Fluid Dynamics And Mass Transfer Study On Commercial Multi-pass Tray

A theoretical model to describe the flow and mass transfer phenomenon inmulti-pass distillation column has been proposed by combining computational fluiddynamics and mass transfer. Using this model, this work primarily focuses on study ofthe influences of liquid flow pattern and vapor/liquid mixing state on the trayefficiency. Specifically, there are two important aspects mainly concerned, as follows:For one part, the influence of the liquid flow configuration on the tray efficiency isanalyzed. Firstly, for a-type tray, the existence of recirculation region and the strongvelocity difference between the arc-shaped region and the main flow region onleft-side tray induces its lower separation efficiency. In order to optimize the trayconfiguration and thereby improve the tray separation efficiency, several modifiedconfigurations are designed. Calculated results reveal that the flow pattern becomesmuch more uniform on these modified trays and thereby the tray efficiency isincreased compared with their traditional counterparts. Also the stable trend ofvariance in tray efficiency for the modified trays (especially modified tray II and III)at different Q1/Q2s shows great promise to ensure the performance of the wholedistillation column even if its internal vapor-liquid distribution is actually not quitewell. Finally, detailed quantitative reasons leading to the enhanced tray efficiency formodified trays have been analyzed. On the one hand, the vapor-liquid residence timebecomes more and more important when the Q1/Q2is increased. On the other hand,the existence of recirculation region on the side tray actually produces a largedecrease in tray efficiency although it is relatively small compared with single-passtrays.For the other part, the effects of vapor/liquid mixing states on the tray efficiencyare also investigated. To that end, four cases are designed and simulated. In case I,simulated results show that the tray efficiency is much higher on trays with evennumber than those with odd number, which is primarily due to the existence ofnon-ideal flow pattern on odd-numbered trays. In case II, as a result of non-uniformdistribution for inlet liquid concentration, the separation efficiency is reduced on eachtray compared with case I. Besides, the efficiency loss becomes larger as the increaseof tray number, which is mainly owing to the increase of standard deviation for inlet liquid concentration distribution at larger tray numbers. By comparing case I, case IIand case III, the tray efficiency is largely dependent on the inlet liquid concentrationdistribution and can be enhanced in case of uniform distributions. Finally, based onthese four case studies, the maximum and minimum tray efficiency can be obtained.Furthermore, the mass transfer process inside the real four-pass distillation columncan be simulated with reasonable accuracy.