| The study on the newly developing membrane separation technology - vacuum membrane distillation is carried out with experimental and theoretical research work in this paper. The VMD technology is applied for the separation of diluted chromium aqueous. The transfer resistances and mechanism are presented theoretically.The main research work of this dissertation focuses on the following aspects: ⑴ Study on the preparation of hydrophobic PVDF membrane by phase inversion method, investigate the influences of the sorts of solvent and additives on the performances of resulting membrane. ⑵ The experimental research of the separation of chromium aqueous solution by VMD technology, especially the effects of operation conditions on membrane flux and chromium rejection. ⑶ Exploring the mass transfer resistances during VMD process. ⑷ Investigate mass transfer mechanism and establish new model equations to describe the mechanism.The preparation and characterization of hydrophobic microporous PVDF membranes are investigated especially for the separation of chromium aqueous solution by VMD process. The following factors, which affect the performances of resulting membranes such as the polymer concentration in the casting solution, the suitable composition of combined solvents, the sort and amount of additives are taken into consideration with theoretical analysis, as well as the preparing technique parameters. The thoroughly experimental investigations of vacuum membrane distillation for separation of chromium aqueous solution are conducted with the employment of PVDF and PTFE porous membrane. The different performances of two polymer membranes in VMD process are observed. The dependence of VMD performances on the membrane material and membrane pore sizes resulted mostly from different oxidation ability of hexavalent chromium and trivalent chromium. The influences of operating conditions including feed solution pH, concentration, flow rate and temperature as well as cold side pressure and temperature on VMD flux and rejection are investigated and the optimum operating conditions are obtained. The results show that under the certain operating condition, chromium can be removed effectively by VMD technique with related high flux and continuously VMD operation is favored.The novel results of VMD experimental and theoretical are depicted here: ①Compared with traditional chromium separation method, applied VMD to separate chromium solution will not produce new pollutants and the whole operation an beproceeded continuously with related high flux and rejection. ②The pressure difference across the membrane is not the only mass transfer force. It is observed that when the pressure difference near to zero, the flux also exists without to fall zero, which is very different from what has been proposed for VMD in literature. This would be attributed to the thermal induced diffusion resulted from the temperature differences across the membrane. The markedly influences of feed solution temperature on VMD flux are observed, VMD flux increases exponentially with the temperature. By analog with Arrhenius equation, the new conception of Apparent Energy Barrier (E) in Mass transfer process is advanced. It can be used as a macroscopic evaluation for the total resistances of mass transfer in VMD process. It depends on the membrane material, membrane pore sizes, and the operating conditions. It will act as an instruction of membrane selecting for practice applications. ③ By experiment designing and employing the stack of two membranes which has almost the same performances, the transfer resistances in liquid and the resistance across membrane are explored. The fraction of mass transfer resistance across membrane is defined (f). It reveals that the mass transfer resistance across membrane decreases with the increasing of pore size and membrane thickness. The total resistance varies according to the operating conditions. At relatively low temperature, mass transfer resistance across membrane is the dominant factors, but at hi...
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