Dual layer asymmetric hollow fiber membranes for gas separation

DUAL-LAYER ASYMMETRIC HOLLOW FIBER MEMBRANES FOR GAS SEPARATION LI DONGFEI NATIONAL UNIVERSITY OF SINGAPORE 2004 DUAL-LAYER ASYMMETRIC HOLLOW FIBER MEMBRANES FOR GAS SEPARATION LI DONGFEI (B. Eng., Dalian University of Technology) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL & BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2004 ACKNOWLEDGEMENT As a milestone, the thesis is by far the most significant achievement in my life. It is not only the result of the five years of my research work but also the fruit concentrated the painstaking efforts of many people who supported me and had faith in me in the past. It could not even be dreamed without the elaborate guidance from Professor Chung Tai-Shung Neal who was my main supervisor in National University of Singapore. I have being greatly benefited from both his deep insight and devoted spirit in science. The thoughts he has offered have enriched my thesis a lot. The things I have learned from him are never just only the sense of research but the mission for ‘never give-up’ and many others. Apart from science, I owe him innumerable gratitude for pushing me closer to the God. It makes me to live a peaceful life. Gratitude also goes to my secondary supervisor Dr. Wang Rong from the Nanyang Technological University for her supervision. Besides of being an excellent supervisor, Dr. Wang Rong is also as close as a generous friend to me. I am glad that I have come to get know her in my life. I am greatly indebted to my former supervisor, Dr. Li Kang who moves to the Imperial College now, for encouraging me to pursue academic career. His recognition is definitely the root for the success I achieved today. It is impossible to forget every single helping hand hid behind my success. I would like to express my gratitude to Dr. Liu Ye and Dr. Ren Jizhong for their valuable contribution, without that my thesis would never be so fruitful. i I sincerely thank Professor D. R. Paul, the member of American Science Academy. He provided me some useful references wrote by the earlier pioneers. Needless to say, that I am grateful to all of my colleagues at IBM Singapore Pte Ltd, Institute of Materials Research & Engineering, Institute of Environmental Science & Engineering, and Department of Chemical & Biomolecular Engineering (NUS) for their support. Especially I am indebted to Mr. S. C. Liang, Dr. Lin Huihui, Mr. Yao Yizhao, Dr. Chen Sixue, Dr. Ma Kuixiang, Dr Cao Yiming, Dr. Cao Chun, Dr Tong Yuejin, Dr. Liu Songlin, Mr. Liang Tee David, Mr. K. P. Ng, Ms Chng Meilin, Ms Low Weiwei, and many others. I would like to thank my wife Li Lintian and my daughter Jessie for their understanding and love during the past few years. The support and encouragement from my family were in the end what made my thesis possible. Li Dongfei March 23, 2004 Singapore ii TABLE OF CONTENTS ACKNOWLEDGEMENT i TABLE OF CONTENTS iii SUMMARY xi LIST OF TABLES xii LIST OF FIGURES xiv LIST OF SYMBOLS xxiv CHAPTER INTRODUCTION 1.1 Definition of a separation membrane 1.2 History of separation membranes 1.3 Classification 1.3.1 Membrane classification 1.3.2 Membrane processes classification 1.3.2.1 Pressure-driven membrane processes 1.3.2.2 Concentration-driven membrane processes 1.3.2.3 Thermally driven membrane processes 1.3.2.4 Electrically driven membrane processes 1.4 Membrane market 10 1.5 Applications of polymeric membranes in the field of gas separations 11 1.5.1 Air separation 13 1.5.2 Hydrogen recovery 14 1.5.3 Natural gas separation 15 1.5.4 Vapor / gas and vapor/vapor separation 17 iii 1.6 Objective and scope 17 1.7 Organization of the thesis 19 1.8 Significance of the thesis 22 CHAPTER LITERATURE VIEW 24 2.1 Membrane materials 24 2.1.1 Experimental approach for membrane material selection 26 2.1.2 Performance prediction based on molecular structure 27 2.2 Formation of asymmetric polymeric membranes for gas separation 31 2.2.1 Asymmetric membranes 32 2.2.2 Phase inversion process 32 2.2.2.1 Thermally induced phase inversion (TIPS) 33 2.2.2.2 Dry process phase inversion 38 2.2.2.3 Wet process phase separation and macrovoid formation 40 2.2.2.4 The polymer-assisted phase-inversion (PAPI) process 45 2.2.3 Composite membranes 45 2.2.4 Membrane modification 48 2.3 Applications of coextrusion approach in the preparation of asymmetric membranes 50 2.3.1 Coextrusion in dual-bath approach 52 2.3.2 Coextrusion in melt spinning process 58 2.3.3 Coextrusion in preparation of ceramic composite hollow fiber membranes 59 2.3.4 Annular structure formation by coextrusion approach 60 2.3.5 Flat composite membranes by coextrusion / co-casting 60 iv 2.3.6 Coextrusion in fabrication of dual-layer asymmetric hollow fiber composite membranes 62 2.4 Hollow fiber membrane modules 68 2.4.1 Advantages 68 2.4.2 Literature survey on membrane modules 68 2.4.3 Aspects of hollow fiber module structures 69 2.4.4 Joule-Thomson effect 70 2.4.5 Packing fractions 72 2.4.6 Tubesheets 73 CHAPTER DEVELOPMENT OF SINGLE-LAYER ASYMMETRIC HOLLOW FIBER MEMBRANES FOR CO2/CH4 SEPARATION 75 3.1 Introduction 75 3.2 Experimental section 77 3.2.1 Material preparation 77 3.2.2 Solubility parameters 77 3.2.3 Fabrication of asymmetric polyimide hollow fiber membranes 78 3.2.3.1 Measurement of spinning solution viscosity 78 3.2.3.2 Spinning procedures 78 3.2.4 Measurement of hollow fiber separation performance 79 3.2.5 Scanning electron microscopy (SEM) 80 3.2.6 Apparent skin layer thickness 80 3.3 Results and discussion 81 3.3.1 Preparation of membrane solution and its rheological characteristics 81 3.3.2 Effect of shear rate on the performance of hollow fiber membranes 84 v 3.3.3 Effect of take-up speed on the performance of hollow fiber membranes 91 3.3.4 Conclusions 93 CHAPTER SUPPRESSION OF CO2-INDUCED PLASTICIZATION IN POLYIMIDE SINGLE-LAYER ASYMMETRIC HOLLOW FIBER MEMBRANES 95 4.1 Introduction 95 4.2 Experimental section 97 4.2.1 Material preparation 97 4.2.2 Fabrication of asymmetric polyimide hollow fiber membranes 97 4.2.3 Measurement of hollow fiber separation performance 98 4.2.4 Scanning electron microscopy (SEM) 92 4.2.5 Thermogravimetric analysis (TGA) 98 4.2.6 Wide-angle X-ray diffraction (WAXD) 98 4.2.7 99 4.2.8 Apparent dense selective-skin thickness 99 4.2.9 Heat treatment procedure 99 4.3 Results and discussion 99 4.3.1 The performance characteristics of 6FDA-2,6 DAT asymmetric hollow H-NMR spectroscopic analysis fiber membranes 4.3.2 99 The effects of membrane solution rheology and spinning process on CO2 induced plasticization 102 4.3.3 The effects of heat treatment on CO2 induced plasticization 103 4.4 Conclusions 108 vi CHAPTER PREPARATION OF PI/PES DUAL-LAYER ASYMMETRIC HOLLOW FIBER COMPOSITE MEMBRANES BY COEXTRUSION APPROACH 110 5.1 Introduction 110 5.2 Experimental section 114 5.2.1 Material selection 114 5.2.2 Spinning solution preparation 116 5.2.3 Dual-layer spinneret design and spinning devices 117 5.2.4 Post-treatment 121 5.2.5 Evaluation of separation performance 121 5.3 Results and discussion 122 5.3.1 Formation of delamination-free dual-layer membranes 122 5.3.2 Fabrication of 6FDA–durene–mPDA/PES dual-layer membranes for gas separation 5.3.3 5.3.4 126 Separation performance of 6FDA–durene–mPDA/PES dual-layer membranes 129 Conclusions 129 CHAPTER MORPHOLOGICAL ASPECTS AND STRUCTURE CONTROL OF DUAL-LAYER ASYMMETRIC HOLLOW FIBER MEMBRANES FORMED BY A SIMULTANEOUS CO-EXTRUSION APPROACH 131 6.1 Introduction 131 6.2 Experimental section 134 6.2.1 Membrane preparation 134 6.2.2 SEM specimen preparation 135 vii 6.2.3 Coating of SEM specimens and the optimal operation of SEM 138 6.3 Results and discussion 140 6.3.1 Integrity of dual-layer asymmetric hollow fiber membranes 140 6.3.2 The outer-layer morphology – the causes of macrovoid-free structure 141 6.3.3 Inner layer morphologies – the control of macrovoids growth 144 6.3.3.1 Influence of the inner membrane solution composition 145 6.3.3.2 Influence of the elongational draw ratio 150 6.3.3.3 Influence of the bore-fluid composition 152 6.3.3.4 Influence of the coagulation and spinneret temperatures 153 6.3.4 154 Interfacial morphology and delamination phenomena 6.3.4.1 Layers’ shrinkage vs. delamination 154 6.3.4.2 Membrane solutions’ chemistry vs. interfacial structure 157 6.3.4.3 Delamination vs. interfacial structure 159 6.4 160 Conclusions CHAPTER CHEMICAL CROSS-LINKING MODIFICATION OF POLYIMIDE/PES DUAL-LAYER HOLLOW-FIBER MEMBRANES FOR GAS SEPARATION 161 7.1 Introduction 161 7.2 Experimental section 162 7.2.1 Membrane materials 162 7.2.2 Fabrication of polyimide/PES dual-layer hollow fibers 163 7.2.3 Chemical cross-linking modification of dual-layer hollow fibers 164 7.2.4 Characterization 164 7.3 Results and discussion 165 viii Middleman, S. 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Filter and Method of Producing Same, US Patent, US1,421,341. 1922. 229 APPENDIX A THICKNESS DEPENDENCE OF MACROVOID EVOLUTION IN WET PHASE-INVERSION ASYMMETRIC MEMBRANES A.1 Introduction Macrovoids often appear in phase-inversion membranes, and their formation mechanisms have been studied and heavily debated in the past decades since Loeb and Sourirajan developed asymmetric cellulose acetate membranes for seawater desalination in the late 1950s.( Loeb and Sourirajan, 1963; Graig et al., 1962; Levich and Krylov, 1969; Matz, 1972; Strathmann et al., 1975, 1977; Cohen et al., 1979; Broens et al., 1980; Uragami et al., 1981; Altena and Smolders, 1982; McHugh and Yilmaz, 1985; Yilmaz and Mchugh, 1986; McDonogh et al., 1987; Reuvers et al., 1987a, 1987b; Yao et al., 1988; Smolders et al., 1992; Shojaie et al., 1994; McKelvey and Koros, 1996; Chung et al., 1997b, 1997c; Pekny et al., 2003). More importantly, the phenomenon of macrovoid formation is of great significance even for normal textile fibers made by wet spinning (Graig et al., 1962; Knudsen, 1963; Takahashi et al., 1964; Epstein and Rosenthal, 1966). Membrane scientists are divided on the origins of macrovoid formation. Several believe that it most likely originates from thermodynamics, so they have investigated the subject from the perspective of chemical potential gradients and phase diagrams with the aid of the Flory-Huggins theory (Cohen et al., 1979; Broens et al., 1980; Altena and Smolders, 1982; McHugh and Yilmaz,, 1985; Yilmaz and Mchugh, 1986; Reuvers et al., 1987a, 1987b; Smolders et al., 1992). Others consider that it more likely starts from local surface instabilities and material and stress imbalances that result in weak points and induce solvent 230 intrusion; thus, their studies emphasize convective flow or nonsolvent penetration, from the aspects of kinetics and dynamics (Graig et al., 1962; Levich and Krylov, 1969; Matz, 1972; Strathmann et al., 1975, 1977; Shojaie et al., 1994; Chung et al., 1997b, 1997c; Pekny et al., 2003). Other mechanisms have also been proposed, such as those based on Marangoni effects (Levich and Krylov, 1969; Shojaie et al., 1994) and osmosis pressure (McKelvey and Koros, 1996). Generally, two different structures, namely, spongelike and macrovoid (including fingerlike) configurations, have often been observed. Membranes that experience instantaneous liquid-liquid demixing tend to exhibit macrovoids, whereas membranes that experience delayed demixing tend to exhibit spongelike structures. Recently, Vogrin et al studied a ternary cellulose acetate/acetone/water system and reported that macrovoid formation is dependent on the membrane thickness (Vogrin et al., 2002). In their study, macrovoids appeared on membranes prepared from a 12.5 wt % casting solution at the thickness of 500 m, but not at thicknesses of 150 and 300 m. In this study, we report, for the first time, that a critical structure-transition thickness has been observed for the transition of membrane morphology from a spongelike to a macrovoid-type structure. This critical structure-transition thickness exists for membranes prepared from polyethersulfone and polyimide. A.2 Experimental Section The membrane materials, polyethersulfone (PES, Radel A-300P, CAS# 25667-42-9) and BTDA-MDI/TDI co-polyimide of 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA)-4,4'-diphenylmethane diisocyanate (MDI, 80%)-2,4-toylene diisocyanate (TDI, 20%) copolymer (BTDA-MDI/TDI or P-84, CAS# 58698-66-1) 231 powder, were supplied by Amoco and Lenzing, respectively. N-Methyl-2-pyrrolidone (NMP, >99.5%, CAS# 872-50-4), supplied by Merck, was used as a solvent. Water as a nonsolvent additive was produced by Milli-Q ultrapure water system. All reagents were used as received without further purification. Two membrane solutions were used to fabricate flat sheet membranes. One was a PES/NMP system, and the other was a BTDA-MDI/TDI (P84)/NMP system with the same polymer concentration of 20 wt % each. The membrane thickness varied from 0.5 to 50 m. The polymer materials were dried at 110 C in a vacuum oven for days. They were then dissolved in NMP solutions by mechanical stirring for days at room temperature. These prepared homogeneous casting solutions were degassed before use. The relative humidity in the casting environment was 70-80%. The flat sheet membranes were fabricated on a glass plate with a casting knife. After casting, the nascent membranes were immediately immersed into a water bath together with the glass plate. Later, the membranes were washed with water for days at room temperature. The cross-sectional morphologies of the prepared membranes were examined by scanning electron microscopy (SEM) using a JSM-6700F instrument. A.3 Results and discussion A.3.1 Effect of membrane thickness on membrane morphology Figure A-1 shows the evolution of the morphology of the PES flat membranes with thickness. The PES flat membranes have a loose nodular-like structure when the thickness is extremely low at about 0.57 m (Figure A-1A). The membrane evolves into a spongelike structure as the membrane thickness increases to 0.76 m (Figure A1B). However, with a further increase in membrane thickness, macrovoids gradually appear. When the membrane thickness is above m, a fingerlike structure is fully 232 0.76 µ 0.57 µ A (× 40,000 B (× 30,000) ) 2.4 µ 3.3 µ 1.9 µ C (× 10,000) D (× 10,000) E (× 10,000) Figure A-1. Effect of membrane thickness on PES membrane structures (membrane solution: PES/NMP; PES 20wt.%; casting temperature: 25°C; coagulant: water) developed (Figure A-1D, A-1E). Similar phenomena were observed for the BTDAMDI/TDI/NMP system, as shown in Figure A-2. The membrane has a spongelike structure when its thickness is about 8.1 m (Figure A-2A). When the thickness reaches 9.5 m, some macrovoids form, as illustrated in Figure A-2B1. Both the number and the size of the macrovoids increase with increasing membrane thicknesses, as shown in Figure A-2B2, A-2B3, and A-2C. A fingerlike structure is fully developed at the thickness of 22 m (Figure A-2D). Clearly, the structure of membranes prepared by the phase-inversion process shows a strong dependency on the membrane thickness. A.3.2 Critical structure-transition thickness Membranes with various thicknesses were cast, and their cross-sectional morphologies were examined by SEM. The thicknesses of both spongelike portions and entire membranes were measured and are summarized in Figures A-3 and A-4. Three regions 233 could be identified along the abscissa corresponding to different membrane structures. B 8.1 µ m 12.4 µ B 11 µ m B A (× 5,000) 9.5 µ m B (× 2,000) 16 µ m 22 µ m C (× 2,000) D (× 2,000) Figure A-2. Effect of membrane thickness on BTDA-MDI/TDI copolyimide membrane structures (membrane solution, BTDA-MDI/TDI/NMP; BTDA-MDI/TDI, 20 wt %; casting temperature, 25 C; coagulant: water). In region I, the thickness of the spongelike portions is the same as the thickness of the entire membranes. It means that the cross sections of the membranes have a fully spongelike structure. The membrane morphology transitions from a spongelike to a fingerlike structure with some degrees of fluctuation in sponge thickness in region II. In region III, the membranes are of mainly fingerlike structure with an almost constant thickness of spongelike portion, which is independent of overall membrane thickness. On the basis of the above analysis, it is reasonable to deduce that there exists a critical structure-transition thickness, Lc, that reflects the transition of membrane morphology 234 Figure A-3. Effect of membrane thickness on the thickness of spongelike portion of PES membranes (dope, PES/NMP; PES, 20 wt %; casting temperature, 25 °C; coagulant, water). Figure A-4. Effect of membrane thickness on the thickness of spongelike portion of BTDA-TDI/MDI membranes (dope, BTDA-MDI/TDI/NMP; BTDA-MDI/TDI, 20 wt %; casting temperature, 25 coagulant, water). C; from a spongelike to a fingerlike structure during the formation of asymmetric flat membranes. Lc is about 1.5±0.4 µm for the 20% PES/NMP membrane solution and about 11±2µm for the 20% BTDA-MDI/TDI/NMP membrane solution. 235 The critical structure-transition thickness Lc is different for different membrane solutions possibly because the solutions have different viscosities, surface energies, phase diagrams, and many other characteristics. Future studies will be focused on the relationship between Lc and the physical chemistry of the membrane solution properties. A.4 Conclusion The thickness dependence of macrovoid evolution during the phase-inversion process of asymmetric flat membranes was studied using 20% PES/NMP and 20% BTDAMDI/TDI/NMP membrane solutions. It was found that the membrane morphology strongly depends on the membrane thickness. A critical structure-transition thickness, Lc, was observed for the two systems, indicating the transition of the membrane morphology from a spongelike to a fingerlike structure with an increase in membrane thickness. 236 [...]... Spinning conditions of dual- layer hollow fiber membranes 120 Table 5-3 Properties of 6FDA durene mPDA dense films 122 xii Table 5-4 Properties of 6FDA durene-mPDA/PES dual layer asymmetric hollow fiber membranes Table 6-1 129 The ID and compositions of the inner and outer layer membrane solutions 135 Table 6-2 Spinning conditions of dual- layer asymmetric hollow fibers 135 Table 7-1 Gas permeance of the... polyimide/PES dual- layer hollow fibers 165 7.3.2 FTIR characterization 165 7.3.3 Effects of cross-linking modification on gas separation properties 170 7.4 Conclusions 175 CHAPTER 8 FABRICATION OF LAB-SCALE HOLLOW FIBER MEMBRANE MODULES WITH HIGH PACKING DENSITY 177 8.1 Introduction 177 8.2 Development of hollow fiber modules for gas separation 178 8.3 Limitations of small modules for performance prediction... sponge-like to a macrovoid structure The morphologies of the interfaces of dual- layer hollow fibers were revealed The uneven shrinkage effect was applied to explain the delamination between inner and outer layers Defect-free, delamination-free, dual- layer hollow fiber asymmetric membranes were successfully demonstrated for gas separation The membrane plasticization caused by CO2 was also studied and... have studied the fabrication of dual- layer asymmetric hollow fiber composite membranes for gas separation The dual- layer composite membranes were prepared by simultaneously extruding a bore fluid and two polymer solutions from a specially designed triple-orifice spinneret This technique offers a platform to construct a novel composite membrane consisted of a high-performance polymer with excellent... 5-12 Cross sections of 6FDA durene-mPDA/PES dual- layer 126 asymmetric hollow fibers, A: near the edge of the outer skin, B: interface, C: inner edge of the inner layer Figure 5-13 127 Surfaces of 6FDA durene- mPDA / PES dual- layer asymmetric hollow fiber, A: outer skin of the outer layer, B: outer skin of the inner layer, C: inner lumen skin of the inner layer Figure 5-14 127 The morphology of the... spinneret design, the research work includes preparation of singlelayer asymmetric hollow fibers, optimization of dual- layer asymmetric hollow fiber spinning, study of macrovoid formation, investigation of delamination phenomenon, as well as fabrication of lab-scale hollow fiber modules Extensive work was introduced to explore the membrane formation induced by phase inversion The concept of critical membrane-structure... the spinneret for batch 1 Table 3-4 The spinning parameters, performance of hollow fiber membranes at different shear rates in the spinneret for batch 2 Table 3-5 85 85 The spinning parameters, performance of hollow fiber membranes at different take-up speeds (batch 1, 6FDA-2,6 DAT/NMP solution) Table 4-1 The performance of hollow fiber membranes as a function of the heat-treatment temperature Table... polyimide/PES dual- layer hollow fibres Modules 1a, 2c, 3c, 4c, and 5c are the same samples as listed in Table 7-1 173 Figure 8-1 Shell-side fed hollow fiber module 179 Figure 8-2 Bore-side fed hollow fiber module 180 Figure 8-3 Shell-side fed hollow fiber module with a central distribution tube 181 Figure 8-4 Hollow fiber fabric bundle with a central distribution tube Figure 8-5 Helical wound hollow fiber. .. high permeation flux for commercial scale gas separation A milestone that facilitated the commercialization of gas separation membranes is the discovery of integrally skinned asymmetric membranes for reverse osmosis process by Loeb and Sourirajan in 1960 (Loeb and Sourirajan, 1964; Matsuura, 2001) Forming by phase inversion, the asymmetric membranes consist of a very thin dense top layer supported by... demonstrated gas separation by permeation through nonporous membranes 3 but also showed that gas mixtures could be separated by permeation through microporous membranes He is therefore called the father of gas separation via membranes (Lonsdale, 1982) In 1909, Knudsen described the geometrical aspect of diffusion of gases through microporous membranes by relating the mean free path of gas molecules . DUAL- LAYER ASYMMETRIC HOLLOW FIBER MEMBRANES FOR GAS SEPARATION LI DONGFEI NATIONAL UNIVERSITY OF SINGAPORE 2004 DUAL- LAYER ASYMMETRIC HOLLOW FIBER MEMBRANES FOR GAS. delamination-free dual- layer membranes 5.3.2 Fabrication of 6FDA–durene–mPDA/PES dual- layer membranes for gas separation 5.3.3 Separation performance of 6FDA–durene–mPDA/PES dual- layer membranes 5.3.4. POLYIMIDE/PES DUAL- LAYER HOLLOW- FIBER MEMBRANES FOR GAS SEPARATION 7.1 Introduction 7.2 Experimental section 7.2.1 Membrane materials 7.2.2 Fabrication of polyimide/PES dual- layer hollow fibers