Influence of unequal component efficiencies on trajectories during distillation of a homogeneous azeotropic mixture

Abstract

The overall objective of this work is to examine the influence of interphase mass transfer on the composition trajectories in homogeneous azeotropic distillation. A total of 38 experiments were carried out in a bubble cap distillation column operated at total reflux with the system: water–ethanol– tert-butanol. The experiments were carried out in the two regions on either side of the distillation boundary connecting the ethanol–water and t-butanol–water azeotropes. In order to model the composition trajectories, a rigorous nonequilibrium (NEQ) stage model is developed. The NEQ model incorporates the Maxwell–Stefan diffusion equations to describe the intraphase transfers in the vapor and liquid phases. The only adjustable parameter in the NEQ model is the size of the vapor bubbles on trays. A choice of a bubble diameter of 4 mm in the developed NEQ model gave the best agreement with the experimental results for all of the 38 experimental runs. The Murphree efficiencies of the constituents in the ternary mixture were found to be significantly different from one another for all the runs. In order to ascertain the influence of unequal component efficiencies on the column composition trajectories, the experimental results were also simulated with an equilibrium (EQ) stage model assuming a uniform, constant efficiency for all components on all the trays. The value of this constant efficiency for any experimental run was obtained by averaging the individual component efficiencies for all the three components on all the trays, calculated by the rigorous NEQ model. The predictions of the EQ model leads to significantly worse predictions of the column composition trajectories for each of the runs, when compared to the NEQ model. It is found that the column composition trajectories are significantly altered due to differences in the component efficiencies. From a design view point, it is shown that for the water–ethanol– tert-butanol system, the attainment of a desired ethanol purity in the top product may require significantly larger number of stages than that anticipated by the EQ model incorporating constant component efficiencies.