Dual layer hollow fiber membrane development for forward osmosis and osmosis power generation

DUAL-LAYER HOLLOW FIBER MEMBRANE DEVELOPMENT FOR FORWARD OSMOSIS AND OSMOSIS POWER GENERATION FU FENG JIANG NATIONAL UNIVERSITY OF SINGAPORE 2014 DUAL-LAYER HOLLOW FIBER MEMBRANE DEVELOPMENT FOR FORWARD OSMOSIS AND OSMOSIS POWER GENERATION FU FENG JIANG (B.Eng., Tianjin University) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 ACKNOWLEDGEMENT First of all, I would like to express my appreciation to my supervisor Prof. Chung Tai-Shung who brings me into the world of membrane research. His guidance, enthusiastic encouragement and invaluable support throughout my master study are invaluable. From him, I have learned and benefited greatly in not only research knowledge but also developed the enthusiasm of a qualified researcher. I would like to express my appreciation to all former and current members of our research group, especially, Dr. Shipeng Sun, Dr. Sui Zhang, Dr. Jincai Su, Dr. Kaiyu Wang, Dr. Peng Wang, Dr Gang Han and Dr. Xue Li for their invaluable help on research experiments. All group members are friendly and helpful to me, which have made my learning experience in NUS enjoyable and unforgettable. I would like to gratefully acknowledge the Singapore National Research Foundation for their financial support through its Environmental & Water Technologies Strategic Research Programme and administered by the Environment & Water Industry Programme Office (EWI) of the PUB for the project entitled “Membrane development for osmotic power generation: Phase 1 :Materials development and membrane fabrication” (grant number: R-279-000-381-279). i TABLE OF CONTENTS ACKNOWLEDGEMENT .............................................................................. i TABLE OF CONTENTS ............................................................................... ii SUMMARY ....................................................................................................v A LIST OF TABLES ................................................................................... vii A LIST OF FIGURES .................................................................................. ix A LIST OF SYMBOLS ................................................................................ xi CHAPTER 1: INTRODUCTION AND OBJECTIVES 1 1.1. Introduction of osmotic process 1 1.2. Background of research 3 1.3. Overall strategies and objectives 7 CHAPTER 2: MATERIALS AND EXPERIMENT METHODOLOGY 10 2.1. Materials 10 2.2. Shear viscosity and phase inversion kinetics of the solutions 11 2.3. Dual-layer hollow fiber spinning process and setup 11 2.4. FO membrane development 12 2.4.1. Preparation of FO membrane dope solutions 12 2.4.2. Spinning conditions for dual-layer hollow fiber FO membrane 13 2.4.3. Post treatment and module fabrication 14 2.5. PRO membrane development 14 2.5.1. Preparation of PRO membrane dope solutions ii 14 2.5.2. Fabrication and evaluation of dual-layer flat-sheet membranes using traditional and universal co-casting methods 16 2.5.3. Fabrication of PBI/POSS-PAN/PVP dual-layer hollow fiber PRO membranes 18 2.5.4. APS post treatment 19 2.6. Membrane characterizations 20 2.6. 1. Morphology, mechanical strength and surface analysis 20 2.6.2. Pure water permeability (PWP), salt rejection, salt permeability, pore size, and pore size distribution 21 2.7. FO tests 23 2.8. PRO performance tests 24 CHAPTER 3: RESULTS AND DISCUSSIONS 3.1. FO membrane experiment result and discussion 26 26 3.1.1. Fabrication of delamination-free PBI-PAN/PVP dual-layer FO hollow fiber membranes 26 3.1.2. Cost-effective and mechanically strong dual-layer hollow fibers 29 3.1.3. Effects of POSS on the morphology of the hollow fibers 32 3.1.4. Effects of POSS on permeability and selectivity of hollow fibers in NF processes 34 3.1.5. Application of annealed PBI/POSS-PAN/PVP membranes in engineered osmosis processes 3.2. PRO membrane experiment result and discussion 36 41 3.2.1. Development of the universal co-casting method for preparing dual-layer flat-sheet membranes iii 41 3.2.2 Optimization of dope formulation for delamination-free dual-layer flat sheet membranes using the universal co-casting method 43 3.2.3. Verification of the universal co-casting method by dual-layer hollow fiber spinning 46 3.2.4. PRO membrane development with APS assisted post-treatment 47 3.2.5. The application of PBI-PAN-PVP6 membranes for osmotic power generation 51 CHAPTER 4: CONCLUSIONS AND RECOMMENDATIONS 54 BIBLIOGRAPHY 58 iv SUMMARY For the first time, polybenzimidazole (PBI)/ Polyacrylonitrile (PAN) dual-layer membranes with ultra-thin outer dense layer (about 1µm) and porous inner support layer were developed for forward osmosis (FO) and pressure retarded osmosis (PRO) applications. In this work, polyvinylpyrrolidone (PVP) incorporation effects on the elimination of membrane delamination; polyhedral oligomeric silsesquioxane (POSS) incorporation effects on the membrane structure and permeability; ammonium persulfate (APS) post treatment effects on the membrane permeability were conducted and drew out some useful conclusions for membrane development. In addition, universal dual-layer co-casting method was developed for the research of the solution for elimination of membrane delamination; with this method, the time consumption for dual-layer delamination-free membrane development had been significantly reduced. In this work, with the optimized POSS concentration, the dual-layer FO membrane shows a maximum water flux 31.37 LMH at room temperature using 2.0 M MgCl2 as the draw solution in the FO process; with the optimized APS concentration of 5 wt%, the post-treated dual-layer PRO membrane shows a maximum power density of 5.10 W/m2 at a hydraulic pressure of 15.0 bar when 1 M NaCl and 10 mM NaCl were used as the draw and feed solutions, respectively. To the best of our knowledge, this is the best phase inversion dual-layer hollow fiber membrane with an outer selective layer for v osmotic power generation. In summary, the newly developed PBI/PAN dual-layer membrane has shown promising results in both FO and PRO processes. With its unique outer dense-selective skin, hydrophilic inner-layer and outer-layer structure, and easy processability, this membrane may have wide applications in the future for osmotic power generation as well as for nanofiltration (NF), ultrafiltration (UF) and other applications. vi A LIST OF TABLES Table 2.1 Structures, solubility parameters & nitrogen content of PBI, PVP, PAN molecules Table 2.2. 10 Spinning conditions for the fabrication of PBI/POSS -PAN/PVP dual-layer hollow fiber FO membranes Table 2.3. Spinning conditions for the fabrication of PBI/POSS -PAN/PVP dual-layer hollow fiber PRO membranes Table 2.4. 15 Co-casting conditions and results of PBI/POSS-PAN/PVP dual-layer flat sheet membranes Table 3.1. 13 15 A comparison of inner and outer dope flow rates, outer layer volume percentage and outer layer thickness in various dual-layer hollow fiber membranes Table 3.2. 30 A comparison of mechanical properties of the PBI-PBI dual-layer and PBI-PAN-P0.5 dual-layer hollow fiber membranes with and without annealing Table 3.3. A comparison of pore size, PWP, rejection and structure of recent papers on PBI membranes Table 3.4. 34 A comparison of FO performance of recent research on PBI membranes Table 3.5. 32 37 Estimated power output per 8-inch module of outer and inner selective membrane modules which are comprised of the hollow fibers with the same dimension, and power density 41 vii Table 3.6. Atomic concentration of PVP polymer and outer surface of outer-layer of membranes analyzed from XPS Table 3.7. 50 A comparison of pore size, PWP, rejection and burst pressure of recent papers on PBI membranes viii 51 A LIST OF FIGURES Fig. 1.1. Fig. 1.1 Illustration of the differences between FO, PRO and RO processes Fig. 1.2. 2 Schematic diagram of dual-layer flat sheet membrane co-casting processes Fig. 2.1. 9 (A) Scheme of the dual-layer spinneret and (B) the hollow fiber spinning line 12 Fig. 2.2. Schematic diagram of APS treatment setup 19 Fig. 2.3. Schematic diagram of customised bench scale PRO performance testing setup 25 Fig. 3.1. Cross-section morphology of hollow fibers 27 Fig. 3.2. (A) Shear viscosity of the PAN/NMP=25/75 wt% solution and PAN/PVP/NMP=16/11/73 wt% solution and (B) the UV absorption curves of membranes cast from both solutions after immersion in water Fig. 3.3. 27 Nitrogen atom distribution as characterized by EDX across the outer edge of (A) the delaminated fiber without PVP addition and (B) the delamination-free fiber with PVP addition Fig. 3.4. 28 Cross-section morphology of PBI/POSS–PAN/PVP hollow fiber membranes as a function of POSS wt% Fig. 3.5. 31 Schematic of the possible hydrogen bonding between PBI and POSS 33 ix Fig. 3.6. Effects of POSS concentration on the NF performance of PBI/POSS-PAN/PVP dual-layer membranes Fig. 3.7. 36 The effects of POSS concentration on FO Performance of PBI/POSS-PAN/PVP membrane with 95°C annealing Fig. 3.8. 37 Effects of draw solution concentration on water permeation flux, and Js/Jw Fig. 3.9. 38 Experimental and computed results of pressurized water flux (A) and power density (B) vs. hydraulic pressure difference in the PRO process Fig. 3.10. Cross-section morphology of PBI/POSS-PAN/PVP flat sheet membranes as a function of PVP wt% Fig. 3.11. 40 43 (A) PVP concentration vs. substrate dope viscosity and (B) PVP concentration vs. PWP and salt rejection of flat sheet dual-layer membranes Fig. 3.12. 44 Morphology of PBI/POSS-PAN/PVP hollow fiber membranes Fig. 3.13. 46 Effects of PVP concentration on the NF performance of dual-layer hollow fiber membranes Fig.3.14. Effects of APS concentration on FO performance of hollow fibers under the PRO mode Fig. 3.15. 49 Color changes of membranes with different APS concentrations Fig. 3.16. 48 50 (A) Water flux and (B) power density of the PBI-PAN-P6-T60 hollow fiber membranes before and after APS post-treatment 52 x A LIST OF SYMBOLS APS ammonium persulfate DMAc N, N-dimethylacetimide DS draw solution FS feed solution FO forward osmosis MW molecular weight MWCO molecular weight cut off NMP n-methyl-2-pyrrolidone NF nanofiltration PAN polyacrylonitrile PAI polyamide-imide PBI polybenzimidazole POSS polyhedral oligomeric silsesquioxane PRO pressure retarded osmosis PVP polyvinylpyrrolidone PWP pure water permeability Js reverse draw solute flux, gMH Jw water flux, LMH ΔP hydraulic pressure difference E power per unit membrane area (power density) ∆Ct salt concentration at the end of the tests Vt feed volume at the end of the tests xi ∆V volumetric change of the feed solution over a predetermined time, liter ∆T a predetermined time of the test, hrs S the effective membrane surface area, m2 rp mean pore radius, nm rs the radius of the neutral solutes, nm σg the geometric standard deviation RT solute rejection, % cp solute concentration in the permeate cf solute concentration in the feed solution xii CHAPTER ONE INTRODUCTION AND OBJECTIVES 1.1. Introduction of osmotic process The osmosis phenomenon was discovered by Nollet in 1748 [1]. When two solutions with different concentrations are separated by a semipermeable membrane, the osmotic pressure, π, arises due to the difference in the chemical potential. Water flows from the low chemical potential side to the high chemical potential side until the chemical potential of both sides become equalized. The increased volume of water in the high chemical potential side builds up a hydrodynamic pressure difference, which is called the osmotic pressure difference Δπ. The osmotic pressure of a solution can be calculated based on van’t Hoff equation [2]: (1) where i is the van’t Hoff factor, c is the concentration of all solute species in the solution, R is the gas constant and T is the temperature. Osmotic processes can be classified into three categories based on the trans-membrane pressure (TMP) difference (ΔP): reverse osmosis (RO), pressure retarded osmosis (PRO) and forward osmosis (FO). Fig. 1.1 illustrates the differences of the three processes. The main advantages of using FO and PRO are: (1) they operate at no hydraulic or low pressures, (2) they have high rejection of a wide range of 1 contaminants, and (3) they may have a lower membrane fouling propensity than RO, which is the pressure-driven membrane process [3]. Because the only pressure involved in the FO process is due to flow resistance in the membrane module (a few bars), the equipment used is relatively simple and the membrane support becomes a minor problem. Furthermore, for food and pharmaceutical processes, FO has the benefit of concentrating the feed stream without requiring high pressures or temperatures that may be detrimental to the feed solution. For medical applications, FO can assist in the slow and accurate release of drugs that have low oral bioavailability due to their limited solubility or permeability [4]. Fig. 1.1 Illustration of the differences between FO, PRO and RO processes PRO is an emerging renewable energy process that is not only environmental friendly but also does not emit CO2. During the PRO process, a low-salinity feed solution such as river or brackish water is drawn through a semipermeable membrane into a pressurised high-salinity solution such as sea 2 water or brine by the osmotic pressure difference between them. Osmotic power can be generated by releasing the pressurised water through a turbine [3, 5-10]. The worldwide unexploited osmotic power is more than 1600 TWh per year, which is equivalent to one-half of the annual power consumption by the European Union [11-14]. 1.2. Background of research With the rapidly growing population, global warming and sharp increases in oil and gas consumption, water and energy have become the two most demanding resources on Earth [3, 15-17]. Although the planet we live on is mostly covered by oceans and other water sources, drinkable water only makes up about 0.8 % of the total amount of water in the world. In addition, the expected energy consumption in the 21st century will triple the amount consumed in the last century [18]. In order to address this challenge, most countries are looking for alternative clean and renewable energy [19-23]. From a manufacturing perspective, water and energy are closely co-dependent. The production of fresh water is an energy-intensive process, while the power generation process consumes a significant amount of water. Forward osmosis (FO) receives global attention because it has the advantages for both water production and power generation by exploiting the osmotic pressure gradient across a semi-permeable membrane as the driving force [3]. However, the major hurdles to fully explore the FO potential for water and energy production are (1) lack of commercial FO membranes with high water flux, 3 low salt reverse flux and low fouling; (2) lack of high-performance draw solutes which can be easily recovered from diluted draw solutions with low energy consumption [7, 24, 25]. Many attempts have been made to develop FO membranes in order to overcome these constraints as summarized by recent reviews [3, 7, 24, 25]. Among various FO applications, PRO is a promising method for power generation [26, 27]. Although Prof. Sidney Loeb pioneered the harvest of osmotic power in 1973, the osmotic driven PRO process was at the infant stage until the opening of the Statkraft's PRO pilot plant in Norway in 2009. The pilot plant has revealed that the key components of an industry-scale PRO plant consist of membranes, membrane modules, pressure exchangers, pre- and post-treatments to remove fouling. Since then, many efforts from both industries and academia have been given to improve the performance of these components [11, 14, 28-32]. However, the semi-permeable membranes for power generation must not only possess high water flux but also withstand high hydraulic pressure. Most conventional FO membranes do not possess these performance requirements because they have been designed to operate at negligible or minimal trans-membrane pressure. Clearly, there is an urgent need to molecularly design PRO membranes via novel material engineering and innovative membrane fabrication. In terms of membranes, both hollow fiber and flat sheet membranes can be used for PRO applications. Although the Statkraft's pilot plant uses flat sheet membranes, hollow fiber membranes and modules are, in some aspects, more appropriate than flat sheet spiral wound modules for PRO applications due to the following reasons: (1) Sivertsen et al. reported that a module design 4 consisting of two inlets and two outlets for fresh water is more efficient for the PRO operation [28]. It is easy to fabricate a hollow fiber membrane module with this configuration. (2) The hollow fiber membrane is self-supporting and does not require membrane spacers on both sides. In addition, the hollow fiber module offers a higher surface area per volume. (3) The elimination of the spacers not only makes the element less sensitive to fouling but also reduces the pressure drop along the module [28, 29]. To date, both phase inversion and thin-film composite (TFC) technologies have been employed to develop forward osmosis (FO) and PRO hollow fiber membranes [8, 22, 27, 33-38]. The TFC membrane, which is composed of a porous support layer and an ultra-thin dense selective layer, has been the focus of most studies since it has shown better PRO performance. However, it is difficult to scale up the interfacial polymerization process for TFC hollow fiber membranes. In addition, the TFC membrane is very sensitive to oxidants such as chlorine. As a consequence, the de-chlorination of feed water and the chemical backwashing of TFC membranes become crucial in PRO processes that would result in additional equipment and operational costs [39-41]. As an alternative, the dual-layer hollow fiber membrane produced by the simultaneous co-extrusion spinning process eliminates the secondary step of depositing a selective layer on the inner or outer surface of the hollow fiber membrane. It is a much straight- forward and cost effective process when comparing with the fabrication of TFC composite membranes [4, 42]. Using this method, we can choose a material with good chlorine resistance and salt rejection properties as the selective layer and a cheap but mechanically strong 5 polymer as the substrate layer to eliminate the problems or difficulties associated with TFC hollow fiber membranes. Among various available materials, polybenzimidazole (PBI) is a strong candidate for the development of FO and PRO membranes. With its excellent thermal stability, super resistance to strong acids and alkalis, and easy film-forming properties, it has the potential to become a good selective layer material for the development of dual-layer FO and PRO membranes [4, 33, 43-45]. However, drawbacks such as high price and brittleness affect its industrial-scale membrane applications. A series of studies have been undertaken to overcome these weaknesses such as the development of single-layer PBI [45, 46] and dual-layer PBI membranes [4]. However, there is still much room for improvement. Polyacrylonitrile (PAN) has been used as the substrate layer material due to its low price, good mechanical properties, and weather and thermal stability, as well as its impressive resistance to sunlight and chemical reagents, such as inorganic acid, bleach, hydrogen peroxide, and general organic reagents [47-49]. However, the major problems in dual-layer PBI-PAN FO and PRO hollow fiber membranes are (1) two disadvantages of PBI price and brittle property (2) delamination between the outer and inner layers and (3) insufficient water permeability. Therefore, the aims of this study are to (1) develop solutions to overcome the high price and brittle property of PBI material (2) overcome the delamination phenomenon between the outer PBI layer and inner PAN layers, and (3) develop PBI-PAN with higher FO and PRO performance. 6 1.3. Overall strategies and objectives Three strategies were employed in this work (i) to modify the PBI dual-layer membrane with enhanced salt rejection and mechanical strength by heat annealing (ii) to modify the PBI dual-layer membrane with enhanced permeability by polyhedral oligomeric silsesquioxane (POSS) incorporation and (iii) to lower its material cost by reducing the outer-layer membrane thickness to minimize PBI usage. For the dense-selective layer, a small amount of POSS was incorporated into the PBI dope to achieve (1) a higher permeate flux and (2) a stronger PBI layer [33]. POSS has a cage-like structure which consists of 8 silicon atoms linked together with oxygen atoms with a formula of [RSiO3/2]n, where n = 6–12 and R could be various chemical groups known in organic chemistry, such as alcohols, amines and epoxides. As a result, POSS molecules have several unique characteristics: (i) high flexibility to be functionalized, (ii) small particle size in the range of 1–3 nm, and (iii) excellent compatibility and dispersibility at the molecular level in diverse polymer matrices [50]. POSS has attracted much attention in the development of nanocomposite materials. It can improve Young’s modulus as much as 70%, tensile strength 30%, and dimensional stability [51]. POSS has been employed as an additive for gas separation and pervaporation membranes recently. Surprisingly, it can simultaneously enhance both permeability and selectivity [50, 52, 53]. To solve the delamination issue, additives such as polyvinylpyrrolidone (PVP) had been added in the inner dope to facilitate molecular interaction between 7 both layers. The delamination was reduced when the PVP concentration reached a certain level but the mechanical strength of the resultant membrane became weaker [33, 42]. Optimal dope and PVP formulations must be found in order to produce high permeability PBI/POSS-PAN/PVP FO membrane and strong PBI/POSS-PAN/PVP PRO membranes that can withstand high pressure PRO operations. Since it takes a lot of time and materials to conduct researches for better dope formulations for dual-layer hollow fibers, a co-casting method developed by He et al. [54-56] as shown in Fig. 1.2(A) was firstly employed to examine its suitability to mimic the dual-layer hollow fiber spinning and help find the optimal formulations. The co-casting method utilizes a customized device consisting of two casting knives with fixed thicknesses to simultaneously cast two different dope solutions into flat-sheet dual-layer membranes. It is a useful tool to evaluate the adhesion between the inner and outer layers before conducting the dual-layer hollow fiber spinning [54]. However, this method suffers from inflexibility of film thickness and incapability of casting highly viscous solutions. It can be only applicable for low concentration dope solutions with low viscosity. For dual-layer PRO hollow fiber membranes, the outer-layer dope concentration is normally high in order to achieve a high salt rejection. Therefore, the one objective of this work is to develop a universal co-casting method with various film thicknesses and solution concentrations that is able to find optimal dope formulations for dual-layer hollow fiber PRO membranes. By doing so, it may save significant time and materials when developing dual-layer PRO hollow fiber membranes. Since the low water permeability of the dual-layer PBI/POSS-PAN/PVP 8 hollow fiber membrane is mainly caused by the high-molecular weight PVP that is entrapped within the substrate layer, PVP must be removed without damaging the selective layer and the interface. Traditionally, sodium hypochlorite has been often used to remove PVP from membranes [14], but it may damage the selective layer and the interface because it is a strong oxidizer, thus decreases salt rejection. A mild removal method was invented recently using ammonium persulfate (APS) at 60 °C to remove PVP from PAN/PVP membranes without much scarifying rejection [57]. Therefore, the second objective of this work is to design and optimize the APS post-treatment process to enhance the water flux of the dual-layer PBI/POSS-PAN/PVP hollow fiber membrane. This work may provide useful insights for the development of outer selective FO and PRO hollow fiber membranes for osmotic power generation as well as for nanofiltration (NF), ultrafiltration (UF) and other applications. Fig. 1.2. Schematic diagram of dual-layer flat sheet membrane co-casting processes. (A). Traditional dual-layer flat sheet membrane co-casting process. (B). Universal dual-layer flat sheet membrane co-casting process 9 CHAPTER TWO MATERIALS AND EXPERIMENT METHODOLOGY 2.1. Materials The PBI polymer was provided by PBI Performance Products Inc. in a solution of 26.2 wt% PBI, 72.3 wt% N, N-Dimethylacetimide (DMAc), and 1.5 wt% lithium chloride (LiCl). The PAN copolymer was kindly provided by Professor Hui-An Tsai from Chung Yuan Christian University, Taiwan. POSS (AL0136) nanoparticles (Hybrid Plastics Inc., USA) and PVP (average molecular weight: 360 kDa; Sigma-Aldrich) were utilised as additives in the PBI and PAN solutions, Table 2.1 Structures, solubility parameters & nitrogen content of PBI, PVP, PAN molecules [58]. Molecule Chemical structure Solubility parameter 1/2 -3/2 1/2 -3/2 14.3% 1/2 -3/2 16.7% PBI 16.48 cal cm PVP 15.03 cal cm PAN 14.39 cal cm APS --- POSS Nitrogen atomic content --- 25.0% --- --- respectively. APS (Sigma-Aldrich) was used for membrane post-treatment. The chemical structures of these polymers are listed in Table 2.1. All of the 10 chemicals, except APS, were vacuum dried for 12 hours before dope preparation. Analytical grade DMAc and n-methyl-2-pyrrolidone (NMP) supplied by Merck were employed to prepare polymer solutions. Sodium chloride (99.5%, Merck) was used to prepare feed and draw solutions. Uncharged neutral solutes of ethylene glycol, glycerol, diethylene glycol, and sucrose (analytical grade, Sigma-Aldrich) were utilised to characterise the membrane structure parameters. 2.2. Shear viscosity and phase inversion kinetics of the solutions The shear viscosities of PAN solutions with and without PVP were measured at shear rates from 0.1 to 1000 s−1 by a rotational cone and plate rheometer (AR-G2 rheometer, TA instruments, USA). A steady-state mode with a 20 mm or 40 mm, 1˚ cone geometry was employed. The phase inversion kinetics of the solutions was studied by light absorption experiments using a UV–vis scanning spectrophotometer (Libra S32, Biochrom Ltd., England). The procedures included casting the solution on a glass slide, quickly immersed it into the coagulant water vertically in a UV cell, and then immediately monitored the absorption at 600 nm. The maximum absorption was used to normalize the absorption curves against time. 2.3. Dual-layer hollow fiber spinning process and setup The setup of the dual-layer hollow fiber spinning line and the schematic 11 diagram of fluid channels within the spinneret are illustrated in Fig. 2.1. Specifically, the outer dope, the inner dope and the bore fluid were fed into the spinneret separately by three ISCO syringe pumps. The outer dope and the inner dope were premixed before exiting the spinneret in order to improve the integration of the two layers. After that, the dopes and the bore fluid met at the tip of the spinneret, and passed through an air gap region before entering the coagulation (water) bath. Finally, the as-spun dual-layer hollow fibers were collected by a take-up drum. The proper spinning parameters for ultra-thin outer selective layer and defect-free dual-layer hollow fiber spinning were worked out after several trials. Fig. 2.1. (A) Scheme of the dual-layer spinneret and (B) the hollow fiber spinning line. Pump A: inner dope solution; pump B: bore fluid; pump C: outer dope solution 2.4. FO membrane development 2.4.1. Preparation of FO membrane dope solutions PBI/DMAc/LiCl/POSS solutions with different POSS concentrations as 12 shown in Table 2.2. were prepared for the outer selective layer. POSS was firstly dissolved in DMAc by continuous stirring at room temperature for 12 hours, and then sonicated for at least 4 hours before being mixed with the PBI solution. The mixture was subsequently stirred at 50 °C for 8 hours to form a homogeneous solution, and finally degassed in a proper sealed container for 24 hours before use. A solution of PAN/PVP/NMP=16/11/73 wt% was prepared for the inner substrate layer as presented in Table 2.2. PAN and PVP were firstly dissolved in NMP by continuous stirring at 60°C temperature for 12 hours to form a homogeneous solution, and then degassed in the air for 24 hours before use. For comparison, a PAN solution without PVP (PAN/NMP = 25/75 wt%) was also used as specified in the context. Table 2.2. Spinning conditions for the fabrication of PBI/POSS-PAN/PVP dual-layer hollow fiber FO membranes Membrane ID PBI-PAN-P0 PBI-PAN-P0.5 PBI-PAN-P1.0 PBI-PAN-P1.5 Outer dope composition (PBI/DMAc/LiCl/POSS, wt%) 24/74.63/1.37/0.0 24/74.13/1.37/0.5 24/73.63/1.37/1.0 24/73.13/1.37/1.5 Inner dope composition (wt%) PAN/PVP360/NMP (16/11/73) Bore fluid composition (wt%) NMP/Water (90/10) Dope and bore fluid temperature (℃) 26±1 Coagulant temperature (℃) 26±1 Solution flow rate (ml/min) Outer dope/Inner dope/Bore fluid (0.06/3.0 /1.5) Air gap (cm) 4 Take-up speed (m/min) 6 External coagulant Tap Water 2.4.2. Spinning conditions for dual-layer hollow fiber FO membrane 13 In order to reduce the expensive PBI material usage and the transport resistance of the membrane selective layer, as well as to assist to eliminate the delamination issue [59], the outer-layer membrane thickness need to be minimised. Upon optimizing the spinning parameters shown in Table 2.2., an outer dope flow rate as low as 0.06 ml/min was achieved for defect-free dual-layer hollow fiber spinning in the FO membrane development process of this work. 2.4.3. Post treatment and module fabrication The as-spun fibers were immersed in tap water for 3 days prior to thermal annealing. The optimized membrane annealing procedures are (1) soaking the fibers in 95°C hot water for 3 minutes, (2) drying in the air for 3 minutes for fiber relaxation, and (3) immersing in room temperature DI water for at least 15 min. After that, the membranes were soaked in a 50 wt% glycerol solution in water for 48 hours and dried in the air at room temperature. For module fabrication, 2 male run tees were connected to each side of a 3/8" perfluoroalkoxy (PFA) tubing and 16 pieces of hollow fibers were bundled into the module housing with an effective length of 13.5 cm. Both ends of the housing were sealed with epoxy. 2.5. PRO membrane development 2.5.1. Preparation of PRO membrane dope solutions A PBI/POSS/DMAc/LiCl solution comprising 0.5 wt% POSS as shown in 14 Table 2.3 was prepared for the outer selective layer. POSS was first dissolved in DMAc by continuously stirring at room temperature for 12 hours, and then sonicated for at least 4 hours before being mixed with the PBI solution. The mixture was subsequently stirred at 50 °C for 8 hours to form a homogeneous solution, and then degassed in a properly sealed container for 24 hours before use. Table 2.3. Spinning conditions for the fabrication of PBI/POSS-PAN/PVP dual-layer hollow fiber PRO membranes Membrane ID PBI-PAN-P3 PBI-PAN-P4 PBI-PAN-P6 PBI-PAN-P8 Inner dope composition (PAN/PVP/NMP, wt%) 21/3/76 21/4/75 21/6/73 21/8/71 Outer dope composition (wt%) Bore fluid composition (wt%) Dope and bore fluid temperature (℃) Coagulant temperature (℃) Solution flow rate (ml/min) Air gap (cm) Take-up speed (m/min) External coagulant Hollow fiber dimension OD/ID (mm) PBI/DMAc/LiCl/POSS (24/74.13/1.37/0.5) NMP/Water (75/25) 26±1 26±1 0.03/0.8 /0.25 0.03/0.8 /0.25 0.03/0.8 /0.25 0.2-0.3 2 2 2 Tap Water 0.75/0.36±0.05 0.77/0.38±0.05 0.78/0.38±0.05 0.02/0.4/0.13 1 0.81/0.39±0.05 Table 2.4. Co-casting conditions and results of PBI/POSS-PAN/PVP dual-layer flat sheet membranes Thicknesses of casting knives Dense layer dope st 1 : 200 µm nd 2 : 250 µm PBI/DMAc/LiCl/POS S (24/74.13/1.37/0.5) Substrate layer Delamination dope formulation (visual check) and amount (PAN/PVP/NMP) (21/0/79); 50g Immediate Salt rejection (%) (21/2/77); 50g After 10 hrs N.A (21/3/76), 50g No 44.3 (21/4/75), 50g No 56.4 (21/6/73), 50g No 74.1 (21/8/71) , 50g No 74.6 N.A PAN/PVP/NMP solutions with different PVP content were prepared as presented in Table 2.3. for hollow fiber membrane spinning. PAN and PVP 15 were firstly dissolved in NMP by continuously stirring at 60 °C for 12 hours to form a homogeneous solution, and then were degassed in air for 24 hours before use. Table 2.4 shows the detailed compositions of the PBI and PAN solutions, which were used for the co-cast of flat sheet membranes. 2.5.2. Fabrication and evaluation of dual-layer flat-sheet membranes using traditional and universal co-casting methods Three parameters were taken into consideration during the optimization of dope formulation: (1) dope viscosity, (2) the integrity of interface between the dense and substrate layer, and (3) nanofiltration (NF) performance. The shear viscosities of PAN solutions as a function of PVP content were measured at shear rates from 0.1 to 1000 s−1 by a rotational cone and plate rheometer (AR-G2 rheometer, TA instruments, USA) using a steady-state mode with a 20 mm or 40 mm, 1˚ cone geometry. Dual-layer flat-sheet membranes were firstly cast from the same PBI dense layer solution and six PAN/PVP/NMP substrate solutions containing different PVP content using the traditional co-casting method as illustrated in Fig. 1.2(A). The co-casting device consists of (1) a cylinder-shape knife to cast the substrate layer; (2) a plate-shape knife to cast the dense layer; (3) two side-plates to fix these two knives together. There are five steps involved in this co-casting method: (1) put the substrate dope on top of the A4-size glass plate; (2) cast an approximately 40 mm long substrate using the traditional 16 co-casting knife; (3) put the dense layer dope on top of the substrate layer; (4) hold the co-casting knives to cast the dual-layer flat sheet membrane; (5) finish the membrane casting and immerse the nascent membrane in the coagulant bath for 10 hours (overnight). The detailed procedures of the universal co-casting method are illustrated in Fig. 1.2 (B). The method utilizes two individual cylindrical-shape casting knives. Each side of the knife can cast membranes with different thicknesses (100, 150, 200, 250 µm). As a result, the dual-layer membrane thickness can be adjusted and the interface between the two layers can be improved. There are five steps involved in this method: (1) put the substrate dope on top of the A4-size glass casting plate, and cast an approximately 40mm long substrate using the 200µm-side of the first casting knife; (2) put the dense layer dope on top of the substrate layer; (3) use the 250µm-side of the second casting knife for the dense layer and hold the two casting knives together to cast the dual-layer flat sheet membrane; (4) finish the membrane casting; (5) immerse the as-cast membrane in the coagulant bath overnight. After the phase separation process is completed, one must visually check the bonding condition between the top dense layer and bottom substrate layer. There are three possible scenarios: (1) the top dense layer is fully separated from the bottom substrate layer, (2) the top dense layer is partially separated from the bottom substrate layer, and (3) the top dense layer and bottom substrate layer are well integrated and could not be separated. The first two cases can be considered as delamination while the last case is deemed as 17 delamination-free. If no delamination can be found by visual check, the following studies are performed to optimise the dope formation: (1) morphological study of the interface between the dense and substrate layers by a scanning electron microscope (SEM JEOL JSM-5600LV) and a field emission scanning electron microscope (FESEM JEOL JSM-6700F), and (2) test the NF performance, i.e., pure water permeability and salt rejection, of the membranes. 2.5.3. Fabrication of PBI/POSS-PAN/PVP dual-layer hollow fiber PRO membranes The dual-layer hollow fiber membranes were fabricated by the co-extrusion technique using a tri-channel dual-layer spinneret. Specifically, the outer dope, the inner dope, and the bore fluid were fed into the spinneret separately by three ISCO syringe pumps. The dual-layer spinneret employed in this work has an indent feature [60]. Therefore, the outer dope and the inner dope were premixed before exiting the spinneret in order to improve the integration of the two layers. After that, the dopes and the bore fluid met at the tip of the spinneret, and then passed through an air gap region before entering the coagulation (water) bath. Finally, the as-spun dual-layer hollow fibers were collected by a take-up drum [33]. The detailed spinning parameters are shown in Table 2.3. The as-spun fibers were immersed in tap water for 3 days to allow solvent exchange to remove the residual solvents in the fibers prior to thermal 18 annealing. For module fabrication, 12 pieces of hollow fibers were bundled into a module housing with an effective length of 13.5 cm. Each module has a filtration area of about 30 cm2. The membrane thermal annealing process was performed according to the FO membrane annealing procedures. Ten modules were fabricated for the experiments. Six of them were used for the APS post-treatment, while the other four were for performance comparison without additional post treatments. Fig. 2.2. Schematic diagram of APS treatment setup 2.5.4. APS post treatment Six fabricated modules were submerged into DI water 1 day before the APS post-treatment. Then they were divided into two groups. The first group includes two modules that were subjected to the conventional APS 19 post-treatment method [57], by immersing the modules in a 5 wt% APS solution at 60°C for 6 hours. The rest four modules were treated with our new APS post-treatment method as described in Fig. 2.2. Four steps were involved in this treatment (1) pump 60°C DI water to the shell side 15 minutes continuously; (2) after 15 minutes, pump 60°C APS solution to the lumen side and recirculate for 1.5hrs; (3) stop the APS pump, but keep the hot water running for another 5 minutes; (4) wash both the lumen and the shell side with DI water for 0.5 hrs. The membranes were treated with APS solutions of four different concentrations, i.e., 3, 4, 5 and 6 wt%. 2.6. Membrane characterizations 2.6. 1. Morphology, mechanical strength and surface analysis The morphology of the hollow fiber membranes was observed by a scanning electron microscope (SEM; JEOL JSM-5600LV) and a field emission scanning electron microscope (FESEM; JEOL JSM-6700F). Before observation, the freeze dried hollow fibers were immersed in liquid nitrogen and fractured into small pieces with tweezers. Then, the small pieces of the fibers were stuck on a sample holder. Finally, the samples were coated with platinum using a JEOL JFC-1300 platinum coater. In addition, the linescan of energy dispersion of X-ray (EDX) was applied during SEM experiments to detect the nitrogen distribution profile across the interfacial region of the dual-layer FO membranes. The tensile strength of hollow fiber and flat sheet membranes was tested by an 20 Instron tension meter (model 5542, Instron Corporation). The membrane sample was clamped at both ends and pulled in tension at a constant elongation rate of 10 mm/min and an initial gauge length of 50 mm. Tensile strength, Young’s modulus, and the extension at break were obtained from the stress-strain curves. Five samples were measured for each membrane and the average was calculated from these results. The APS treated PRO membranes were flushed with DI water in both lumen and shell sides for 0.5hrs, and then immersed in DI water for 2 days to remove contaminants. Thereafter, the PRO membranes were dried in a freeze dryer for further characterizations. X-ray photoelectron spectroscopy (XPS, Kratos AXISUltraDLD spectrometer, Kratos Analytical Ltd) with a Mono Al KαX-ray source was employed to investigate the chemical changes on the PRO membrane surface. 2.6.2. Pure water permeability (PWP), salt rejection, salt permeability, pore size, and pore size distribution Pure water permeability A (or PWP) and salt rejection of the membranes were tested at a constant flow rate of 0.2 L/min (the linear velocities was about 0.2 m/s) and a hydraulic transmembrane pressure of 1, 6 and 10 bar at room temperature with their denser layers facing the feed solution. PWP (LMH/bar) was calculated using the equation: (2) where Q is the water permeation volumetric flow rate (L/h), Am is the effective 21 filtration area (m2), and ΔP is the hydraulic transmembrane pressure (bar). To determine the salt permeability, a 1000 ppm NaCl solution was used. The concentrations of salt in the feed (cf) and the permeate (cp) were determined by conductivity measurements. The salt rejection (RT) was calculated as follows: (3) Accordingly, the salt permeability B can be calculated based on Eq.(4) (4) where ∆P is the trans-membrane hydraulic pressure applied and ∆π is the osmotic pressure difference between the feed and permeate [61]. Pore size distributions of hollow fibers were tested by using a bench-scale NF setup that has been described elsewhere [62]. The feed solutes were ethylene glycol, glycerol, diethylene glycol, glucose, sucrose (five neutral solutes with progressively increased molecular weights). All NF experiments were conducted at a hydraulic transmembrane pressure of 1.0 bar at room temperature and the permeate water was collected from the lumen side of the membrane module because the outer layer is the selective layer. The salt concentrations in the feed and permeate solutions were measured using an electric conductivity meter (Lab 960, Schott) and the concentrations of the neutral solutes were determined using a total organic carbon analyser (TOC-VCSH, Shimadzu, Japan). The solute rejection (RT, %) was calculated using Eq. (3). The pore size distributions of hollow fiber membranes were determined by the 22 solute transport method that has been described elsewhere [62]. The radii (rs, nm) of the neutral solutes (ethylene glycol, glycerol, glucose, and sucrose) can be expressed by their molecular weights (MW) through Eq. (5): (5) The rejections of the neutral solutes were measured using the aforementioned NF setup. Then, the rejections of the solutes were related to their solute radii by the established log normal probability function, from which the molecular weight cut off (MWCO), mean pore radius (rp, nm), and the geometric standard deviation (σg) were obtained. MWCO refers to the lowest feed solute molecular weight in which 90% of the solute in the feed solution was retained by the membrane, where rp is equal to the rs at RT=50%, and σg is defined as the ratio of rs at RT =84.13% to that at RT = 50%. 2.7. FO tests The modules were tested in FO processes using a bench-scale FO setup [46] using 1.0 M NaCl as the draw solution and DI water was as the feed solution. The draw solution went through the shell side of the membrane in the module, and the draw solution and DI water were counter-currently flowed through the module. The water flux (Jw; LMH) was calculated using Eq. (6): (6) Where ∆V (litre) is the volumetric change of the feed solution over a predetermined time (∆t; hrs), and Am (m2) is the effective membrane surface area. 23 The salt reverse flux (Js, gMH) from the draw solute to the feed solution was determined by the increased conductivity of the feed solution when DI water was used as the feed solution as follows: (7) where ∆Ct and Vt are the salt concentration and the feed volume at the end of the tests, respectively. 2.8. PRO performance tests The modules were subjected to a PRO test using a customised bench-scale PRO setup, as illustrated in Fig. 2.3. A high-pressure piston pump (Hydra cell pump, Minneapolis, MN) was used to re-circulate the draw solution at 0.5 L/min (the linear velocities was about 0.2 m/s). A peristaltic pump (Masterflex, EW-07554-95) was used to re-circulate the feed solution at 0.2 L/min (the linear velocities was about 0.2 m/s). Two regulators in the bypass piping system were used to stabilise the system pressure. The pressure fluctuation of the system was minimised with this type of PRO setup during operation. In order to minimise the temperature effect on the system, a cooling circulator was installed to maintain the DS temperature at approximately 26±0.5°C. A 1.0 M NaCl solution was used as the draw solution to simulate seawater brine, and a 10 mM NaCl solution was used as the feed solutionto simulate river water. The active layer of the membrane was always facing the draw solution for the PRO tests. The PRO experiments were started from zero hydraulic pressure and then gradually increased. The membrane module permeate flux 24 was determined at predetermined time intervals (0.75hrs) by measuring the weight changes of the feed tank with a digital mass balance connected to a computer data logging system. The power per unit membrane area (power density), W is given by the following: (8) where ΔP (bar) is the hydraulic transmembrane pressure, and Jw (LMH) is the water flux through the membrane, which can be obtained from Eq. (6). Fig. 2.3. Schematic diagram of customised bench scale PRO performance testing setup. 25 CHAPTER TREE RESULTS AND DISCUSSIONS 3.1. FO membrane experiment result and discussion 3.1.1. Fabrication of delamination-free PBI-PAN/PVP dual-layer FO hollow fiber membranes Delamination is a common problem in dual-layer hollow fiber membranes. It is caused by uneven shrinkages between inner and outer layers due to material incompatibility and different phase inversion rates during the phase inversion process [59]. Fig. 3.1 shows a comparison of the PBI-PAN and the PBI-PAN/PVP dual-layer hollow fiber membranes. It can be clearly seen from Fig. 3.1(A) that the PBI-PAN hollow fiber has an inner layer full of finger-like macrovoids. In addition, serious delamination occurs between the outer and inner layers. Interestingly, the addition of PVP not only eliminates the macrovoids in the inner PAN support layer, but also results in an integrated and delamination-free dual-layer hollow fiber membrane, as shown in Fig. 3.1 (B). The improved morphology is possibly due to the following reasons: (1) It can be noticed from Fig. 3.2 (A) that the PVP-containing inner dope has a higher shear viscosity than the PAN solution without PVP. As a result, the former is more inclined to induce delayed demixing during the phase inversion process as revealed by the UV absorption curves in Fig. 3.2(B). It is known that delayed demixing favors the formation of the sponge-like structure and therefore, a macrovoid-free structure is observed across the PAN/PVP inner layer [59, 63]. (2) The prolonged phase inversion with the aid of PVP allows 26 A B Fig. 3.1. Cross-section morphology of hollow fibers prepared from (A) an outer layer dope of PBI/DMAC/LiCl (24:74.63:1.37) and an inner layer dope of PAN/NMP (25/75) and (B) an outer layer dope of PBI/DMAC/LiCl (24:74.63:1.37) and an inner layer dope of PAN/PVP360/NMP (16/11/73) B A Fig. 3.2. (A) Shear viscosity of the PAN/NMP=25/75 wt% solution and PAN/PVP/NMP=16/11/73 wt% solution and (B) the UV absorption curves of membranes cast from both solutions after immersion in water. more adequate interpenetration at the interface between the outer and inner layers. As the solidification rates of both layers may get closer, stresses are also more evenly distributed at the interface which stabilizes the interpenetrated dual-layer hollow fiber and eliminates the delamination phenomenon. (3) The PVP molecules may play the role of compatibilizers in 27 this process as well to facilitate the effective linkage and interpenetration between PBI and PAN polymers. Table 2.1 presents the structures and solubility parameters of the three polymers [64]. The solubility parameter of PVP lies in between the other two, making the polymer a quite suitable agent for bridging the other two polymers. (4) Hydrogen bonding might be formed between the carbonyl groups of PVP and the -NH groups of PBI before precipitation is completed [65, 66]. Elemental analyses by EDX across the interfacial regions in Fig. 3.3 confirm the hypothesis. Different from the nitrogen distribution profile of the delaminated hollow fiber where nitrogen content is the highest at the interface, the delamination-free membrane shows A B Fig. 3.3. Nitrogen atom distribution as characterized by EDX across the outer edge of (A) the delaminated fiber without PVP addition and (B) the delamination-free fiber with PVP addition. 28 slightly decreased nitrogen content in the transition area from the PBI layer to the interface, and then the nitrogen content grows higher in the PAN/PVP layer. Since the theoretical nitrogen content of PVP is the lowest as shown in Table 2.1, it can be concluded that PVP molecules accumulate near the interfacial region and act like a compatibilizer to assist in the formation of a fully integrated transition layer. 3.1.2. Cost-effective and mechanically strong dual-layer hollow fibers Comparing to single-layer membranes, dual-layer membranes have two major advantages. Since most functional materials for separation are much more expensive than the support materials, one can significantly reduce material costs by designing the membrane with an ultrathin outer selective layer made of the functional materials and an inner support layer made of low cost materials. The percentage of material cost saving can be estimated from the mass ratio of the outer layer to the inner layer if the inner layer material is really cheap. In addition, the dual-layer membrane has the ability to minimize the transport resistance by lowering the polymer concentration in the inner-layer dope for the support layer while maintaining a high concentration in the outer-layer dope for the selective layer. Many attempts have been carried out previously to either decrease the outer layer thickness or lower the inner dope concentration in order to enhance the performance of dual-layer hollow fiber membranes. 29 Table 3.1. A comparison of inner and outer dope flow rates, outer layer volume percentage and outer layer thickness in various dual-layer hollow fiber membranes. Membrane (dual-layer) Fi (inner dope flowrate, ml/min) Fo (outer dope flow rate, ml/min) α [Fo/(Fi+Fo)] To (µm)(outer layer thickness) Reference 3 0.06 2.0% 1.2±0.3 This Work 0.6 0.06 10.0% 2 [60] PBI/PEI 4 0.5 12.5% 16 [66] PBI/P84 2.4 0.6 25.0% 20 [67] PBI/PES 3 0.3 10.0% NA [4] PVDF-PAN-Nanoclay 2 0.5 25.0% 49.6 [68] PVDF-PTFE-Nanoclay 2 0.3 15.0% 25 [69] PBI-PAN-P0.5 Ultem/P84 PAI/CA 3 0.5 16.7% 7 [70] Matrimid/PSF 0.5 0.13 20.6% 11 [71] PAI/PES 4.6 1.7 27.0% 50 [13] As shown in Table 3.1, previously the thinnest outer layer was achieved by Widjojo et al who developed Ultem/P84 dual-layer hollow fibers with an Ultem outer selective layer of 2 µm thick [60]. As shown in Fig. 3.4 and Table 3.1, an ultrathin outer PBI layer with a thickness of 1.2±0.3 µm has been spun in this work by manipulating the spinning conditions as described in the experiment section. Table 3.1 also compares the material ratio of the outer layer to inner layer as defined in Eq. (9) between this work and other dual-layer hollow fibers which have been developed for water desalination, gas separation and pervaporation in recent years [13, 45, 60, 67-71]. (9) where Fo is the outer layer dope flow rate (ml/min), Fi is inner layer dope flow rate (ml/min). As can be seen in Table 3.1, α in this study is the smallest among all fibers and its outer layer is also the thinnest. This means the material saving of this work 30 is the most compared to all other membranes. The ultra-thin outer layer is also advantageous in lowering water transport resistance and increasing water permeability. A B C D Fig. 3.4. Cross-section morphology of PBI/POSS–PAN/PVP hollow fiber membranes as a function of POSS wt%.(A) PBI-PAN-P0 (no POSS), (B) PBI-PAN-P0.5 (C) PBI-PAN-P1.0 and (D) PBI-PAN-P1.5. In addition to the advantage of material saving, the dual-layer configuration combining with thermal annealing also improves the mechanical properties of PBI membranes. Table 3.2 compares the mechanical properties of PBI/POSS-PAN/PVP hollow fibers with and without thermal annealing as well as a dual-layer PBI membrane where PBI is used as both the outer and inner layers. Both annealed and un-annealed PBI/POSS-PAN/PVP hollow fibers show drastically improved mechanical properties comparing to the PBI-PBI dual-layer fibers. The PBI-PBI dual-layer membrane (designated as 31 ―PBI-PBI annealed‖) has the lowest tensile strength, Young’s modulus and elongation at break. By introducing PAN as the supporting material, the ―PBI-PAN-P0.5 as spun‖ hollow fiber possesses much stronger mechanical properties. The annealed PBI/POSS-PAN/PVP fiber (designated as ―PBI-PAN-P0.5 annealed‖) shows an impressive Young’s modulus of 340.9 ± 21.9 MPa due to the densified packing of polymer chains after thermal annealing. Clearly, the annealed PBI/POSS-PAN/PVP dual layer membrane is significantly stronger than the PBI-PBI fiber and can greatly expand the practical applications of the PBI material. Therefore, all fibers are annealed prior to applications in osmosis processes. Table 3.2. A comparison of mechanical properties of the PBI-PBI dual-layer and PBI-PAN-P0.5 dual-layer hollow fiber membranes with and without annealing. Hollow fiber dimension OD/ID (mm) Tensile strength (MPa) Young's modulus, (MPa) Elongation at break (%) PBI-PBI annealed* 0.95/0.58 1.3 ±0.3 121.9 ±24.0 2.3 ±0.7 PBI-PAN-P0.5 as spun 1.00/0.65 3.8 ±0.9 137.4 ±8.4 94.3 ±17.1 PBI-PAN-P0.5 annealed 0.85/0.43 5.1 ±0.9 340.9 ±21.9 32.0 ±6.6 Membrane *Spinning parameters of the PBI-PBI dual-layer membrane: outer layer dope composition: PBI/DMAC/LiCl (24/74.63/1.37); inner layer dope composition: PBI/DMAC/LiCl (22/76.63/1.37); bore fluid composition: DMAc/water (86/14 wt%); air gap 4.0 cm; take-up speed: 6.0 m/min; outer/inner/bore fluid flow rate 0.06/3.0/1.5 ml/min 3.1.3. Effects of POSS on the morphology of the hollow fibers After eliminating the delamination between the interface and macrovoids in the inner layer, we found that adding a small amount of POSS into the PBI-containing outer layer may also improve the morphology of the outer 32 layer. Fig.3.4 shows the cross-section morphology of PBI/POSS-PAN/PVP hollow fiber membranes as a function of POSS content increasing from 0 (PBI-PAN-P0) to 1.5 wt% (PBI-PAN-P1.5). As can be seen, the newly developed dual-layer membranes have delamination-free interfaces and outer layers with a thickness around 1.2±0.3µm, regardless of the POSS content added in the dope formation. Fig. 3.5. Schematic of the possible hydrogen bonding between PBI and POSS. However, the number of macrovoids of the outer layer decreases with increasing POSS content from 0 to 1.5%. This is probably due to the hydrogen bonding interaction between the -NH group of the PBI and the hydroxyl group of POSS [50, 52, 72], as illustrated in Fig.3.5. The hydrogen bonding interaction may reduce the mobility of polymer chains and hinder the torsional motions of chain segments. Hence the stiffness of polymer chains may increase with increasing POSS loading. Such entangled network structure possibly makes the nascent fibers stronger to balance shrinkage stresses, 33 hinders the nonsolvent intrusion, and hence eliminates the macrovoids. Therefore, the number of macrovoids is reduced. To our best knowledge, the cross-section structure of most PBI hollow fibers fabricated by the dry-jet wet spinning process in previous studies are full of finger-like macrovoids [4, 45, 46, 65], as listed in Table 3.3. This study provides a good starting point to make macrovoid-free PBI hollow fibers by adding a small amount of POSS into the dope. Meanwhile, characterized by FESEM, the thickness of the dense selective layer of the outer PBI layer is reduced by almost half from 310 nm to 160 nm with increasing POSS concentration from 0 to 1.5 wt%. This may be because the nanosized POSS molecules tend to move to the outer edge of the fibers and accumulate at the membrane-air interface to lower down the overall energy [26, 73]. The accumulation of nanoparticles might reduce the dense layer thickness [74]. Table 3.3. A comparison of pore size, PWP, rejection and structure of recent papers on PBI membranes. Membrane type rp (nm) σp MWCO (Da) PWP (LMH/bar) MgCl2 NaCl Rej. (%) Rej. (%) PBI out-layer macrovoid Reference PBI dual-layer (PBI-PAN-P0.5) 0.27 1.22 178 0.58 92.3 81.6 small tear-drop This work PBI Single-layer 0.32 1.28 293 -- -- -- big finger-like [46] PBI Single-layer 0.41 NA 993 2.43 75.0 39.0 big finger-like [45] PBI cross linked 0.29 NA 354 1.25 92.5 61.0 big finger-like [45] PBI dual-layer 0.40 1.16 338 1.74 87.2 40.0 big finger-like [4] PBI Hydrophilized 0.33 0.75 886 -- -- -- big finger-like [74] Note: Rej.: rejection 3.1.4. Effects of POSS on permeability and selectivity of hollow fibers in NF processes 34 Fig.3.6 shows the effects of POSS on water permeability (A) and salt permeability (B) of the thermally annealed hollow fiber membranes against NaCl and MgCl2 under NF tests. As displayed in Fig.3.6 (A), the A increases with increasing POSS loading. Interestingly, the A is almost tripled at the initial increase of POSS loading from 0 to 0.5 wt%, and then the slope of the curve is gradually flattened. Meanwhile, Fig.3.6 (B) shows both NaCl and MgCl2 salt permeability (B) increase with an increase in POSS content. Nevertheless, the B is increased only slightly when the POSS loading is 0.5 wt%, but a further increase in POSS leads to a much sharper increment in B. Clearly, the addition of POSS within the dense layer enhances both water and salts permeability across the membrane. This phenomenon may be due to two factors. As mentioned above, the thickness of the outer dense selective layer decreases with increasing POSS concentration. As a result, permeability increases with a decrease in dense layer thickness. On the other hand, water and salt permeability is proportional with defects and pore size. Since POSS has a cage like nanoscopic structure with a Si-Si distance of 0.5 nm and an R-R distance of 1.5 nm [26], the accumulation of POSS nanoparticles in the dense layer may create defects and large pore sizes, and results in higher water and salts permeability across the membrane. In summary, a 0.5 wt% POSS loading is the optimal concentration to get the best balance of water permeability and salt rejection for the membranes. The pore size distribution of the PBI-PAN-P0.5 membrane is then studied. A comparison of the current membrane with previous publications is tabulated in Table 3.3. The PBI-PAN-P0.5 membrane has a mean pore radius of only 0.27 35 nm, which is smaller than previous PBI FO membranes [4, 75, 76]. As a result, a much higher NaCl rejection (81.6%) is obtained, which offers potential FO and PRO applications as discussed in the following sections. A B Fig. 3.6. Effects of POSS concentration on the NF performance of PBI/POSS-PAN/PVP dual-layer membranes. (A) Water permeability A vs. POSS Loading, (B) salt permeability B vs. POSS Loading. Membrane: annealed PBI-PAN-P0.5; Operating pressure: 1 bar; Salt concentration: 200ppm 3.1.5. Application of annealed PBI/POSS-PAN/PVP membranes in engineered osmosis processes 3.1.5.1. Effects of POSS concentration on FO performance Fig.3.7 shows the effects of POSS concentration on FO performance of the newly developed dual-layer membranes. Both PRO and FO modes are studied. The draw solution and feed solution are 2.0 M MgCl2 and DI water, respectively. In the PRO mode, the water flux goes up sharply from 21.36 LMH with no POSS loading to 31.37 LMH as 0.5 wt% POSS is loaded into the membrane. The maximum water flux is achieved at 36.55 LMH with 1.0 wt% POSS content. 36 The water flux starts to decrease when POSS concentration is further increased due to the rapidly increased salt permeability of the membranes. The A B Fig. 3.7. The effects of POSS concentration on FO Performance of PBI/POSS-PAN/PVP membrane with 95°C annealing. (A) Water flux vs. POSS concentration, (B) salt reverse flux vs. POSS concentration. Draw solution: 2.0 MgCl2; feed solution: DI water; lumen side flow rate: 200ml/min; shell side flow rate: 300ml/min. Table 3.4. A comparison of FO performance of recent research on PBI membranes. Membrane type Membrane Orientation PRO Flux (LMH) (DS: 2.0M MgCl2) 31.37 Flux (LMH) (DS: 2.0M NaCl) 25.37 This work PBI Single-layer PRO 9.02 3.84 [45] PBI Single-layer PRO 25.0 -- [4] PBI cross linked PRO 5.0 -- [4] PBI dual-layer PRO 20.0 -- [50] PBI dual-layer (PBI-PAN-P0.5) REF DS: draw solution salt reverse flux goes up slightly when POSS loading increases from 0 to 0.5 wt% and then dramatically increases. The trends of water and salt fluxes in the FO mode are similar to those of the PRO mode. The data are generally in good 37 accordance with the NF results. The membrane comprising a 0.5 wt% POSS loading is again found to have the most balanced combination of both high water flux and low salt reverse flux. As compared in Table 3.4, this newly developed membrane exhibits the highest water flux among all PBI membranes recently developed for forward osmosis. B A Fig. 3.8. Effects of draw solution concentration on water permeation flux, and Js/Jw. Membrane orientation: PRO mode; Membrane: PBI-PAN-P0.5; Feed: DI water; Lumen side flow rate: 200ml/min; Shell side flow rate: 300ml/min. 3.1.5.2. Effects of draw solution concentration on the FO performance Fig.3.8 shows the effects of MgCl2 and NaCl concentrations on water flux of the PBI-PAN-P0.5 membrane and the ratio of salt reverse flux to water flux (Js/Jw, g/l) in the PRO mode. For both MgCl2 and NaCl draw solutions, the water flux steadily increases when the draw solution concentration increases from 0.5 M to 2 M due to the increased effective osmotic pressure as the driving force. However, the Js/Jw exhibits different trends for MgCl2 and NaCl draw solutions. The Js/Jw slightly declines from 0.32 g/l to 0.23 g/l using MgCl2 draw solutions, but steadily increases from 1.54 g/l to 1.88 g/l using 38 NaCl draw solutions. This phenomenon is due to the fact that the weakly positive charged PBI membrane exhibits higher rejections to divalent cations and lower rejections to monovalent cations. In addition, the stoke radius of the hydrated divalent Mg2+ ion is larger than that of the monovalent Na+ ion (0.43 vs. 0.36 nm) [77]. As a result, the membrane exhibits a high rejection to Mg2+ whereas a low rejection to Na+, which is attributed to the steric (size exclusion) effect and electrostatic partitioning interaction (Donnan exclusion) between the membrane and the draw solution [45]. The lower rejection of NaCl than that of MgCl2 may result in a higher salt leakage and a severer effect of internal concentration polarization, which reduces the effective osmotic pressure difference across the membrane when the draw solution concentration is increased. 3.1.5.3. The application of PBI-PAN-P05 membranes for osmotic power generation. From Table 3.3, the PBI-PAN-P05 membrane, which is loaded with 0.5 wt% POSS and thermally annealed, exhibits a high NaCl rejection of 81.6%, which is the best among previous PBI membranes. In the meantime, it is able to sustain a hydraulic pressure up to 7.5 bar. Fig.3.9 shows the experimental water flux and power density as a function of hydraulic pressure in the PRO process for osmotic power generation. The water flux (Jw) decreases almost linearly when the hydraulic pressure increases as a result of the reduced effective driving force. Interestingly, the slope of water flux decline between 0 and 7.0 bar is only 0.94 LMH/bar, which is much lower than previous 39 publications where serious flux declines were observed with increasing hydraulic pressure [36, 37, 78]. One possible reason is that unlike the previous flat-sheet membranes and inside-out hollow fiber membranes which deform severely under high pressures, the current outside-in hollow fiber is strong enough to resist compaction and cracking. The power density calculated by Eq.5 is steadily increased with hydraulic pressure and the maximum power density is 2.47 W/m2 at 7.0 bar. B A Fig. 3.9. Experimental and computed results of pressurized water flux (A) and power density (B) vs. hydraulic pressure difference in the PRO process. Membrane: 95°C annealed PBI-PAN-P0.5; draw solution: 1 M NaCl; feed solution: 0.01 M NaCl; draw flow rate: 300ml/min; feed flow rate: 200ml/min Table 3.5 compares the estimated power output from 8-inch modules made of outer-selective and inner-selective membranes. Each module comprises the same amount of hollow fiber membranes and each hollow fiber has the same dimension and power density as listed in Table 3.5. The 8-inch module made of outer selective membranes has an energy output of 269.23 W/module, while that of inner-selective membranes has only 136.20 W/module due to a smaller inner surface area. In other words, the inner-selective membrane needs about 2 40 times amount of modules and footprint to produce the same amount of energy as the outer-selective membrane. Even though the power density of the newly developed membrane in this study is not very high, its power density is equivalent to 4.88 W/m2 of the inner-selective membrane module, which fabricated with same packaging density and effective membrane area as outer selective membrane module. Additional advantages of the current outer-selective membrane include (1) it has both hydrophilic inner support and outer selective layer, which is favourable for less fouling under real industrial operations compared to most thin-film composite (TFC,) polyamide membranes and (2) it is fabricated in one step and therefore bears a lower fabrication cost comparing to TFC membranes. Table 3.5. Estimated power output per 8-inch module of outer and inner selective membrane modules which are comprised of the hollow fibers with the same dimension, and power density. Membrane type of modules Packing density [79] Fiber (mm) OD/ID Effective area (m2) Outer-selective 50% 0.85/0.43 109.0 2.47 269.23 Current work Inner-selective 50% 0.85/0.43 55.1 2.47 136.20 Hypothetical Power density Power output (w/m2) per module (w) Membrane Module specific dimensions: length: 2.0 m; diameter: OD/ID=219/192 mm; effective length: 1.76 m 3.2. PRO membrane experiment result and discussion 3.2.1. Development of the universal co-casting method for preparing dual-layer flat-sheet membranes Delamination is a typical issue in the fabrication of dual-layer hollow fiber 41 membranes. Due to material immiscibility and different phase inversion rates during the phase inversion process, uneven contraction (i.e., shrinkage rates) between the inner and outer layers is the main cause of delamination [42]. Resolving this problem in the hollow fiber spinning line is a time-consuming work. In order to save time and overcome this issue, we first conducted the traditional co-casting method, as shown in Fig.1.2 (A), which was developed by T. He in 2002.Two issues were found with this method; namely, (1) the top layer thickness could not be adjusted and (2) the 24 wt% PBI solution is too viscous to form a homogeneous top layer, as shown in the top right image of Fig.1.2 (A). This may arise from the following reasons: (1) the PAN/PVP concentration is so high that a serious die-swell phenomenon happens after passing through the first casting knife; (2) the plate-shape casting knife with a sharp edge cannot properly make the PBI/POSS dope and the swelled PAN/PVP substrate layer evenly integrated. As a result, the thickness of the PBI top layer is not uniform and the resultant defective membrane cannot be further tested by NF experiments. Clearly, the traditional co-casting method may only be suitable for the co-casting of lower polymer concentration dopes where the die-swell effect is small and the top layer can be evenly casted on top of the substrate. It may not be suitable for the search of dope formulations for the PRO dual-layer hollow fiber membrane because high dope concentrations are required for both outer layer and inner layer in order to produce membranes with both high mechanical strength and high salt rejection. A universal co-casting method was therefore developed to provide a simple tool 42 for dual-layer membranes. The detailed experiment procedure has been descripted in section 2.3. As shown in Fig. 1.2(B), a smooth and uniform PBI layer can be formed on the PAN substrate without delamination and cracks. A series of dual-layer flat-sheet and hollow fiber membranes were therefore fabricated from the same dope formulations and their properties were tested. The results will be discussed in later sections. In summary, the new casting device can effectively identify proper dope formulations for dual-layer membranes and save significant time and materials during the trial and error of the tedious hollow fiber spinning process. A C B D Fig. 3.10. Cross-section morphology of PBI/POSS-PAN/PVP flat sheet membranes as a function of PVP wt%. 3.2.2 Optimization of dope formulation for delamination-free dual-layer flat sheet membranes using the universal co-casting method Dual-layer flat sheet membranes with various PVP concentrations were prepared from the universal co-casting method. The as-cast membranes were checked visually and then investigated by FESEM, Table 2.4 and Fig. 3.10 43 show the results. From Table 2.4, immediate delamination happened for the membrane spun from the substrate dope without containing PVP. When 2 wt% PVP was added into the substrate dope, the delamination happened after it was immersed in DI water for 10 hrs. When the PVP concentration was further increased to over 3 wt%, no delamination was visually observed. These membranes were also investigated by FESEM after freeze drying. As shown in Fig. 3.10, partial delamination happened after freeze drying for dual-layer membranes spun from the substrate dope containing 3 wt% PVP. When the PVP concentration is above 4 wt%, all of the membranes become delamination-free. A B Fig. 3.11(A). PVP concentration vs. substrate dope viscosity and (B) PVP concentration vs. PWP and salt rejection of flat sheet dual-layer membranes. Operating pressure: 1 bar; salt concentration: 200 ppm. Clearly, the PVP concentration plays an important role to mitigate the delamination issue. Therefore, dope viscosity, PWP and salt rejection were measured in order to understand the science behind it. Fig. 3.11(A) shows the viscosity of the substrate dope increases sharply with an increase in PVP concentration. The interception of two extrapolation lines between viscosity vs. low and high PVP concentrations is at 5.5 wt% PVP which is the critical 44 concentration where chain entanglement becomes serious [31]. Based on the previous study to form macrovoid-free hollow fibers [80], membranes spun from dopes with a critical polymer concentration and above have tendency to form a sponge-like morphology. Thus, 6.0 wt% PVP was selected. Fig. 3.11(B) displays the NF performance of the membranes as a function of PVP % in the substrate dope. The salt rejection dramatically increases to 74.1% when the PVP concentration is increased from 3 to 6 wt%. The salt rejection remains almost the same if further increasing the PVP concentration. Similarly, PWP increases sharply from 0.65 to 0.82 LMH/bar when the PVP concentration is increased from 3.0 to 4.0 wt%, and then PWP declines slowly from 0.82 to 0.64 when the PVP concentration is further increased to 8.0 wt%. The high flux but low rejection at 3-4% PVP is due to the fact that the top dense layer and the bottom substrate layer are not integrated properly as proved by the FESEM morphology. The delamination-free structure and improved salt rejection with increasing PVP concentration are possibly due to the following reasons (1) the addition of hydrophilic PVP not only increases the dope viscosity but also induces delayed demixing and extends phase inversion time [33]. These two effects may facilitate interpenetration at the interface between the selective and the substrate layers. As the viscosity of the top and bottom layers is getting closer, shear stresses are more evenly distributed at the interface, which stabilises the interface and eliminates the delamination; (2) As shown in Table 2.1 [58], the solubility parameter of PVP lies in between those of PBI and PAN, making PVP has good miscibility with both PBI and PAN [31, 79]. This makes PVP 45 as an agent suitable for bridging these two polymers; and (3) As the PVP concentration increases, more hydrogen bonding might be formed between the carbonyl groups of PVP and the -NH groups of PBI before precipitation is completed [33, 72, 80]. As a result, PVP molecules accumulate near the interfacial region and act as compatibilizers to assist the formation of a fully integrated interface layer. Therefore, with the increase of PVP concentration, the interface layer may become denser, which accounts for the increase of salt rejection and the decrease of PWP [81].The PAN/PVP/NMP (6/21/73) is therefore chosen for hollow fiber membrane fabrication. A B Fig. 3.12. Morphology of PBI/POSS-PAN/PVP hollow fiber membranes. (A). General morphology of PBI/POSS-PAN/PVP dual-layer hollow fiber membranes. (B). Cross-section morphology of PBI/POSS-PAN/PVP hollow fiber membranes as a function of PVP concentration. 3.2.3. Verification of the universal co-casting method by dual-layer hollow fiber spinning The results obtained from the universal co-casting method were verified by 46 spinning dual-layer hollow fiber membranes with the same dope solutions. Fig. 3.12(A) shows the overall morphology of the dual-layer hollow fiber membranes. The membranes spun from four inner layer dopes containing different PVP concentrations all have porous inner surfaces and dense outer surfaces. As shown in Fig. 3.12(B), when the PVP concentration is 3 wt%, the interface layer has slight delamination. As the PVP concentration further increases, the delamination is eliminated and the interface layer becomes denser. As shown in Fig. 3.12(B), such morphological evolution causes the salt rejection dramatically raises from 61.3% to 94.1% when the PVP concentration is increased from 3.0 wt% to 6.0 wt%. The salt rejection is only slightly increased to 94.7% when the PVP concentration is further increased from 6 to 8 wt%, but the PWP drops from 0.42 to 0.36 LMH/bar. Therefore, the NF test results indicate that PAN/PVP/NMP (6/21/73) is the best substrate dope formulation for the dual-layer hollow fiber membrane fabrication. This conclusion is consistent with the results from the universal co-casting method for dual-layer flat sheet membranes. 3.2.4. PRO membrane development with APS assisted post-treatment From the above observation, PVP serves a critical role in eliminating delamination and increasing salt rejection. However, the entrapped large PVP molecules are adverse to membrane flux. As shown in Fig. 3.13, the PWP of the optimized dope formulation is only 0.42 LMH/bar. By immersing the membrane modules in a 5 wt% APS solution for 6 hours at 60°C with the conventional APS post-treatment method, the pure water permeability was 47 increased to 1.54 LMH/bar at 1 bar, but the PBI dense thin layer was damaged when the hydraulic pressure was raised to 5 bar during the permeability test. This was probably due to the decomposition of PVP molecules in both of the substrate and the selective layers due to the reaction between APS and the pyrrolidone ring of PVP [57] and the adhesion between the selective and the substrate layers becomes weak. Therefore, a thin PBI selective layer of about 1.2 µm can be easily damaged under high hydraulic pressure tests such as the PRO test. In our customized APS post-treatment method, the 60°C APS solution was flowed in the lumen side while 60°C DI water was flowed in the shell side at a pressure of 0.5 bar as described in section 2.5 and Fig. 2.2. With this method, most PVP molecules in the substrate are degraded and removed during by the APS post treatment. However, the pressurization from the shell side by the 60°C DI water may minimize the PVP degradation in the outer and interfacial layers as well as maintain the integrity of the dual-layer membrane during the PVP leaching process in the substrate layer. Fig. 3.13. Effects of PVP concentration on the NF performance of dual-layer hollow fiber membranes. 48 Fig. 3.14 illustrates the effect of APS concentration on FO performance under the PRO mode (the active-layer faces the draw solution using1M NaCl as the draw solution and DI water as the feed solution). The water flux of the resultant membranes increases with an increase in APS content. The highest water flux (22.4 LMH) occurs when the APS concentration is 5.0 wt%. However, the salt reverse flux also increases slowly with increasing APS concentration from 0 to 5.0 % but sharply jumps at 6.0 wt% APS. Therefore, 5.0 wt% is the best APS concentration for PVP removal that offers the highest water flux and a reasonable salt reverse flux. B A Fig.3.14. Effects of APS concentration on FO performance of hollow fibers under the PRO mode. (A) FO Water flux vs. APS concentration; (B) Salt reverse flux vs. APS concentration. The degradation and removal of PVP can be proven by XPS analyses and the membrane colour changes, as presented in Table 3.6 and Fig. 3.15, respectively. Table 3.6 shows that the N content of neat PBI membranes and PVP are 3.81 % and 10.04 % respectively. The N content of PBI-PAN-P6 membranes after annealing at 60 °C hot water (designed as PBI-PAN-P6-T60) 49 is significantly increased to 9.03% which is quite close to the N content of PVP. However, after treating by 6 wt% APS, the N content of PBI-PAN-P6 Fig. 3.15. Color changes of membranes with different APS concentrations: (A) 0 wt% APS solution, (B) 4.0 wt% APS solution, (C) 6.0 wt% APS solution, (D) Single-layer PBI hollow fiber membrane. Table 3.6. Atomic concentration of PVP polymer and outer surface of outer-layer of membranes analyzed from XPS. Atomic concentration PBI single layer membrane1 PVP PBI-PAN-P6-T602 PBI-PAN-P6-APS63 C (%) 81.23 79.71 79.58 80.23 N (%) 3.81 10.04 9.03 4.88 O (%) 14.96 10.25 11.39 14.89 1. PBI single layer membrane spun from PBI/DMAc/LiCl/POSS (24/74.13/1.37/0.5) which has the same as the dual-layer PBI-PAN-P6 hollow fiber membrane. 2. The PBI-PAN-P6 hollow fiber membrane annealed at 60°C hot water was designated as PBI-PAN-P6-T60 3. The PBI-PAN-P6 hollow fiber membrane treated at a 60°C hot 5 wt% APS solution was designated as PBI-PAN-P6-APS6 membranes (designed as PBI-PAN-P6-APS6) decreases to 4.88 % which is close to the N content of PBI single layer membranes. In other words, part of PVP in the outer layer of the membranes has been removed by the APS 50 post-treatment. The higher the APS concentration, the more the PVP degradation is. Similarly, the degradation may also happen at the interface layer. This may weaken the integrity of the interface and result in a lower salt rejection. Fig. 3.15 shows that the membrane colour becomes darker with an increase in APS concentration. The colour of the PBI-PAN-P6-APS6 membrane is close to the colour of single-layer PBI membranes because of the removal of PVP. This phenomenon further proves the XPS findings. Table 3.7. A comparison of pore size, PWP, rejection and burst pressure of recent papers on PBI membranes. rp (nm) σp MWCO (Da) PWP (LMH/bar) NaCl Rej. (%) Tensile strength (Mpa) Burst Pressure (bar) Power density (w/m2) 0.26 1.23 169 0.42 94.2 21.2±1.2 17.0±1 3.8±0.2 This work 0.31 1.26 220 1.28 61.2 21.1±1.2 17.0±1 5.1±0.3 This work PBI dual-layer 0.27 1.22 178 0.58 81.6 5.1 ±0.9 7.5 2.47 [33] PBI Single-layer 0.32 1.28 293 -- -- -- -- [46] PBI Single-layer 0.41 NA 993 2.43 39.0 -- -- [45] PBI cross linked 0.29 NA 354 1.25 61.0 -- -- [45] PBI dual-layer 0.40 1.16 338 1.74 40.0 -- -- [4] PBI hydrophilized 0.33 0.75 886 -- -- -- -- [74] Membrane type PBI dual-layer (PBI-PAN-P6-T60) PBI dual-layer* (PBI-PAN-P6-APS5) Reference 3.2.5. The application of PBI-PAN-PVP6 membranes for osmotic power generation Table 3.7 shows a comparison of pore size, PWP, rejection, and burst pressure of recently developed PBI membranes in literatures [4, 33, 45, 46, 75]. The PBI-PAN-P6-T60 membrane, which is loaded with 6.0 wt% PVP and thermally annealed, exhibits the highest NaCl rejection of 94.2%. This is the best rejection among previous PBI single- and dual-layer membranes. However, its PWP 0.42 LMH/bar is the lowest, and so it may not be a good 51 choice for PRO application. After post-treatment by 5 wt% APS, the PBI-PAN-P6 membrane (designated as PBI-PAN-P6-APS5) possesses a much higher PWP of 1.28 LMH/bar even though the salt rejection drops to 61.2%. Both of these two PRO membranes have high burst pressures and good tensile strengths. B A Fig. 3.16. (A) Water flux and (B) power density of the PBI-PAN-P6-T60 hollow fiber membranes before and after APS post-treatment. Draw solution: 1 M NaCl; feed solution: 0.01 M NaCl; draw flow rate: 300 ml/min; feed flow rate: 200 ml/min. Temperature 26°C. Fig. 3.16 displays the PRO performance of both PBI-PAN-P6-T60 and PBI-PAN-P6-APS5 membranes. The water fluxes (Jw) of both membranes decrease almost linearly with an increase in hydraulic pressure as a result of reduced effective driving forces across the membranes. The power densities of both membranes increase steadily as the hydraulic pressure increases. The maximum power densities are 5.1 w/m2 at 15.0 bar for the PBI-PAN-P6-APS5 membrane and 3.7 W/m2 at 15.0 bar for the PBI-PAN-P6-T60 membrane. Due to the improved water permeability, the membrane with the APS treatment has better PRO performance than that without treatment. Both membranes can sustain a hydraulic pressure up to 16 52 bar. Although the membranes do not burst at 17bar, the water fluxes sharply decrease after 16.0 bar. This phenomenon indicates that their selective layers have been damaged due to high operation pressures. These results confirm that the 5.0 wt% APS post-treatment could effectively remove PVP, enhance the water flux, augment membrane PRO performance and produce a much higher power density than the membrane without the APS post-treatment. 53 CHAPTER FOUR CONCLUSIONS AND RECOMMENDATIONS We have developed novel dual-layer hollow fiber membranes consisting of a PBI/POSS outer-selective layer and a PAN/PVP inner layer for forward osmosis and power generation applications. The roles of PVP on macrovoids and interfacial integration as well as the effects of POSS concentration on membrane morphology and osmotic performance have been investigated. The following conclusions can be made from this study: 1) The addition of PVP effectively eliminates the finger-like macrovoids in the PAN support layer and resolves the interfacial delamination problem of the PBI/PAN dual-layer hollow fibers, which may be attributed to the increased dope viscosity and the function of PVP as a compatibilizer between PAN and PBI layer. 2) A minimum concentration of 4 wt% PVP is required to eliminate delamination in both dual-layer flat-sheet and hollow fiber membranes. As the PVP concentration further increases, the salt rejection improves while the PWP decreases. 6 wt% is the optimum PVP concentration to produce delamination-free dual-layer membranes with the highest water permeability and salt rejection. 3) The incorporation of POSS into the PBI layer may reduce the dense layer thickness and eventually reduce the macrovoids in the PBI layer. 54 4) Increasing POSS loading in the PBI dope enhances both water and salt permeability across the membranes, while the salt rejection stays almost the same at a low POSS loading and then drops dramatically at higher POSS content. A POSS loading of 0.5 wt% has been identified through NF and FO tests as the optimal concentration in this study. 5) A universal co-casting method was developed to replace the traditional co-casting method. By utilizing two individual casting knives with different thicknesses, the new method provides possibility to cast homogeneous dual-layer flat sheet membranes from highly concentrated dopes. With the aid of the universal co-casting method, the consumption of time and materials can be dramatically reduced in the development of delamination-free dual-layer hollow fiber membranes. 6) Continuously flowing an APS solution in the lumen side of hollow fiber membranes can degrade PVP molecules entrapped in the substrate layer. As the APS concentration increases, both water flux and salt leakage increase. The5.0 wt% APS solution has been identified as the optimum concentration for the post-treatment. 7) The newly developed PBI-PAN-P05 FO membrane with the optimized POSS loading shows a maximum water flux 31.37 LMH at room temperature using 2.0 M MgCl2 as the draw solution in the FO 55 process. 8) The newly developed PBI-PAN-P6-APS5 PRO membrane has shown promising results for osmotic power generation. It has a power density of 5.1 w/m2 using 1 M NaCl and 10 mM NaCl as the draw solution and feed solution, respectively. To the best of our knowledge, this is the best PRO performance of outer-selective dual-layer hollow fiber membranes made from phase inversion methods. In summary, the newly developed PBI-PAN dual-layer membranes have shown promising results in both FO and PRO processes. With its unique outer dense-selective skin, hydrophilic inner-layer and outer-layer structure, and easy processability, this membrane may have wide applications in the future for osmotic power generation as well as for nanofiltration (NF), ultrafiltration (UF) and other applications. The key challenge for this kind of PBI dual-layer membrane development is that how to effectively reduce PVP content in the substrate layer in order to get higher water permeability and better mechanical strength. One possible solution is that separating the substrate layer to two layers, one ultra-thin layer with PVP additive is used to integrate with the outer selective layer, while another layer without PVP is used for support layer only. This kind of design could significantly reduce the PVP content in the substrate layer. Therefore, higher water permeability and good mechanical strength could be achieved without additional post treatment for PVP removal. 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Sci, 243 (2004) 45-57. 68 [...]... dual- layer PBI/POSS-PAN/PVP hollow fiber membrane This work may provide useful insights for the development of outer selective FO and PRO hollow fiber membranes for osmotic power generation as well as for nanofiltration (NF), ultrafiltration (UF) and other applications Fig 1.2 Schematic diagram of dual- layer flat sheet membrane co-casting processes (A) Traditional dual- layer flat sheet membrane co-casting... flat-sheet dual- layer membranes It is a useful tool to evaluate the adhesion between the inner and outer layers before conducting the dual- layer hollow fiber spinning [54] However, this method suffers from inflexibility of film thickness and incapability of casting highly viscous solutions It can be only applicable for low concentration dope solutions with low viscosity For dual- layer PRO hollow fiber membranes,... dope viscosity and (B) PVP concentration vs PWP and salt rejection of flat sheet dual- layer membranes Fig 3.12 44 Morphology of PBI/POSS-PAN/PVP hollow fiber membranes Fig 3.13 46 Effects of PVP concentration on the NF performance of dual- layer hollow fiber membranes Fig.3.14 Effects of APS concentration on FO performance of hollow fibers under the PRO mode Fig 3.15 49 Color changes of membranes with... to develop forward osmosis (FO) and PRO hollow fiber membranes [8, 22, 27, 33-38] The TFC membrane, which is composed of a porous support layer and an ultra-thin dense selective layer, has been the focus of most studies since it has shown better PRO performance However, it is difficult to scale up the interfacial polymerization process for TFC hollow fiber membranes In addition, the TFC membrane is... strong candidate for the development of FO and PRO membranes With its excellent thermal stability, super resistance to strong acids and alkalis, and easy film-forming properties, it has the potential to become a good selective layer material for the development of dual- layer FO and PRO membranes [4, 33, 43-45] However, drawbacks such as high price and brittleness affect its industrial-scale membrane. .. 2.3 for hollow fiber membrane spinning PAN and PVP 15 were firstly dissolved in NMP by continuously stirring at 60 °C for 12 hours to form a homogeneous solution, and then were degassed in air for 24 hours before use Table 2.4 shows the detailed compositions of the PBI and PAN solutions, which were used for the co-cast of flat sheet membranes 2.5.2 Fabrication and evaluation of dual- layer flat-sheet membranes... for defect-free dual- layer hollow fiber spinning in the FO membrane development process of this work 2.4.3 Post treatment and module fabrication The as-spun fibers were immersed in tap water for 3 days prior to thermal annealing The optimized membrane annealing procedures are (1) soaking the fibers in 95°C hot water for 3 minutes, (2) drying in the air for 3 minutes for fiber relaxation, and (3) immersing... dual- layer hollow fibers were collected by a take-up drum The proper spinning parameters for ultra-thin outer selective layer and defect-free dual- layer hollow fiber spinning were worked out after several trials Fig 2.1 (A) Scheme of the dual- layer spinneret and (B) the hollow fiber spinning line Pump A: inner dope solution; pump B: bore fluid; pump C: outer dope solution 2.4 FO membrane development. .. delamination phenomenon between the outer PBI layer and inner PAN layers, and (3) develop PBI-PAN with higher FO and PRO performance 6 1.3 Overall strategies and objectives Three strategies were employed in this work (i) to modify the PBI dual- layer membrane with enhanced salt rejection and mechanical strength by heat annealing (ii) to modify the PBI dual- layer membrane with enhanced permeability by polyhedral... dual- layer hollow fiber PRO membranes The dual- layer hollow fiber membranes were fabricated by the co-extrusion technique using a tri-channel dual- layer spinneret Specifically, the outer dope, the inner dope, and the bore fluid were fed into the spinneret separately by three ISCO syringe pumps The dual- layer spinneret employed in this work has an indent feature [60] Therefore, the outer dope and the inner .. .DUAL-LAYER HOLLOW FIBER MEMBRANE DEVELOPMENT FOR FORWARD OSMOSIS AND OSMOSIS POWER GENERATION FU FENG JIANG (B.Eng., Tianjin University) A THESIS SUBMITTED FOR THE DEGREE OF... PBI/POSS-PAN/PVP hollow fiber membrane This work may provide useful insights for the development of outer selective FO and PRO hollow fiber membranes for osmotic power generation as well as for nanofiltration... thickness in various dual-layer hollow fiber membranes Table 3.2 30 A comparison of mechanical properties of the PBI-PBI dual-layer and PBI-PAN-P0.5 dual-layer hollow fiber membranes with and without