苦咸水淡化用聚偏氟乙烯微孔蒸馏膜的研制

【作者】 张永波；

【导师】 顾振亚；

【作者基本信息】 天津工业大学 ， 纺织工程， 2002， 博士

【摘要】 目前世界各地淡水资源匮乏日益严重，尤其在我国西部地区，该问题更为突出。解决淡水资源危机的有效战略途径就是水的淡化。海水／苦咸水淡化技术经过半个多世纪的发展，从技术上讲已经比较成熟，目前水的淡化以多级闪蒸和反渗透为主，其它有多效蒸馏、电渗析、压汽蒸馏。上述方法需要高温或高压，设备复杂，技术要求高，对处理水质量要求高，一般需要进行水的预处理，这使得淡化水成本居高不下。目前出现的膜蒸馏方法则克服了上述缺点，在水淡化领域越来越受到重视。在膜蒸馏过程中，被加热的咸水产生水蒸汽，水蒸汽透过膜，膜的特点是允许蒸汽透过而液态水却不能通过，透过的水蒸汽遇到温度较低的表面后冷凝，冷凝的水滴由于膜的特殊性不能透过膜而返回，将其进行收集便得到淡水。膜蒸馏法所用设备简单，维护方便，除盐率高，较低的温度下即可进行，淡化后的浓缩水浓度高，作为副产品可用于盐的生产，这些都有可能使得淡化水的成本降低。虽然膜蒸馏法处理海水／苦咸水具有诸多的优点，但目前为止仍未取得商业上的成功，其重要原因在于蒸馏膜的性能，膜的成本、蒸馏通量、强度、疏水性、耐老化性、抗污染性等还有待进一步的改善和提高。本课题从上述角度出发，进行了大量研究工作，其主要内容包括以下几部分： 1．PVDF单膜性能的研究。课题的初步阶段，去除支撑体的影响，改变制膜条件，制得不同结构的PVDF微孔膜，然后对膜的形态结构及其性能进行研究，找出制膜条件与膜的形态结构及性能之间的关系，为制备出性能优良的蒸馏膜打下基础。首先对利用相转化法制备的微孔膜形态结构进行研究，确定传导阻力较小的理想结构模型，致密层在能阻止微孔被浸润的情况下尽可能薄；指状孔在膜厚方向所占比例尽可能大；孔径大小适宜；海绵状组织结构疏松。然后通过改变PVDF溶解温度，铸膜液的溶剂，PVDF的分子量，PVDF用量，添加剂氯化锂用量，涂膜时预挥发时间，凝固浴温度以及凝固浴中无机盐氯化钠、溶剂丙酮等影响因素，考察这些因素对膜形态结构及孔径，孔隙率，水通量，透湿量等性能的影响规律，初步确定了铸膜液配方及涂膜工艺条件。实验结果表明，利用优化工艺所制得PVDF微孔膜孔隙率在75％以上，平均孔径在1μm以下，透湿量较高，形态结构接近于理想模型的微孔膜。 2．PVDF复合膜性能的研究。在初步确定的铸膜液配方及涂膜工艺的基础之上，为提高PVDF单膜的强度，引入支撑体制得复合膜。实验结果表明，支撑体的加入在一定程度上改变了PVDF膜的形态结构，使得海绵状组织减少，结构疏松，指状孔变得贯通，与课题所设计的理想模型比较吻合。在完成膜蒸馏通量测试装置的基础之上，本课题对PVDF复合膜的脱盐性能进行了研究，考察了料液温度，料液盐浓度，料液流量，操作压力，操作时间，气隙距离，支撑体种类，料液中离子种类对蒸馏通量，除盐率及蒸发效率的影响规律。实验结果表明，除盐率的衰减与膜的浸润直接相关，对膜进行拒污整理能明显提高膜的抗污染性，。使得蒸馏膜的除欢率提高，衰减缓慢。 3 传质传热模型的建立。膜蒸馏传质传热机理的研究对于认识该过程的本质及其影响因素，强化忧化过程和设计计算有着重要指导意义。在大量实验的基础之上，本课题对气隙膜蒸馏过程的传质传热进行了分析研究。通过对使用PVDF复合膜的气隙膜蒸馏过程的传热分析，建立了传热模型，根据蒸馏通量及部分操作条件可以求得PVDF微孔膜两侧的温度，进一步得知作为膜蒸馏过程推动力的膜两侧蒸汽压差及重要参数膜蒸馏系数。同样，在得知膜蒸馏系数的情况下可以计算出理论上的蒸馏通量值。通过对膜蒸馏过程的传质分析，建立了计算膜蒸馏系数的数学模型，利用所得膜蒸馏系数及膜蒸馏过程的操作条件结合传热模型可以计算出蒸馏通量。理论计算与实测结果较为接近，验证了模型的可靠性，传质，传热模型的建立对膜蒸馏实际生产能够起到一定的指导作用。 以卜的实验研究及分析表明，本课题所制备接近通孔结构的PVDF微孔复合。膜对水蒸汽的传导阻力较小，支撑体涂膜前的防水整理及复合膜的拒污整理提高。了脱的抗污染件，比饺适合于利用膜蒸馏法对苦成水进行脱盐，传质传热模型的建立对实际生产具有指导意义。更多还原

【Abstract】 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更多还原