Homogeneous Azeotropic Distillation: Entrainer Selection

Abstract

We examine the simplest homogeneous azeotropic distillation sequence of industrial relevance, where we add an entrainer to a binary azeotrope in order 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. - In some cases the only feasible separation yields the intermediate component as a pure distillate while the bottom product contains the light and heavy components. - In some cases the only feasible separation yields the intermediate component as a pure bottom product while the distillate contains the light and Hay components. While these unusual features can be regarded as curiosities, they are essential for proper entrainer selection and design. When designing a homogeneous azeotropic sequence which separates a binary azeotrope into two pure products, we must first choose the entrainer. Currently available entrainer selection criteria, are inadequate: They contradict one another and often lead to incorrect conclusions. Indeed, for a minimum boiling azeotrope, the existing entrainer selection rules state that, one should use a high boiling component that introduces no additional azeotrope (Benedict & Rubin 1945), an intermediate boiling component that introduces no additional azeotrope (Hoffman 1964), a component which introduces no distillation boundary between the azeotropic constituents (Doherty & Caldarola 1985), and either a low boiling component that introduces no additional azeotrope or a component that introduces new minimum boiling azeotropes (Stichlmair, Fair & Bravo 1989). By taking advantage of the curious aforementioned features, we have been able to understand when these criteria, are correct, or incorrect. In the case of homogeneous azeotropic distillation, separability at finite reflex and at infinite reflux are not equivalent and must be examined separately. By analyzing in detail the profiles of columns operated at infinite reflux, we have: - shown that a binary azeotrope can be separated with only one distillation column. We present a necessary and sufficient condition that identifies such situations; - found a necessary and sufficient condition for separability in a two-column sequence. When separation is feasible, this condition indicates the flowsheet of the corresponding separation sequence; - shown that separation is very often feasible in a three-column separation if the two azeotropic constituents are located in adjacent distillation regions. Then, we examine two situations where separation is feasible at finite reflux but not at infinite reflex. Finally, we present practical solutions (in the case of entrainers that add no azeotropes to two problems of industrial relevance: Given a binary azeotrope that we want to separate into pure components, and a set of candidate entrainers, how do we determine which one is the best? Also, for each of these entrainers, what is the flowsheet of the feasible separation sequence(s)? We obtain these solutions by analyzing in detail the mechanisms by which heavy, intermediate and light entrainers make separation feasible, using the new notions of equivolatility curves, of isovolatility curves and of local volatility order. We show that the second question finds an easy solution from the volatility order diagram. This analysis shows that a good entrainer is a component that breaks the azeotrope easily (i.e., even when its concentration is small) and yields high relative volatilities between the two azeotropic consituents. Because these attributes can be easily identified in an entrainer from the equivolatility curve diagram of the ternary mixture azeotropic component #1 - azeotropic component #2 - entrainer, we can easily compare entrainers by examining the corresponding equivolatility curve diagrams. We also demonstrate the validity and limits of this method with numerous examples.