| Membrane separation technology was wildly used in wastewater treatment, sea water desalinization and healthcare fields. Immersion-precipitation phase-inversion process was the most wildly used process to prepare ultrafiltration (UF) membrane, but it is difficult to quantitatively describe the kinetics of membrane formation via phase-inversion process because it is a co-process of thermodynamics and kinetics, which is the main obstacle to the development of the study on the mechanism of membrane formation. It is limited to widen the UF membrane application fields, because mechanism of membrane formation unable to effectively guide the practical production, which resulting in membrane pore size cannot be effectively controlled, the molecular weight cut off (MWCO) range is wide and the separation efficiency is poor. Compared with the traditional separation method, UF has incomparable advantage, but it is easy to be polluted if the feed liquid containing macromolecule materials such as protein and pectin, resulting in processing capacity decline and operation costs raise, so the application of UF membrane is greatly restricted. Currently, there are two methods to solve membrane pollution, one is to prepare of membrane tube with resistance-pollution ability, the other is to study reasonable cleaning method to control membrane fouling. Therefore, enhance the performance of flux, separation efficiency, resistance-pollution ability and hydrophilic modification on PSF membrane are the current research hotspots of organic membrane modification.In this paper, to mature the process of polysulfone (PSF) UF membrane preparation, the content of casting solution and membrane preparation conditions were optimized, using poly(vinyl pyrrolidone) (PVP) as pore former and dimethyl-acetamide (DMAc) as dissolvant in PSF UF membrane preparation via phase-inversion process. Basing on the single factor experimentation, the best content of casting solution and membrane preparation conditions of PSF flat UF membrane with MWCO of 60000Dal were ascertained:PSF concentration was 18wt%, PVP concentration was 10wt%, pre-evaporation time was 10s and the coagulation bathe was water with temperature of 20℃. A new method used to calculate apparent diffusion coefficient (Da) in PSF membrane formation was successfully established on the base of coagulation time measurement accurately by dynamics instrument of membrane formation. The relationship among content of casting solution and membrane preparation conditions, Da, the thermodynamic property, microstructure and performance of membrane was investigated via Da as a link. The effect of content of casting solution and membrane preparation conditions on coagulation dynamics, microstructure and performance of PSF flat membrane was investigated and three conclusions were obtained:(1) the rejection rate increases but the flux decreases as PSF concentration increases, the flux increases but the rejection rate is essentially invariant as PVP concentration increases. (2) the rejection rate increases but the flux decreases as pre-evaporation time extends because pore diameter minish and the pore number decrease, the flux increase and the rejection rate decreases as coagulation bath temperature increase. (3) The membrane formation process is a combination of thermodynamics and kinetics, membrane formation time and the square of membrane thickness is proportional, meanwhile, porosity and apparent diffusion coefficient is proportional. Based on the conclusion of membrane formation mechanism of UF membrane, two kinds of PSF flat UF membranes with MWCO of 20000 Dal and 10000 Dal were designed and prepared respectively by the technology of design and regulation of membrane pore dimaeter. The best content of casting solution and membrane preparation conditions of PSF flat UF membrane with MWCO of 20000 Dal were ascertained:PSF concentration was 21wt%, PVP concentration was 5wt%, pre-evaporation time was 20s and the coagulation bathe was water with temperature of 15℃. Synchronously, the best content of casting solution and membrane preparation conditions of PSF flat UF membrane with MWCO of 20000 Dal were ascertained:PSF concentration was 22wt%, PVP concentration was 5wt%, pre-evaporation time was 10s and the coagulation bathe was water with temperature of 20℃. Subsequently, three kinds of PSF hollow fiber UF membranes were prepared basing on the content of casting solution and membrane preparation conditions of flat membrane.In this paper, TiO2 nano-powder was modified firstly, subsequently, the modified TiO2 nano-powder was introduced to the casting solution to improve the resistance-pollution ability of PSF UF membrane. Three kinds modified PSF hollow fiber UF membranes with MWCO of 60000,20000 and 10000 Dal were prepared and then UF membrane tubes were cast. It was found that the average pore diameter decrease, the purified water flux increase 64.7、32.7、52.1 percent respectively when the operate pressure was 0.2MPa. the water contact angle decrease 32.1、30.7、28.5 degrees respectively, so it was obvious that the resistance-pollution ability of PSF UF membrane was modified conspicuously because of introduction of modified TiO2 nano-powder.The resistance-pollution ability of the two kinds of modified PSF UF membrane were evaluated in the processes of oily wastewater treatment and sweet potato protein separation, and then, the best parameters of operation and cleaning of nano TiO2/PSF organic-inorganic hybrid UF membrane in the two kinds of processes. The effect of emulsification oily wastewater treatment by modified PSF hollow fiber UF membrane with MWCO of 60000 Dal was excellent and the best operation parameter were obtained. The permeate flux of modified membrane increased 22% than non-modified membrane and the emulsification oil in the wastewater was rejected totally. Using modified membrane tube at the condition of 0.1 MPa, the rejection rate of emulsification oil was greater than 90%. It was found that surfactant SDS with concentration of 0.5% was the best chemical cleaning agent and the flux recovery rate of modified membrane is greater than 94% at the conditiongs of the operational cycle time is 90 min and the cleaning time is.30s. The effect of sweet potato protein separation by modified PSF hollow fiber UF membrane with MWCO of 10000 Dal was excellent and the best operation parameters were obtained. The permeate flux of modified membrane increased 17% than non-modified membrane and the sweet potato protein in the sweet potato starch processing wastewater was rejected totally. Using modified membrane tube at the condition of 0.2 MPa, the rejection rate of sweet potato protein was greater than 99%. It was found that alkali protease is the best chemical cleaning agent and the flux recovery rate of modified membrane is greater than 90% at the conditiongs of the concentration of alkali protease is 0.3% and the cleaning time is 4 hours. |
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