| Membrane distillation (MD), which has been applied to purification of water and concentration of thermally labile substances, is an innovative membrane separation technique. To investigate the mechanism and further improve the transfer process of MD are important both in theoretic study and in practice.This work is a part of the Australia-China Institutional Link Program "Treatment of brackish water in the northwest of China by membrane distillation". Batch operation was adopted in this program and membrane distillation mechanism for high concentration of NaCl solution was studied. It was found that when keep the same pressure on both the feed and the permeate sides, transfer mechanism of vapor in membranes was analogous to that of gas in lacunose material. Therefore the Poiseuille flow can be ignored. For the temperature range from 0 C to 100 C, the mean free molecular path of the gaseous water molecules was about 0.2um, of the same size as the pore diameter of experiments membrane. As a result, in the apertures, collisions between molecule - molecule and molecule - aperture wall have to be taken into consideration. In other words, DCMD mass transfer process is an inter-gradation diffusion, which includes Knudsen diffusion and molecular diffusion. Both theoretical and experimental investigations were carried out on direct contact membrane distillation (DCMD) for comparison of different models. The effect of temperature and concentration of the feed on A H were not considered for DCMD.Our study on DCMD showed that when the feed was concentrated by batch operation, water fluxes decreased as the concentration of NaCl solution increased. When the concentration of NaCl solution was about 25% (close to saturation), water fluxes decreased sharply; while concentration of NaCl solution approached saturation, water fluxes reached a steady level gradually. Variation of membrane-fouling resistance accounts for the above observations. Concentrations of NaCl solution, operation temperatures and flow rates etc. can affect the speed of membrane fouling. Among them, concentration is the most important factor. When the concentration of NaCl solution was close to saturation, membrane fouling increased sharply, then reached a steady level.Feed temperatures, permeation temperatures, feed concentrations and flow rates were studied thoroughly for high concentration NaCl aqueous solutions. Fluxes were improved when spacers were put in the membrane module. The density of spacersaffected fluxes as follow: When the density of spacers was increased, degree of turbulence increased, thus fluxes increased. However, when the density exceeded a certain value, the resistance of fluid increased sharply, the degree of turbulence decreased, thus fluxes decreased. The effect of the turbulent degree for the feed on fluxes was more severe than that for the permeate. Fluxes of fine and coarse spacers put in the module were smaller than that of coarse spacers and larger than that of fine spacers. Feasibility of concentrating penicillin solution by DCMD and membrane fouling for high concentration salts solution were also studied.The key point of improving fluxes for MD is to get advanced hydrophobic micro-membranes. There are two methods available. Firstly, pore size and porosity can be improved by changing additives species and concentrations for PVDF membrane. Secondly, pore size and porosity can be improved by gel velocities difference of PVDF and PS for PVDF/PS blends membranes.(1) Preparation of hydrophobic PVDF micro-membranesFirst of all, the effect of the polymer concentration on membrane performance was studied. The suitable concentration of PVDF should fall into the range of 12% to 20% from abundant experiments. The interactions among the polymers, solvents and gel solutions were affected by additives. The effects of salt additives and macro-molecular additives on membrane performance were also investigated. Based on all this work, advanced PVDF micro-membranes were prepared by using blends additives.It was observed that the po...
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