Rate-Based Mixed-Pool Model of a Reactive Distillation Column

Abstract

This paper presents a rate-based mixed-pool model for a reactive distillation column. In the model the tray is divided horizontally into cells where reaction and mass transfer takes place. The basic mass-transfer model applied is the Maxwell−Stefan model, which is implemented in several variants. Various vapor flow patterns can be can approximated by connecting the vapor flows rising from the tray below in different ways. That way it is possible to model the case where the vapor is completely mixed between the trays, the case where vapor is not mixed between the cells and liquid flows always in the same direction with respect to the vapor flow over the trays, and finally the case where vapor is unmixed between the cells and the liquid flow direction is reversed when it flows from a tray to the next tray below. The model is used to test the effect of various vapor flow patterns in a reactive distillation system where the reaction equilibrium is kinetically limited. The model shows a significant effect of the flow pattern on the concentration profiles on the tray. However, the effect on the overall performance of the column depends heavily on the application. When the model is applied to a relatively tall column with dilute feed and moderately exothermal reaction, the effect of the vapor flow pattern is small. On the other hand, the computation cost increases significantly when the number of equations increases. When the model is applied to a reactive distillation system with very high reaction enthalpy, concentrated feed, and a small number of stages, the effect of the lateral concentration profiles is very significant. In the case of a short column, the cost of computation is also not as serious a problem as that with a tall one. Thus, the most likely application for a model of this type in the near future is simulation of absorbers with highly exothermal reactions.