbons,
Proceedings of the 2
nd
Annual Gas Processing Symposium, Doha, Qatar, January
10-14, 2010, Published in Elsevier book series “Advances in gas processing”.
7. W.S. Loh, K.A. Rahman, A. Chakraborty, B.B. Saha, K.C. Ng, W.G. Chun,
Evaluation and simulation of a waste heat driven pressurized solid-sorption chiller,
Proceedings of the 5
th
Asian Conference on Refrigeration and Air-conditioning
(ACRA), Tokyo, Japan, June 7-9, 2010, Paper No. 033.
8. W.S. Loh, K.A. Rahman, K.C. Ng, The storage of methane using sorption
method, The 2
nd
International Meeting of Advances in Thermofluids, Jakarta,
Indonesia, November 16-19, 2009.
9. W.S. Loh, K.A. Rahman, K.C. Ng, B.B. Saha, A. Chakraborty, Parametric
studies of charging and discharging in adsorbed natural gas vessel using activated
carbon, The Third International Symposium on Physics of Fluids, Jiuzhaigou,
China, June 15-18, 2009.
10. K. A. Rahman, M. M. Alam, Pioneering of a prediction method for wind speed
and validation for local site records, Proceedings of the International Conference
on Mechanical Engineering, Dhaka, Bangladesh, December 29-31, 2007, Paper
No. 070.
Table of Contents
vi
Table of Contents
Acknowledgements i

List of Publications iii
Table of Contents vi
Summary xi
List of Figures xiii
List of Tables xix
Nomenclature xx
Chatper 1 Introduction 1
1.1 Overview of the ANG Storage System 1
1.2 Motivation for this Research 4
1.3 Research Objectives 6
1.4 Organization of the Thesis 7
Chatper 2 Literature Review 9
2.1 Introduction 9
2.2 Adsorption Fundamentals 9
2.2.1 Adsorption Mechanisms 9
2.2.2 Adsorption Equilibrium 10
2.2.3 Adsorption Kinetics 11
2.2.4 Adsorption Thermodynamics 12
2.3 Development of Adsorbents for the ANG Storage System 13
2.3.1 Microporous Adsorbents 14
2.3.2 Activated Carbons 15
2.3.2.1 Precursor materials and synthesis of activated carbons 16
2.3.2.2 Features of a good carbon adsorbent for natural gas storage 17
2.3.2.3 Types of activated carbons 17
2.3.3 Metal Organic Frameworks 19
2.4 Advances in the ANG Storage System 21
2.4.1 Theoretical Studies on the ANG Storage System 21
Table of Contents
vii
2.4.1.1 Theoretical efforts on storage capacity improvement 22

2.4.1.2 Theoretical efforts on effective thermal management 23
2.4.2 Experimental Investigations on the ANG Storage System 26
2.4.2.1 Experiments for storage capacity improvement 26
2.4.2.2 Experiments for effective thermal management 28
2.5 Conclusions 31
Chatper 3 Adsorption Characteristics of Methane onto Activated Carbons 33
3.1 Introduction 33
3.2 Materials 34
3.2.1 Methane 34
3.2.2 Activated Carbon 34
3.3 Experiments 37
3.3.1 Measurement of Adsorbents Properties 37
3.3.2 Measurement of Adsorption Isotherms 39
3.3.2.1 Apparatus 39
3.3.2.2 Procedures 41
3.3.2.3 Data reductions 42
3.3.3 Measurement of Adsorption Kinetics 43
3.3.3.1 Apparatus 43
3.3.3.2 Procedures 45
3.4 Theoretical Models 46
3.4.1 Adsorption Isotherm 46
3.4.2 Adsorption Kinetics 48
3.5 Results and Discussion 51
3.5.1 Adsorption Isotherms 51
3.5.1.1 Regression of uptake data with the Langmuir and Tóth isotherm models
51
3.5.1.2 Regression of uptake data with the D-A isotherm model 54
3.5.1.3 Comparison of the current uptake results with the earlier studies 57
3.5.1.4 Selecting an adsorbent sample for the ANG storage study 59
3.5.2 Adsorption Kinetics 61

3.5.2.1 Temperature and pressure profiles during adsorption kinetics 61
Table of Contents
viii
3.5.2.2 Regression of transient uptake data with the modified kinetics model63
3.5.3 Heat of Adsorption (H
ads
) 65
3.5.4 Henry’s Law Coefficient (K
H
) 67
3.6 Conclusions 68
Chatper 4 Adsorption Isotherms of Methane onto Maxsorb III at
Temperatures of Cryogenic Ranges 70
4.1 Introduction 70
4.2 Materials and Experiments 71
4.2.1 Activated Carbon and Methane 71
4.2.2 Experimental Apparatus 72
4.2.2.1 Volumetric apparatus 72
4.2.2.2 Cryostat 74
4.2.3 Instrumentation 76
4.2.4 Procedures 76
4.2.5 Data Reductions 78
4.3 Results and Discussion 82
4.3.1 Adsorption Isotherms 82
4.3.2 Isosteric Heat of Adsorption 88
4.3.3 Henry’s Law Coefficient 89
4.4 Conclusions 90
Chatper 5 Adsorption Thermodynamics of Methane/Activated Carbon Systems
92
5.1 Introduction 92

5.2 Thermodynamic Frameworks 93
5.2.1 Adsorption Equilibrium Model 94
5.2.2 Heat of Adsorption (H
ads
) 95
5.2.3 Adsorbed Phase Specific Heat Capacity (c
p,a
) 97
5.2.4 Adsorbed Phase Entropy (s
a
) 98
5.2.5 Adsorbed Phase Enthalpy (h
a
) 100
5.3 Results and Discussion 104
5.3.1 Adsorption Isotherms 104
5.3.2 Adsorbed Phase Specific Volume 106
Table of Contents
ix
5.3.3 Heat of Adsorption 107
5.3.4 Adsorbed Phase Specific Heat Capacity 111
5.3.5 Adsorbed Phase Entropy and Enthalpy 113
5.4 Conclusions 115
Chatper 6 Theoretical and Experimental Study of the ANG Storage Prototype
117
6.1 Introduction 117
6.2 Theoretical Modeling of the ANG Storage System 118
6.2.1 Thermodynamic Properties of Methane 120
6.2.2 Adsorption Equations 120
6.2.3 ANG Storage Modeling 121

6.2.3.1 Adsorbent bed 122
6.2.3.2 Vapor in cylinder void 124
6.2.3.3 Cylinder wall 125
6.2.3.4 Fins 125
6.2.3.5 Metal tube 126
6.2.3.6 Coolant (water) 126
6.3 Simulation Results 130
6.3.1 Charge Cycle 132
6.3.1.1 Pressure profile 132
6.3.1.2 Bed temperature history 134
6.3.1.3 Temperature history for fin and tube 136
6.3.1.4 Storage capacity 138
6.3.2 Discharge Cycle 139
6.4 Description of the Experimental Storage Assembly 141
6.4.1 The Storage Cylinder with End Covers 144
6.4.2 The Activated Carbon Bed inside the Storage Cylinder 144
6.4.3 The Gas Charge/Discharge Assembly 147
6.4.4 The Water Circulation Assembly 149
6.4.5 The Data Acquisition System 150
6.4.6 Instrumentation 151
6.5 Results and Discussion 151
Table of Contents
x
6.5.1 Charge Cycle 152
6.5.1.1 Cylinder vapor pressure 152
6.5.1.2 AC bed temperatures 153
6.5.1.3 Adsorbed mass of methane in the AC bed 156
6.5.2 Discharge Cycle 159
6.6 Conclusions 161
Chatper 7 Conclusions 163

7.1 Major Findings of this Research 163
7.2 Limitations and Recommendations 166
References 168
Appendix A Determination of Regeneration Temperature for the Activated
Carbons ………………………………………………………… 185
Appendix B Experimental Adsorption Uptake Data………………….…… 188
Appendix C Drawings and Dimensions of the Cryostat……………………… 191
Appendix D Drawings and Dimensions of the Storage Cylinder and AC Bed
Heat Exchanger…………………………………………………… 193
Appendix E Details of Leak Test of the Storage Cylinder and AC Bed Heat
Exchanger………………………………………………………… 195
Summary
xi
Summary
The extensive usage of the adsorbed natural gas (ANG) storage system is mainly
hindered by the thermal effects during its cyclic operations due to the adsorption and
desorption processes. In this research, the ANG storage system is comprehensively
studied both experimentally and theoretically for enhanced storage capacity and
effective thermal management of the adsorbent bed.
The adsorption characteristics of methane, which is the major component of
natural gas, with different types of activated carbon are determined for temperatures
ranging from (278 to 348) K and pressures up to 2.5 MPa. Among the carbon samples
used in isotherm measurement, the type Maxsorb III provides higher adsorption
capacity owing to its higher surface area and pore volume. The measured uptake data
are regressed with different isotherm models and the Dubinin-Astakhov isotherm
model is found to be the most suitable in describing the experimental uptakes due to its
(i) accountability of the heterogeneity parameter and (ii) consideration of the adsorbed
phase volume. The adsorption kinetics are also measured for the methane/Maxsorb III
pair and the transient uptake values are regressed with a modified approach of the
Linear Driving Force (LDF) model. This modified LDF model takes into account the

non-isothermal behavior of the adsorption process and is observed to be well-fitted
with the experimental kinetics data. These adsorption characteristics, both the isotherm
and kinetics, are the key information in designing and analysing the ANG system.
The adsorption isotherms of the methane/Maxsorb III pair are also measured for
cryogenic temperatures ranging from (120 to 265) K and pressures up to 1.5 MPa
using a purpose-built cryostat. The Dubinin-Astakhov isotherm model is used to fit the
Summary
xii
measured uptake values which are useful in the ANG study when gas at near cryogenic
temperatures from the LNG terminal is considered to charge. In addition, the
thermodynamic analysis of the ANG system is more precise when isotherms for both
the sub- and supercritical states of methane are available. The theoretical frameworks
for the adsorbed phase thermodynamic properties are developed from the rigor of
adsorption thermodynamics by incorporating the micropore filling theory approach,
where the effect of adsorbed phase volume is considered. The derived thermodynamic
quantities exhibit strong dependence on the adsorbate uptake along with the pressure
and temperature and these values are functional in the analysis of charge and discharge
processes of the ANG storage system.
The ANG storage system with internal thermal control based on finned type heat
exchanger in the adsorbent bed is theoretically modeled taking the inertial and viscous
effects of the heat and fluid flow through the porous activated carbon. The simulated
results show that the cyclic processes are notably enhanced by shortening the charge
period because of the quick removal of adsorption heat as well as by maximizing the
gas delivery due to the heat supply to the adsorbent bed during discharge cycle.
Experiments are also performed for a prototype of the ANG storage system with
fins and tubes embedded in the activated carbon bed. The results confirmed that the
storage capacity is increased by 17 % with charge period of less than 10 minutes when
there is water circulation through the heat exchanger tubes to remove the adsorption
heat. Similarly, the gas delivery is enhanced by 7 % during the discharge process due
to the heat supply to the adsorbent bed. Thus, it can be said that the storage capacity as

well as the thermal effects of the adsorbent bed can be enhanced with the heat
exchanger arrangement in the packed activated carbon bed.
List of Figures
xiii
List of Figures
Figure 1.1 A typical adsorption process providing relative comparison of the
adsorbate density between the gaseous phase and the adsorbed phase 2
Figure 2.1 Adsorption isotherm, C = f (P) at T 10
Figure 2.2 Scheme of different precursor materials and activation processes for
PACs and ACFs in the study of Lozano-Castelló et al. (2002c) 18
Figure 2.3 Schematic diagram of two-step operation of an ANG storage system: α
is the bypass flow fraction during discharge and (1–α) fraction is heated
before passing to guard bed (Esteves et al., 2005) 22
Figure 2.4 An ANG storage system with multi-tube tank where gas circulates
through the tank and the heat exchanger (Santos et al., 2009) 25
Figure 2.5 An ANG storage cylinder with a perforated tube inserted at the center of
the adsorbent bed (Chang and Talu, 1996) 28
Figure 2.6 The cross-section of a multi-cell ANG vessel with internal heating
elements (Vasiliev et al. 2000) 29
Figure 2.7 An ANG storage cylinder with U-shaped heat exchanging pipe (Yang et
al., 2005) 30
Figure 3.1 SEM photographs of Maxsorb III sample at magnifications of (a) 1000
and (b) 6000 36
Figure 3.2 SEM photographs of ACF (A-20) sample at magnifications of (a) 1000
and (b) 6000 36
Figure 3.3 SEM photographs of Chemviron sample at magnifications (a) 1000 and
(b) 6000 36
Figure 3.4 Pore size distributions of the activated carbon samples 37
Figure 3.5 Schematic diagram of the experimental apparatus for the measurement
of adsorption isotherms 39

Figure 3.6 Pictorial views of the adsorption isotherm apparatus and its components
40
List of Figures
xiv
Figure 3.7 Typical Pressure and Temperature profiles for the adsorption cell of the
methane/Maxsorb III pair during adsorption: O, Pressure (left ordinate);
◊, Temperature (right ordinate) 42
Figure 3.8 Schematic diagram of the experimental apparatus for measurement of
adsorption kinetics 44
Figure 3.9 Pictorial view of the pneumatically actuated pressure regulator with the
electronic controller 44
Figure 3.10 Adsorption isotherms of methane onto Maxsorb III with 5 % error bars;
Solid lines are from the Langmuir isotherm model and broken lines are
from the Tóth isotherm model 53
Figure 3.11 Adsorption isotherms of methane onto ACF (A-20) with 5 % error bars;
Solid lines are from the Langmuir isotherm model and broken lines are
from the Tóth isotherm model 53
Figure 3.12 Adsorption isotherms of methane onto Chemviron with 5 % error bars;
Solid lines are from the Langmuir isotherm model and broken lines are
from the Tóth isotherm model 54
Figure 3.13 Adsorption isotherms of methane onto Maxsorb III with 5 % error bars;
The D-A isotherm model predicts the solid lines for α = 1/T and the
broken lines for α = 0.0025 55
Figure 3.14 Adsorption isotherms of methane onto ACF (A-20) with 5 % error bars;
The D-A isotherm model predicts the solid lines for α = 1/T and the
broken lines for α = 0.0025 56
Figure 3.15 Adsorption isotherms of methane onto Chemviron with 5 % error bars;
The D-A isotherm model predicts the solid lines for α = 1/T and the
broken lines for α = 0.0025 56
Figure 3.16 Comparison of the current adsorption isotherm data with the earlier

studies: (a) Maxsorb III; (b) ACF (A-20); (c) Chemviron 58
Figure 3.17 Comparison of the adsorption uptake capacity at 25 °C for the three
activated carbon samples studied 60
Figure 3.18 Charging cell (solid line) and adsorbent (dashed line) temperature
versus time during the adsorption process at P
*
=9.1 bar, T
*
=30 °C 62
List of Figures
xv
Figure 3.19 Charging cell (solid line) and adsorption cell (dashed line) pressure
versus time during the adsorption process at P
*
=9.1 bar, T
*
=30 °C 62
Figure 3.20 Comparison of experimental adsorption uptake (solid lines) with
predicted adsorption uptake (dashed lines) using the modified kinetics
correlation 63
Figure 3.21 Pressure dependent effective mass transfer coefficient (k
s
a
v
) versus
equilibrium pressure (P
*
) at different adsorption process temperatures
(T
*

) 64
Figure 3.22 Temperature dependent effective mass transfer coefficient (β) versus
equilibrium pressure (P
*
) at different adsorption process temperatures
(T
*
) 64
Figure 3.23 The uptake and temperature dependent heat of adsorption calculated
from the Clausius-Clayperon equation along with the correction term 66
Figure 4.1 (a) Schematic diagram of the volumetric apparatus (b) Schematic
diagram of the cryostat 73
Figure 4.2 Pictorial views of the cryo-isotherm measurement facility 75
Figure 4.3 Typical pressure and temperature profiles for the adsorption cell during
the adsorption process 77
Figure 4.4 Adsorption isotherms of methane onto Maxsorb III with error bars of 5
%; Solid lines are predicted from the Tóth isotherm model 82
Figure 4.5 Adsorption isotherms of methane onto Maxsorb III with 5 % error bars;
Solid lines are predicted from the D-A isotherm model with the
adsorbed phase volume correction 85
Figure 4.6 Adsorption isotherms of methane on Maxsorb III with 5 % error bars;
Solid lines are predicted from the D-A isotherm model with no
consideration of the adsorbed phase volume 85
Figure 4.7 Comparison of the differences between the experimental uptake and the
predicted uptake from the isotherm models: □, Tóth model; ○, D-A
model with the adsorbed phase volume correction; ∆, D-A model with
no consideration of the adsorbed phase volume 86
Figure 4.8 Estimation of the charging pressure for low temperature gas; ○, uptake
at 298.15 K for the methane/Maxsorb III pair from previous Chapter . 87
List of Figures

xvi
Figure 4.9 The heat of adsorption for methane/Maxsorb III pair at different
isosteric conditions; Horizontal broken line is from the Tóth isotherm
model 88
Figure 4.10 The Henry’s law coefficients at different temperatures 90
Figure 5.1 Demonstration of a typical adsorption process 93
Figure 5.2 Adsorption uptake data of the methane/Maxsorb III pair for
temperatures (120 to 350 K) with error bars of 7 %. Solid lines are
predicted from the D-A isotherm model 104
Figure 5.3 Temperature dependence of (a) the adsorbed phase specific volume (v
a
)
and (b) the relative surface loading 106
Figure 5.4 Heat of adsorption (H
ads
) for the methane/Maxsorb III pair against
adsorbate surface loading (C/C
0
) at (a) Subcritical and (b) Supercritical
temperatures 108
Figure 5.5 Heat of adsorption (H
ads
) for methane adsorption onto different types of
activated carbon at temperature of 298 K 110
Figure 5.6 Adsorbed phase specific heat capacity (c
p,a
) of methane/Maxsorb III
system at both sub- and supercritical states 111
Figure 5.7 Comparison of c
p,a

values evaluated for methane adsorption onto
different types of activated carbon; (a) Isobars at P = 5 bar (b) Isosters
at C/C
0
= 0.5 112
Figure 5.8 Adsorbed phase entropy versus Temperature (s
a
– T) plot at different
isosters for adsorption of methane onto Maxsorb III 114
Figure 5.9 Pressure versus Adsorbed phase enthalpy (P – h
a
) plot at different
temperatures for adsorption of methane onto Maxsorb III 115
Figure 6.1 Schematic diagram of an ANG cylinder and adsorbent bed with heat
exchanger arrangement 122
Figure 6.2 Simulated profiles for the methane vapor pressure (solid line) and the
gas charging rate (dashed line) as function of time during the charge
cycle 133
Figure 6.3 Simulated profiles for the gas pressure inside the porous adsorbent bed
along the width during the charge cycle 134
List of Figures
xvii
Figure 6.4 Temperature surfaces of the adsorbent bed during charge cycle along
bed width at (a) z = 110 mm, (b) z = 400 mm and z = 750 mm 135
Figure 6.5 Temperature contour plots of the fin at z = 400 mm with the tubes at
different times of the charge cycle 137
Figure 6.6 Stored amount of methane both in ANG and CNG vessel 138
Figure 6.7 Methane vapor pressure inside the cylinder (solid line) and deliverable
volumetric capacity of methane (dashed line) as function of time during
the discharge cycle 140

Figure 6.8 Temperature surface of the adsorbent bed along the width at z = 400
mm and y = 20 mm location during the discharge cycle 140
Figure 6.9 Schematic diagram of the experimental assembly of the ANG storage
prototype to study the charge and discharge cycles 142
Figure 6.10 Pictorial view of the experimental assembly of the ANG storage
prototype to study the charge and discharge cycles 143
Figure 6.11 The fin and tube type heat exchanger before the activated carbon is
filled 145
Figure 6.12 Pictorial views of the activated carbon bed heat exchanger 146
Figure 6.13 Pictorial view of the gas charge/discharge assembly 148
Figure 6.14 Pictorial view of the water circulation assembly 149
Figure 6.15 Pictorial view of the data acquisition system 150
Figure 6.16 Methane vapor pressure inside the storage cylinder during charge 153
Figure 6.17 Transient AC bed temperatures at three different longitudinal positions:
z = 108 mm (solid lines); z = 402 mm (dashed lines); z = 753 mm
(dotted lines); [For all z-positions: x=56 mm and y=20 mm] 154
Figure 6.18 Transient AC bed temperatures at three different axial positions: x = 26
mm (solid lines); x = 56 mm (dashed lines); x = 86 mm (dotted lines);
[For all x-positions: y = 20 mm and z = 408 mm] 154
Figure 6.19 Comparison of simulation results with experimental data for the AC bed
temperatures at different longitudinal positions [For all z-positions: x =
56 mm and y = 20 mm] 156
Figure 6.20 Adsorbed amount of methane in the AC bed during the charge cycle 157
Figure 6.21 Simulated volumetric capacity versus packing density of the AC bed158
List of Figures
xviii
Figure 6.22 Average AC bed temperatures measured for different operating
conditions of the discharge process 159
Figure 6.23 Delivered amount of methane from the AC bed for the discharge
processes with different operating conditions 160

Figure 7.1 A provisional configuration of the ANG vessel for the LNG charge . 167
Figure A.1 Computrac Max 5000 Moisture Analyzer ………………………… 185
Figure A.2 The percentage and the rate of adsorbate removal as function of time
……………………………………………………………………… 186
Figure A.3 The ratios of final mass and initial mass of the Maxsorb III sample at
different higher temperatures ……………………………………… 187
Figure C.1 Cryostat for the Adsorption Cell (Top View) 191
Figure C.2 Cryostat for the Adsorption Cell (Front Sectional View) 192
Figure D.1 Drawings and dimensions of the Storage Cylinder 193
Figure D.2 Drawings and dimensions of the AC Bed Heat Exchanger 194
Figure E.1 Pressure test arrangement for the Storage Cylinder 196
Figure E.2 Pressure test arrangement for the AC Bed Heat Exchanger 196
Figure E.3 Leak test report of the Storage Cylinder charged with pure helium 197
List of Tables
xix
List of Tables
Table 3.1 The thermophysical properties of the activated carbon samples 35
Table 3.2 Adsorption parameters (C
0
, k
0
, H
ads
and t) for the Langmuir and the Tóth
isotherm models 52
Table 3.3 Adsorption parameters for the D-A isotherm (W
0
, E and n) model with the
adsorbed phase volume correction 55
Table 3.4 The Henry’s law coefficients, K

H
(1/ MPa) 68
Table 4.1 Experimental uptake data for methane adsorption onto Maxsorb III 80
Table 4.2 Adsorption parameters (C
0
, k
0
, H
ads
and t) for the Tóth isotherm model 83
Table 4.3 Adsorption parameters (W
0
/C
0
, E and n) for the D-A isotherm model 84
Table 5.1 Comparative study of the thermodynamic framework between the present
study and the study by Chakraborty et al. (2009) 102
Table 5.2 The Surface characteristic of different activated carbon samples and the
adsorption parameters (W
0
, E, n, α)
*
of the D-A isotherm model for
adsorption of methane onto these carbon samples 109
Table 6.1 Adsorption characteristics for methane adsorption onto Maxsorb III 127
Table 6.2 Thermophysical parameters used in the mathematical models 128
Table 6.3 Physical dimensions of the ANG storage cylinder assembly 130
Table 6.4 Experimental operating conditions during the charge and the discharge
cycles 132
Table B.1 Experimental uptake data for adsorption of methane onto Maxsorb III 188

Table B.2 Experimental uptake data for adsorption of methane onto ACF (A-20) 189
Table B.3 Experimental uptake data for adsorption of methane onto Chemviron 190
Nomenclature
xx
Nomenclature
A
Adsorption potential [kJ/kg]
C
Adsorption equilibrium uptake [kg/kg]
C
0
Maximum equilibrium adsorption uptake [kg/kg]
C
eq
Adsorption equilibrium uptake [kg/kg]
C
v
Valve coefficient
C
N
Valve constant
c
p
Specific heat capacity [kJ/kg.K]
D
s
Diffusivity [m
2
/s]
D

so
Diffusivity at zero activation energy [m
2
/s]
D
*
so
Pre-exponential constant [m
2
/s]
D
bed
Adsorbent bed depth or height [m]
D
fin
Fin depth or height [m]
D
H
Hydraulic diameter [m]
dV
Differential pore volume [cm
3
/g]
dr
Differential pore radius [Å]
E
Adsorption characteristics energy [kJ/kg]
E
a
Activation energy [kJ/kg]

F
0
Constant in surface diffusion equation
Ġ
Mass flux [kg/m
2
.s]
G
g
Specific gravity of methane with respect to air
H
ads
Heat of adsorption [kJ/kg]
h
Specific enthalpy [kJ/kg]
h
fg
Enthalpy of vaporization [kJ/kg]
h
fin,s
or h
m,s
Convection coefficient for solid adsorbent to tube or fin [W/m
2
.K]
h
vapor
Convection coefficient of gas or vapor [W/m
2
.K]

h
amb
Convection coefficient of ambient air [W/m
2
.K]
h
w
Convection coefficient of water [W/m
2
.K]
k
Conductivity [W/m.K]
Nomenclature
xxi
k
0
Equilibrium constant of the Langmuir and Tóth isotherm models
[1/MPa]
K
eff
Overall effective mass transfer coefficient which is function of both
equilibrium pressure and temperature [1/s]
k
s
a
v
Effective mass transfer coefficient which is function of equilibrium
pressure [1/s]
K
H

Henry’s law coefficient [1/MPa]
L
bed
Adsorbent bed length [m]
L
cyl
Cylinder length [m]
L
tube
Tube length [m]
m
Mass of adsorbate or adsorbent [kg]
ṁ
Mass flow rate [kg/s]
N
Number of data points or Number of grid points
n
Index of the Dubinin-Astakhov isotherm model
P
Pressure [kPa]
P*
Equilibrium process pressure [kPa]
P
o
Reference pressure [kPa]
p
f
Fin spacing [m]
Q
Volumetric flow rate [m

3
/s]
R
Universal gas constant [kJ/kg.K]
R,r
Radius [m]
s
Specific entropy [kJ/kg.K]
T
Temperature [K]
T*
Equilibrium process temperature [K]
T
o
Reference temperature [K]
t
Time [s] or Heterogeneity parameter of the Tóth isotherm model
u
Velocity [m/s]
V
Volume [m
3
]
v
Specific volume [cm
3
/g]
v
μ
Micropore volume of adsorbent [cm

3
/g]
W
Volumetric adsorption equilibrium uptake [cm
3
/g]
W
0
Maximum volumetric adsorption equilibrium uptake [cm
3
/g]
W
bed
Adsorbent bed width [m]
W
fin
Fin width [m]
Nomenclature
xxii
x
Axial direction or distance [m]
y
Axial direction or distance [m]
z
Longitudinal direction or distance [m]
Greek
α
Thermal expansion coefficient for the adsorbed phase [K
-1
]

β
Effective mass transfer coefficient which is function of equilibrium
temperature [1/s]
ε
Porosity
δ
Fin thickness [m]
Δ
Directional operator
ρ
Density [kg/m
3
]
θ
Surface coverage
μ
Viscosity [kPa.s]
ψ
Darcy’s law coefficient [m
2
/kPa.s]
Subscripts
a, adsorbed
Adsorbed phase
ac
Activated carbon
ads_cell
Adsorption cell
amb
Ambient

avg
Average
b
Adsorbent bed or boiling point
bed
Adsorbent bed
b&cw
Between adsorbent bed and cylinder wall
cri
Critical point
cyl
Cylinder
cw
Cylinder wall
charge
Charge process
discharge
Discharge process
experiment
Experimental data point
f
Liquid phase
Nomenclature
xxiii
fin
Fin
final
Final condition
g
Gaseous phase

in
Inlet
initial
Initial condition
i
Inner dimension
0
Outer dimension
l, r
Left, right
m
Metal tube
max
Maximum
model
Isotherm model
p
Particle
packing
Packing of activated carbon
s, solid
Solid adsorbent
sat
Saturation point
t
Total
tube
Metal tube
vapor
Vapor phase

w
Water
void
Cylinder inside void space