Study On The Preparation Of Polypiperazine Trimesoyl Amide/Polysulfone Nanofiltration Composite Membrane Of Ultra Low Operating Pressure And High Permeating Flux

The study gave an overall analysis of the characteristic and mechanism of the nanofiltration membrane. It set up the development of the world recognized high performance nanofiltration composite membrane of aromatic polyamide, on the basis of reviewing the development, the importance in the membrane separation technology, the classification, the fabrication technique and the current development of the research and the application of the nanofiltration membrane. Polysulfone was chosen as the material for making the porous substrate because of its excellent compress-to-denseness resistance, its cheap price and its great availability. By fabricating an ultra thin polyamide functional layer on the polysulfone substrate through interfacial polymerization taking piperazine as the monomer in aqueous phase and trimesoyl chloride as the monomer in organic phase, high performance nanofiltration composite membranes of ultra low pressure and ultra high permeating flux were prepared. Using 0.1% MgSO4 as the feed liquid, the plate composite membrane (I) prepared has a salt rejection of 96.1% and a permeating flux of 85.2 L-m"2.h-1 under an operating pressure of 0.4MPa, while the other one (II) has a salt rejection of 23.7% but a high permeating flux of 397.8 L-m-2-h-1. A hollow fiber composite membrane (III) has a salt rejection above 93% and a permeating flux above 40 L-m-2-h-1 under a pressure of 0.6MPa.The study conducted research on the following four fields: firstly, the characteristics of the interfacial polymerization system of piperazine aqueous solution/trimesoyl hexane solution were systematically investigated. The results show that at a certain molar ratio between the two monomers, piperazine and trimesoyl, at the two phase interface, a dense functional layer of ultra low pressure, high permeating flux and high salt rejection formed by controlling the time of interfacial polymerization, such as the composite membrane (I), the concentration in water phase is 0.4%, and 0.1% in organic phase, and the polymerization time is 1 minute. High performance nanofiltration composite membrane with varied permeating flux and salt rejection was obtained by changing the monomer concentrations in both phases properly. In the case of composite membrane (II), the concentration of organic phase was enhanced to 0.5%. A heat treatment to compositemembrane, preferred at a 70 hot air bath for 20 minutes can improve the salt resistance of the composite membrane.It can be concluded that the performance differences between the plate membrane and the hollow fiber membrane can be attribute to difference of the stresses in the membranes when they were under pressure and the difference of the strain caused in the membrane, besides the possible reason that the surface curvature difference between a plate substrate and a hollow fiber one may cause structure difference in the dense functional layer when coating. The stresses in a hollow fiber composite membrane under an outside pressure are quite different from that in a plate one in that they exert on the hollow fiber in both the radial and circular directions, and the compress strain of the hollow fiber is thus much more complex, which may cause the top layer crinkle and a decreased salt rejection.A quantitative analysis on the surface of the plate composite membranes with various polymerization tune was made on the basis of the feature absorption to IR by means of attenuated total reflection-Fourier transform infrared spectroscopy. The results showed that polyamide functional layer grew thick with the polymerization time going on. This indicates that the polyamide functional layer formed on the interface cannot separate the W/O phases to stop the reaction. On the contrary, because the molecular weight of piperazine monomer is far below the molecular weight cut-off range of nanofiltration, it can easily permeate the functional layer into the W/O interface to keep reacting with trimesoyl molecules. While with the increase hi the thickness of the functional layer, the diffusion resi...