Membrane materials and fabrication for gas separation

CHAPTER 9 CONCLUSIONS AND RECOMMENDATIONS……….217 9.1 Conclusions………217 9.1.1 Chemical Cross-linking Modification of Polyimide Membranes for Gas Separation………..218 9.1.2 Separation of CO2

MEMBRANE MATERIALS AND FABRICATIONS FOR

GAS SEPARATION

TIN PEI SHI

NATIONAL UNIVERSITY OF SINGAPORE

2005

MEMBRANE MATERIALS AND FABRICATIONS FOR

GAS SEPARATION

TIN PEI SHI

(B Eng (Chemical) (Hons.), University Technology Malaysia)

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMICAL AND BIOMOLECULAR

ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2005

ACKNOWLEGEMENTS

I wish to take this opportunity to express my sincere appreciation to all the contributors whose cooperation and assistance were essential in helping me to gradually acquire sharper tools toward the completion of my PhD study reported herein:

First of all, I especially wish to record my deepest appreciation and thanks to my immediate advisor, Professor Neal Chung Tai-Shung for his invaluable guidance, advice, patience, constructive comments and challenges that helped me actualize and sharpen my professional skills I am also indebted to my co-advisors, Dr Wang Rong and Dr Liu Ye for their keen efforts and consistent consultation throughout my candidature

I would like to gratefully acknowledge the research scholarship offered to me by the National University of Singapore (NUS), which provided me a positive, conducive and professional atmosphere for researching Sincere thanks to Institute of Materials Research and Engineering (IMRE) for the characterization instrument I also wish to express my recognition to Agency for Science, Technology and Research (A*Star) and National Research Council Canada (NRC) for their financial support that enables this work to be successfully completed

I would like to convey my gratitude to Dr Pramoda Kumari Pallathadka, Dr Dharmarajan Rajarathnam and Mr Lim Poh Chong for their various assistances in operating characterization instrument and equipment I may also like to thank Dr

Anita J Hill for her collaboration in conducting the PALS to characterize the carbon membranes, as well as Dr Liu Songlin for sharing his expertise in the mixed gas permeation tests My gratitude is extended to efforts of Mr Ng Kim Poi, who fabricated and contributed expert advice in equipment setup and machinery

Of course, personal thanks go to all members of our research group, especially Dr Cao Chun and Mr Xiao Youchang for many good times, discussion and sharing of technical expertise Special thanks to Ms Chng Mei Lin for all her kindest cooperation and handy help during my days in the laboratory Also worth mentioning are my friends that have been kind and helpful to me, which have made my study in

NUS enjoyable and memorable

I must express my deepest love and hearties gratefulness to my family for their endless support, enduring patience and positive encouragement that brighten up every phase of my life I can never sufficiently thank or acknowledge them for their unwavering and unconditional love

Last but not least, acknowledgements are due to all those who have assisted me in any way throughout the period of my PhD study, for both directly and indirectly continued assistance and support that I received

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENT……… i

TABLEOF CONTENTS……… iii

SUMMARY……… x

NOMENCLATURE………xiii

LIST OF TABLES……… xix

LIST OF FIGURES………xxii

CHAPTER 1 INTRODUCTION AND PERSPECTIVE OF GAS SEPARATION MEMBRANE…… ……….1

1.1 Membrane-Based Gas Separations……….4

1.1.1 Scientific Milestones……… 5

1.1.2 Advantages of Membrane Gas Separation……….9

1.2 Industrial Applications of Membrane Gas Separation……….10

1.3 Engineering Principles for Membrane Gas Separation………14

1.3.1 Membrane Materials Selection……… 15

1.3.1.1 Organic (Polymeric) Materials……….….17

1.3.1.2 Inorganic Materials………19

1.3.1.3 Mixed Matrix Membranes……….20

1.3.2 Membrane Fabrication and Modification……….22

1.3.3 Membrane Characterization and Evaluation………24

1.3.4 Membrane Modules and Design Considerations……… 25

1.4 Gas Transport Mechanisms of Membrane Separation……….29

1.4.1 Poiseuille Flow……….31

1.4.2 Knudsen Diffusion……… 31

1.4.3 Molecular Sieving………32

1.4.4 Solution-Diffusion………33

1.5 Research Objective and Organization of Dissertation……….34

CHAPTER 2 POLYMERIC MEMBRANES FOR GAS SEPARATION………38

2.1 Principles of Membrane Gas Separation……… 38

2.2 Theory of Gas Transport in Nonporous Glassy Polymeric Membranes…… 44

2.2.1 Polymers Free Volume and Occupied Volume………44

2.2.2 Dual-Mode Sorption Model ……… 45

2.2.3 Dual-Mode Model for Permeation ……… 48

2.2.4 Effect of Upstream Pressure on Gas Sorption and Permeation………50

2.2.5 Effect of Temperature on the Gas Transport Properties……… 52

2.3 Membrane Materials for Gas Separation-Polyimides……… 54

2.4 Advanced Modification of Polymeric Membranes……… 58

CHAPTER 3 CARBON MOLECULAR SIEVE MEMBRANES FOR GAS SEPARATION………63

3.1 Introduction……… 63

3.2 Fabrication of Carbon Molecular Sieve Membranes……… 64

3.2.1 Selection of Polymeric Precursors and Membranes Preparation…….65

3.2.2 Pyrolysis of Polymeric Precursors……… 67

3.2.3 Modification of Carbon Membranes………73

3.2.3.1 Pretreatment……… 73

3.2.3.2 Post-treatment……… 76

3.2.4 Configurations of Carbon Membranes……….79

3.3 Microstructure of Carbon Membranes……….82

3.4 Mechanisms of Gas Transport in Carbon Membranes……….85

CHAPTER 4 MATERIALS AND EXPERIMENTAL PROCEDURES……… 89

4.1 Materials……… 89

4.1.1 Polymers……… 89

4.1.2 Molecular Sieves……… 91

4.2 Preparation of Polymeric Membranes……… 92

4.2.1 Polymeric Dense Film Formation………92

4.2.2 Chemical Cross-linking Modification……… 92

4.3 Fabrication of Carbon Membranes……… 93

4.3.1 Polymeric Dense Film Formation….………93

4.3.2 Pretreatment……… 93

4.3.2.1 Chemical Cross-linking Modification……… 93

4.3.2.2 Nonsolvent Pretreatment……… 94

4.3.3 Pyrolysis Process……… 94

4.4 Fabrication of Carbon-Zeolite Composite Membranes………96

4.4.1 Preparation of Polymer-zeolite Mixed Matrix Membranes (MMMs) 96

4.4.2 Pyrolysis Process……… 96

4.5 Characterization of Physical Properties……… 97

4.5.1 Measurement of Gel Content……… 97

4.5.2 Fourier Transform Infrared Spectrometer (FTIR)………98

4.5.3 Differential Scanning Calorimetry (DSC)………98

4.5.4 Modulated Differential Scanning Calorimetry (MDSC)……… 98

4.5.5 Dynamic Mechanical Analysis (DMA)………99

4.5.6 Thermomechanical Analysis (TMA)………99

4.5.7 Elemental Analysis……….100

4.5.8 Measurement of Density………100

4.5.9 Thermogravimetric Analysis (TGA)……… 101

4.5.10 TGA-FTIR……… 101

4.5.11 Wide-angle X-ray Diffraction (WAXD)………102

4.5.12 Surface Area and Pore Size Analyzer………102

4.5.13 Positron Annihilation Lifetime Spectroscopy (PALS)……… 103

4.5.14 Scanning Electron Microscope (SEM)……… 103

4.6 Characterization of Gas Transport Properties………104

4.6.1 Constant Volume-Variable Pressure Method……….104

4.6.2 Pure Gas Permeation Tests……….106

4.6.3 Mixed Gas Permeation Tests……… 109

4.6.4 Pure Gas Sorption Tests……….113

CHAPTER 5 CHEMICAL CROSS-LINKING MODIFICATION OF POLYIMIDE MEMBRANES FOR GAS SEPARATION…… 115

5.1 Introduction……… 115

5.2 Results and Discussion……… 118

5.2.1 Characterization of Cross-Linked Matrimid Membranes………… 118

5.2.2 Mechanisms of Chemical Cross-linking Reaction……… 121

5.2.3 Pure Gas Transport Properties of Matrimid®5218………125

5.2.4 Effect of Cross-Linking on Plasticization Phenomenon………134

5.2.5 Permeation Transport of Mixed Gases……… 136

5.3 Conclusion……… 139

CHAPTER 6 SEPARATION OF CO 2 /CH 4 THROUGH CARBON MOLECULAR SIEVE MEMBRANES DERIVED FROM POLYIMIDES……… 141

6.1 Introduction……… 141

6.2 Results and Discussion……… 144

6.2.1 Characterization of Carbon Membranes……….144

6.2.2 CO2/CH4 Permeation Performance of P84-derived Carbon Membranes……….151

6.2.3 A Comparison of Gas Separation Performance between P84-derived CMSMs and Other Commercially Available Polyimides-derived CMSMs……… 153

6.2.4 Separation of CO2/CH4 Binary Mixture………158

6.3 Conclusion……….160

CHAPTER 7 NOVEL APPROACHES TO FABRICATE CARBON MOLECULAR SIEVE MEMBRANES BASED ON CHEMICAL MODIFIED AND NONSOLVENT-TREATED POLYMIDES………162

7.1 Introduction………162

7.2 Pretreatment I: Chemical Cross-Linking Modification……… 166

7.2.1 Results and Discussion……… 167

7.2.1.1 Characterization of Carbon Membranes……… 167

7.2.1.2 Effects of Pyrolysis on Pure Gas Permeation Properties…173

7.2.1.3 Effects of Cross-linking Degree on the Gas Permeation

Properties of CMSMs Derived from Matrimid Precursor 176 7.2.1.4 Pure Gas Permeation Properties of CMSMs Based

on the Methanol Pretreated Polyimides……… 177 7.3 Pretreatment II: Nonsolvent Pretreatment……… 181

7.3.1 Results and Discussion……… 181

7.3.2.1 Thermal Behavior of Nonsolvent Treated

7.3.2.2 Characterization of Carbon Membranes……… 182 7.3.2.3 Effects of Nonsolvent Pretreatment on the

Pure Gas Permeation Properties of Resultant

8.2 Results and Discussion……… 206

8.2.1 Characterization of Polymer-Zeolite Mixed Matrix

Membranes (MMMs)……….206 8.2.2 Characterization of Carbon-Zeolite Composite Membranes……… 208

8.2.3 Pure Gas Permeation Properties of Carbon-Zeolite

Composite Membranes……… 211 8.3 Conclusion……… 215

CHAPTER 9 CONCLUSIONS AND RECOMMENDATIONS……….217

9.1 Conclusions………217

9.1.1 Chemical Cross-linking Modification of Polyimide Membranes for Gas Separation……… 218

9.1.2 Separation of CO2/CH4 through Carbon Molecular Sieve Membranes (CMSMs) Derived from Polyimides………219

9.1.3 Novel Approaches to Fabricate Carbon Molecular Sieve Membranes Based on Chemical Modified and Nonsolvent-Treated Polyimides……….220

9.1.4 Carbon-Zeolite Composite Membranes for Gas Separation……… 222

9.2 Recommendations for Future Work……… 222

9.2.1 Chemical Cross-linking Modification of Polyimide Membranes 223

9.2.2 Fabrication of Carbon Molecular Sieve Membranes……….224

9.2.3 Fabrication of Carbon-Zeolite Composite Membranes……… 226

REFERENCES……….228

APPENDICES Appendix A Derivations of the Average Diffusion Coefficient and the Effective Diffusion Coefficient Base on the Definition of Permeability………266

Appendix B Calculations of the Volume of Downstream Compartments in a Gas Permeation Cell………268

Appendix C Calculations of the Fractional of Free Volume (FFV)……… 269

Appendix D Calculations of Solubility Parameter (δsp)………275

SUMMARY

The purpose of this work is to evaluate the use of commercially available polyimides

as the precursor to prepare the high performance membranes for gas separation A comprehensive research study, which covers the fabrication and characterization of three types of membranes, particularly cross-linked polymeric membrane, carbon molecular sieve membrane (CMSM) and carbon-zeolite composite membrane, is presented Various instruments were employed to screen the physical properties and gas separation performance of these membranes Emphases were put on the separation of CO2/CH4, O2/N2 and He/N2 because of their high market impact

Firstly, the effectiveness of chemical cross-linking modification in improving the gas separation capability of polyimide membranes was investigated An extremely simple room-temperature chemical cross-linking modification was performed on Matrimid®

5218 The influence of cross-linking modification on thermal and gas transport properties of Matrimid membranes were studied The gas permeability of cross-linked membranes decreased with immersion time (The time for membranes being immersed in cross-linking reagent) after achieved the maximum value at 1-day immersion time On the other hand, the ideal selectivity for O2/N2, CO2/CH4 and

CO2/N2 remain almost constant with cross-linking reaction, except for He/N2, where the ideal selectivity increased with cross-linking density Experimental results showed that the proposed cross-linking modification was capable to enhance the anti-plasticization characteristics of polymeric membranes (This work was published in

the J Membr Sci., 2003, 225, 77-90)

Secondly, carbon molecular sieve membranes were prepared through pyrolysis of commercially available polyimides with excellent intrinsic separation properties WAXD, density measurement and TGA-FTIR were performed to characterize the morphology of carbon membranes and the degradation of precursor during pyrolysis The permeation properties of single and equimolar binary gas mixture through carbon membranes were measured and analyzed A comparison of permeation properties among carbon membranes derived from 4 commercially available polyimides showed that the P84 carbon membranes exhibited the highest separation efficiency for

CO2/CH4 separation The mixed gas permeation data confirmed the underestimation

of separation efficiency by pure gas permeation measurement (This work was

published in the Carbon, 2004, 42, 3123-3131)

One of the most significant contributions of this work is development of two novel pretreatment approaches in enhancing the separation capability of CMSMs These two approaches were chemical cross-linking modification and nonsolvent pretreatment The permeation properties of carbon membranes derived from cross-linked polyimide precursors were characterized as a function of cross-linking density The improved separation efficiency of CMSM was achieved at low degree of cross-linking, as compared to untreated CMSM Besides, an extensive study was conducted

to investigate the effect of nonsolvent (methanol, ethanol, propanol and butanol) pretreatment on the separation properties of resultant CMSMs Various characterization methods verified that the nonsolvent pretreatment appears to be an effectual approach to produce highly selective CMSMs (This work was published in

the (1) Macromol Rapid Commun., 2004, 25, 1427-1250; (2) Microporous and

Mesoporous Materials, 2004, 73, 151-160; and (3) Ind Eng Chem Res., 2004, 43,

6476-6483 A US Provisional Patent Application has been filed on 26 Feb 2004)

Lastly, a new membrane material, carbon-zeolite composite membrane with excellent separation performance is introduced This heterogeneous or hybrid membranes comprising of zeolite entities dispersed in carbon matrix were prepared through the pyrolysis of zeolite-filled polymeric mixed matrix membranes The morphology and pure gas permeation properties of carbon-zeolite composite membranes were characterized The experimental results revealed that the improved ideal selectivity was obtained with the correct selection of polymer/zeolite pair The composite membranes possessed good separation properties with combining the advantages of carbon membranes and zeolite materials (This work was published in the Carbon,

2005, 43, 2025-2027 A US Provisional Patent Application has been filed on 22 Oct

2004)

NOMENCLATURE

A Effective are of the membrane (cm2)

b Langmuir affinity constant (1/atm)

C Local penetrant concentration in the membrane

(cm3(STP)/cm3(polymer))

C D Henry sorption concentration (cm3(STP)/cm3(polymer))

C i Local penetrant concentration of species i in the membrane

(cm3(STP)/cm3(polymer))

C H Langmuir sorption concentration (cm3(STP)/cm3(polymer))

C H ’ Langmuir capacity constant (cm3(STP)/cm3(polymer))

C1 Local penetrant concentration at downstream side

D app Apparent diffusion coefficient, (cm2/s)

D avg Average diffusion coefficient, (cm2/s)

D A Diffusion coefficient of component A (cm2/s)

D B Diffusion coefficient of component B (cm2/s)

H

∆ Enthalpy of solution (kJ/mol)

J Permeation flux (cm3/cm2-s)

J D Henry’s diffusional flux (cm3/cm2-s)

J H Langmuir’s diffusional flux (cm3/cm2-s)

D

C b k

k D Henry’s dissolution constant (cm3(STP)/cm3(polymer)-atm)

P B Permeability coefficient of a membrane to gas B (1 Barrer = 1 x 10

p1 Downstream pressure of the penetrant (cmHg)

p2 Upstream pressure of the penetrant (cmHg)

∆p Pressure difference between the upstream and the downstream of a

S Solubility coefficient (cm3(STP)/cm3(polymer)-cmHg)

S app Apparent solubility coefficient (cm3(STP)/cm3(polymer)-cmHg)

S A Solubility coefficient of component A (cm3(STP)/cm3(polymer)-cmHg)

S B Solubility coefficient of component B (cm3(STP)/cm3(polymer)-cmHg)

S0 Pre-exponential factor for the apparent enthalpy of solution

(cm3(STP)/cm3(polymer)-cmHg)

δsp Solubility parameter ((cal/cm3)1/2)

∆δsp Difference of solubility parameter ((cal/cm3)1/2)

T Absolute temperature of measurement (K)

T d Degradation temperature of polymer which corresponding to 5%

weight loss (°C) Glass transition temperature (°C)

V Van der Waals volume (cm3/g)

v Average molecular velocity (m/s)

W 0 Original weight of cross-linked films (g)

W 1 Insoluble fraction weight of cross-linked films after extraction (g)

W p Weight of polymer (g)

W sp Weight of sample pan (g)

w o Membrane weight in air (g)

w1 Membrane weight in solvent (g)

x Distance between the upstream and the downstream of a

membrane (cm)

x A Upstream mole fractions of component A

x B Upstream mole fractions of component B

2

CO

x CO2 molar fraction in the feed gas

x i Upstream mole fractions of component i

y A Downstream mole fractions of component A

y B Downstream mole fractions of component B

2

CO

y CO2 molar fraction in the permeate gas

y i Downstream mole fractions of component i

σeff Effective molecular diameter (Å)

σk Kinetic molecular diameter (Å)

σLJ Lennard-Jones collision diameter (Å)

θ Time lag (s)

ϑ Diffraction angle (°)

τ1 Positron lifetimes (s)

φ Volume fractions

Abbreviations

2,6-DAT 2,6-diamino toluene

6FDA 2,2’-bis(3,4’-dicarboxyphenyl) hexafluoropropane dianhydride

ATR Attenuated Total Reflection

BTDA 3,3’4,4’-benzophenone tetracarboxylic dianhydride

CTE Coefficient of Thermal Expansion

DAPI diamino-phenylindane

DMA Dynamic Mechanical Analysis

DSC Differential Scanning Calorimetry

Durene 2,3,5,6-tetramethyl-1,4-phenylenediamine

FFV Fractional of Free Volume

FTIR Fourier Transform Infrared Spectroscopy

MDI Methylene dianiline

MDSC Modulated Differential Scanning Calorimetry

oPs Ortho-positronium

PALS Positron Annihilation Lifetime Spectroscopy

SEM Scanning Electron Microscope

TDI Methylphenylene-diamine

TGA Thermogravimetric Analysis

TGA-FTIR Thermogravimetric Analysis-Fourier Transform Infrared Spectroscopy TMA Thermomechanical Analysis

WAXD Wide-angle X-ray Diffraction

LIST OF TABLES

Table 1.1 Development of Membrane Processes Market……… 2

Table 1.2 Future Market of Membrane Gas Separation……….3

Table 1.3 Scientific Developments of Membrane Gas Transport……… 7

Table 1.4 Industrial Applications of Gas Separation Membranes………11

Table 1.5 Materials for Gas Separation Membranes………16

Table 1.6 Qualitative Comparisons of Various Membrane Modules……… …29

Table 2.1 List of Companies Active in Patenting……….56

Table 2.2 Membrane Modification Methods………59

Table 4.1 Chemical structures and properties of Matrimid® 5218 and P84……90

Table 4.2 Main Characteristics of Zeolites……… 91

Table 5.1 Gas Separation Properties of Untreated and Cross-Linked Matrimid Dense Films………126

Table 5.2 Gas Diffusion Coefficient and Solubility Coefficients of Cross-Linked Polyimides……… 130

Table 5.3 Molecule Diameters of Gases Tested in This Study……… 131

Table 5.4 Comparison of Transport Performance for Matrimid Membranes between the Mixed gas and Pure Gas Measurements………138

Table 6.1 Elemental Analysis of P84 and Matrimid Precursors and Their Pyrolyzed Membranes………145

Table 6.2 Density of P84-derived Carbon Membranes……… 146

Table 6.3 Gas Separation Properties of P84 Precursor and Derived Carbon Membranes………153

Table 6.4 Chemical Structures and Properties of Commercially Available

Polyimides……… 154 Table 6.5 Comparison between the Gas Permeation Properties of P84-

and Matrimid-Derived CMSMs……….155 Table 6.6 Comparison of Transport Performances for P84-derived Carbon

Membranes between Mixed Gas and Pure Gas Measurements…….159 Table 7.1 Elemental Analysis of Matrimid Precursor and Pyrolyzed

Table 7.2 Density of CMSMs Derived from Untreated and Cross-linked

Matrimid……….169 Table 7.3 Gas Separation Properties of Matrimid Precursor and Carbon

Membranes……….174 Table 7.4 Gas Separation Properties of CMSMs Derived from

Methanol-treated Matrimid……….… 178 Table 7.5 Glass Transition Temperatures (T g) of Matrimid and P84

Membranes……….182 Table 7.6 Chemical Structures and Properties of Nonsolvents Used for

Pretreatment………185 Table 7.7 Solubility Parameter of Polyimides and Nonsolvents………186 Table 7.8 Positron Annihilation Lifetime Spectroscopy Parameters for

Selected CMSMs………189 Table 7.9 Gas Permeation Properties of Carbon Membranes Derived from

Nonsolvent-treated Matrimid……….191 Table 7.10 Gas Permeation Properties of Carbon Membranes Derived from

Nonsolvent-treated P84……….….192

Table 7.11 Gas Transport Properties of Carbon Membranes Derived from

Methanol- and Ethanol-treated Matrimid at Various Immersion Time………193 Table 7.12 Gas Diffusion and Sorption Coefficients of CMSMs Derived

Table 7.13 Sorption and Diffusion Selectivities of CMSMs Derived from

Matrimid……….197 Table 8.1 Gas Separation Properties of Matrimid Precursor and

Matrimid-Zeolite Mixed Matrix Membranes……….211 Table 8.2 Gas Separation Properties of Carbon-Zeolite Composite

Membranes……….212

LIST OF FIGURES

Figure 1.1 Schematic diagram of gas separation process by a membrane……… 5

Figure 1.2 Milestones in the development of membrane gas separations……… 9

Figure 1.3 Schematic of the specific volume of polymer as a function of

temperature……… 17 Figure 1.4 Trade-off line curve of oxygen permeability and oxygen/nitrogen

selectivity……… 19 Figure 1.5 Schematic of a mixed matrix membrane…….……….21

Figure 1.6 Typical membrane morphology………….……… 24

Figure 1.7 Schematic drawing of a plate-and-frame module……….26

Figure 1.8 Spiral-wound elements and assembly……… 27

Figure 1.9 Hollow fiber separator assembly……… 28

Figure 1.10 Schematic presentation of main mechanisms of membrane-based

Figure 2.1 Gas membrane separations……… 38

Figure 2.2 Aromatic characteristics in polyimides contribute to their

Figure 3.1 Carbon membrane fabrication process……….65

Figure 3.2 Conceptual model for “pore” structure evolution and reorganization

in PFA-derived CMSM………70 Figure 3.3 Configurations of carbon membranes……… 80

Figure 3.4 Structure of turbostratic carbon………83

Figure 3.5 Idealized structure of a pore in a carbon material………84

Figure 4.1 Chemical structure of p-xylenediamine……… 92

Figure 4.2 Steps involved in pyrolysis process at final temperature of

(a) 550 °C, (b) 650 °C and (c) 800 °C……… 95 Figure 4.3 Schematic diagram of a gas permeation cell……… 105 Figure 4.4 Pressure versus time plot (transient and steady state permeation)….108 Figure 4.5 Schematic diagram of dense membrane mixed gas permeation

test apparatus……… 112 Figure 4.6 Schematic diagram of the microbalance sorption cell…… ……….113 Figure 5.1 MDSC thermograph of original and modified Matrimid

membranes……… 119 Figure 5.2 DMA results for original and modified Matrimid membranes.…….120 Figure 5.3 Plot of TMA for original and modified Matrimid membranes…… 121 Figure 5.4 A comparison of FTIR spectra for Matrimid: (a) Original sample;

(b) and (c) cross-linked sample obtained by immersing in cross-linking reagent for 32 days and 61 days, respectively……… 122 Figure 5.5 Chemical cross-linking modification by p-xylenediamine

methanol reagent solution……… 123 Figure 5.6 Mechanism of chemical cross-linking modification and the

possible cross-linking structure……… 124 Figure 5.7 Influence of immersion time on the pure gas permeabilities of

Matrimid……….127 Figure 5.8 Gas separation properties of Matrimid dense films in methanol……127 Figure 5.9 Influence of immersion time on the decreasing order of gas

permeabilities……….129

Figure 5.10 Apparent diffusion coefficient as a function of square of gas molecule

diameters for (a) untreated Matrimid, and (b) 7-days cross-linked Matrimid……….132 Figure 5.11 Apparent diffusion coefficients as a function of square of

effective diameter……… 133 Figure 5.12 Influence of immersion time on the ideal gas selectivity………… 134 Figure 5.13 Pure CO2 permeability for untreated and cross-linked films as a

function of upstream pressure………135 Figure 6.1 Thermogravimetric analysis of Matrimid and P84 precursors…… 147 Figure 6.2 Selected IR spectra of products from TGA for P84 precursor…… 148 Figure 6.3 Selected IR spectra of products from TGA for Matrimid

precursor……….149 Figure 6.4 Wide-angle X-ray diffraction for P84-derived CMSMs

carbonized at 550 °C, 650 °C and 800 °C……… 150 Figure 6.5 Wide-angle X-ray diffraction for Matrimid-derived CMSM

carbonized at 800 °C……… 151 Figure 6.6 Separation properties of CO2/CH4 for P84-based and

Matrimid-based CMSMs with respect to upper-bound curve………157 Figure 7.1 Thermogravimetric analysis of Matrimid precursors……….170 Figure 7.2 Selected IR spectra of the products from TGA for (a) original

Matrimid precursor, (b) 1-day cross-linked Matrimid and (c) 1-day methanol treated Matrimid……… 172 Figure 7.3 Wide-angle X-ray diffraction of CMSMs derived from

Matrimid® 5218……….173

Figure 7.4 Gas permeabilities of Matrimid-derived CMSMs vs kinetic

diameters of gas molecules……….175 Figure 7.5 Effect of immersion time on the ideal gas selectivity of

Matrimid-derived CMSMs……….177 Figure 7.6 Separation properties of CO2/CH4 for Matrimid-derived

CMSMs and precursor with respect to upper-bound curve…………180 Figure 7.7 Thermal degradation of untreated and nonsolvent-treated

Matrimid films analyzed by TGA ……… 183 Figure 7.8 DSC thermograms of nonsolvents treated Matrimid membranes… 184 Figure 7.9 Thermal degradation of untreated and non-solvents treated P84

membranes analyzed by TGA………186 Figure 7.10 Wide-angle X-ray diffraction for CMSMs derived from

untreated and nonsolvent-treated Matrimid® 5218 pyrolyzed

at 800 °C……….187 Figure 7.11 Wide-angle X-ray diffraction for CMSMs derived from

untreated and nonsolvent-treated P84 pyrolyzed at 800°C….…… 188 Figure 7.12 PALS timing histograms for untreated Matrimid and ethanol-

pretreated Matrimid CMSMs showing the two component fit giving positron lifetimes τ1 and τ2……… 189 Figure 7.13 CO2, CH4, O2 and N2 sorption isotherms in CMSMs derived

from Matrimid precursors at 35 °C………195 Figure 7.14 Permeabilities of Matrimid CMSMs as a function of kinetic

diameter……… 197 Figure 7.15 Separation properties of CO2/CH4 for Matrimid derived

CMSMs and precursor with respect to the upper-bound curve…… 198

Figure 7.16 Separation properties of CO2/CH4 for P84 derived CMSMs

and precursor with respect to the upper-bound curve………199 Figure 8.1 Scanning electron micrographs (cross-section) of (a) Zeolite

Beta-filled mixed matrix membrane (Matrimid-Beta-MMM), and (b) Zeolite 4A-filled mixed matrix membrane

(Matrimid-4A-MMM), (c) Zeolite silicalite-1-filled mixed matrix membrane (Matrimid-silicalite-1-MMM) and (d) Zeolite

KY-filled mixed matrix membrane (Matrimid-KY-MMM)……… 207 Figure 8.2 WAXD patterns of (a) Carbon-Beta-CM, (b) Carbon-4A-CM,

(c) Carbon-Silicalite-1-CM and (d) Carbon-KY-CM………209 Figure 8.3 Scanning electron micrographs (cross-section) of

(a) Carbon-Beta-CM, (b) Carbon-4A-CM, (c) Carbon-Silicalite-1-CM, and (d) Carbon-Beta-CM at the magnification of 3,000 and 10,000……….210 Figure 8.4 Separation properties of CO2/CH4 for Matrimid-KY-derived

membranes with respect to upper-bound curve……… 215

In recent years, membrane separation process has been utilized as separation tool and supplemented the conventional mass separation techniques such as distillation, crystallization, absorption, adsorption, solvent extraction, etc (Scott, 1990; Mohammadi, 1999) It is recognized as an energy efficient and economical tool in solving many mass separation tasks

Membrane technology for the separation of liquid/liquid and liquid/solid streams has been practiced in industry for many years in reverse osmosis, ultrafiltration, microfiltration, pervaporation, hemodialysis, electrodialysis, controlled release of drugs, gas separation and so on (Yoshiharu, 1992) The worldwide

membrane market in 1988 can be summarized as follows (Nunes and Peinemann,

2001; Strathmann, 2001):

• Sales of membranes and modules > US$ 4.4 billion

• Sales of membrane systems > US$ 15 billion

• Market growth is 8-10 % per year

The market of membrane separation is extremely heterogeneous and growing fast,

where requires different membrane structures and processes for specific application

The membranes and module sales is growing at a rate of 10 %/year to US$ 4.8 billion

at year 2000 (Yampolskii et al., 2002) The development of membrane processes in

the end of century is reviewed as shown in Table 1.1

Table 1.1 Development of Membrane Processes Market

Sales (US$ Million) Membrane Process

Table 1.2 Future Market of Membrane Gas Separation (Baker, 2001)

Membrane Market (US$ million, 2000 dollars) Separation

2000 2010 2020

Conclusively, although gas separation is a relatively young technology, it

accounts for about US$ 250 million/year and is growing relatively fast with a rate of

15 %/year With the development of new membranes with enhanced separation

properties and stability, the importance of membrane-based gas separation to solve

difficult mass separation problems will certainly increase in the future, especially in

responding to the market demand for industrial applications Currently, the major

application of gas membrane separation is the separation of noncondensable gases,

such as nitrogen from air, carbon dioxide from methane, and hydrogen from nitrogen,

methane Nevertheless, membrane gas separation technology in refinery,

petrochemical and natural gas industries are predicted to be a great potential market

for membrane technology Table 1.2 illustrates the prediction of future membrane gas separation market made by Baker (2001)

Membrane technology for gas mixtures separation has rapidly grown from being a laboratory curiosity to becoming a commercially viable separation approach within the last two decades (Nunes and Peinemann, 2001) Membrane gas separation has emerged as one of the most significant new unit operations in the chemical industries in the past 25 years (Prasad et al., 1994) At least 20 companies worldwide offer membrane-based gas separation systems for a variety of industrial applications Membrane gas separation plays an increasing important role in the separation industry with over $125 million in module sales with an annual growth rate of around 10 % over the next decade (Crull, 1997) Thus, there is a large potential for this separation technology to capture a significant slice of the separation market Membrane gas separation is an area of considerable current research interest as the number of applications is expected to expand rapidly over the next decade

Membrane in a gas permeation process act as a selective barrier, usually thin, interposed between two phases, which obstructs gross mass movement between the phases but permits passage of certain species from one phase to the other with various degrees of restriction (Koros and Fleming, 1993; Mulder, 1996) Generally, in membrane gas separation processes, the bulk phases are gas mixtures Gas Permeation is a physical phenomenon where certain gas components selectively pass through a membrane The membrane is selective to one of the gas species, where one

of the species in the mixture is allowed to be exchanged in preference to others One bulk phase is enriched in one of the species while the other is depleted of it Separation of a gas mixture occurs since each type of molecules diffuses at a different rate through the membrane (Geankoplis, 1995) This movement of any species across the membrane is caused by one or more driving forces These driving forces arise from a gradient of chemical potential due to concentration gradient or pressure gradient or both Figure 1.1 shows schematic diagram of a two-phase gas separation system separated by a membrane

Phase 1 Membrane Phase 2

by 10 gases through natural rubber balloons (Mitchell, 1830; 1833) At approximately the same time, A Fick, an outstanding physiologist postulated the concept of diffusion and formulated the well-known Fick’s first law by studying the gas transport through nitrocellulose membranes (Fick, 1855)

However, many significant scientific observations about membrane separation, such as the first quantitative measurement of the rate of gas permeation were accomplished by Sir Thomas Graham, the discoverer of Graham’s law of gas effusion He proposed “solution-diffusion” mechanism for gas permeation through a membrane by repeating Mitchell’s experiments with the films of natural rubber in

1866 (Graham, 1866) Approximately 13 years later in 1879, Von Wroblewski quantified Graham’s model and defined the permeability coefficient as the permeation flux multiplied by the membrane thickness divided by the transmembrane pressure (Wrobleski, 1879) He also characterized the permeability coefficient as a product of diffusivity and solubility coefficients, which soon became an important model in membrane permeation A decade later in 1891, H Kayser demonstrated the validity

of Henry’s law for the absorption of carbon dioxide in rubber (Kayser, 1891)

The progress of membrane separation techniques was very slow in the early stage Nevertheless, many fundamental scientific works and contributions related to gas separation membranes were carried out in the twentieth century, as summarized in Table 1.3 (Kesting and Fritzsche, 1993) Partucularly, H.A Daynes developed the time lag method from nonsteady-state transport behavior of gases via a membrane to determine diffusion coefficient (Dayness, 1920)

Table 1.3 Scientific Developments of Membrane Gas Transport (Kesting and

Fritzsche, 1993)

Graham (1829) First recorded observation

Mitchell (1931) Gas permeation through natural rubbers

Fick (1855) Law of mass diffusion

von Wroblewski (1879) Permeability coefficient product of diffusion and

absorption coefficient Kayser (1891) Demonstration of validity of Henry’s Law for the

absorption of carbon dioxide in rubber Lord Rayleigh (1900) Determination of relative permeabilities of oxygen,

nitrogen and argon in rubber Knudsen (1908) Knudsen diffusion defined

Shakepear (1917-1920) Temperature dependence of gas permeability independent

of partial pressure difference across membranes Daynes (1920) Developed time lag method to determine diffusion and

solubility coefficient Barrer (1939-1943) Permeabilities and diffusivities followed Arrhenius

equation Matthes (1944) Combined Langmuir and Henry’s law sorption for water

in cellulose Meares (1954) Observed break in Arrhenius plots at glass transition

temperature and speculated about two modes of solution

in glassy polymers Barrer, Barrie and Slater

(1958)

Independently arrived at dual mode concept from sorption of hydrocarbon vapors in glassy ethyl cellulose Michaels, Vieth and

Barrie (1963)

Demonstrated and quantified dual mode sorption concept

Vieth and Sladek (1965) Model for diffusion in glassy polymers

Paul (1969) Effect of dual mode sorption on time lag and permeability

Petropoulos (1970) Proposed partial immobilization of sorption

Paul and Koros (1976) Defined effect of partial immobilizing sorption on

permeability and diffusion time lag

The above fundamental works provide the foundation in membrane processes, which conduce to the commercialization of membrane separation technology in industrial applications Following the first breakthrough of asymmetric phase-inverted membranes made of cellulose acetate for reverse osmosis by Loeb and Sourirajan in 1960 (Loeb and Sourirajan, 1960; 1962; 1964), membrane gas separation appeared to be a competitive separation tool for industry processes in the 1970’s The first commercially viable gas separation membrane, Prism® was produced at 1980 subsequent upon the method of repairing pinhole size defects in the thin layer of asymmetric membranes by Henis and Tripodi (Henis and Tripodi , 1980) As a consequence, the successful application of the first commercial gas separation membrane has accelerated the development of novel membrane materials

as it offer an attractive alternative for specific separation applications Figure 1.2 displays the important milestones in the history and scientific development of membrane gas separation technology (Baker, 2002)

Figure 1.2 Milestones in the development of membrane gas separations (Baker, 2002)

1.1.2 Advantages of Membrane Gas Separation

Today, a large scale membrane gas separation system has found acceptance

in many industrial sectors Membrane technology compares favorably with other conventional separation techniques due to its multidisciplinary character, which is often faster, more capital and energy efficient The specific features and inherent advantages of membrane separation process can be recapitulated as follows:

1 Simplicity of operation and installation (Paul and Yampol’skii, 1994)

2 Lower capital outlay and large reduction in power (electricity and fuel, etc) consumption No additional utilities/additives are required for membrane systems unless a compressor is needed (Berry, 1981; Mulder, 1996)

3 Economic viability even at high system-capacity Membrane processes are flexible, where the modules can be simply arranged in stages to accommodate higher capacity and scaled to small sizes (Berry, 1981; Paul and Yampol’skii, 1994)

4 Membrane devices and systems are always compact in size and modulus, which generally are space and weight efficient (Spillman and sherwin, 1990)

5 Membrane processes can be carried out under mild conditions, for example, air separation able to be operated at atmosphere pressure and room temperature instead of a cryogenic condition in distillation of air (Berry, 1981; Mulder 1996)

6 Membrane separation can be carried out continuously (Mulder, 1996)

7 Membranes can be “tailor-made” to a certain extent, thus their separation properties are viable and can be adjusted to a specific separation task (Strathmann, 1981; Mulder 1996)

8 Membrane processes can easily combined with other separation processes for effective hybrid processing (Mulder, 1996)

Membrane gas separation process becomes an emerging technology on industrial scale in the late seventies when Prism® was introduced in 1978 However, the utilization of membrane technology in gas separation has rapidly expanded and

observed the broad usage/interest in industrial application Membrane gas separation impacts the separation business with US$250 million a year The multitude applications of gas separation membranes are listed in Table 1.4 The major applications for gas separation membranes are discussed

Table 1.4 Industrial Applications of Gas Separation Membranes (Spillman 1989,

Noble and Stern, 1995)

Gas separation Application

O2/N2 Oxygen enrichment, nitrogen (Inert gas) generation

H2/Hydrocarbons Refinery hydrogen recovery

H2/CO Syngas ratio adjustment

H2/N2 Ammonia purge gas

CO2/Hydrocarbons Acid gas treatment enhanced oil recovery, landfill gas

upgrading

H2S/Hydrocarbons Sour gas treating

H2O/Hydrocarbons Natural gas dehydration

He/Hydrocarbons Helium separations

Hydrocarbons/Air Pollution control, hydrocarbon recovery

worldwide Membrane-based gas separation seems to be extremely attractive for the enrichment of nitrogen from air The nitrogen purity up to 95%, which is acceptable

in many industrial applications, can be produced economically through membrane separation The market share of membrane technology in producing nitrogen is growing and currently producing 30% of total nitrogen However, oxygen-enriched air generated by membrane separation yet to achieve the significant value in the separation market Ultra-pure oxygen is usually needed in many commercial and industrial applications Cryogenic distillation (99.999%) and vacuum swing adsorption (95%) dominated the current gaseous oxygen market (Puri, 1996) The current membranes process can only achieve a maximum purity of 45-50% (Koros and Mahajan, 2000) Ideally, the new membrane materials with desire permeability (~250 barrers) and the oxygen separation factor of 4-6 are needed to increase the practicability of membrane technology for industrial oxygen separation (Puri, 1996)

2 Hydrogen Separation

Hydrogen separation is the first large-scale commercial application of membrane gas separation process It has accounted about 25% of membrane technology in separating hydrogen from nitrogen, methane, coal, adjusting the ratio of H2 and CO in syn gas, as well as the hydrogen recovery in ammonia synthesis and a number of refinery operations (Crull, 1998; Zolandz and Fleming, 1992) Hydrogen separation has been effectively performed through polymeric membranes separation, because of the extremely high diffusion coefficient of hydrogen relative to other gas molecules (Koros, 1991) Nevertheless, the drawback of poor reliability, especially fouling and plasticization problems of polymeric membranes have inhibited the application of membranes separation in refineries (Baker, 2002)