【作者】 杜军；

【导师】 徐楚韶；

【作者基本信息】 重庆大学 ， 钢铁冶金， 2002， 博士

【摘要】 本论文对新型的膜分离技术—减压膜蒸馏技术进行了全面的实验研究和理论研究。采用减压膜蒸馏技术进行了分离含铬溶液的实验研究，并对减压膜蒸馏过程的传递阻力和传质机理进行了理论探索。论文主要开展了以下几个方面的研究工作：⑴ 相转化法制备疏水性PVDF微孔膜的研究，特别是溶剂体系、添加剂等选择及其对成膜性能影响的考察；⑵ 减压膜蒸馏技术对含铬溶液的分离效果，以及分离过程中膜通量、截留率随实验操作因素变化的规律的认识；⑶ 减压膜蒸馏过程中的传质阻力的研究；⑷ 减压膜蒸馏过程中的传质机理及模型方程的建立。针对所研究的含铬溶液的减压膜蒸馏过程，本文首先开展了适用于该过程的疏水性聚偏氟乙烯（PVDF）微孔膜的制备以及膜孔结构表征的研究。采用湿法相转化法制膜，深入探讨了铸膜液中PVDF含量、复合溶剂的选择及比例、致孔剂种类及比例、制膜工艺条件对成膜质量和性能的影响。采用混合溶剂体系—N，N二甲基乙酰胺和磷酸三乙脂可以得到外观平整的膜。致孔剂的选择有无机盐氯化锂和有机物甘油、聚乙二醇、聚乙烯基吡咯烷酮等，致孔剂的种类和用量变化时分别对成膜的平均孔径产生不同的影响，但最终成膜孔径的大小并非完全与致孔剂的用量成正比，实验确定了最佳的膜制备条件。在膜制备研究和膜孔结构表征基础上，本文选择PVDF微孔膜和聚四氟乙烯（PTEF）微孔膜，采用减压膜蒸馏技术进行了从溶液中分离铬的研究。详细考察了两种膜材料在减压膜蒸馏技术分离含铬溶液过程中的应用性能，发现由于溶液中六价铬和三价铬的氧化性不同，从而对膜材料和膜孔径的要求也有所不同；系统地研究了进料液pH值、进料液浓度、进料液流速、进料液温度、冷侧压力等因素对减压膜蒸馏过程分离效果的影响行为和规律，得出了实验条件下分离含铬溶液的最佳工艺条件。 通过对减压膜蒸馏过程进行深入细致的实验研究和总结分析，本文在减压膜蒸馏的实验研究方面和理论研究方面获得新的研究结果：⑴与传统的分离铬的方法相比，采用减压膜蒸馏技术处理含铬溶液，对溶液中的铬离子有良好的去除效果和较高的膜通量，不会引入新的污染物，可连续进行操作，在节能方面有较好的优势。⑵ 本文实验呈示当膜两侧压差接近零时膜通量不为零，这是由于膜两侧温度差引起热扩散而表现出的必然现象，表明减压膜蒸馏过程同时存在压力驱动和热驱动；进料温度对减压膜蒸馏过程具有显著的影响，膜通量随温度的增大而指数式上升；本文提出了减压膜蒸馏过程的“表观能垒”概念，它是对膜过程阻<WP=5>力的宏观量度，综合体现了膜材料、膜孔性能、实验操作条件所对应的流体动力学行为等因素影响减压膜蒸馏过程的信息，对膜的实际选择和评价具有一定的指导价值。⑶ 采用不同孔径的PTEF微孔膜，通过设计实验，考察了液相传质阻力和膜内传质阻力，定义并推导出了减压膜蒸馏过程中膜内传质阻力分数，从而简单、有效而直观地得到了过程阻力及其相对大小。研究表明，随膜孔径增大和膜厚度减小，膜内的传质阻力减小；传质过程中起主导作用的阻力因素随操作条件的变化而变化；在温度较低时，膜内传质起主导作用；温度较高时，热侧汽化阻力起主导作用。⑷在本文减压膜蒸馏的实验条件下，通过膜的蒸汽分子的平均运动自由程与微孔膜的孔径大小相当，因而此时过程的传质机理是界于努森流机理和粘性流机理之间的过渡流机理。在努森流和粘性流模型的基础上，结合减压膜蒸馏的传质过程和传热过程之间的偶合关联，引入反映实验操作条件的权重系数，构筑出了新的过渡流模型，以描述减压膜蒸馏的传质机理，并可通过数值计算对减压膜蒸馏过程的膜通量进行理论预测。比较计算结果和实验结果表明，本文所建立的过渡流模型能较好地描述减压膜蒸馏过程的传质机理和预测实验结果。更多还原

【Abstract】 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: ① Compared with traditional chromium separation method, applied VMD to separate chromium solution will not produce new pollutants and the whole operation an be<WP=7>proceeded 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更多还原