Crossing of the Distillation Boundary in Homogeneous Azeotropic Distillation:  Influence of Interphase Mass Transfer

Residue curve maps can be used to predict composition trajectories for packed and trayed distillation columns, provided that vapor and liquid phases are in thermodynamic equilibrium and the column is operating at total reflux. In the case of ternary azeotropic distillations, distillation boundaries divide the composition space into two, or more, regions. It has been mentioned in the literature that distillation boundaries that are straight lines cannot be crossed during actual column operation. The major objective of this paper is to show that interphase mass transfer in ternary azeotropic distillation leads to differences in the component Murphree efficiencies and that such differences can allow even straight-line distillation boundaries to be crossed. Experiments were carried out in a bubble-cap distillation column operated at total reflux. The investigated homogeneous azeotropic system water (1)−ethanol (2)−acetone (3) has a binary minimum-boiling azeotrope for the water−ethanol mixture, which leads to a simple distillation boundary between the azeotrope and pure acetone. Even though the distillation boundary is nearly straight, our experiments clearly demonstrate boundary-crossing phenomena. 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. 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. It is concluded that for reliable design of azeotropic distillation columns we must take interphase mass-transfer effects into account in a rigorous manner.