Fundamentals of hollow fiber formation for gas separation

FUNDAMENTALS OF HOLLOW FIBER FORMATION FOR GAS SEPARATION PENG NA NATIONAL UNIVERSITY OF SINGAPORE 2009 FUNDAMENTALS OF HOLLOW FIBER FORMATION FOR GAS SEPARATION PENG NA (B. Eng, Dalian University of Technology, P. R. China) A THESIS SUBMITTED FOT THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 Acknowledgement I wish to take this opportunity to express my gratitude to all those who gave me the support to complete this thesis. I would like to express my heartful acknowledgement to my supervisor Prof. Chung Tai-Shung. Prof. Chung has taught me a lot of skills and given me a lot of valuable advices. He was always approachable and patient whenever I had questions and difficulties in my study. Besides the knowledge and skills, I also leant the dedication and diligence from him, which makes me become a qualified researcher. I would like to thank to National University of Singapore (NUS) and Department of Chemical and Biomolecular Engineering (ChBE) for providing the research scholarship and facilities for my PhD study. I would like to thank the Singapore National Research Foundation (NRF) for her support on the Competitive Research Programme for the project entitled "Molecular Engineering of Membrane Materials: Research and Technology for Energy Development of Hydrogen, Natural Gas and Syngas" with grant number of R-279-000-261-281, A-Star for funding this project with the grant numbers of and R-279-000-218-305,  UOP, Mitsui and Merck for the financial support via the grant number of WBS N-279-000-010-001, as well as Institute of Materials Research & Engineering (IMRE) for providing the high-quality instrument. I wish to acknowledge Prof. W. B. Krantz, Prof. M. R. Mackley and Prof. S. B. Chen for their valuable advices on membrane formation and rheological study. I would like to i    thank Dr. K. Y. Wang, Dr. Q. Yang, Dr. Y. Li, Dr. L. Shao for their kindly patience, help, and discussions during my study. I would also like to thank to my lovely colleagues, especially Dr. M. M. Teoh, Dr. N. Widjojo, Ms B. T. Low, and Ms M. L. Chng, for their valuable support and precious friendship. Finally, I have to express my most sincere appreciation to my family for their unfailing love! ii    TABLE OF CONTENTS ACKNOWLEDGEMENT………………………………………………………….…….i TABLE OF CONTENTS…………………………………………………………… .…iii SUMMARY…………………………………………………………………………… ix LIST OF TABLES…………………………………………………………………… .xii LIST OF FIGURES………………………………………………………………… .xiv Chapter Introduction and Literature Review on Hollow Fiber Formation for Gas Separation………… .……………………………….1 1.1 Formation of hollow fiber membranes by phase inversion………………………… .2 1.1.1 The effect of dope composition and coagulations………………………… 1.1.2 The effects of spinneret design on hollow fiber formation……………… 1.1.3 The effects of air-gap on hollow fiber formation……………………………9 1.1.4 Effect of take-up speed…………………………………………………….11 1.1.5 Post-treatment and additional coating…………………………………… .12 1.1.6 Effect of dope rheology……………………………………………………13 1.2 Membrane-based gas separation…………………………………………………… 16 1.2.1 Basic concepts and mechanism………….……………………………16 1.2.2 Terminology in gas transport………………………………………………19 1.2.3 History of membranes for gas separation………………………………….22 1.2.4 Application of gas separation membranes………………………………24 1.3 Formation of desirable membrane structure…………………………………………26 iii 1.3.1 The evolution of macrovoids in asymmetric polymeric membranes………26 1.3.2 The formation of defect-free and ultra-thin dense-selective layer for gas separation hollow fiber membranes………………………………………… .…31 1.4 Reference…………………………………………………………………………….35 Chapter Fundamental Theory on Phase Inversion in Membrane Formation…………………………………………………………………………… 45 2.1 Types of phase inversion process……………………………………………………46 2.1.1 Precipitation induced by solvent evaporation…………………………… .46 2.1.2 Vapor-induced precipitation……………………………………………….46 2.1.3 Precipitation by controlled evaporation……………………………………47 2.1.4 Thermal precipitation………………………………………………………47 2.1.5 Immersion precipitation……………………………………………………48 2.2 Thermodynamics and kinetics of phase inversion………………………………… .48 2.2.1 Nucleation and growth…………………………………………………… 50 2.2.2 Spinodal decomposition ……………………………………………………51 2.3 Kinetics of phase inversion………………………………………………………… 52 2.4 The limitation of Flory-Huggins theory in hollow fiber spinning………………… 54 2.5 Reference…………………………………………………………………………….58 Chapter Experimental………………………………………………………… .60 3.1 Polymer materials……………………………………………………………………60 3.2 Rheology measurement………………………………………………………………66 3.3 Hollow fiber spinning and post-treatment………………………………………… .67 3.4 Gas permeation test………………………………………………………………… 68 iv 3.4.1 Pure gas permeation test…………………………………………… … .68 3.4.2 Mixed gas permeation test…………………………………………………70 3.5 Other characterizations………………………………………………………………71 3.5.1 Scanning Electron Microscope (SEM) and Field Emission Scanning Electron Microscope (FESEM)………………………………………………… 71 3.5.2 X-ray Diffraction (XRD)………………………………………………… 71 3.5.3 AFM……………………………………………………………………… 71 3.5.4 Polarizing Optical Microscope PLM………………………………………72 3.5.5 TGA……………………………………………………………………… 72 3.5.6 FTIR……………………………………………………………………… 72 3.5.7 Particle size measurements……………………………………………… .73 3.6 Reference…………………………………………………………………………….74 Chapter Macrovoid Evolution and Critical Factors to Form Macrovoid-Free Hollow Fiber Membranes……………………………….… 77 4.1 Introduction………………………………………………………………………… 77 4.2 Experimental…………………………………………………………………………79 4.2.1 Spinning conditions……………………………………………………… 79 4.2.2Morphology study of the hollow fibers…………………………………….80 4.3 Results and discussion……………………………………………………………….81 4.3.1 Effects of polymer concentration………………………………………… 81 4.3.2 Effects of take-up speed ………………………………………… .……….86 4. 3.3 Effects of air gap distance…………………………………………………89 4.3.4 The observation of the critical acceleration of stretch……………………93 v 4. Conclusions……………………………………………………………………… 96 4.5 References………………………………………………………………………… 97 Chapter The Effects of Spinneret Dimension and Hollow Fiber Dimension on Gas Separation Performance of Ultra-thin Defect-free Torlon ® Hollow Fiber Membranes……………………….…… … . 103 5.1 Introduction…………………………………………………………………………103 5.2. Spinning conditions ……………………………………………………………….107 5.3 Results and discussions…………………………………………………………… 109 5.3.1 The effects of spinneret dimension and hollow fiber dimension on membrane morphology…………………………………………………………109 5.3.2 The effects of draw ratio and elongation stress on O2 /N separation performance…………………………………………………………………….112 5.3.3 The effects of spinneret dimension and hollow fiber dimension on O2/N2 selectivity……………………………………………………………………….114 5.3.4 Relationship between spinneret dimension and draw ratio to yield the maximum O2/N2 selectivity…………………………………………………….115 5.3.5 The effects of spinneret dimension and hollow fiber dimension on denseselective thickness………………………………………………………………118 5.3.6 The effects of air-gap distance on O2/N2 permselectivity… …………….121 5.4 Conclusions…………………………………………………………………………123 5.5 References …………………………………………………………………………125 vi Chapter The Rheology of Torlon® Solutions and Its Role in the Formation of Ultra-thin Defect-free Torlon® Hollow Fiber Membranes for Gas Separation…………………………………………………………………131 6. Introduction……………………………………………………………………… .131 6.2 Dope formulation and hollow fiber spinning……………………………………….134 6.3 Results and discussion…………………………………………………………… .135 6.3.1The rheology of Torlon® solutions………………………………… ……135 6.3.2 The effect of dope temperature on membrane morphology………………138 6.3.3 The effect of dope temperature on O2/N2 selectivity…………………….142 6.3.4 The role of Torlon® dope rheology on hollow fiber formation………… 144 6.3.5 The effect of draw ratio on dense layer formation at different dope temperatures…………………………………………………………………….147 6. Conclusions……………………………………………………………………… .148 6.5 References………………………………………………………………………… 150 Chapter The Role of Additives on Dope Rheology and Membrane Formation of Defect-free Torlon® Hollow Fibers for Gas Separation.155 7.1 Introduction…………………………………………………………………………155 7.2 Dope formulation, solubility parameters, and spinning conditions…………… .157 7.3 Results and discussion…………………………………………………………… .160 7.3.1 The effect of additives and temperature on dope rheology……………….160 7.3.2 The effect of additives on membrane morphology……………………….170 7.3.3 The effect of nonsolvent concentration and additive chemistry on gas separation performance of fibers spun at different temperatures……………….172 vii 7.4 Conclusions…………………………………………………………………………177 7.5 References ………………………………………………………………………… 179 Chapter Summary…………………………………………………… 185 Publication list…………………………………………………… 186 viii According to the phase diagram in Fig. 7.1, the dope containing 10 wt% methanol is relatively closer to the cloud point than that containing 10 wt% ethanol. However, it is surprised to observe that the former results in many macrovoids, while the latter leads to a macrovoid-free morphology. One reason can be attributed to the viscosity effect on the macrovoid formation. The addition of 10 wt% ethanol obviously increases the dope viscosity and enhances the polymer chain entanglement, so that the nonsolvent intrusion and convection, nonsolvent super-saturation, as well as the mutual diffusion of solvent and nonsolvent during the coagulation (which are critical factors for macrovoid formation) are retarded [31-32, 35-43]. As opposed to ethanol, since methanol possesses very low viscosity itself, it would dilute the dope viscosity and chain-chain interaction, and hence make the nascent hollow fiber vulnerable to the nonsolvent intrusion and promote the macrovoid formation. A similar rationale is applicable to explain the macrovoid formation on fibers spun from dopes containing water or THF. THF has a similarly low viscosity with methanol, so that it also dilutes the dope viscosity, enhances the mutual solvent and nonsolvent exchange, and results in more macrovoids. Another reason may be the effect of solubility parameter on the kinetics of the macrovoid formation. Since water/NMP (2.5/69.5 wt%) and ethanol/NMP (10/62 wt%) mixtures have good solubility parameters and hydrogen bonding to Torlon® polymers, the mutual diffusion between water/NMP or ethanol/NMP and external coagulant would be hindered during the phase inversion and eventually the macrovoids were suppressed [34]. To verify the above viewpoints, the convection, precipitation and solidification fronts of water into the Torlon® flat membrane (with a thickness about 167 µm) were video- 171 recorded under an Olympus BX50 polarizing optical microscope and illustrated in Fig. 7.6. The water intrusion front is very clear and obvious in the film containing 10% methanol, whereas the water intrusion front in the film with 10% ethanol is hardly observable. Fig. 7.6 Observation of water intrusion in Torlon® flat membranes under PLM at 1s and 5s. (the thickness of the polymer solution: 167 μm) 7.3.3 The effect of nonsolvent concentration and additive chemistry on gas separation performance of fibers spun at different temperatures Fig. 7.7 illustrates the effect of nonsolvent content on O2/N2 selectivity and O2 permeance. Hollow fibers spun from dopes with a smaller amount of additives show up-and-down patterns for O2/N2 selectivity and a trend of increasing O2 permeance as a function of 172 dope temperature. Similar performance patterns have been observed and discussed for the Torlon® hollow fibers without any additives in chapter 5. However, fibers spun from dopes with a larger amount of additives show increasing patterns for O2/N2 selectivity. Generally, a reasonably high spinning temperature is indispensable to produce defect-free hollow fiber membranes from a highly concentrated solution. Since the viscosity is high and/or hydrogen bonding is strong at low temperatures, it is difficult to completely orient the polymer chains. Only until the dope temperature is increased, under which the hydrogen bonding is weakened and the mobility of polymer chains is restored, the polymer chains can be effectively oriented. This is the essential criterion to result in hollow fibers with good gas pair selectivity. 173 Fig. 7.7 The effect of the additive amount on gas separation performance of Torlon® fibers (A) water; (B) THF; (C) Methanol; (D) Ethanol 174 The above statement can be supported by Fig. 7.7 (A1), where an increase in water amount in dope solution deteriorates the O2/N2 selectivity but increases the O2 permeance. In addition, as the water concentration increases, the dope temperature needed to fabricate defect-free hollow fibers also increases. For the dope containing 1% water, a spinning temperature of 50 oC would be enough for the formation of defect-free skin layer; however, for the dope containing 2.5% water, the high selectivity can only be regenerated when a spinning temperature of 85.7 oC is used. Clearly, if more water is added, a higher degree of hydrogen bonding would be induced, and hence a higher temperature is required to weaken the hydrogen bonding, free polymer chains for being oriented and thus regenerate gas permselectivity. Another reason that dopes containing higher amounts of water would result in fibers with overall low O2/N2 selectivity may arise from the fast stress relaxation rate of polymer chains as discussed in Section 7.3.1. Such a fast chain relaxation may destruct the shear-induced and elongation-induced molecular orientation, and finally deteriorate the gas pair selectivity. Fibers produced from dopes containing wt% water also follow the hypothesis that the higher the water content, the higher the spinning temperature to make the defect-free dense-selective layer. However, since the dope containing wt% water shows some instability during spinning, the rheological behavior of this dope during spinning may be different from the above discussions. Similar phenomena and principles can be observed and applied to Fig. 7.7 (C1) and (D1) for methanol and ethanol added dopes. Hollow fibers spun from a dope containing 5% methanol show a much lower O2 permeance than the control. This is partially due to the 175 fact that methanol has a higher vapor pressure than ethanol and water, a faster solidification would therefore occur at the outer skin of the nascent fiber because of the quick increase in local polymer concentration. Even though THF possesses a high vapor pressure, its addition does not vary the gas separation performance of hollow fibers significantly in terms of both O2/N2 selectivity and O2 permeance, as shown in Fig. 7.7 (B1). Except those samples spun at room temperature show a bit lower selectivity and those samples spun at 90ºC show slightly higher selectivity, they have comparable O2 permeance for all fibers spun at various dope temperatures. This interesting phenomenon may be attributed to the lack of hydrogen bonding contribution and the poor solubility of Torlon® in the spinning solutions containing THF as shown in Table 7.3. As a result, Torlon® polymer chains cannot be well dispersed and effectively oriented in the THF/NMP mixture. Even though some of the THF may evaporate during the spinning and lead to the local increase in polymer concentration, the poor solubility may create a relatively loose dense layer that compensates the thicker skin formation but maintain O2 permeance and selectivity similar to the control one. To confirm the reliability of the high O2/N2 selectivity of Torlon® fibers shown in Fig 7.7, some samples were tested in mixed gases using compressed air as the feed. Table 7.6 shows the both selectivity and permeance in mixed gas tests are lower than those in pure gas tests, and the decrease in fibers spun from the dope containing 1.5 wt% water is more pronounced. Since the hollow fiber modules were tested by pure gases first and then 176 tested by the air, one possible reason is that the additives may also have distinct effects on the sorption site (Henry and Langmuir modes) distribution and aging properties. We are studying this issue and future work will be published on this topic. Table 7.6 Comparison of the pure gas and mixed gas separation performance of Torlon® fibers Pure gas (200 psi) ID P(O2)/GPU P(N2)/GPU Air (200 psi) α(O2/N2) P(O2)/GPU P(N2)/GPU α (O2/N2) 5% Methanol 0.46 70ºC 0.06 9.97±0.80 0.45 0.05 9.00±0.45 1.5% Water 70ºC 0.35 8.29±0.16 1.25 0.17 7.12±0.30 2.81 7.4 Conclusions We have studied the effects of solution rheology on hollow fiber formation and revealed some of integrated sciences bridging polymer fundamentals (such as polymer cluster size, shear and elongational rheology, molecular orientation, and stress relaxation) and phase inversion processes (such as spinning conditions, coagulant chemistry, dynamic membrane formation, microstructure morphology, and separation performance). A series of defect-free ultra-thin Torlon® gas separation hollow fiber membranes have been fabricated with impressive performance. The following conclusions can be made: (1) Hydrogen bonding and solubility parameters play very important roles in rheological characteristics of Torlon® solutions. Since water and ethanol can form hydrogen bonds with Tolron®, the addition of water and ethanol could enhance polymer chain 177 entanglement and form big polymer clusters. Those kinds of microstructure may retard the nonsolvent intrusion and the mutual diffusion between the solvent and nonsolvent during phase inversion and facilitate the formation of macrovoid-free morphology. The addition of methanol and THF works in the opposite way because they possess quite different solubility parameters from Torlon® and low viscosities. Therefore, the macrovoid formation is promoted by the low dope viscosity, the lack of polymer chain entanglement, and high mutual diffusion during the phase inversion. (2) The elongational viscosity of Torlon® solutions, which is the most significant force in determining the molecular orientation in moderate-speed hollow fiber spinning, has been examined. Dopes containing water or ethanol show strain hardening because of the formed hydrogen bonding. Such kind of rheological character could resist the chain sliding from each other during the elongation induced deformation and hence maintain polymer chain orientation in a finer manner for gas separation with good selectivity. (3) The rate of stress relaxation and pressure release is another characteristic property of spinning dopes, which is influenced by both the hydrogen bonding and the compressibility of the additives. Dopes containing water show very fast rates of pressure relaxation at room temperature, which may create difficulties to maintain fine molecular orientation to produce fibers with good gas selectivity. (4) Hydrogen bonding, polymer cluster size, shear/elongational viscosity, stress relaxation and pressure release are independent but correlative rheological factors that determine the hollow fiber formationWith balanced dope chemistry and dope rheology, we can simply use water as additive instead of THF or other solvent mixture to produce desirable hollow fiber membranes in terms of good selectivity and permeance. 178 7.5 Reference [1] L. H. Sperling, Introduction to physical polymer science, Wiley, New York, NY, 2001. [2] C. Macosko, Rheology - Principles, Measurements, And Applications, Wiley, New York, NY, 1994 [3] F. T. Trouton, On the coefficient of viscous traction and its relation to that of viscosity, Proc. Roy. Soc. A, 77 (1996) 426. [4] G. Fano, Contribution to the study of thread-forming materials, Archivio di Fisiologio (1908) 365. [5] C. J. S. Petrie, One hundred years of extension flow, J. Non-Newtonian Fluid Mech. 137 (2006) 1. [6] R. K. Gupta and T. 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Pur. Technol. 63 (2008) 415. [43] X. Y. Fu, T. Sotani, H Matsuyama, Effect of membrane preparation method on the 183 outer surface roughness of cellulose acetate butyrate hollow fiber membrane, Desalination, 233 (2008) 10. 184 Chapter Summary The mechanism of hollow formation is very complicated because there are many parameters to be considered simultaneously. This dissertation aims at studying the fundamentals to produce hollow fiber membranes with a macrovoid-free morphology and an ultra-thin, defect-free dense-selective layer for gas separation by examining the effect of high take-up rate, spinneret dimension, as well as the rheology of polymer solution containing various additives at different temperatures. Some of the most significant findings of this dissertation include: 1) high-speed spinning (50 m/min) could facilitate the formation of macrovoid-free hollow fiber membranes from various polymer materials; 2) using the similar spinning conditions, spinneret dimension greatly affects the membrane structure and gas separation performance; and 3) the rheology of polymer dope solutions is a fundamental but very informative subject to disclose the hollow fiber formation mechanism. Elongational viscosity has been found to be more dominant than shear viscosity in determining the membrane structure, and polymer cluster size, hydrogen bonding also contribute to the membrane formation. However, it will be impressive if quantitative descriptions and correlations of the above parameters can be made. To achieve this, more fundamental works on solution rheology under different spinning conditions, advanced instrumental characterization of dope solution in various state and mathematical calculation of the phase inversion pathways would be proposed for future work. 185 Publication list [1] N. Peng, T. S. Chung, K. Y. Wang, Macrovoid evolution and critical factors to form macrovoid-free hollow fiber membranes, J. Membr. Sci., 318 (2008) 383. [2] N. Peng, T. S. Chung, The effects of spinneret dimension and hollow fiber dimension on gas separation performance of ultra-thin defect-free Torlon® hollow fiber membranes, J. Membr. Sci., 310 (2008) 455. [3] N. Peng, T. S. Chung, J. Y. Lai, The rheology of Torlon® solutions and its role in the formation of ultra-thin defect-free Torlon® hollow fiber membranes for gas separation, J. Membr. Sci., J. Membr. Sci., 326 (2009) 608. [4] Y. Wang, S. H. Goh, T. S. Chung, N. Peng, Polyamide-imide/polyetherimide duallayer hollow fiber membranes for pervaporation dehydration of C1–C4 alcohols, J. Membr. Sci., J. Membr. Sci., 326 (2009) 222. [5] N. Peng, T. S. Chung, Y. L. Kwok, The role of additives on dope rheology and membrane formation of defect-free Torlon® hollow fibers for gas separation, J. Membr. Sci., J. Membr. Sci., accepted 186 [...]... material in the rest of the work The formation of defect-free as-spun hollow fiber membranes with an ultra-thin denseselective layer is an extremely challenging task because of the complexity of phase ix inversion process during the hollow fiber fabrication and the trade-off between the formation of an ultra-thin dense-selective layer and the generation of defects The second part of this dissertation... release properties of various dopes…… 170 Table 7.6 Comparison of the pure gas and mixed gas separation performance of Torlon® fibers……………………………………………………………………………………178 xiii LIST OF FIGURES Fig.1.1 Schematic diagram of hollow fiber spinning………………………………… .3 Fig.1.2 General curve of dope viscosity as a function of polymer concentration… ….5 Fig.1.3 A qualitative description of phase separation kinetics... material in the rest of the work because it has very good thermal stability and high inherent selectivities for various gas pairs The formation of defect-free as-spun hollow fiber membranes with an ultra-thin ix dense-selective layer is an extremely challenging task because of the complexity of phase inversion process during the hollow fiber fabrication and the trade-off between the formation of an ultra-thin... corresponding dense layer thickness of hollow fibers spun from each spinneret…………………………………………… 111 Table 5.3 Calculated shear rate, shear stress at the outermost point of the spinneret outlet and separation performance of wet spun hollow fibers……………………………… 116 Table 6.1 Dope formulation and spinning conditions………………………………… 134 Table 6.2 Gas separation performance of Torlon® 4000T-MV fibers at different dope temperatures……………………………………………………………………………142... 7.6 Observation of water intrusion in Torlon® flat membranes under PLM at 1s and 5s (the thickness of the polymer solution: 167 μm)……………………………………172 Fig 7.7 The effect of the additive amount on gas separation performance of Torlon® fibers (A) water; (B) THF; (C) Methanol; (D) Ethanol……………………………… 174 xvi    Chapter 1 Introduction and Literature Review on Hollow Fiber Formation for Gas Separation Since... ultra-thin dense-selective layer and the generation of defects The second part of this dissertation studies the effects of spinneret dimension and hollow fiber dimension on hollow fiber formation for O2/N2 separation and it has been discovered: (1) As the spinneret dimension increases, a higher elongation draw ratio is required to produce defect-free hollow fiber membranes; (2) The bigger the spinneret dimension,... exhibiting significant chain entanglement is one of the key requirements to produce hollow fibers with minimum defects If the dope composition is below the critical point, the resultant hollow fibers may have too many defects for gas separation to be properly repaired by the silicone rubber coating [22] The optimal polymer concentration for gas separation hollow fiber membranes may be located at 1-2 wt %... hollow fibers……………………………… 117 Table 6.1 Dope formulation and spinning conditions………………………………… 135 Table 6.2 Gas separation performance of Torlon® 4000T-MV fibers at different dope temperatures……………………………………………………………………………142 Table 6.3 CO 2 /CH 4 separation performance of defect-free Torlon ® hollow fiber membranes………………………………………………………………………… …146 Table 7.1 Dope formulation and spinning conditions…………………………………... Torlon® fibers……………………………………………………………………………………177 x    LIST OF FIGURES Fig.1.1 Schematic diagram of hollow fiber spinning………………………………… .3 Fig.1.2 General curve of dope viscosity as a function of polymer concentration… ….5 Fig.1.3 A qualitative description of phase separation kinetics and membrane morphology …………………………………………………………………………… 6 Fig 1.4 Schematic illustration of a membrane in gas separation ………………….….17... the generation of defects The second part of this dissertation studies the effects of spinneret dimension and hollow fiber dimension on hollow fiber formation for gas separation and it has been discovered: (1) As the spinneret dimension increases, a higher elongation draw ratio is required to produce defect-free hollow fiber membranes; (2) The bigger the spinneret dimension, the higher the selectivity; . FUNDAMENTALS OF HOLLOW FIBER FORMATION FOR GAS SEPARATION PENG NA NATIONAL UNIVERSITY OF SINGAPORE 2009 FUNDAMENTALS OF HOLLOW FIBER. Literature Review on Hollow Fiber Formation for Gas Separation ……… ………………………………. 1 1.1 Formation of hollow fiber membranes by phase inversion………………………… 2 1.1.1 The effect of dope composition. coagulations………………………… 3 1.1.2 The effects of spinneret design on hollow fiber formation …………… 7 1.1.3 The effects of air-gap on hollow fiber formation …………………………9 1.1.4 Effect of take-up speed…………………………………………………….11