| Nanofiltration (NF) is a new kind of membrane separation technology , which was developed about 20 years ago. Compared to reverse osmosis (RO), the operating pressure for NF is much lower, and the monovalent inorganic salt can permeate the NF membrane, while the organics with molecule weight more than 200 g/mil are retained. For this reason, NF was thought to lie between reverse osmosis and ultrafiltration (UF).As a new separation technology, NF will be widely used in future industry, even though its industrialization scale at present is much smaller than RO or UF. However, the industrial use of membrane separation just began in China and the research in NF was later than that in west countries. And there are only a few factories could produce NF membranes. Compared with the performance of NF membrane in the world market, our membranes are not satisfactory. So the research and development of new NF membranes is worth doing.In this paper a new method was proposed to fabricate NF membrane. The purpose is to coat a thin polypiperazine-amide (PPA) layer onto the polysulfone (PS) membrane by interfacial polymerization (IP), which has the morphology and functionality of NF. The experiment was designed to cast a PS membrane as a support by phase inversion first, and then to make a NF membrane by IP method.In order to make UF support membrane, PS was chosen as membrane material, NMP as solvent, and pure water as non-solvent. Experimental parameters such as the concentration of PS, polyvinylpyrrolidone, and acetone, and the evaporating time of the solvent in the air were investigated by using orthogonal experiment.As for the preparation of polypiperazine-amide composite NF membrane, the concentration of monomers, surfactant (SDS), alkaline, and the reaction time of IP were carefully studied for determining the characterization ofmembrane.Besides the experimental study, the theory of non-equilibrium thermodynamics was used to establish a mathematical model for describing mass transfer of salt and water across NF membrane. Special experiments were done to obtain the parameters in the model.It is well known that the pore sizes of PS support membrane greatly affect the feature of NF membrane. If the pore is too large, it is hard to coat a fine thin layer of PPA onto it by IP technique, and if too small, the flux of electrolytes across NF membrane will be very small. Based on the experiment results, it was found that the higher the concentration of PS, and the longer the time of solvent evaporating, the smaller the flux of pure water across membrane; and that the higher the concentration of acetone or polyvinylpyrrolidone, the larger the flux. The optimized conditions for PS support membrane preparation were as follows:PS concentration in casting solution (wt%) 14%;Evaporating time (s) 10Acetone concentration in casting solution (wt%) 0.3%Polyvinylpyrrolidone concentration(wt%) 0.25%.On the basis of conditions above, PS support membrane was made, and tested in the UF apparatus. The results show that there is a linear relation between the pressure difference and the water flux, and that the retention to polyvinyl alcohol solution with concentration beyond 500ppm is more than 98%.The experiments for NF membrane fabrication by IP method were carried out and following optimized conditions were obtained:Iso-phthaloyl chloride concentration(mol/l) 0.12;Piperazine concentration(mol/l) 0.1 ;Alkali concentration (wt%) 0.3%,SDS concentration (wt%) ,0.1 %,Reaction time of IP (s) 210Under such condition, NF membrane made had a good performance. The retention coefficient was about 30-50% to NaCl, nearly 90% to Na2SO4, and almost 100% to polyvinyl alcohol. As for operating conditions, the experimental results sh...
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