| Membrane separation technique is novel and effective,as one of the membraneseparation processes, Microfiltratin (MF) is mostly used in industry. Because of widepore size distribution, the microporous membranes which are commercially availableare unfit for new style membrane separation processes such as membrane extraction,membrane distillation, membrane absorption and so on. In this work, a new type offlat sheet microporous membrane was studied and developed, which can be used tothese membrane processes comfortably. The membrane prepared must be narrow poresize distribution, cheaper, with good hydrophobic performance, chemical stability andbetter separation performance. The membrane distillation performance was studied ofthe resultant membranes in this paper.The flat sheet microporous membranes which pore size distribution was almostfrom 0.02 to 0.1μm were prepared via thermally induced phase separation (TIPS) withmaterial isotactic polypropylene (iPP), the diluent was soybean oil. The membranepreparing technology of iPP-soybean oil system was studied. The mechanicalperformances, thickness, ash, the contact angle of water, thermal performance and thepore parameters were determined, the pore structure of the resultant membranes wasinvestigated by scanning electron microscope (SEM).Basing on thermodynamics and crystallization kinetics theory, the mechanism ofpreparing membranes via thermally induced phase separation (TIPS) was analyzedand many problems during this process were solved such as crystal morphology, poresize distribution and control and so on. The effect of the factors and levels duringmembrane′s preparing on the structure and the pure water flux of the resultantmembranes was studied in this work, which were the concentration and the melt indexof iPP, diluents and their concentration, nucleating agents and their concentration, thetemperature of quench bath and so on. The structure and flux of pure water of theresultant membranes were studied also, the pure water flux 359kg/(m~2·h) was obtainedwhen gauge pressure was 0.15MPa.The vacuum membrane distillation (VMD) performance of prepared membranemodule was studied, and orthogonal test was adopted to design the optimumtechnology for preparing iPP flat sheet microporous membranes for VMD via TIPS inthis study. The optimum technology was: the diluent was soybean oil, theconcentration of iPP in casting solutions was 27wt.%, melt-index (MI) of iPP was16.08g/10min, the quench temperature of water bath was 20℃ , the nucleating agentwas adipic acid and the concentration was 0.5wt.%, the extractant was hexane, thetemperature of casting solutions was 190 ℃ and the temperature of steel plate was210℃ .Both Poiseuille flow and Knudsen diffusion were considered to establish themathematical model for pure water VMD of the iPP flat sheet membrane moduleprepared with optimum technology. Experimental results of pure water VMD wereused, the turbulence heat transfer coefficient of the membrane module in feed sidewas determined, which was used to the mathematical model of aqueous NaCl solutionVMD. The temperature polarization phenomena and its effect on the heat and masstransfer of aqueous NaCl solution VMD was analysed at the same time, simulatedresults have showed a good accordance with the experimental. The salt rejection ofproduction water was higher than 99.9% during this process.Desalination by VMD was carried out first time using the resultant flat sheetmembranes module. The feed was retreated seawater of Bohai gulf in Tianjin city,permeat flux 10.56 kg/(m2·h) was reached when the pressure in vacuum side was 3kPa,feed temperature was 323.15K and feed flux was 50L/h, and the chloride eliminationwas higher than 99.8% of the production water. A good respect was showed of theresultant membranes module in use of desalination industry.
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