Computational Fluid Dynamics Modeling And Validation Toportray The Liquid Flow Behavior For Multiphase

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.