【作者】 许媛媛；

【导师】 袁景淇； Günter Wozny；

【作者基本信息】 上海交通大学 ， 控制理论与控制工程， 2010， 博士

【摘要】 规整填料塔分离效率的预测是提高该单元操作经济效益的一个关键。影响塔效率的因素是多尺度的,既包括宏观的操作变量如进料量、进料成分、塔内压强与温度等,也包含了微观的液相局部流态与分布。尤其是在实际流程工业中,难以避免的会出现汽-液-液三相精馏,液相间的相互影响使得液相的流动形态呈现多样性和随机性,从而使塔的分离效率也呈现很大的不确定性。因此,有必要从微观尺度上来研究流体的流动形态及影响塔效率的相关因素。从控制和操作角度讲,为提高规整填料塔的分离效率,就必须增加气液传质面积和停留时间。已经知道,在精馏过程中当液相以封闭液膜方式流动时,气液相间的接触面积是最大的。为此,就需要找出影响封闭液膜流动的因素。而这也正是本文的研究重点之一。由于规整填料塔内液相流动形态的改变往往只发生在几英寸大小的填料片上,所以本文以简化的规整填料片(倾斜板)为对象,采用计算流体力学(Computational Fluid Dynamics, CFD)的方法对倾斜板上的局部气-液、气-液-液流动进行了三维建模与仿真,通过对流体流动特点的研究来寻找影响其流动形态的主要因素。研究结果表明,对气-液流动而言,影响封闭液膜流动的因素包括液相负载、液相表面张力(包括接触角的影响)以及气液逆流时的相间曳力。这几个因素相互耦合,相互制约:液相负载越大,尽管可以使液相以封闭液膜方式流动,但液相的流速也会相应变大,从而会使得气液相间的接触时间减小;相间曳力越大,尽管可以减小液相的流动速度,但会使低液体负载区产生液泛,所以通气量又必须与液相负载量相对应;表面张力越小(接触角越小),可以使液相以封闭液膜方式流动,但液相越不稳定,受曳力影响越明显,甚至会产生液泛、夹带。对气-液-液流动而言,除了上面的影响因素外,液相的进料次序也是影响其流动形态的一个重要因素。这些影响因素的研究,对规整填料塔的优化设计、过程控制提供了直接的理论指导。本文是以递进方式进行研究的,论文的主要研究结果包括:1.基于CFD的流体体积法(Volume of Fluid, VOF),并结合Brackbill提出的连续表面力方法(Continnum Surface Force, CSF),对倾斜板上的单液相流动(气-液两相流)进行三维建模与仿真。研究了倾斜板上单液相的流动形态,包括封闭液膜流动、液膜破裂及溪流和滴流的形成,并指出液相负载及表面张力(包括接触角)对液相流动形态的影响。另一方面,也开展了相应的实验研究,来确定模型参数、验证模型的有效性。2.利用压力降模型,将气-液逆流下的相间曳力引入到VOF模型中,从而建立了气-液两相逆流下的数学模型。采用该改进的模型,对逆流操作下、倾斜板上的单液相流动进行了三维仿真,研究了逆向气流对板上的封闭液膜流动及溪流流动的影响,并指出当前的最大有效气速(uge=5.0m/s)下,可减小水相的流动速度且保证其流动形态不发生变化,从传质传热角度讲,是有利的。另外,也给出了与文献中实验数据的对比,从而验证该改进模型的可行性。3.在改进的VOF模型基础上初步建立了逆流操作下的气-液两相吸收传质模型。采用该吸收传质模型,研究了垂直光滑平板上、封闭液膜的瞬态流体力学参数与传质的特点。研究结果与文献中的实验观察定性一致。另外,也开展了相应的实验,通过测量和分析出口处的浓度来验证该吸收传质模型是有效的。4.将确定参数的VOF模型进行扩展,对倾斜板上的气-液-液三相流体流动进行了三维建模与仿真,研究了互不相溶的两液相并流情况下的流动形态与特点。研究结果表明,互不相溶的多液相流动是集合了溪流、滴流和封闭液膜流动的复杂流动:一液相以近似封闭液膜状流动,而另一液相则以溪流或滴流的方式流动;两液相中不同的进料次序可得到不同的流动形态,有可能会导致夹带的产生;对近似封闭液膜相而言,第二个液相的引入可降低封闭液膜相的流动速度。另外,也开展了相应的实验研究,通过图形与参数的仿真和实验对比,从定性和定量两个角度来验证该扩展模型是可行的。5.将改进的VOF模型进行扩展,通过与压力降模型的结合,建立了逆流操作下的气-液-液三相流体流动的数学模型。采用该模型,数值研究了逆向气流对两两互不相溶的液相流体流动的影响。研究结果表明,不同的进料次序,会导致气相对两液相的影响不同、相间的稳定性不同;同液体流量下的单液相流动与两液相流动相比较而言,逆向气流对两液相流动下的影响要小很多,从而表明液-液相间存在很强的相互作用。但是这些结论还有待于进一步的实验验证。更多还原

【Abstract】 The prediction of the separation efficiencies for the structured packed columns is the key to improve the economic benefit of the unit operations. There are many factors to affect the separation efficiencies of packed columns, including the macroscopic variables (e.g.the feed rate, the material, the pressure, the temperature) and the microscopic liquid flow behavior and distribution. Especially in the process industries, the appearance of the second liquid is inevitable. Due to the interaction between the two liquids, the liquid flow behavior is diverse and time-varying which leads to the uncertainties of the separation efficiencies for the structured packed columns. Therefore, it is necessary to investigate the local flow behavior of the liquid phase and the influencing factors of the separation efficiencies.From the viewpoint of control and operation, the interfacial area for mass transfer and the residence time should be increased in order to improve the separation efficiencies of the structured packed columns. It has been known that the film flow provides a maximum interfacial area between the gas and liquid phases comparing to the other flow behavior in distillations. Therefore, a better knowledge on the influencing factors for the liquid flow behavior is needed, which is one of the focuses of this dissertation.Since the liquid flow behavior such as film flow, film breakup with rivulet and droplet flow takes place only in a few inches inside packings, first investigations are carried out on an inclined plate. In this dissertation, three-dimensional modeling and simulation are conducted to investigate the liquid flow behavior for gas-liquid and gas-liquid-liquid flows. The results show that the liquid flow rate, the surface tension (including the contact angle) and the drag force between a liquid and a counter-current gas phases play major roles for film flow in the case of gas–liquid flow. Furthermore, they are mutually related to each other and mutually restrict each other. In the case of gas-liquid-liquid flow, the feed sequence is another important factor for the liquid flow behavior besides the influencing factors above. The investigations on these factors provide the theoretical guidance for the optimization design and process control of the structured packed columns.The main contributions of this dissertation including:1. A basic CFD model, VOF model, is applied to investigate the flow behavior of a liquid phase flowing on an inclined plate in the case without a counter-current gas flow. Simulations are carried out in three-dimension to describe the film flow and the film breakup with rivulets or droplets flow. The results show that the liquid flow rate and the surface tension (including the contact angle) have an important influence on the liquid flow behavior. On the other hand, experiments are performed to validate the model.2. A drag force between a liquid phase and a counter-current gas flow derived from the pressure drop model is included in the VOF model. With the developed model, the influences of a gas phase on a liquid phase including film flow and rivulet flow are investigated in three-dimension. The results show that the current gas velocity will not change the liquid flow behavior but could decrease the liquid velocity, which are beneficial for the mass transfer in structured packed columns. In addition, the developed model is further validated with the experimental data found in the literature.3. A three-dimensional CFD model taking the local absorption mechanism into account is developed for a film flow on a vertical plate with a counter-current gas flow. With the model, the instantaneous hydrodynamics and mass transfer characteristics of the liquid phase are investigated quantitatively. Experiments are carried out with the same geometry and operation system used in simulations. A comparison of the outlet concentration between the experimental data and simulation results is presented to validate the model.4. An extension of the VOF model is considered to investigate the flow behavior of two liquids flowing on an inclined plate in the case without a counter-current gas flow. The results show that one liquid forms a film on the plate, the other one forms rivulet or droplet and moves below or above the film surface. It also illustrate that the feed sequence is another important factor for the multi-liquid flow behavior. Besides, the appearance of the second liquid phase could reduce the velocity of the film flow. Experiments are carried out to validate the extension of the VOF model.5. An extension of the developed model based on the VOF model and pressure drop model is performed to numerically investigate the influence of the gas phase on the two liquid phases. The simulation results illustrate that the feed sequence could affect the flow characteristic and stability of the two liquid phases. For the same the liquid flow rate, the influence of the counter-current gas phase on the liquid phase in the case of two liquids flow is smaller than that in the case of one liquid flow. However, the simulation results need to be validated with the experimental data in the future.更多还原