减压膜蒸馏过程微观特征的模拟研究

发布时间：2019-02-15 20:12

【摘要】：膜蒸馏是蒸馏过程与膜过程相结合的新型分离技术,能够利用低品位、低热值的廉价热源作为过程推动力是膜蒸馏的主要优点之一。膜蒸馏技术也是唯一可以直接分离得到结晶产物的膜技术。中空纤维减压膜蒸馏同其它几种形式的膜蒸馏(直接接触式膜蒸馏、气隙式膜蒸馏、气扫式膜蒸馏等)相比,具有膜通量大、操作方式灵活、膜充填密度高、能量利用效率高等优点,在海水淡化、工业废水处理、化工、食品医药等行业具有广泛应用,对膜蒸馏过程进行深入全面的研究和了解具有重要意义。本文采用模拟软件FLUENT对中空纤维减压膜蒸馏过程进行模拟研究,建立了三维模拟计算模型,编写用户自定义程序加载到计算过程中以模拟膜表面气液相变的热量转移和质量转移过程,并进行了间接的实验验证,结果表明模拟通量值和实验值具有很好的一致性,所建立的模拟模型能够科学客观的反映真实的膜蒸馏过程。在此基础上系统全面的分析了纤维膜表面微观层面的流动参数、传热传质参数的分布规律,研究了操作条件如料液流速、料液温度和真空压力等对热质传递参数分布的影响。研究显示,在纤维膜表面的热质传递参数分布与膜表面的位置密切相关,中空纤维减压膜蒸馏过程的膜通量主要来自于纤维左右两侧流速和传热系数较大的区域;纤维前后两侧均有较厚的边界层存在,不利于膜表面整体热质传递速率的提高;纤维膜表面存在气液混合现象,相变产生的蒸汽在料液流动的影响下向纤维后侧迁移;料液流速能显著影响纤维表面的温度、蒸发速率、传热系数等分布情况,增大流速对强化膜蒸馏过程有较好的效果;真空压力的提高虽然强化了膜通量,但是热侧膜表面的对流传热系数会降低。本论文的研究成果丰富了膜蒸馏过程的基础理论,为膜蒸馏过程强化提供了理论依据。此外还通过模拟模型讨论了膜蒸馏膜的孔结构参数(孔隙率和曲折因子)对膜通量的综合影响,为高效膜蒸馏用膜的研制提供了方向性参考。
[Abstract]:Membrane distillation is a new separation technology which combines the distillation process with the membrane process. It is one of the main advantages of membrane distillation that the cheap heat source with low grade and low calorific value can be used as the driving force of the process. Membrane distillation is also the only direct separation of crystalline products. Compared with other kinds of membrane distillation (direct contact membrane distillation, air gap membrane distillation, air sweep membrane distillation, etc.), hollow fiber vacuum membrane distillation has high membrane flux, flexible operation mode and high membrane filling density. Because of its high efficiency of energy utilization, it has been widely used in seawater desalination, industrial wastewater treatment, chemical industry, food and medicine, etc. It is of great significance to study and understand the membrane distillation process thoroughly and comprehensively. In this paper, the vacuum membrane distillation process of hollow fiber was simulated by FLUENT, and a three-dimensional simulation model was established. In order to simulate the heat transfer and mass transfer process of gas-liquid phase transition on the membrane surface, a user-defined program is written to simulate the heat transfer and mass transfer process. The results show that the simulated flux value is in good agreement with the experimental value. The simulation model can reflect the real process of membrane distillation scientifically and objectively. On this basis, the distribution of flow parameters and heat and mass transfer parameters on the surface of the fiber membrane are analyzed systematically and comprehensively, and the effects of operating conditions such as feed flow rate, feed temperature and vacuum pressure on the distribution of heat and mass transfer parameters are studied. The results show that the distribution of heat and mass transfer parameters on the membrane surface is closely related to the position of the membrane surface, and the membrane flux of hollow fiber vacuum membrane distillation mainly comes from the region of high flow rate and heat transfer coefficient on the left and right sides of the fiber. There is a thick boundary layer on both sides of the fiber, which is not conducive to the increase of the overall heat and mass transfer rate on the membrane surface, the gas-liquid mixing phenomenon exists on the surface of the fiber membrane, and the steam produced by the phase change migrates to the back of the fiber under the influence of the flow of the feed liquid. The distribution of temperature, evaporation rate and heat transfer coefficient on the fiber surface can be significantly affected by the flow rate of the feed liquid, and the increase of the flow rate has a better effect on the enhanced membrane distillation process. Although the increase of vacuum pressure strengthens the membrane flux, the convection heat transfer coefficient of the hot side membrane surface will decrease. The research results of this paper enrich the basic theory of membrane distillation process and provide theoretical basis for membrane distillation process strengthening. In addition, the effects of pore structure parameters (porosity and zigzag factor) on membrane flux were discussed by the simulation model, which provided a directional reference for the development of membrane for high efficiency membrane distillation.
【学位授予单位】：天津大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TQ028.8

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