Desalination by membrane distillation fabrication of high performance membranes

DESALINATION BY MEMBRANE DISTILLATION: FABRICATION OF HIGH PERFORMANCE MEMBRANES SINA BONYADI (B Eng (Chemical) (Hons.), Amirkabir University of Technology) A THESIS SUBMITED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL & BIOMOLECUALR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2008 Acknowledgement First of all, I would like to express my deepest heartfelt appreciation to my supervisor Prof TaiShung Chung Neal, in Dept of Chemical & Biomolecular Engineering of NUS, for his excellent guidance, enthusiastic encouragements and invaluable suggestions throughout my two year master study From him, I have learnt a great deal on both research knowledge and active work spirits I am especially grateful to Prof William B Krantz Isac mayor professor in Dept of Chemical & Biomolecular Engineering of NUS for his wise guidance regarding my first paper in the journal of membrane science Special thanks are also due to Mr NG Kim Po the workshop chief at NUS Chemical Engineering department for the fabrication of membrane distillation experimental set up I also want to take this opportunity to give my sincere thanks to all the colleagues in my research group for their kind assistance My association with them is a memorable part of my experience and study at NUS I gratefully acknowledge A*STAR for providing me an opportunity to pursue my Master degree and research scholarship i Table of Contents Acknowledgement……………………………………………………………………………… i Summary………………………………………………………………………………………… vi List of Tables…………………………………………………………………………………….viii List of Figures………………………… …………………………………………………… ix List of symbols………………………………………………………………………………… xii Chapter Background Review and Objectives 1.1 Introduction…………………………………………………………….……………… 1.2 Desalination Processes………………………………………………….……………… 1.3 Alternative desalination processes………………………………… .6 1.4 Introduction to membrane distillation as an alternative desalination approach…… .7 1.5 Membrane distillation configurations……………………………………………………8 1.6 Temperature polarization phenomenon…………………… 1.7 Wetting phenomenon………………………………………………………………… 1.8 Applications of membrane distillation……………………………………………… 10 1.9 Membrane distillation advantages and drawbacks………………………………… ….10 Chapter Literature Review 2.1 Overview on MD literature…………………………………………………………… 12 2.2 Literature review on membrane fabrication for MD…………………………………….13 2.3 Research objectives…………………………………………………………………… 17 Chapter Theory and model development 3.1 Mass transfer…………………………………………………………………………… 19 ii 3.2 Heat transfer…………………………………………………………………………… 22 3.3 Characteristics of a high performance MD membrane………………………………… 24 3.3.1 High membrane permeability…………………………………………………… 24 3.3.2 High membrane wetting resistance and long term stability…………………… 26 3.3.3 Suitable membrane geometry and dimensions……………………………………26 3.3.4 Hydrophilic layer porosity and thermal conductivity…………………………….27 Chapter Experimental 4.1 Materials……………………………………………………………………………… 28 4.2 Dope preparation……………………………………………………………………… 29 4.3 Fabrication of flat sheet membranes…………………………………… .30 4.4 Fiber spinning……………………………………………………………………………30 4.5 Morphology study of hollow fibers by SEM…………………………… .31 4.6 Contact angle measurements…………………………………………… 31 4.7 Porosity measurement……………………………………………………………………32 4.8 Pore-size distribution measurement…………………………………………………… 33 4.9 Gas permeation test………………………………………………………………………33 4.10 Polymer flow observation by a high magnification camera…………………………… 34 4.11 Video microscopy flow visualization…………………………………… 34 4.12 Module Fabrication……………………………………………………… 34 4.13 DCMD experiments…………………………………………………………………… 35 Chapter Fabrication of Dual Layer Hydrophilic-Hydrophobic Hollow Fibers 5.1 Effect of coagulant on the surface morphology of PVDF membranes………………… 37 5.2 First batch of fiber spinning…………………………………………………………… 39 5.2.1 Membrane morphology………………………………………………………… 40 iii 5.2.2 5.3 5.4 DCMD performance…………………………………………………………… 41 Second batch of hollow fiber spinning………………………………………………… 41 5.3.1 Hollow fibers morphology ……………………………………… 43 5.3.2 Pore-size distribution…………………………………………… .45 5.3.3 Gas permeation and porosity measurement tests………………… 45 5.3.4 Contact angle measurements…………………………………………………… 45 5.3.5 DCMD results…………………………………………………………………….46 Summary……………………………………………………………………………… 50 Chapter Investigation of Corrugation Phenomenon in the Inner Contour of Hollow Fibers during the Non-solvent Induced Phase-Separation Process 6.1 Introduction………………………………………………………………………………51 6.2 Experimental observations……………………………………………… .53 6.3 Discussion……………………………………………………………………………… 58 6.3.1 System description………………………………………………………………… 59 6.3.1.1 Phases I1, I2 or O1, O2…………………………………….……………………60 6.3.1.2 Phase I3 or O3…………………………………………………………………60 6.4 Possible instability mechanisms……………………………………………………… 61 6.4.1 Hypothesis (Mass transfer and hydrodynamic instability)……………………… 62 6.4.2 Hypothesis (Elastic and Buckling Instability)………………… 64 6.5 Effect of air-gap distance……………………………………………………………… 66 6.6 Effect of bore fluid composition…………………………………….………………… 68 6.7 Effect of external coagulant………………………………………….………………… 68 6.8 Effect of take-up speed……………………………………………… 68 6.9 Effect of dope concentration………………………………………… 69 6.10 Summary……………………………………………………………………………… 69 iv Chapter Conclusion………………………………………………………………………… 70 Bibliography…………………………………………………………………………………… 72 Appendix…………………………………………………………………………………………84 v Summary For the first time, co-extrusion was applied for the fabrication of dual layer hydrophilichydrophobic hollow fibers especially for the direct contact membrane distillation (DCMD) process The effect of different non-solvents on the morphology of the PVDF membranes was investigated and it was found that weak coagulants such as water/methanol (20/80 wt%) can induce a 3-dimensional porous structure on PVDF membranes with high surface and bulk porosities, big pore size, sharp pore size distribution, high surface contact angle and high permeability but rather weak mechanical properties Hydrophobic and hydrophilic clay particles were incorporated into the outer and inner layer dope solutions, respectively, in order to enhance mechanical properties and modify the surface tension properties in the membrane inner and outer layers Different membrane characterizations such as pore size distribution, gas permeation test, porosity and contact angle measurements were carried out as well Ultimately, the fabricated hollow fibers were tested for the DCMD process and flux as high as 55 kg/m2hr and energy efficiency of 83% at 90 °C was achieved in the test The obtained flux is much higher than most of the previous reports, indicating that the application of dual layer hydrophilic-hydrophobic hollow fibers may be a promising approach for MD In the second part of this research, by proposing a novel mechanism, we revealed one of the most controversial issues in the hollow fiber fabrication process regarding the instability leading to the deformed cross-section of fibers fabricated through nonsolvent induced phase separation We analyzed possible instability mechanisms based on our experimental observations and then postulated that the principal instability occurs in the external coagulation bath where the rigid precipitated polymer shell in the dope and bore fluid interface is buckled due to a generated pressure The pressure is postulated to be induced in the nascent fiber outer layer as a result of vi diffusion/convection, precipitation, densification and shrinkage In addition, the effect of some spinning conditions such as air-gap distance, bore fluid composition, take-up speed, external coagulant and dope concentration on the final shape of the fiber cross-section have been investigated The proposed mechanism was in good qualitative agreement with all our observations vii List of Tables Table.2.1 Summary of commercial membranes applied by some studies in the literature Table.4.1 Specifications of Cloisite particles Table.5.1 Total solubility parameter (δt) of water, methanol, NMP and PVDF Table.5.2 Spinning conditions applied for the first batch of spinning Table.5.3 DCMD operating conditions and the obtained flux for the fabricated fibers Table.5.4 Comparison of the maximum flux obtained in this study with the literature for DCMD processes with a hollow fiber configuration Table.6.1 Spinning conditions of hollow fiber membrane fabrication viii List of Figures Fig.1.1 Distribution of Earth’s water Fig.1.2 Fresh water resources Fig.1.3 Schematic presentation of a Multi-Stage Flash desalination plant Fig.1.4 Schematic diagram of world’s desalination plants capacity percentage by 1998 Fig.1.5 Typical Costs for a Reverse-Osmosis Desalination Plant Fig.1.6 Schematic diagram representing the separation mechanism involved in MD Fig.1.7 Schematic diagrams representing different configurations of the MD process Fig.1.8 Schematic diagram of temperature polarization phenomenon in DCMD Fig.2.1 Schematic diagram representing a typical melt spinning process Fig.2.2 A composite hydrophilic-hydrophobic membrane before the MD test (left) A composite hydrophilic-hydrophobic membrane during the MD test (right) Fig.2.3 Schematic picture of a dual layer hydrophilic-hydrophobic fiber Fig.3.1 Schematic DCMD process with dual layer hydrophilic-hydrophobic hollow fibers Fig.4.1 Chemical structure of PVDF polymer Fig.4.2 Chemical structure of PAN polymer Fig 4.3 Schematic representation of the fiber spinning line Fig.4.4 Schematic diagram showing the steps involved in fabrication of lab scale membrane modules Fig.4.5 Schematic experimental set-up applied for direct contact membrane distillation process Fig.5.1 SEM pictures from the top surface (facing the coagulant) of the PVDF flat sheet membranes for different coagulant compositions Fig.5.2 SEM pictures representing the structure of first spinning batch fibers Fig.5.3 Delamination phenomenon during the DCMD process using the first batch of fibers Fig.5.4 The spinning conditions applied in the second batch of spinning process Fig.5.5 Cross section morphology of fibers fabricated through the second batch of spinning ix Chapter Conclusion The following conclusions can be drawn out of this study: 1- Very different membrane morphologies can be formed in PVDF membranes by tailoring the strength of coagulation bath Highly porous membrane structure with three dimensional networks is formed in PVDF membranes using weak coagulants such as methanol/water mixtures Such membrane morphologies possess a very high gas permeability , leading to a high flux in membrane distillation process 2- PVDF surface tension and mechanical properties can be modified by blending the PVDF polymer with hydrophobic or hydrophilic clay particles The addition of hydrophobic particles to the hydrophobic layer further increases the contact angle of the functional hydrophobic layer by increasing the membrane surface roughness 3- The surface tension of PVDF polymer can be reduced significantly by blending the PVDF polymer with hydrophilic PAN as well as the hydrophilic lay particles 4- Co-extrusion looks to be an efficient approach to fabricate hydrophilic-hydrophobic hollow fiber membranes to be applied in MD process This process provides the freedom to easily tailor membrane properties such as porosity, pore size distribution and thickness 5- Delamination between the two hydrophilic and hydrophobic layers of the fibers can be prevented by carefully formulating the dope composition of the two layer in a way that the two layers are compatible with each other   70   6- High flux and high energy efficiency can be obtained using the dual layer hydrophilichydrophobic fibers 7- The non-uniform cross section of hollow fibers can be attributed to an instability occurring during the fabrication of hollow fibers It was postulated that the main instability happens in the coagulation bath where the pressure induced in the nascent fiber outer layer as a result of diffusion/convection, precipitation, densification and shrinkage will buckle the rigid elastic shell formed in the interface between the bore fluid and the dope solution It is also inferred that since it takes only a fraction of second for a nascent fiber traveling through the air gap region, the hydrodynamic instability may not have enough time to magnify its effects Therefore, it is postulated that the hydrodynamic instability may be the initial stage of causing instability and the elastic and buckling instability is the magnifying stage for the development of non-uniform cross section of hollow fibers 8- The proposed buckling mechanism can effectively explain all our experimental observations regarding the effect of different spinning parameters on the final shape of the hollow fibers 9- The proposed qualitative mechanism can be applied to quantitatively model the corrugation phenomenon 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Dual Layer Hydrophilic-Hydrophobic Hollow Fiber Membranes, Journal of Membrane Science, 306 (2007) 134–146 S Bonyadi, T.S Chung, W.B Krantz, Investigation of the Corrugation Phenomenon in the Inner Contour of Hollow Fibers during Non-solvent Induced Phase Separation Process, Journal of Membrane Science, 299 (2007) 200-210 M.M Teoh, S Bonyadi, T.S Chung, J.J Shieh, Investigation of different module designs for flux enhancement in the membrane distillation process, Journal of Membrane Science, 311 (2008) 371–379 S Bonyadi, T.S Chung, K.Y Wang, Fabrication of High Performance Dual Layer Hydrophilic-Hydrophobic Hollow Fibers for Membrane Distillation Process, a US Patent filed, March 2007 S Bonyadi, T.S Chung, W.B Krantz, Investigation of the Corrugation Phenomenon in the Inner Contour of Hollow Fibers during Non-solvent Induced Phase Separation AIChE Annual Meeting (2007) Salt Lake City, Utah   S Bonyadi, T.S Chung, Flux Enhancement in Membrane Distillation by Fabrication of Dual Layer Hydrophilic-Hydrophobic Hollow Fiber Membranes AIChE Annual Meeting (2007) Salt Lake City, Utah   84   ... spinning 29 4.3 Fabrication of flat sheet membranes In order to investigate the effect of different coagulants on the surface porosity and roughness of the PVDF membranes, PVDF flat sheet membranes. .. review on membrane fabrication for MD The membranes commonly applied in membrane distillation literature have been the commercially available membranes actually fabricated for other membrane applications... in MD process by fabrication of novel high performance membranes In this respect, we have devoted this chapter to review membrane distillation literature regarding membranes applied in this process