Design, fabrication, and characterization of high-performance membranes for osmotic energy generation
Date of Issue2017
School of Civil and Environmental Engineering
Singapore Membrane Technology Centre
Concerning the rapid increase in global energy consumption and severe environmental issues, sustainable energy is being increasingly investigated as an alternative to fossil fuels. Osmotic energy, as one form of sustainable energy, can be harnessed by pressure retarded osmosis (PRO) technology, and the semi-permeable membrane is employed as an essential element in the PRO process. However, it is challenging to achieve highly efficient PRO operations due to internal concentration polarization (ICP) effect, mechanical deformation and fouling issues regarding the PRO membranes. Developing membranes that could maintain continuous energy generation with various feed and draw streams is critical for the real-world PRO applications. The objectives of this thesis are to design and fabricate novel PRO membranes with stable power output using various feed and draw solutions. Firstly, this thesis covers the study of substrate structure design and performance evaluation of a novel thin film composite (TFC) flat sheet PRO membrane with excellent mechanical stability under high hydraulic pressures. Subsequently, a TFC membrane in the configuration of hollow fiber with excellent performance was synthesized, which could be applied in a mild fouling environment and maintain the energy output after a low-fouling surface modification. Finally, a specially-designed integral membrane that could be operated in active layer facing feed (AL-FS) orientation under high pressure was fabricated and could produce stable energy generation in a severe fouling environment. To begin with, TFC polyetherimide (PEI) flat sheet membranes were fabricated and characterized. Firstly, three substrates with various morphologies were developed, and subsequently, an interfacial polymerization method was conducted on top of the substrates to form a thin-film selective skin layer for solutes rejection. The membrane performance was evaluated in terms of water flux, power density and reverse solute flux, and these factors were found to vary with the substrate structure. Among three as-prepared TFC membranes, TFC-PEI#2 membrane was chosen for subsequent stability tests due to its relatively high performance as well as satisfactory mechanical strength. The membrane intrinsic properties in terms of water and salt permeability altered with hydraulic pressure were studied for two ascending and descending pressure cycles. Both the intrinsic properties and PRO performance could reach similar values at each applied pressure within the two cycles, which indicated a reversible deformation within 0~17.2 bar. With a 300-min period of PRO test performed at 17.2 bar, a stable 12.8 W·m-2 power density was achieved. In the second study, thin-film composite polyetherimide (TFC-PEI) hollow fiber membranes with excellent performance were synthesized by phase inversion and interfacial polymerization method. The resultant membranes consisted of small-dimension fibers which bestowed excellent mechanical properties to withstand high pressure without any supporting spacers. The membrane surface facing feed stream was modified by depositing polyelectrolytes to increase surface negative charge and hydrophilicity. The introduction of this method was then proven to be effective in reducing protein and poly-alginate fouling while maintaining mechanical properties and intrinsic separation properties of the membranes. The modified TFC-PEI-M membranes resulted in almost no flux reduction at 15 bar with alginate as feed foulants. It demonstrated that osmotic power generation of TFC membranes could be well maintained after polyelectrolytes-deposited low-fouling modification with a feed stream containing organic foulants. Finally, a novel integral polyamide-imide (PAI) membrane was designed and fabricated in order to maintain performance using a feed stream consisting of a complex fouling system. Two PAI substrates with various morphologies were acquired by adjusting the internal coagulant solution during the spinning process. Subsequently, a surface cross-linking post-treatment using poly(allylamine hydrochloride) (PAH) was conducted to synthesize the selective layer. The PRO performances of the two resultant integral membranes were evaluated in the active layer-facing-feed (AL-FS) orientation. The membrane with a sponge-like substrate could produce higher power output using deionized water (DI) as feed, and 1 M MgCl2 as the draw stream compared to that with the other substrate structure and was selected for further optimization. The NaCl rejection of this membrane was then further improved by a secondary cross-linking procedure using glutaraldehyde (GA). The as-prepared integral membrane was characterized by a series of protocols to investigate the pore size and functional groups on the surface. Fouling tests were conducted with a real wastewater reverse osmosis (RO) retentate adopted as the feed. With 1 M NaCl as the draw stream, the membrane could produce a stable power density 4.3 W·m-2 at 12~13 bar hydraulic pressure. Although the AL-FS orientation has been recognized as the orientation with low fouling occurrence, the mechanical robustness of traditional TFC membranes is weak in this orientation and impossible to be operated under high pressures. Fabricating an integral membrane with promising performance and stability in AL-FS mode demonstrated great potential for this low-fouling strategy in PRO process. The future work for the next stage includes four parts. Firstly, the effect of mono and divalent ions in the feed solution will be studied for the low-fouling modified thin film composite membrane Secondly, the PRO performance of the integral membrane is recommended to be further improved by exploring different cross-linking chemicals and conditions. Thirdly, the stability tests of the integral cross-linking membranes and long-term fouling tests will be conducted to further study the viability in real-world applications. Fourthly, the substrates with low cost and suitable for chemical cross-linking will be investigated to explore the possible applications.