Model for prediction of multicomponent distillation efficiencies

Abstract

This paper presents a model for the calculation of Murphree point efficiencies in distillation on non-ideal multicomponent fluid mixtures from information on tray hydrodynamics and diffusivity data for the constituent binary pairs in the mixture. A two-zone model is used to describe the hydrodynamic behaviour of the tray. Zone I consists of a formation zone, just above the tray floor, with the gas issuing as a cylindrical gas jet through the perforations. Above this zone is the bulk froth zone, Zone II, where the gas is dispersed in the form of rigid spherical bubbles of varying size populations. The interfacial mass transfer rate relations are based on the generalized Maxwell-Stefan diffusion equations and take proper account of differences in the binary pair Maxwell-Stefan diffusion coefficients in the vapour and liquid phases. Non-equimolar transfer effects are taken into account by invoking an energy balance at the vapour—liquid interface. Calculations based on the model are used to demonstrate the strong influence of froth hydrodynamics (bubble sizes, size distributions and rise velocities) on the relative values of the component efficiencies. Using a particular example for distillation of ethanol—tert butanol—water, it is shown depending on the bubble size and residence time, the efficiency of tert butanol, could attain efficiency values in the range —20% to + 50%. Channelling of the gas phase via fast-rising bubbles is also shown to have an effect on the extent of diffusional coupling experienced in the system. The model developed in this paper could form the basis of an a priori method for the estimation of multicomponent distillation efficiencies.