| In recent years there have been chronic freshwater shortages around the world, especially in West China. The inexpensive and stable securing of "water" is becoming increasingly important in a variety of different areas. The best way is desalination to resolve the problem of water scarcity. Technology of desalination has been developed for more than 50 years. Currently implemented technology for converting seawater/saline water into freshwater, such as distillation, reverse osmosis and electric dialysis has problems of difficult maintenance like pretreatment of seawater, high temperature or pressure, the need for engineers with specialist knowledge and so on. The method of Membrane Distillation (MD) appeared recently overcomes those shortages above. Membrane distillation uses a special membrane to bring out the water vapor generated from collected seawater, collecting it as freshwater, and it is a revolutionary new method different from its predecessors. This special membrane is a porous hydrophobic membrane, which means that water vapor can pass through it, but drops of water cannot. By passing hot seawater through one side of the membrane, only the water vapor generated from the seawater passes through, the water vapor is then cooled by the cool water on the other side of the membrane, forming water drops, resulting in freshwater. MD has many advantages, such as simple equipments, maintenance free, running at low temperature, high condensation of remain, high performance in separating and by-product salts. All mentioned above can keep running costs of desalination low. But MD has had demonstrated no commercial success. Because the properties of MD membrane, such as cost, flux, strength, hydrophobic and anti-dust ability need to be improved. Based on such problems, quantity of research work has been done. Research on sheet PVDF membrane propertiesAt the beginning of this project, different sheet PVDF microporous membranes without support were prepared. Then research on structure and properties of those membranes were carried out to find out the relationship between them. On the basis of study on structure and mass transfer, ideal model of membrane structure was built up, in which skin layer should be as thin as possible ensure resisting water to pass though membrane; finger pore should be as long as possible in the membrane; proper pore size; loose sponge tissue. The effects of some important factors on structure, pore size, porosity, flux, weight of vapor transfer, etc. have been investigated by changing temperature of PVDF resolution, types of solvent, molecular weight of PVDF, weight of additive, time of solvent pre-evaporation and so on. The optimal process of makingmembrane and recipe of solution were made out. Experiments indicate that the micropore membrane made by the optimal process and recipe of solution has porosity > 75%, average pore size <1m, the similar structure to ideal model. Research of composite PVDF membrane propertiesComposite PVDF membrane was produced via adding support in order to improve the strength of membrane. Support changed the structure of PVDF membrane: sponge tissue reducing and loosening, finger pore lengthening. The desalination properties of membrane were discussed through equipment of MD. Factors effecting on flux of distillation, desalination efficient, heat efficient were analyzed. Conclusion can been drawn that reducing of desalination efficient was related to membrane soakage. Soil-resistance finishing improved anti-dust property. The model of mass transfer and heat transferStudy on principle of mass transfer and heat transfer is significant for process. According to heat transfer model and part operating conditions, temperature of membrane, vapor pressure difference of both sides and distillation coefficient can be calculated out. Obviously, MD flux can be gotten through other factors. Mass transfer model for calculating distillation coefficient was built up by means of the analysis to mass transfer of MD, flux can be worked out using abo...
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