| For membrane distillation and pervaporation technology, optimization of preparation conditions of polyvinylidene fluoride(PVDF) hollow fiber membrane, preparation of polysiloxane imide(PSI) composite membranes interfacial resistance of dual-layer asymmetric hollow fiber pervaporation membranes by assisting tutor were carried out in this research work. The specific work is as follows:Non-solvent induced phase inversion technology has been applied to develop porous PVDF hollow fiber membranes in this study and the membrane was applied in direct contact membrane distillation (DCMD). Application of Taguchi experimental design and statistical analyses was attempted for identifying the relationship between the hollow fiber membrane formation conditions selected and the membrane performance. Membrane morphologies and microstructures were examined using FESEM and XRD; mechanisms for the membrane morphologies observed were discussed. Pore size test by gas permeation revealed that surface pore size correlated positively with the mass transfer in both DCMD and gas permeation. Similar pattern of the two types of mass transport gave the hint that the skin resistance was not negligible in DCMD for these membranes. Analysis of variance (ANOVA) showed that among the three fabrication variables, content of non-solvent additive (i.e. glycerol) in the dope had the most significant effects on both membrane distillation coefficient Cm and thermal efficiency EE, while external coagulant composition is least influential. The highest water flux achieved in current work in desalinating3wt.%NaCl aqueous solution was higher than20kg/m2·h at average feed temperature of64.5℃with99.9%rejection. Experiments were carried out to predict the membrane under other conditions and the error between predicted values and experimental values was less than17%. The results of the long time running test under the optimal condition achieved the anticipated effect.The PSI were synthesized from3,3,4,4-biphenyltetracarboxylic dianhydride (S-BPDA) and reactive siloxane with amino group. The characterization of PSI synthesized was analysed by fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) testing means. In order to improve mechanical properties of the PSI single-layer membrane, the three-layer composite membrane of PSI/PEI/non-woven fabric was prepared. Pervaporation results for separation of5wt.%ethanol solution at60℃showed that the flux and separation factor was more than0.5kg/m2·h and2.9, respectively. The examination of composite membrane morphologies and microstructures by examined with field emission scanning electron microscope (FESEM) and energy dispersive X-ray detector (EDX) implied that the leakage between each layer is not serious.Interface is critical for dual-layer membranes fabricated by co-extrusion and dry-jet wet spinning. In this work, for the first time, the importance of interface structure in overall membrane transport property was revealed by using Polysulfone (outer layer)/Matrimid (inner) dual-layer hollow fibers as a demonstration system. Due to the dope formulations of the two layers, dense skins came into formation at the shell side of Matrimid inner layer facing the interface. The Matrimid inner layer obtained from the dual-layer hollow fiber with the thinnest Polysulfone outer layer exhibited a flux around1.0×10-3kg/m2·s and a separation factor of-800in tert-butanol dehydration (feed flow rate30L/h, temperature80℃, permeate pressure200Pa, the same for the other tests). An estimation based on resistance model clearly indicated the dominance of Matrimid inner layer in the overall mass transfer of corresponding dual-layer membrane. As for hollow fibers with the thickest Polysulfone outer layer, the bulk substrate comprising the interfacial dense skin of Matrimid inner layer also displayed significant resistance and appreciable selectivity.In this paper, there are28figures,23tables and120references. |
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