The need for using rigorous rate-based models for simulations of ternary azeotropic distillation

Abstract

Experiments were carried out in a bubble cap distillation column operated at total reflux with the system: water-ethanol-methylacetate. This system has two binary azeotropes (water-ethanol and water-methylacetate), which gives a simple distillation boundary connecting the two azeotropes. All experiments were restricted to the homogenous region without liquid phase splitting. For certain starting compositions the measured distillation composition trajectories clearly demonstrate that crossing of the distillation boundary is possible. In order to rationalize our experimental results, we develop a rigorous nonequilibrium (NEQ) stage model, incorporating the Maxwell-Stefan diffusion equations to describe transfer in either fluid phase and a fundamental description of tray hydrodynamics. The developed NEQ model anticipates the boundary crossing effects, and is in excellent agreement with a series of experiments carried out in different composition regions. 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 vapor phase diffusivities Ð_y,ij. It is concluded that for reliable design of azeotropic distillation columns we must take interphase mass transfer effects into account in a rigorous manner.