Preparation And Application Of High Flux And Anti-fouling Superhydrophobic Membrane For Membrane Distillation

Membrane distillation(MD) is a thermally driven process that depends on the partial pressure difference across a porous hydrophobic membrane. MD process is a high efficiency, energy saving, and environmental friendly process, which combines the advantages of traditional distillation and membrane separation, the rejection for non-volatile component can reach up to 100%, and can make use of low-level heat energy. However, no large scale MD plant has been implemented after developed for about nearly half a century. One of the major problems is the lack of tailor-made MD membranes. Therefore, preparation of dedicated MD membrane becomes the first problem must to be solved for the industrialization of MD process. Thermal conductivity, hydrophobicity, and the structure of the membrane are key factors to decide the MD performance.Hydrophilic flat sheet polysulfone(PSf), blended with polyvinylpyrrolidone(PVP) membranes were prepared by dual-bath coagulation phase inversion, where N-methylpyrrolidone(NMP)/water mixtures were used as the first coagulant before immersion into a water bath. CF4 plasma modification was utilized to render the originally hydrophilic membranes hydrophobic. Results demonstrated that the first coagulation bath acted as a dense skin layer remover in comparison to a PSf membrane prepared in a single water bath. CF4 plasma modified porous PSf membranes using pure NMP as the first coagulant showed a contact angle of 144 o and a DCMD flux of 53.33 kg/m2·h(Tf = 70.3 oC), which was 80% higher than a commercial PVDF membrane with comparable performance stability. Heat and mass simulation modeling results demonstrated that the tortuosity of the PSf membranes is close to 1 in contrast to 2.5 for the commercial PVDF membranes. This result testifies that using of hydrophilic membrane as the base structure can increase the interconnectivity of the pores, lower the mass transfer resistance, and improve the MD flux.In FO-MD integrated process, MD was used to concentrate the diluted draw solution of FO. Virgin and plasma modified PVDF membranes were used to study the advantages of superhydrophobic membrane in dealing with high salt concentration solution. The results showed that, superhydrophobic membrane had higher flux and rejection than virgin hydrophobic membrane, and because of the anti-sticking property of superhydrophobic surface, the yielded crystal on the surface produced during the MD process was fewer than virgin membrane. In addition, the superhydrophobic membrane showed better stability and repeatability in recycling experiment.Finally, the virgin and modified PVDF membranes were used to dispose the emulsified oily wastewater. Results demonstrated that the virgin hydrophobic membrane was easily wetted, and resulted in decreasing in flux and rejection. However, the superhydrophobic membrane had better performance, and the wetting behavior emerged only in long term experiment, which certified the superior property of anti-wetting for superhydrophobic membrane.In summary, a plasma surface modification approach is described in this thesis which can convert hydrophilic membrane to hydrophobic membrane, or hydrophobic membrane to superhydrophobic membrane, and yield higher flux, better stability and anti-fouling property in MD process. The use of plasma modification broadened the scope of selection of membrane materials, increased MD efficiency and showed promise in large scale application.