Fabrication Of Hydrophobic Microporous Membranes And Their Application In Membrane Distillation

As a novel and high efficiency process, membrane distillation (MD) is often employed in desalination of sea water, preparation of superpure water, concentration and recovery of chemicals, and so on. In recent years, the development of MD technique requires a higher performance for the membrane materials. First and foremost, the membrane for MD should be highly hydropbobic. Moreover, these materials should possess excellent chemical resistance, high gas permeability, good physical and thermal stability. In this work, poly(vinylidence fluoride) (PVDF) and polyethersulfone (PES) were used to prepare hydrophobic membranes for MD process. The preparation and modification of MD membranes were investigated. The main work are divided into three aspects: the structure control of PVDF membranes prepared by immersion precipitation method, the preparation and MD properties of PVDF composite membranes, the hydrophobic modification and MD properties of PES porous membrane.The dynamic process of PVDF membrane-forming system was studied via light transmission experiments. The structure and performances of the obtained PVDF membranes were characterized using scanning electronic microscope (SEM), mercury porometer, contact angle instrument etc. The influences of the main factors including polymer concentration, additive concentration, coagulant composition and casting temperature on membrane structure and properties were investigated. It was found that, with the increase of polymer concentration, CaCl₂ concentration in casting solution and DMAc concentration in coagulation bath, the system tended to delayed de-mixing. The resultant membranes had a dense skin layer and a sponge-like sub-layer. When added amount of H₂O in the casting solution and casting temperature ascended, the system tended to instantaneous de-mixing and the obtained membranes had a porous skin and finger-like sub-layer. When PVDF concentration in the casting solution was 10 wt.%, CaCl₂ concentration was 3 wt.%, H₂O concentration was 2 wt.%, and the room temperature water was used as the coagulation, the porosity of the obtained membrane was 85.14 % and the nitrogen gas flux was 0.5 cm³/cm²Â·s·cmHg.The PVDF composite membranes were prepared and used in vacuum membrane distillation(VMD) for desalination, the effects of feed temperature, feed concentration, flow rate and vacuum pressure on distillation flux and dealination rate of membranes were investigated. It was found that the distillation flux went up with the feed temperature and pressure difference. The distillation flux of the membrane reached 5.39 L/m²Â·h when the feed temperature was 65℃. With the increase of feed concentration, the distillation flux slightly declined. The distillation flux increased with the flow rate, while it ascended slightly when the flow rate increased.The hydrophobic surface modification of PES microporous membranes was achieved via low temperature perfluohexane plasma treatment. X-ray photoelectronic spectroscopy (XPS) results confirmed the existence of perfluohexane segments on the membrane surfaces and the content of fluorin reached 49.3 %. Water contact angle of the membranes increased from 92.0° to 123.0°, suggesting the hydrophobicity of the membranes was improved significantly. Furthermore, SEM images showed that the surface of the modified membrane was covered by a lamella which contained fluorin, but the main structure was not changed. The modified membranes were applied in VMD for desalination. The experimental results showed that, under a low Re constant, the desalination ratio was 99.99 % and the distillation flux reached 4.53 L/m²h.