【作者】 丁忠伟；

【导师】 马润宇；

【作者基本信息】 北京化工大学 ， 化学工程， 2001， 博士

【摘要】 膜蒸馏是一种新型的膜分离技术，可用于海水和苦咸水淡化、热敏物质的浓缩等诸多方面。研究用于膜蒸馏过程的膜组件性能，进而解决它在实际应用中存在的相关问题，具有重要的理论和实际意义。本文以研究膜蒸馏跨膜传质过程为起点，重点研究用于膜蒸馏过程的直接接触式平面膜组件、气隙式平面膜组件和中空纤维膜组件的性能，并将研究结果用于考察动态膜蒸馏过程。 以膜蒸馏的跨膜传质过程为对象，分析了气体在多孔介质中的传递机理，由此得到了膜蒸馏系数(MDC)随膜内平均温度的变化关系，发现对于孔径较大的膜，MDC值随温度的升高明显升高，这是Poiseuille流动对跨膜传质有重要贡献的外在表现。三种膜直接接触式膜蒸馏的实验结果证实了上述结论。据此，提出了考虑Poiseuille流动贡献的三参数跨膜传质模型，即KMPT模型。用该模型拟合实验数据，得到了很好的拟合结果。KMPT模型中在后面膜组件的数值模拟研究工作中被用来计算跨膜传质通量。 在用实验方法得出自行设计膜组件传热系数关联式的基础上，又测定了一种膜的KMPT模型参数。这些工作不仅再次验证了KMPT模型的可靠性，同时也为大型膜组件的模型化工作准备好了微元段模型。在上述工作的基础上，建立了直接接触式膜蒸馏(DCMD)多层平面膜组件的数学模型，并对数值模拟结果进行分析和讨论，明确了各种因素对组件通量、产量和最大能量回收率的影响。对大型膜组件所表现出的与小型组件在性能上的差异进行了分析。 对小型气隙式(AGMD)平面膜组件进行了实验研究，重点考察流体流量、温度和气隙厚度对组件通量的影响，发现流量和温度对组件通量的影响程度与DCMD相比相差甚远，而气隙厚度对组件通量的影响非常大。这些结果都可归因于气隙的存在构成了膜蒸馏过程的主要传递阻力。为便于对比，建立了相应的数学模型，对组件通量模拟计算值低于实验测定值的原因进行了分析。在此基础上，建立了多层AGMD平面膜组件的数学模型，数值模拟计算结果表明，AGMD组件对各种影响因素的反应远不如DCMD组件灵敏，通量水平与DCMD相比也有较为明显的差距。但AGMD也具有能使其工艺流程得以简化的两大特点，即直接得到产品和能量自回收。 从理想中空纤维组件的数学模型出发，用数值模拟的方法重点研究了纤维丝内径的多分散性和纤维丝在组件壳程分布的不均匀性对组件性能的影响。发现只要纤维丝内径的标准差不是特别大，其多分散性对组件通量的影响可以忽略不计。对流体沿纵向流过按一定规则排列的管束充分发展层流问题进行了求解，得到了北京化工大学博士学位论文摩擦因数一雷诺数因子的计算方法;在此基础上，用Voronoi网格法确定了实际组件局部封装分率取值的概率，并进而确定了壳程流量分配原则。计算结果表明，流体在组件壳程的流量分配确会由于纤维丝的随机分布而不能均匀一致，由此而引起的沟流效应非常严重，组件通量值明显低于理想组件。本着忽略纤维丝内径的多分散性、充分考虑沟流效应的原则，用数值模拟的方法考察了流体流量、温度、组件膜面积、组件封装分率等对组件通量和最大能量回收率的影响。 进行了两种类型的动态膜蒸馏实验，即间歇恒温操作和间歇降温操作。间歇恒温操作实验结果表明，膜蒸馏过程能够在料液浓度接近于饱和的情况下正常进行，组件通量随料液浓度的上升而下降的幅度令人可以接受。该类间歇操作模拟计算结果表明，组件通量随料液浓度的升高而下降主要是由于水的蒸汽压随浓度的升高而下降，低浓度时浓度极化的影响可以忽略;随着浓度的升高，浓度极化的作用越来越明显。间歇降温操作的通量值明显低于恒温操作，这是料液浓度升高和温度下降双重作用的结果。建立了以太阳能为热源的动态膜蒸馏系统的数学模型，利用数学模型的求解结果分析了各种因素对膜蒸馏产量的影响，太阳能接收面积和系统能量回收率对膜蒸馏产量都有着非常明显的影响作用;而膜蒸馏启动延迟时间和初始料液量则对最终产量没有什么影响。 上述研究结果可为用于膜蒸馏过程的膜组件及整个膜蒸馏装置的优化设计和优化操作提供理论依据和指导。关键词:膜蒸馏;跨膜传质;平面膜组件;中空纤维膜组件;动态膜蒸馏过程更多还原

【Abstract】 Membrane distillation (MD), which is being developed for desalination and concentration of thermally labile substances, is a relatively new separation technique. Study on the performance of membrane modules employed in MD has much academic and practical sense. This research work started from the mechanism of mass transfer across the membrane, and mainly focused on the performance of MD module of flat sheet and hollow fiber membrane, and the results of these works were applied in the study of dynamic MD process.On analyzing mass transfer mechanism in porous media, the effect of temperature within membrane on membrane distillation coefficient (MDC) was determined. The obvious increase of MDC with temperature in the membrane of large pore size indicates that Poiseuille flow plays a very important role in the mass transfer process of MD. This conclusion was verified by direct contact membrane distillation (DCMD) experiment, in which three kinds of flat-sheet membrane were employed. Based on these results, a three-parameter model was presented to describe mass transfer process across the membrane in MD. The model parameters were obtained by fitting experiment data, and the result seemed to be good. The KMPT model was applied in the following modeling of MD module.DCMD experiment was performed on another membrane module for empirical relation of heat transfer coefficient, and the KMPT parameters for another membrane were obtained. These works prepared the segment model for large module. Based on these works, the mathematical model of multi-layer flat sheet DCMD module was set up. The model was solved by numerical method, and some analysis and discussion were performed, by which the effect of various conditions on module flux, production rate and maximum heat recovery rate was determined. Here, the large module shows several different properties with the small one employed in our MD experiment.Experimental study on air gap membrane distillation (AGMD) showed that flow rate and temperature have less effect on module flux compared with DCMD, and air gap thickness can largely affect module flux. These results can be attributed to the existence of air gap in membrane module, which form the main resistance of transfer process. Forcomparison, mathematical model of this process was set up. The modeling ol multi-layer flat sheet AGMD module was performed. The simulation result shows that AGMD module is less sensitive to various operating conditions compared with DCMD, and its flux is also much less than DCMD. But AGMD has some properties, which make its flow-sheet much brief than DCMD.Starting at the modeling of ideal hollow fiber module, the effect of polydispersity of hollow fiber radius on module performance was studied, and it can be concluded that this polydispersity has little influence on module performance on condition that the variation of hollow fiber radius is not so large. For the problem of longitudinal fully developed laminar flow between cylinders arranged in regular array, the momentum equation was solved and the method to calculate friction factor-Reynolds numberfactor( f ?Re) was obtained. Based on this result, the probability of local packing densityof practical module was obtained by Voronoi tesselation technique, and then the rule for the distribution of flow in the shell side of a randomly packed hollow fiber module was decided. The calculating result shows that the ununiformity of fiber distribution on the shell side can lead to the serious maldistribution of flow on this side, and significant channeling effect can be observed, the flux of practical hollow fiber module is much less than that of the ideal one. Neglecting the polydispersity of fiber radius and with the regard of channeling, the module performance was studied by numerical simulation.For study dynamic MD process, two kinds of batch operation were carried out. namely, batch operation with constant feed temperature and batch operation with decreasing feed temperature. The former experiment showed that MD process could be normally performed under a更多还原