THE STUDY OF 6FDA-POLYIMIDE GAS SEPARATION MEMBRANES CAO CHUN NATIONAL UNIVERSITY OF SINGAPORE 2003 THE STUDY OF 6FDA-POLYIMIDE GAS SEPARATION MEMBRANES CAO CHUN (M.Eng,CAS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY OF ENGINEERING DEPARTMENT OF CHEMICAL & ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2003 ACKNOWLEGEMENT First of all, I would like to extend my deepest appreciation and thanks to my supervisors, Professor Neal Chung Tai-Shung and Dr. Wang Rong for their invaluable and intellectual-stimulating guidance throughout my candidature, which has shaped my research philosophy. I am gratefully for the Research Scholarship of the National University of Singapore (NUS) that enables me to pursue my Ph. D degree. I am also indebted to the Institute of Materials Research and Engineering (IMRE) of Singapore for the equipment and the top-up financial support. Thanks are also due to my fellow students and the researchers in our group, Mr. C. Zhou, Dr. Y. M. Cao, Mr. D. F. Li, Dr. J. Z. Ren, Dr. Y. Liu, Dr. S. L. Liu, Ms. M. L. Chng, Ms. W. W. Loh for all the handy helps, technical supports, invaluable discussion and suggestions to my work. I also wish to take this opportunity to thank Dr. K. P. Pramoda, Dr. W. H. Lin, Dr. S. X. Cheng, Dr. V. Rohitkumar, Dr. K. X. Ma and Dr. D. L. Wang for the various assistances. Special thanks are due to Mr. K. P. Ng in the Department of Chemical and Environmental Engineering at NUS for the help in fabrication and machinery. i Last but not least, I am most grateful to my wife, Wang Lu, and my family members, Parent, Sister, Parent-in-law and Brother-in-law for their love, encouragement and support. This thesis would not have existed without them. ii TABLE OF CONTENTS Page ACKNOWLEDGEMENT i TABLE OF CONTENTS iii SUMMARY ix NOMENCLATURE xii LIST OF FIGURES xvi LIST OF TABLES xx CHAPTER INTRODUCTION 1.1 General background of membrane and membrane process for gas separation 1.1.1 History background 1.1.2 Classification of membranes and membrane processes for gas separation 1.1.3 Application of membrane-based gas separation 12 1.2 Membrane material selection 17 1.3 Membrane formation and characterization 26 1.4 Module fabrication and system design 32 1.5 Goals and organization of this research 34 CHAPTER THEORETICAL BACKGROUND 37 2.1 37 Gas transport mechanism in membrane 2.1.1 General principle 37 2.1.2 Gas transport mechanism in glassy polymer membrane 39 iii 2.1.2.1 Sorption in glassy polymers 39 2.1.2.2 Diffusion in glassy polymers 40 2.1.2.2.1 Diffusion coefficients in the Henry and Langmuir modes (DD and DH ) 40 2.1.2.2.2 Average diffusion coefficient Davg 41 2.1.2.2.3 Effective diffusion coefficient Deff 42 2.1.2.2.4 Diffusion coefficients derived from the time-lag method (Dapp and DD,t ) 44 2.2 45 Pressure and temperature dependences of gas performances in glassy polymeric membrane 2.3 Characterization of gas performance for hollow fiber membranes 48 CHAPTER EXPERIMENTS 56 3.1 Characterization on diffusion coefficients in 6FDA-6FpDA polyimide dense 56 membranes 3.1.1 Materials 56 3.1.2 Preparation of dense membranes 57 3.1.3 Measurements of pure gas permeation 58 3.1.4 Measurements of gas equilibrium sorption 62 3.2 64 The pressure and temperature dependences of gas transport properties of 6FDA-2, 6-DAT polyimide dense membranes 3.2.1 Material 64 3.2.2 Preparation of dense membranes 65 3.2.3 Measurements of pure gas permeation 65 3.2.4 Measurements of gas equilibrium sorption 66 3.3 66 Formation of high-performance 6FDA-2, 6-DAT asymmetric composite iv hollow fiber membranes for CO2/CH4 separation 3.3.1 Dope preparation and viscosity measurements 66 3.3.2 Spinning process and post-treatment 68 3.3.3 Scanning electron microscope (SEM) 69 3.3.4 Membrane module fabrication and silicone rubber coating 69 3.3.5 Evaluation of separation performance 71 3. The chemical cross-linking modification of 6FDA-2,6-DAT hollow fiber 73 membranes for natural gas separation 3.4.1 Fabrication of 6FDA-2, 6-DAT asymmetric polyimide hollow fibers 73 3.4.2 Chemical cross-linking modification and coating procedure 75 3.4.3 Characterization and evaluation of cross-linked 6FDA-2, 6-DAT hollow 75 fiber membranes CHAPTER The characterization of various diffusion coefficients 77 of 6FDA–6FpDA polyimide membranes 4.1 Introduction 77 4.2 Result and discussion 79 4.2.1 Sorption isotherms of 6FDA–6FpDA polyimide 79 4.2.2 Permeability of 6FDA–6FpDA dense membranes 82 4.2.3 Various diffusion coefficients 84 4.2.3.1 Diffusion coefficients in the Henry and Langmuir modes (DD and DH ) 84 4.2.3.2 Average diffusion coefficient Davg and effective diffusion coefficient Deff 85 4.2.3.3 Diffusion coefficients derived from the time-lag method (Dapp and DD,t ) 85 4.2.3.4 Pressure dependence of various diffusion coefficients 91 v 4.3. Conclusions CHAPTER Pressure and temperature dependences of gas 93 94 transport properties of 6FDA-2, 6-DAT polyimide dense membranes 5.1 Introduction 94 5.2 Results and discussion 96 5.2.1 Sorption behavior of 6FDA-2, 6- DAT dense membranes 96 5.2.2 Diffusion behavior of 6FDA-2, 6- DAT dense membranes 105 5.2.3 Permeation behavior of 6FDA-2, 6- DAT polyimide 112 5.2.4 Plasticization study of 6FDA-2, 6-DAT dense membranes 115 5.3 Conclusions 116 CHAPTER Formation of high-performance 6FDA-2, 6-DAT 118 asymmetric composite hollow fiber membranes for CO2/CH4 separation 6.1 Introduction 118 6.2 Results and discussion 120 6.2.1 Separation performance of 6FDA-2, 6-DAT asymmetric composite hollow 120 fibers 6.2.2 Aging study of 6FDA-2, 6-DAT asymmetric composite hollow fiber 126 membranes vi 6.2.3 Morphology of 6FDA-2, 6-DAT asymmetric hollow fiber membranes 129 6.3 131 Conclusions CHAPTER Chemical cross-linking modification of 6FDA-2, 6- 132 DAT hollow fiber membranes for natural gas separation 7.1 Introduction 132 7.2 Results and discussion 135 7.2.1 Characterization and mechanism of the chemical cross-linking reaction using 135 FTIR 7.2.2 Effects of p-xylenediamine induced cross-linking modification on gas 141 separation performance 7.2.3 Resistant improvement to plasticization of cross-linked 6FDA-2, 6-DAT 146 hollow fibers 7.2.4 Effects of m-xylenediamine induced cross-linking modification on 6FDA-2, 148 6-DAT hollow fibers 7.3 Conclusion 149 CHAPTER CONCLUSIONS 150 8.1 The characterization of various diffusion coefficients of 6FDA–6FpDA 150 polyimide membranes 8.2 Pressure and temperature dependences of gas transport properties of 6FDA-2, 151 6-DAT polyimide dense membranes vii 8.3 Formation of high-performance 6FDA-2, 6-DAT asymmetric composite 152 hollow fiber membranes for CO2 / CH4 separation 8.4 Chemical cross-linking modification of 6FDA-2, 6-DAT hollow fiber 153 membranes for natural gas separation REFERENCES 155 APPENDICE A 176 viii Hachisuka, H., T. 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Phys., 37, pp. 12511265, 1999b. 175 APPENDIX A Derivations of the average diffusion coefficient and the effective diffusion coefficient based on the definition of permeability (Wang et al, 2002a) The permeability (P) of a membrane for a given gas is defined as the flux (N) normalized for the pressure difference across the membrane and membrane thickness (l). One can derive the following equation for the permeability without any assumption: P= N N = ∆p l (p − p1 ) l (A.1) where p2 and p1 are the pressures at the upstream and downstream of the membrane, respectively. The gas transport through the membrane obeys Fick’s law as N = − D(C) dC dx (A.2) where D(C) is a local concentration-dependent diffusion coefficient of a penetrant at any arbitrary point between the membrane and dC/dx is the local concentration gradient at the same point in the membrane. By combining equations (A.1) and (A.2) and integrating over the membrane thickness with a negligible down stream pressure p1, one can obtain the following equation: P= p2 ∫ C2 (A.3) D(C) dC . Equation (A.3) may be rewritten as P = = D ∫ C2 avg D(C)dC C2 C2 p2 (A.4) S 176 where Davg is defined as ∫ C2 D(C)dC C , the solubility coefficient S is equal to C2/p2, as the downstream pressure is negligible. The differentiation of equation (A.3) with respect to the concentration at any arbitrary point within the membrane yields:   C2 d  C2   ∫ D(C) dC + D(C) dC    p dC  ∫0  p2  dp C d  C2 dC D(C) dC + = −  ∫0 D(C) dC  ∫  dC p dC p dC  dP d = dC dC dp Pp = − p dC = − dC D(C ) + p2 dC (A.5) P dp D(C ) dC + p dC p2 dC At the membrane surface facing the upstream pressure, i.e., C=C2, the above equation becomes: dP dC C2 =− P dp p dC C2 + D(C2 ) p2 (A.6) The above equation can be rearranged as dp dP C + p2 dC dC dP dp = (P + p ) ( ) dp p dC p D(C2 ) = P C2 (A.7) 177 [...]... with the dual-mode sorption model, with the magnitudes of dual-mode sorption parameters kD and b of these four gases following the order of N2 < O2 < CH4 < CO2 Gas solubility coefficients decrease with either increasing pressure or increasing temperature At the same time, the absolute values of the heats of sorption for O2, N2 and CH4 decrease with increasing pressure in the whole pressure range with the. .. dependence of solubility coefficient for 6FDA- 2, 6-DAT 103 polyimide at 35 °C xvii Figure 5.4 Permeation isotherms of N2 for 6FDA- 2, 6-DAT polyimide at 30, 107 35, 40, 45 and 50 °C Figure 5.5 Permeation isotherms of O2 for 6FDA- 2, 6-DAT polyimide at 30, 108 35, 40, 45 and 50 °C Figure 5.6 Permeation isotherms of CH4 for 6FDA- 2, 6-DAT polyimide at 30, 109 35, 40, 45 and 50 °C Figure 5.7 Permeation isotherms of. .. Schematic representation of transport mechanisms of membranebased gas separation (Koros and Fleming, 1993) The first type of separation, based on Knudsen diffusion, appears in such membranes containing pores in the barrier layer, whose diameters are smaller than the gas mean free path (the average traveling distance of a molecule in the gas phase between collisions) Knudsen-flow separation membranes are not... obtained the oxygen-enriched air with oxygen concentration of 46.6% by permeation through nonporous natural rubber films, but also proposed a series of gas transport theories in membrane-based gas separation, which includes: (1) the gas permeation through rubber membranes abides by the “solution-diffusion” mechanism, i.e the gas molecules in the upstream gas side (high-pressure side) first sorb into the. .. of the apparatus for gas permeation 71 measurements Figure 3.9 Apparatus of mixed gas permeation tests 72 Figure 3.10 SEM pictures of 6FDA- 2, 6-DAT / NMP hollow fiber membranes 74 Figure 4.1 Sorption isotherm for 6FDA- 6FpDA polyimide at 35oC 80 Figure 4.2 Permeability vs Pressure for 6FDA- 6FpDA polyimide at 35 oC 83 Figure 4.3 The pressure dependence of diffusion coefficients for O2 87 Figure 4.4 The. .. diluted situation, the difference among Deff, Davg and Dapp caused by the concentration dependence of diffusion coefficients will vanish, whereas the value ix of Deff becomes closer to the value of DD or DD,t when the upstream pressure is high enough Based on a study of the pressure and temperature dependences of 6FDA- 2, 6-DAT dense films, we built a database whereby: 1) the sorption isotherms are fairly... from the characterization of the intrinsic properties for dense film to the optimization of a spinning system and finally the chemical cross-linking modification on the resultant hollow fibers to withstand the plasticization of CO2 for CO2/CH4 separation, has been presented in this thesis The work on various diffusion coefficients, which is based on permeation and sorption experiments of 6FDA- 6FpDA polyimide. .. penetrant-scale transient gaps and achieve the penetrants diffusion from the feed stream to permeate stream Therefore, the “solution-diffusion” mechanism consists of three steps, that is, the gas molecules in the upstream gas side (high-pressure side) first sorb into the membrane surface, then diffuse across the membrane, finally desorb from the membrane surface on the downstream gas side (low-pressure side) (2)... general, there are five types of membranes for gas separation: dense film membranes (symmetric membranes) , asymmetric membranes, asymmetric composite membranes, microporous composite membranes and matrix composite membranes, as shown in Figure 1.2 (Chung, 1996) 7 Figure 1.2 Different type of membranes (Chung, 1996) A symmetric dense membrane composed of a relatively uniform morphology across the membrane... Fleming, 1993) Separation is achieved as a consequence of the difference in the relative transport rates of different penetrating gas molecules, i.e components that permeate faster will be enriched in the permeate stream, while the other components will become concentrated in the retentive stream 1 1.1 General background of membrane and membrane process for gas separation 1.1.1 History background The earliest . THE STUDY OF 6FDA- POLYIMIDE GAS SEPARATION MEMBRANES CAO CHUN NATIONAL UNIVERSITY OF SINGAPORE 2003 THE STUDY OF 6FDA- POLYIMIDE GAS SEPARATION MEMBRANES. Measurements of pure gas permeation 58 3.1.4 Measurements of gas equilibrium sorption 62 3.2 The pressure and temperature dependences of gas transport properties of 6FDA- 2, 6-DAT polyimide dense membranes. 8.1 The characterization of various diffusion coefficients of 6FDA 6FpDA polyimide membranes 150 8.2 Pressure and temperature dependences of gas transport properties of 6FDA- 2, 6-DAT polyimide