Experimental verification of the Maxwell-Stefan formulation in describing composition trajectories during azeotropic distillation

Abstract

Experiments were carried out in a bubble cap distillation column operated at total reflux with the quaternary mixture: water (l)-ethanol (2)-methanol (3)-acetone (4). This system has a binary minimum-boiling azeotrope for the water-ethanol mixture and the distillation boundary is represented by a surface with its corners at pure acetone, pure methanol and the water-ethanol azeotrope. For certain starting compositions the measured distillation composition trajectories clearly demonstrate that crossing of the distillation boundary is possible. In order to rationalize the experimental results, the authors develop a rigorous nonequilibrium (NEQ) stage model, incorporating the Maxwell-Stefan diffusion equations to describe transfer in either fluid phase. The developed NEQ model anticipates the boundary crossing effects, and is in excellent agreement with a series of experiments. In sharp contrast, an equilibrium (EQ) stage model fails even at the qualitative level to model the experiments. The differences in the NEQ and EQ trajectories emanates from differences in the component Murphree efficiencies, which in turn can be traced to differences in the binary pair vapour phase diffusivities. It is concluded that for reliable design of azeotropic distillation columns interphase mass transfer effects must be taken into account in a rigorous manner.