Control And Optimization Of Membrane Morphology In Phase Inversion Process

The immersion precipitation is one of the most important methods for preparation of UF membrane. In this technology the membrane was formed from a casting solution of polymer with solvent by immersing the casting solution into a non-solvent bath. During this process phase inversion happened with mass and heat transfer simultaneously. Nowadays, we cannot understand this highly coupled process quite well so that new membranes are still developed under an empirical way. Besides, instead of a polymer-solvent-nonsolvent three-component solution being a casting solution in early stage of membrane technology, composed solvent or nonsolvent was used in order to regulate the structure of the membrane and decrease the defect on the skin.In this investigation, the effect of all kinds of factors on the membrane properties in preparation process has been studied analytically and experimentally. As a result, the forming technology has been optimized and different membranes were obtained.Firstly, the mechanism for membrane formation by immersion precipitation processes was reviewed based on the thermodynamics, mass transfer dynamics and mechanism of phase separation.The polyethersulfone ultrafiltration membranes with small size pore were prepared by the phase inversion process from a casting solution containing dimethylacetamide as solvent, diethylene glycol dimethyl ether as a weak nonsolvent additive and polyethylene glycol as an additive. Water was used as a coagulant in this membrane formation process. The influence of polymer mass fraction, additives in the casting solution, as well as the effect of the casting conditions on the membrane properties was investigated. It was found that an increase in polymer concentration causes a decrease in the flux of pure water and an increase in the solute rejection rate of membrane. By adding DGDE into the polymer solution, the pure water flux will lower and the solute rejection rate will rise. The effect of different PEG molecular weights and different PEG content in casting solution on the membrane property arealso studied. Increasing the evaporation temperature and evaporation time will decrease the flux of the membrane, and the rejection rate will rise. Adding DMAc into the precipitation bath will change the properties of membrane. While the influence of precipitation temperature on the membrane properties is much complex.The model equation has been established based on the data analysis of the uniform design experimental results. The model is in good agreement with experimental data. The optimal condition was obtained by multivariate regression method. On the basis of those studies, a membrane with low molecular weight cut-off (PEG6000) was manufactured.The surface area and pore size analyzer was used to observe the pore size and distribution in the membrane. It was shown that the skin of the PES membrane was much denser than that of the PSF membrane.The polyetherimide (PEI) asymmetric nanofiltration (NF) membranes were developed by the dry/wet phase inversion methods in order to overcome some drawbacks of the composite membrane. The regression equation has been established based on the uniform design. Based on the simulation, it was found that the higher the concentration of polyetherimide (PEI) and the longer the evaporation time, the smaller the flux of pure water of membrane. By adding DGDE into the polymer solution, the solute rejection ratio will rise. Increasing H3PO4 and PEG1000 in the casting solution will increase the flux of the membrane.