The curious behavior of homogeneous azeotropic distillation—implications for entrainer selection

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Abstract
We examine the simplest homogeneous azeotropic distillation sequence of industrial relevance, where an entrainer is added to a binary azeotrope to recover both azeotropic constituents as pure products. Despite its apparent simplicity, such distillation columns can exhibit an unusual behavior not observed in zeotropic distillation: For some mixtures, separation as a function of reflux goes through a maximum. At infinite reflux, no separation is achieved. In some cases, achieving the same specifications with a larger number of trays requires a larger reflux. Sometimes the only feasible separation yields the intermediate component as a pure distillate, while the bottom product contains the light and heavy components. Sometimes the only feasible separation yields the intermediate component as a pure bottom product while the distillate contains the light and heavy components. While these unusual features can be regarded as curiosities, they are essential for proper entrainer selection and design. For a minimum boiling azeotrope, the existing and conflicting entrainer selection rules state that one should use a component that introduces no distillation boundary between the azeotropic constituents (Doherty and Caldarola, 1985), and either a low or high boiling component that introduces no additional azeotrope or a component which introduces new minimum boiling azeotropes (Stichlmair et al., 1989). By taking advantage of the curious aforementioned features, as well as our experience involving more than 400 mixtures, we have been able to analyze the assumptions behind these criteria, show when those assumptions break down, and therefore understand the limitations of the criteria.