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

168 Appendix A Determination of Regeneration Temperature for the Activated Carbons ………...185 Appendix B Experimental Adsorption Uptake Data……….……...188 Appendix C Drawings and Dimensions

ON ADSORBED NATURAL GAS STORAGE SYSTEM USING ACTIVATED CARBONS

KAZI AFZALUR RAHMAN

NATIONAL UNIVERSITY OF SINGAPORE

2011

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

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 beenimpossible 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 mysupervisor during the course of this research work have been invaluable I would like

to thank National University of Singapore for the financial support throughout mycandidature

I would like to express my special thanks and gratitude to Dr Loh Wai Soongfor 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 thetechnical 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 warmrelationship 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 JayaprakashSaththasivam, Mr Aung Myat, Ms Filian Arbiyani, Mr Muhammad Wakil Shahzad,

Mr Li Ang, Mr Azhar Bin Ismail for their kind cooperation and insightfulsuggestions, which have been greatly helpful for the advancement of my research.

Last but not least, I record my heartfelt thanks to my respected parents, mybeloved 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

Patent

1 NG Kim Choon, LOH Wai Soong, Kazi Afzalur RAHMAN, Bidyut Baran

SAHA, Method and System for Storing Natural Gas, US Provisional Patent, PCTApplication 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 andMass 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 CH4 on activated carbon fromIndonesian 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 atsub- 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.

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 andactivated 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, TheForth 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 theInnovative 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 3rdInternational 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 systemusing activated carbon, Proceedings of the 9th International Conference on

Sustainable Energy Technologies, Shanghai, China, August 24-27, 2010, Paper

No CO-32

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 usingvolumetric method, Proceedings of the 5th 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 2nd 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 5th 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 2nd 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 activatedcarbon, 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

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

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

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

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

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

The extensive usage of the adsorbed natural gas (ANG) storage system is mainlyhindered by the thermal effects during its cyclic operations due to the adsorption anddesorption processes In this research, the ANG storage system is comprehensivelystudied both experimentally and theoretically for enhanced storage capacity andeffective thermal management of the adsorbent bed

The adsorption characteristics of methane, which is the major component ofnatural gas, with different types of activated carbon are determined for temperaturesranging from (278 to 348) K and pressures up to 2.5 MPa Among the carbon samplesused in isotherm measurement, the type Maxsorb III provides higher adsorptioncapacity owing to its higher surface area and pore volume The measured uptake dataare regressed with different isotherm models and the Dubinin-Astakhov isothermmodel 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 adsorbedphase volume The adsorption kinetics are also measured for the methane/Maxsorb IIIpair and the transient uptake values are regressed with a modified approach of theLinear Driving Force (LDF) model This modified LDF model takes into account thenon-isothermal behavior of the adsorption process and is observed to be well-fittedwith the experimental kinetics data These adsorption characteristics, both the isothermand kinetics, are the key information in designing and analysing the ANG system.The adsorption isotherms of the methane/Maxsorb III pair are also measured forcryogenic temperatures ranging from (120 to 265) K and pressures up to 1.5 MPausing a purpose-built cryostat The Dubinin-Astakhov isotherm model is used to fit the

measured uptake values which are useful in the ANG study when gas at near cryogenictemperatures from the LNG terminal is considered to charge In addition, thethermodynamic analysis of the ANG system is more precise when isotherms for boththe sub- and supercritical states of methane are available The theoretical frameworksfor the adsorbed phase thermodynamic properties are developed from the rigor ofadsorption thermodynamics by incorporating the micropore filling theory approach,where the effect of adsorbed phase volume is considered The derived thermodynamicquantities exhibit strong dependence on the adsorbate uptake along with the pressureand temperature and these values are functional in the analysis of charge and dischargeprocesses of the ANG storage system.

The ANG storage system with internal thermal control based on finned type heatexchanger in the adsorbent bed is theoretically modeled taking the inertial and viscouseffects of the heat and fluid flow through the porous activated carbon The simulatedresults show that the cyclic processes are notably enhanced by shortening the chargeperiod because of the quick removal of adsorption heat as well as by maximizing thegas 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 withfins and tubes embedded in the activated carbon bed The results confirmed that thestorage capacity is increased by 17 % with charge period of less than 10 minutes whenthere is water circulation through the heat exchanger tubes to remove the adsorptionheat 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 aswell as the thermal effects of the adsorbent bed can be enhanced with the heatexchanger arrangement in the packed activated carbon bed

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 heatedbefore 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

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

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 arefrom 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 arefrom 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 arefrom 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

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 kineticscorrelation 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 theadsorbed 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 noconsideration 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-Amodel 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

Figure 4.9 The heat of adsorption for methane/Maxsorb III pair at different

isosteric conditions; Horizontal broken line is from the Tóth isothermmodel 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 arepredicted 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) Supercriticaltemperatures 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 chargecycle 133

Figure 6.3 Simulated profiles for the gas pressure inside the porous adsorbent bed

along the width during the charge cycle 134

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

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

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

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]

D bed Adsorbent bed depth or height [m]

D fin Fin depth or height [m]

D H Hydraulic diameter [m]

dV Differential pore volume [cm3/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/m2.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/m2.K]

h amb Convection coefficient of ambient air [W/m2.K]

h w Convection coefficient of water [W/m2.K]

k Conductivity [W/m.K]

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 [m3/s]

R Universal gas constant [kJ/kg.K]

v μ Micropore volume of adsorbent [cm3/g]

W Volumetric adsorption equilibrium uptake [cm3/g]

W 0 Maximum volumetric adsorption equilibrium uptake [cm3/g]

W bed Adsorbent bed width [m]

W fin Fin width [m]

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

b Adsorbent bed or boiling point

bed Adsorbent bed

b&cw Between adsorbent bed and cylinder wall

cri Critical point

cw Cylinder wall

charge Charge process

discharge Discharge process

experiment Experimental data point

packing Packing of activated carbon

s, solid Solid adsorbent

sat Saturation point

tube Metal tube

void Cylinder inside void space

Chatper 1 Introduction

Natural gas (NG) has emerged as an alternative energy source in transportation sector

as it provides clean combustion and hence lowers exhaust pollution It is mostlycomposed of methane (CH4) which has the highest heating value per unit mass (55.2MJ/kg) of hydrocarbon fuels (e.g butane, diesel fuel, gasoline, etc.) (Talu, 1992).Moreover, its competitive price and copious availability makes NG a tangible fuel inenergy sectors According to the U.S Energy Information Administration (EIA), theconsumption of dry NG is about 25 % of the total primary energy consumptions forrecent years More than half of the world’s known reserves of NG are found in thePersian Gulf and Russia The conventional method of storing and supplying NG iseither in the compressed form of natural gas (CNG) at high pressures up to 30 MPa orthe liquefied natural gas (LNG) at cryogenic temperature (–163 °C) In recent years,adsorbed natural gas (ANG) storage system has attracted considerable attention as apossible alternative to the CNG and the LNG methods for energy storage andtransportation purposes (Vasiliev et al., 2000; Bastos-Neto et al., 2005a; Mota, 2008).The ANG storage system provides high energy density but operates at much lowerpressure (usually 2 to 4 MPa) than the CNG method Also, the ANG system does notrequire costly cold energy to store gas in the liquid phase as does LNG

1.1 Overview of the ANG Storage System

In ANG storage system, the gas is pressurized and stored in a vessel which iscompacted with suitable adsorbent materials According to adsorption principles, thegas (adsorbate) molecules are captured in the pores of the adsorbent due to the strong

attractive surface forces known as van der Walls forces The molecular distances insidethe pores of the adsorbent are much shorter than in the gaseous phase for similarpressure and temperature conditions and thus the adsorbate density in adsorbed phasebecomes liquid-like (Suzuki, 1990) In this way, NG can be stored in an adsorbed statewith high energy density but at much lower pressure compared to that required by thecompressed gas method To provide a better understanding of the adsorbate density inthe adsorbent pores, a typical illustration is given in Figure 1.1 where the adsorbatemolar density in the adsorbent pores is denser than in the gaseous phase.

AdsorbentPore

Multilayer Adsorption

Adsorbate molecules in Adsorbed Phase

Figure 1.1 A typical adsorption process providing relative comparison of the

adsorbate density between the gaseous phase and the adsorbed phase

Therefore, the storage capacity of the ANG storage system depends on theporous structure of the adsorbent material The most promising adsorbents are themicroporous activated carbons with relatively high packing densities (Quinn andMacdonald, 1992; Alcãz-monge et al., 2009) and high specific surface area (Menonand Komarneni, 1998) Considerable efforts have been made for the development ofsuitable activated carbons for the ANG storage (Molina-Sabio et al., 2003; Lozano-Castelló et al., 2002a; Prauchner and Rodríguez-Reinoso, 2008) and it is found that the

activated carbons with average pore diameter of less than 20 Å can adsorb gas in anamount which is proportional to its pore volume (Biloé et al., 2001a; Mota, 2008).Activated carbons are usually produced from several precursor materials, such ascoals, agricultural products (coconut shells, rice husks, cherry stones, corn cobs, etc.),and polymeric materials, by some physical and chemical activation processes.Nevertheless, the appropriate raw materials and the optimum synthesis procedures arewell-identified for microporous activated carbons with excellent adsorption capacity.Besides the adsorption capacity of activated carbon, there are several factors thatcan affect the storage capacity and the gas deliverability of the ANG system Theadsorbent textural characteristics, the packing density of the adsorbent bed, and the gascomposition play significant role on the storage performance (Lozano-Castelló et al.,2002b; Pupier et al., 2005; Bastos-Neto et al., 2007; Rios et al., 2009) High packingdensity and adsorption affinity are desirable in order to maximize the stored volumeand good textural characteristics (with micro- and mesopores) of the adsorbent arerequired for faster diffusion of adsorbate molecules.

Additionally, there is another important issue concerning the thermal effects due

to both heat of adsorption and desorption during the charge and discharge processes ofthe ANG vessel (Chang and Talu, 1996; Vasiliev et al., 2000; Santos et al., 2009).Adsorption process being exothermic, the adsorbent temperature rises during thecharge cycle that results a reduction in gas storage capacity compared to an isothermalcharge Similarly, the adsorbent temperature drops during the endothermic dischargeprocess which increases the adsorbate retention lessens the volume of delivered gas.Therefore, an effective cooling/heating arrangement is necessary for enhanced thermalmanagement of the ANG system during both the charge and discharge processes

1.2 Motivation for this Research

Due to the increasing demand of NG throughout the world, the ANG storagesystem has become popular in the field of developing efficient gas storage andtransportation technologies Although the conventional storage techniques, such as theCNG and LNG methods, which are widely used for gas storage and transportationpurposes, are associated operational complexity For example, the CNG vessel storesgas at very high pressure (20 to 30 MPa) that incurs high manufacturing and fillingcosts and also represents a safety concern On the other hand, the LNG methodrequires huge energy for cryogenic cooling of the gas at a temperature of −163 °C andneeds specialized equipment for re-gasification However, the ANG vessel can bedesigned to store NG at relatively low pressure (2 to 4 MPa) in a lightweight cylinderfilled with porous adsorbent which allows good design flexibility in tank configurationand also the storage tank can be filled with an inexpensive single-stage compressor.These attractive features of the ANG storage system over the conventional storagemethods are the prime motivating factors for the ongoing developments of thistechnology Wegrzyn and Gurevich (1996) provided a comparative cost analysisassociated with each technology and showed the superiority of the ANG storagemethod because of lower costs in gas refuelling and tank fabrication

The performance indicator of the ANG storage is the lume of adsorbed NG,measured at standard conditions (25 °C and 1 atm), per unit volume of storage vessel(V/V) It has been reported that the break-even ANG storage factors are 78 V/V forlight-duty vehicles and 120 V/V for heavy-duty vehicles (Talu, 1992) However, thestorage target has recently been set by the U.S Department of Energy (DOE) as 180V/V at 35 bar and ambient temperature so that the energy density of the ANG method

becomes comparable to that of the CNG storage (Ma et al., 2008) There are currentlyseveral commercial activated carbons that can adsorb gas at 35 bar the same amountthat is stored by compression at a pressure of 2.5 times higher (Mota, 2008) andhitherto, the highest volumetric capacity obtained for activated carbon is 165 V/V at 40bar and 25 °C for KOH activated anthracite with a packing density of 640 kg/m3(Lozano-Castelló et al., 2002c) With the molecular simulation of methane adsorption

in a model carbon adsorbent made of parallel graphite plates, it is found that the ANGvessel can reach a storage capacity of 195 V/V if the carbon is compacted to form amonolith (Matranga et al., 1992)

Nevertheless, the usage of the ANG storage system is hindered by the thermalmanagement during the adsorption and desorption processes and hence, an effectivethermal enhancement is essential for the development of this technology Severalmethods and techniques are reported in the literature to overcome this problem Anumber of studies have been performed to enhance heat transfer from the wall to thecenter of the ANG cylinder by inserting a perforated tube (gas diffuser) into the center

of the adsorbent bed (Chang and Talu, 1996; Vasiliev et al., 2000; Bastos-Neto et al.,2005a) The diffusion resistance for mass transport of the gas molecules isconsiderably reduced by altering the gas diffusion in radial direction, rather than inaxial direction However, the thermal effects are not significantly enhanced duringcharge and discharge of the ANG cylinders due to poor thermal properties (e.g heatconductivity, specific heat capacities) of the adsorbents Biloé et al (2001b) hasstudied for highly conductive adsorbent matrix where the activated carbon isconsolidated with high conductive material, i.e expanded natural graphite (ENG).Although the thermal conductivity of the adsorbent composite block is increased about

30 times, this technique lessens the amount of gas stored for same cylinder volume asthe binder material occupies some volumes of the adsorbent bed Considering theseproblems this research is motivated to study the ANG storage system where theadsorbent bed consists of a fin and tube arrangement in order to achieve enhancedthermal effects during both the charge and discharge cycles Since the adsorptioncharacteristics are the key information for such study, the isotherms and kinetics arerequired to be measured for adsorption of methane onto activated carbons.

1.3 Research Objectives

This research is mainly targeted to perform a thorough study of the ANG storagesystem in order to enhance the storage capacity as well as to achieve an effectivethermal control of the adsorbent bed The specific objectives are:

a) to measure the adsorption uptake capacity of methane/activated carbon systems

at assorted pressures and temperatures of the ANG operating ranges The uptakevalues are important in designing the adsorbent storage system For this purpose,three different types of activated carbons are selected those are powdered,fibrous, and granular types

b) to measure the adsorption kinetics of methane in the activated carbon Thekinetics information are required to determine the diffusion rate of gas molecules

in a packed adsorbent bed Therefore, the Maxsorb III sample, which ispowdered type and provides highest adsorption capacity, is chosen for thekinetics measurement at different pressures and temperatures

c) to measure the adsorption isotherms for methane/Maxsorb III pair at cryogenictemperature ranges that are maintained using a purpose-built cryostat These data

are important to study the ANG storage system when low temperature naturalgas is considered to charge.

d) to perform a theoretical analysis of the thermodynamic quantities for adsorption

of methane onto activated carbons The thermodynamic frameworks aredeveloped from the rigor of adsorption thermodynamics by incorporating themicropore filling theory and considering the adsorbed phase volume Thequantities are evaluated from the experimental uptake data and these are useful inthe thermodynamic analysis of the ANG storage system

e) to provide a detailed theoretical modeling of the ANG storage system withinternal thermal control of the activated carbon bed based on finned type heatexchanger Simulation is performed using the theoretical models developed toevaluate the thermal behavior and the storage capacity of the ANG storagesystem during its cyclic processes and

f) to conduct experiments of an ANG storage prototype mainly for the charge anddischarge processes The storage cylinder assembly fabricated in the laboratoryconsists of an activated carbon bed embedded with fins and tubes for enhancedthermal management The experimental data are compared with the simulationresults to verify the developed theoretical models

1.4 Organization of the Thesis

This thesis comprises seven chapters A comprehensive literature review isfurnished in the Chapter 2, which has three sections The first section provides a briefdescription of the adsorption fundamentals In the second section, developments ofadsorbents for the ANG storage system are described following by a thorough review

of the activated carbons Finally, an extensive literature review on advances in theANG storage system both in theory and experiment is provided.

Chapter 3 describes experimental details of measuring adsorption characteristics,such as isotherms and kinetics for adsorption of methane onto different types ofactivated carbons The Methods of describing experimental adsorption data with thetheoretical models are also provided

Chapter 4 describes experimental methods and procedures of measuringadsorption isotherms for methane/Maxsorb III pair at cryogenic temperatures Theprocedure of estimating gas filling pressure in ANG vessel is discussed in case of lowtemperature methane vapor from the LNG

Chapter 5 provides a theoretical analysis for the thermodynamic quantities of anadsorption system The adsorption uptake data for methane/activated carbon systemsare used in this chapter to evaluate the adsorbed phase thermodynamic quantities atvarious pressure, temperature and uptake conditions

Chapter 6 presents the detailed theoretical study and experimental investigations

of an ANG storage prototype mainly for charge and discharge processes Thetheoretical models for the elements of an ANG storage assembly are described and thesimulation results are compared with the experimental findings

Chapter 7 summarizes the major findings of this thesis along with furtherrecommendations

Chatper 2 Literature Review

2.1 Introduction

A comprehensive review of the literature for the adsorbed natural gas (ANG)storage technology is presented in this chapter Being an attractive alternative forenergy storage and transportation purposes, a number of efforts have been made byseveral researchers Most of these efforts mainly conducted investigations on (i) thedevelopment of suitable adsorbents for the ANG storage, (ii) the methods andtechniques to increase the storage capacity, and (iii) the design and implementation for

an effective thermal management for the storage vessel The reviews are presentedhere in chronological manner for these research scopes of the ANG technology Based

on the reviews, the developments targeted in our study are mentioned at the conclusion

of this chapter Since the ANG storage technique works on the principle of adsorption,

a brief discussion of the adsorption fundamentals is provided before approaching to theother reviews

2.2 Adsorption Fundamentals

2.2.1 Adsorption Mechanisms

In the physical adsorption process, the adsorbate (gas or liquid) molecules attractonto the adsorbent (solid) surfaces by the van der Waals forces and they are held at themicropores (mostly) and mesopores (to some extent) of the solid adsorbent (Ruthven,1984) These phenomena are physical in nature and hence, this process is reversible(using heat, pressure, etc.) On the other hand, chemical adsorption involves reactions

between adsorbate and adsorbent resulting in chemical bond formation (Rouquerol etal., 1999) and hence, it may not be completely reversible In the ANG technology, thegas molecules are adsorbed onto the adsorbent surfaces by the physical adsorption anddesorbed reversibly by altering the physical parameters, such as pressure andtemperature The physical adsorption is an exothermic process where heat is releaseddue to the change in energy level of the adsorbate molecules between gaseous andadsorbed phases (Suzuki, 1990).

2.2.2 Adsorption Equilibrium

The adsorbate molecules start to gather onto the adsorbent surfaces when theyare exposed to the solid adsorbent and it takes a long period for the adsorbate andadsorbent to reach an equilibrium state The amount of adsorbate that accumulates ontothe adsorbent surface at equilibrium condition is known as the equilibrium adsorbate

uptake (C) and it is a function of equilibrium pressure (P) and equilibrium temperature (T), i.e C = f (P, T) When the temperature is kept constant, the change in equilibrium adsorbate uptake (C) against the equilibrium pressure (P) is called the adsorption isotherm, i.e C = f (P) at T, as demonstrated in Figure 2.1.

The adsorption isotherm of an adsorbate/adsorbent pair is one of the importantcharacteristics in designing any system that involves adsorption processes Therefore,measurement of adsorption isotherms for methane/activated carbon pair is essential tostudy the storage capacity of the ANG storage system To measure the adsorptionisotherms, there are different techniques available in the literature which are mainlyvolumetric, gravimetric and gas flow techniques (Suzuki, 1990) Among them thevolumetric technique is widely used and popular because of its simplicity andreasonable accuracy in measurement In this research, the volumetric technique isemployed to measure the adsorption isotherms of methane onto different types ofactivated carbon samples.

Adsorption isotherms are described in mathematical forms based on theadsorption equilibrium or adsorption isotherm models A number of adsorptionequilibrium models are found in the literature to describe the adsorption isotherm datafor different adsorbate-adsorbent pairs (Do, 1998) Three different adsorption isothermmodels, namely those of Langmuir, Tóth and Dubinin-Astakhov, are used in this study

to correlate the experimental adsorption uptake data of methane/activated carbon pairs

2.2.3 Adsorption Kinetics

The diffusion process of the adsorbate molecules to the interior of the adsorbentpores is governed by the adsorption kinetics The mechanism of the diffusion processdepends on the porous structure of the adsorbent and the adsorption conditions, such astemperature and concentration range (Ruthven, 1984) In adsorbent particles with bi-dispersed pore structures, such as activated carbon, macropores usually act as a pathfor the adsorbate molecules to reach the micropores of the adsorbents and thus, theadsorption rate is determined from the overall diffusion properties (Suzuki, 1990)

When the performance of an adsorption system such as the ANG storage systemdepends on both the adsorption capacity and adsorption rate, it is necessary to measurethe adsorption kinetics for the working pair employed in that system.

To evaluate the adsorption kinetics, the time dependent uptake data for anadsorbate-adsorbent pair are usually measured either gravimetrically or volumetrically.The Linear Driving Force (LDF) model (Glueckauf, 1955) or pseudo-first orderreaction model (Latham and Burgess, 1981) is widely used to describe the adsorptionkinetics The rate parameters of the kinetics model are evaluated through regression ofthe transient adsorption uptake data

2.2.4 Adsorption Thermodynamics

In the adsorption process, the adsorbate molecules are more stabilized on theadsorbent surface than in the gaseous phase and it is because of the reduction in energylevel of the adsorbate molecules that accumulate in the pores of adsorbent with a phasetransformation The transformed phase of the adsorbate molecules is called asadsorbed phase and it is treated as a distinguishable phase in thermodynamicviewpoint, even though the precise location of the phase boundary is uncertain(Ruthven, 1984) Therefore, it is considered that the thermodynamic states of theadsorbed phase are not only function of pressure and temperature like gaseous phasebut also depend on adsorption uptake (Chua et al., 2003; Chakraborty et al., 2009) Theevaluation of adsorbed phase thermodynamic quantities, such as heat of adsorption,specific heat capacity, internal energy, enthalpy and entropy, are essential forthermodynamic analysis of any adsorption system The thermodynamic formulations

of these adsorbed phase quantities are achieved in this research and the property valuesare evaluated from the experimental adsorption uptake data

2.3 Development of Adsorbents for the ANG Storage System

The adsorption capacity and the rate properties are the principal criteria inselecting adsorbents for the ANG storage system The feasibility and performance ofthe ANG method depend on how the adsorbents behave in both adsorption equilibriumand adsorption kinetics For example, an adsorbent with higher uptake capacity butslower kinetics is not suitable for the ANG method because of the longer charge period

to reach the equilibrium state Thus, a good adsorbent must provide high adsorptioncapacity as well as fast kinetics, and hence, the adsorbents must be accomplished withthe following requirements

(i) The adsorbents must have reasonably larger surface area with a highmicropore volume

(ii) The adsorbents must have relatively large pore network for the transport of

adsorbate molecules to the interior of the particles

The surface characteristics and pore structure are the key properties indetermining the adsorption equilibrium and kinetics which are the essential data indesigning and performance evaluation of the ANG storage system The surface areaand the porosity of adsorbents control the adsorptive capacity while the porosity andthe pore size distribution determine the selectivity of the adsorbents The ultimate goal

is to find out such adsorbents which have higher surface area with smaller pore sizeand reasonable porosity This suggests that the best solid adsorbents to be used in theANG storage are those with pores in the micro (< 20 Å in diameter) and macro (>500

Å in diameter) porous ranges (Do, 1998)

2.3.1 Microporous Adsorbents

Based on the microporous structure, the adsorbents are roughly classified intothree groups: amorphous materials (e.g silica gel, alumina, clays, other oxides, etc.),zeolites, and activated carbons (Ruthven, 1984)

Silica gel is a synthetic amorphous solid with continuous network of sphericalparticles of colloidal silica It is more popular for adsorption of (–OH) groups andwater due to its strong hydrophilic nature (Yang, 1987) and therefore, there is noample work reported in the literature for adsorption of hydrocarbons on silica gel.Zeolites, which can be found naturally or made synthetically, have beenextensively used in the separation of hydrocarbons and as catalysts in petroleumrefining, synthetic fuels and petrochemical production, etc (Ruthven, 1984) In earlierdays, zeolites were looked upon as a potential means for the ANG storage systemwhen the procedures for synthesizing high surface area activated carbons were notfamiliar (Karger and Ruthven, 1992) Several researchers have studied adsorption ofmethane onto different types of zeolites for possible use in methane storage technology(Ding et al., 1988; Zhang et al., 1991; Malbrunot et al., 1996) However, zeolites arenot as popular as activated carbons because of their smaller micropore volumes andextreme hydrophilic nature, which may largely affect their adsorption capacity formethane molecules (Arash et al., 2011) Furthermore, due to the possibility ofmodifying large micropore volumes, activated carbons provide superior storagecharacteristics compared to zeolites

For these reasons, activated carbons are chosen in this research as potentialadsorbents for the ANG storage system The details of activated carbons are discussedhere in the perspective of natural gas storage application The adsorption behaviours of

porous metal-organic frameworks (MOF) are also briefly reviewed as this adsorbentmaterial has recently been investigated for possible use in the adsorptive gas storagesystem.

2.3.2 Activated Carbons

Activated carbons are the adsorbents with the most favourable characteristics forthe ANG storage system due to their textural properties such as high surface area andlarge micropore volume (Quinn and MacDonald, 1992; Parkyns and Quinn, 1995;Sircar et al., 1996; Menon and Komarneni, 1998; Lozano-Castelló et al., 2002a;Alcañiz-Monge et al., 2009) They are hydrophobic in nature and having strongaffinity for organic substances (Cook et al 1999) Large internal area and chemicallyinert graphite surface area of the activated carbons are the basic reasons for their strongadsorption capacity of non-polar molecules like methane and weakly polar molecules(Kim et al., 2003) Besides, activated carbons can be efficiently compacted into apacked bed with (Lozano-Castelló et al., 2002b; Balathanigaimani et al., 2008) orwithout (Inomata et al., 2002; Guan et al., 2011) binder, and can cheaply bemanufactured in large quantities (Bagheri and Abedi, 2011)

Over the past few decades, a larger number of studies have been conducted onthe preparation and characterization of the activated carbons focused on the methanestorage to reach an optimum adsorption capacity (Molina-Sabio et al., 2003; Azevedo

et al., 2007; Bastos-Neto et al., 2007; Dai et al., 2008; Namvar-Asl et al 2008; Yeon etal., 2009; Dai et al., 2009; Mu and Walton, 2011) These revealed that activatedcarbons can be synthesized with high microporosity in the carbon matrix and with highsurface area formed by an intricate network of small pores, where the density ofadsorbed phase methane is liquid-like and thus provide a high storage density