Kaiyu Wang
Email Address
chewkai@nus.edu.sg
Organizational Units
COLLEGE OF DESIGN & ENG
faculty
ENGINEERING
faculty
40 results
Publication Search Results
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Publication Novel polyamide-imide/cellulose acetate dual-layer hollow fiber membranes for nanofiltration(2010-11) Sun, S.P.; Wang, K.Y.; Peng, N.; Hatton, T.A.; Chung, T.-S.; CHEMICAL & BIOMOLECULAR ENGINEERINGA novel dual-layer nanofiltration (NF) hollow fiber membrane was fabricated by the simultaneous co-extrusion of Torlon® polyamide-imide and cellulose acetate dopes through a triple-orifice spinneret in a dry-jet wet phase inversion process. For the first time, the nanopores of dual-layer hollow fiber membranes were molecularly designed by controlling the phase inversion process with the aid of various non-solvent additives into the polymer solutions. Compared to ethanol and 2-propanol, the addition of methanol into the dope led to a significantly decreased pore size but dramatically increased pure water permeability. The improved NF performance may be attributed to (1) a controllable thin selective outer layer; (2) a less resistant interface between the outer and inner layers; and (3) a fully porous substructure with reduced transport resistance. In addition to non-solvent additives, spinneret temperature also plays an important role in designing dual-layer hollow fiber membranes with desirable NF performance. When the spinneret temperature was increased from 25°C to 50°C, the mean effective pore radius and the pure water permeability were simultaneously decreased, which was due to the formation of a denser surface skin and a more compact interface between the two layers. In addition to exhibiting a higher rejection of divalent anions than monovalent anions, and a lower rejection of divalent cations at pH 7.0, the newly developed NF dual-layer hollow fiber membranes with methanol as additive has a relatively high pure water permeability of 11.93lm-2bar-1h-1 with a mean effective pore radius of 0.63nm. These concepts hold great potential for the design of tailor-made NF membranes for various industrial applications. © 2010 Elsevier B.V.Publication The effects of flow angle and shear rate within the spinneret on the separation performance of poly(ethersulfone) (PES) ultrafiltration hollow fiber membranes(2004-09-01) Wang, K.Y.; Matsuura, T.; Chung, T.-S.; Guo, W.F.; CHEMICAL & BIOMOLECULAR ENGINEERINGThe effects of dope flow rate and flow angle within a spinneret during spinning hollow fiber membranes on the morphology, water permeability and separation performance of poly(ethersulfone) ultrafiltration hollow fiber membranes were investigated. For this purpose, two spinnerets with different flow angles were designed and used. The dope solution, containing polyethersulphone (PES)/N-methyl-2-pyrrolidone (NMP)/diethylene glycol (DG) with a weight ratio of 23/41/36, which was very close to its cloud point (binodal line), was used in order to speed up the coagulation of nascent fibers so that the relaxation effect on molecular orientation was reduced. The wet-spinning process was purposely chosen to fabricate the hollow fibers without extra drawing. Therefore, the effects of gravity and elongation stress on fiber formation could be significantly reduced and the orientation induced by shear stress within the spinneret could be frozen into the wet-spun fibers. Experimental results suggest that higher dope flow rates (shear rates) in the spinneret produce UF hollow fiber membranes with smaller pore sizes and denser skin layers due to the enhanced molecular orientation. Hence, the pore size and the water permeability decrease, but the solute separation increases. Hollow fibers spun from a conical spinneret have smaller mean pore sizes with larger geometric standard deviations, thus exhibiting lower water flux and greater solute separation than hollow fibers spun from a traditional straight spinneret. In addition, SEM studies indicate macrovoids response differently for the 90°straight and 60°conical spinnerets when increasing the dope flow rate. Macrovoids can be significantly suppressed and almost disappear in the 90°spinneret at high dope flow rates. This phenomenon cannot be observed for the 60°conic spinneret. © 2004 Elsevier B.V. All rights reserved.Publication Highly water-soluble magnetic nanoparticles as novel draw solutes in forward osmosis for water reuse(2010-06-16) Ling, M.M.; Wang, K.Y.; Chung, T.-S.; CHEMICAL & BIOMOLECULAR ENGINEERINGHighly hydrophilic magnetic nanoparticles have been molecularly designed. For the first time, the application of highly water-soluble magnetic nanoparticles as novel draw solutes in forward osmosis (FO) was systematically investigated. Magnetic nanoparticles functionalized by various groups were synthesized to explore the correlation between the surface chemistry of magnetic nanoparticles and the achieved osmolality. We verified that magnetic nanoparticles capped with polyacrylic acid can yield the highest driving force and subsequently highest water flux among others. The used magnetic nanoparticles can be captured by the magnetic field and recycled back into the stream as draw solutes in the FO process. In addition, magnetic nanoparticles of different diameters were also synthesized to study the effect of particles size on FO performance. We demonstrate that the engineering of surface hydrophilicity and magnetic nanoparticle size is crucial in the application of nanoparticles as draw solutes in FO. It is believed that magnetic nanoparticles will soon be extensively used in this area. © 2010 American Chemical Society.Publication The observation of elongation dependent macrovoid evolution in single-and dual-layer asymmetric hollow fiber membranes(2004-11) Wang, K.Y.; Li, D.F.; Chung, T.-S.; Chen, S.B.; CHEMICAL & BIOMOLECULAR ENGINEERINGWe have reported, for the first time, that macrovoids in asymmetric hollow fiber membranes may be completely eliminated at high elongational draws. The evolution of macrovoids vs. elongational draw was observed for both single- and dual-layer hollow fiber membranes. The number of macrovoids and the number of macrovoid layer decrease with an increase in elongational draw ratio, while the dimension of macrovoids varies with increasing elongational draw ratio until the macrovoids are fully eliminated. This study indicates that the elongational stress may play a much more important role than our original thoughts on hollow fiber membrane morphology. © 2004 Elsevier Ltd. All rights reserved.Publication Molecular design of the cellulose ester-based forward osmosis membranes for desalination(2011-05-01) Zhang, S.; Wang, K.Y.; Chung, T.-S.; Jean, Y.C.; Chen, H.; CHEMICAL & BIOMOLECULAR ENGINEERINGThis work has investigated the fundamental science of phase inversion and formation mechanism of cellulose ester membranes at the interface between polymer and casting substrate. It also explores the desired membrane preparation conditions for forward osmosis (FO) applications. With the aid of positron annihilation lifetime spectroscopy (PALS), the similarity in physicochemical properties between the polymer and the substrate was found to play a significant role in determining the porosity of the bottom interfacial layer. The structure of the dense interfacial layer was also strongly dependent on membrane thickness and solvent composition. Experimental results surprisingly reveal that the original pore size of the as-cast membrane plays a critical role determining the final performance of the subsequent annealed membrane independently of annealing temperature and time. In addition, since a threshold pore size exists during annealing above which pores become difficult to downsize, we have found that a thin dense selective layer integrated in an asymmetric membrane may not always be the best option for FO. A balanced membrane structure consisting of a thin porous support and a thin dense selective layer has been developed for FO, which shows a low internal concentration polarization (ICP) and a relatively high water flux when seawater was employed as the feed. © 2011 Elsevier Ltd.Publication Forward osmosis processes: Yesterday, today and tomorrow(2012-11-15) Chung, T.-S.; Zhang, S.; Wang, K.Y.; Su, J.; Ling, M.M.; CHEMICAL & BIOMOLECULAR ENGINEERINGThe purpose of this short communication is to share our perspectives on future R & D for FO processes in order to develop effective and sustainable technologies for water, energy and pharmaceutical production. © 2010 Elsevier B.V.Publication The investigation of irregular inner skin morphology of hollow fiber membranes at high-speed spinning and the solutions to overcome it(2006-10-05) Santoso, Y.E.; Chung, T.S.; Wang, K.Y.; Weber, M.; NUS NANOSCIENCE & NANOTECH INITIATIVE; CHEMICAL & BIOMOLECULAR ENGINEERINGIn the fabrication of hollow fiber membranes, a high-speed spinning is important to maximize the productivity and minimize the production cost. However, the high-speed spinning process has been shown to influence the morphology of the resultant fibers, especially inner layer of the fibers. For the first time, we report that the increase in the take-up speed results in the deformation of inner shapes of the fibers. This phenomenon is not desirable as it results in the formation of low mechanical strength fibers. It is hypothesized that the phenomenon occurs due to the formation of vacuum condition in the lumen side of the fiber, which may be due to rapid formation of dense skin inner layer. With increasing take-up speed the inner surface becomes denser leading to insufficient bore fluid supply which promotes more vacuum condition in the lumen side. Four possible solutions were proposed to eliminate the inner layer deformation phenomena, i.e. (1) by increasing the bore fluid flow rate, (2) by chemical modification of the bore fluid, (3) by simultaneously increasing dope and bore flow rates while maintaining a constant ratio between them and (4) by simultaneously increasing bore and dope flow rates in the same order with the increase in take-up speed. The increase in bore fluid flow rate can effectively eliminate the deformation. The addition of the solvent to the bore fluid can also eliminate the deformation when 70 wt.% NMP/water was used as bore fluid. No inner layer deformation was observed when dope and bore flow rates were increased up to 4 and 3 ml/min, respectively. However, the first three approaches may have negative side effects. The fourth approach gives us the most promising results. Even after increasing take-up speed up to 470% from its original take-up speed, the resultant fibers show no irregular formation at inner layer morphology and no changes in their inner and outer diameter. Thus, it can be said that by utilizing this approach, more flexible and easy control over fibers can be achieved. © 2006 Elsevier B.V. All rights reserved.Publication Dual-layer hollow fibers with enhanced flux as novel forward osmosis membranes for water production(2009-04-15) Yang, Q.; Wang, K.Y.; Chung, T.-S.; CHEMICAL & BIOMOLECULAR ENGINEERINGWe have demonstrated in this work the prospect of dual-layer polybenzimidazole-polyethersulfone (PBI-PES) nanofiltration (NF) hollow fiber membranes in the forward osmosis (FO) process for water production: The state-of-the-art for dual-layer membrane fabrication via coextrusion technology could produce the resultant membrane consisting of an ultrathin selective skin, fully porous water channels underneath, and a microporous sponge-like support structure. Together with its sharp pore size distribution and self-charged PBI selective membrane surface, the dual-layer hollow fiber forward osmosis membrane can achieve a water flux as high as 33.8 L m-2hr-1 and a salt flux less than 1.0 g·m-2·hr-1 at room temperature of 23 °C using 5 M MgCl2 as the draw solution. A comprehensive literature review of previous efforts on identifying suitable membranes and appropriate draw solutions in the FO process for water production and seawater desalination have also been conducted. It shows that the water fluxes of the dual-layer hollow fiber FO membrane developed in this work utilizing MgCl2 as the draw solutions generally surpasses those FO processes utilizing RO membranes and is comparable to most FO processes using commercial FO membrane and employing other salts or sugar instead of MgCl 2 as the draw solutions. © 2009 American Chemical Society.Publication Employing a green cross-linking method to fabricate polybenzimidazole (PBI) hollow fiber membranes for organic solvent nanofiltration (OSN)(Elsevier, 2021-01-15) ZHAO BAIWANG; SHI GUIMIN; WANG KAIYU; Juin-Yih Lai; CHUNG TAI-SHUNG,NEAL; CHEMICAL & BIOMOLECULAR ENGINEERINGA solvent resistant polybenzimidazole (PBI) hollow fiber membrane has been designed and fabricated by a dry-jet wet-spinning process and subsequently cross-linked with a K2S2O8 aqueous solution at 35 °C. This green cross-linking process not only dramatically improves the membrane stability in harsh solvents including N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF) and dimethylsulfoxide (DMSO), but also enhances the mechanical properties of the PBI hollow fibers. A cross-linking duration of 14 h is found to produce PBI hollow fibers with most balanced separation performance. The newly developed cross-linked PBI hollow fiber membrane shows a rejection of Rose Bengal (RB) more than 99% in acetone, exhibiting very good prospects for solvent resistant nanofiltration (SRNF) applications. To effectively mitigate hollow fibers from damage due to membrane swelling during organic filtration, it is recommended that the module configuration for SRNF has one end of hollow fibers free from the module housing. A simple backwash cleaning process has also been developed to restore the membrane performance of a fouled membrane.Publication Macrovoid evolution and critical factors to form macrovoid-free hollow fiber membranes(2008-06-20) Peng, N.; Chung, T.-S.; Wang, K.Y.; CHEMICAL & BIOMOLECULAR ENGINEERINGThe origins of macrovoid and the ways to eliminate it have received great attention and heavy debates during the last five decades, but no convincing and agreeable comprehension has been achieved. We have discovered, for the first time, that there should be critical values of polymer concentration, air gap distance and take-up speed, only above all of which the macrovoid-free hollow fibers can be successfully produced from a two-component (one-polymer and one-solvent) system. This observation has been confirmed for hollow fibers spun from different materials such as polysulfone, P84 and cellulose acetate, and may be universally applicable for other polymers. The major mechanisms why these critical parameters can effectively suppress macrovoids have been elaborated. The concept of acceleration of stretch was proposed and a quantitative relationship was observed to relate it with the number of macrovoids per unit area at the critical velocity and critical air gap distance. © 2008 Elsevier B.V. All rights reserved.