Experiments were carried out in a bubble cap distillation column operated at total reflux with the system: water (1) - ethanol (2) - methanol (3). This system has a binary azeotrope for the water-ethanol mixture, which leads to a slightly curved simple distillation boundary between the azeotrope and pure methanol. For certain starting compositions the measured distillation composition trajectory clearly demonstrate that crossing 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. 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.
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