The influence of mass transfer and mixing on the performance of a tray column for reactive distillation

Abstract

We develop a generic steady-state design model for reactive distillation tray columns involving arbitrary liquid-phase reactions. The main features of this model are: 1. The generalized Maxwell-Stefan diffusion equations are used to model the transfer in the vapour and liquid phases. 2. The interphase energy transfer relations are properly taken into account. 3. Chemical reactions taking place both in the diffusion "film" and bulk liquid phase are allowed. 4. For description of the cross-flow of vapour and liquid phases at any given stage, a multi-cell modelling approach is adopted. By choosing the number of cells in the direction of the flow of the vapour and liquid phases, conditions of plug flow, well-mixed and intermediate mixing characteristics of either fluid phases can be realized,. 5. Tray hydraulics and mass transfer relations are incorporated into the model and the program can be run in a true design mode, whereby the initial tray specification and layout using guess values of internal flows are updated using the actual flows as iterations proceed. The usefulness of the developed nonequilibrium cell model is demonstrated by means of a case study involving hydration of ethylene oxide to ethylene glycol using kinetic data from the literature (Ciric and Miao, Ind. Engng. Chem. Res. 33, 2738-2748, 1994). We confirm the existence of multiple steady states, reported earlier by Ciric and Miao (1994) in a study using a model assuming that the vapour and liquid phases are in thermodynamic equilibrium. Introduction of interphase mass transfer resistance is seen to decrease the production of ethylene glycol and at the same time increases the formation of the by-product di-ethylene glycol. The formation of di-ethylene glycol is reduced when we increase the degree of staging in the liquid phase by increasing the number of well-mixed cells along the liquid flow path. This case study underlines the importance of tray hardware design on the reaction selectivity.