| hen designing a homogeneous azeotropic sequence which separates a binary azeotrope into two pure products, we must first choose the entrainer.;In the case of homogeneous azeotropic distillation, separability at finite reflux 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: (1) shown that a binary azeotrope can be separated with only one distillation column. We present a necessary and sufficient condition that identifies such situations; (2) 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; (3) 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 reflux.;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 constituents. Because these attributes can be easily identified in an entrainer from the equivolatility curve diagram of the ternary mixture azeotropic component... |
