In this article, we present practical solutions (in the case of entrainers which add no azeotropes) to two problems of industrial relevance: Given a binary azeotrope which 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 details 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. W e show that the second question finds an easy solution from the volatility order diagram.This analysis shows that a good entrainer is a component which "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 #1 -azeotropic component #2 -entrainer, we can easily compare entrainers by examining the corresponding equivolatility curve diagrams. Finally, we demonstrate the validity and limits of this method with examples. Dans cet article, nous prCsentons des solutions pratiques (dans le cas de composants d'entrainement n'ajoutant pas d'azCotropes) B deux probltmes d'importance industrielle: considCrant un azkotrope binaire voulant se separer en deux composants purs, et un ensemble d'Nentraineurs. possibles, comment dCterminer le meilleur? Par ailleurs, pour chacun de ces entraineurs, quel est le diagramme de procCdC de la sCquence ou des sequences de separation possibles? Nous obtenons ces solutions en analysant en dttail les m6canismes par lesquels des entraineurs lourds, intermdiaires et ICgers rendent la sCparation possible, en utilisant les nouvelles notions de courbes CquivolatilitC, des courbes d'isovolatilit6 et de I'ordre de volatilitC local. Nous montrons que la deuxikme question trouve une rCponse facile dans le diagramme de I'ordre de volatilid. Cette analyse montre qu'un bon entraineur est un composant qui rbrise* I'aztotrope facilement (c.a.d. mCme lorsque sa concentration est petite) et produit des volatilitCs relatives ClevCes entre les deux constituants azCotropes. Etant donnC que ces qualitks peuvent Ctre facilement reconnues dans un entraineur B partir du diagramme d'CquivolatilitC du mClange ternaire composant azCotropique #1 -composant azkotropique #2 -entraineur, nous pouvons facilement comparer les entraineurs en examinant les diagrammes d'CquivolatilitC correspondants. Enfin, nous dCmontrons la validiti et les limites de cette mtthode et donnons de nombreux exemples.

(Stichlmair et al., 1989

In this article we study multiple steady states in ternary homogeneous azeotropic distillation. We show that in the case of infinite reflux and an infinite number of trays one can construct bifurcation diagrams on physical grounds with the distillate flow as the bifurcation parameter. Multiple steady states exist when the distillate flow varies non-monotonically along the continuation path of the bifurcation diagram. We derive a necessary and sufficient condition for the existence of these multiple steady states based on the geometry of the distillation region boundaries. We also locate in the composition triangle the feed compositions that lead to these multiple steady states. We further note that most of these results are independent of the thermodynamic model used. We show that the prediction of the existence of multiple steady states in the case of infinite reflux and an infinite number of trays has relevant implications for columns operating at finite reflux and with a finite number of trays. Using numerically constructed bifurcation diagrams for specific examples, we show that these multiplicities tend to vanish for small columns and/or for low reflux flows. Finally, we comment on the effect of multiplicities on column design and operation for some specific examples.

In this article we study multiple steady states in ternary homogeneous azeotropic distillation. We show that in the case of infinite reflux and an infinite number of trays one can construct bifurcation diagrams on physical grounds with the distillate flow as the bifurcation parameter. Multiple steady states exist when the distillate flow varies non-monotonically along the continuation path of the bifurcation diagram. We derive a necessary and sufficient condition for the existence of these multiple steady states based on the geometry of the distillation region boundaries. We also locate in the composition triangle the feed compositions that lead to these multiple steady states. We further note that most of these results are independent of the thermodynamic model used. We show that the prediction of the existence of multiple steady states in the case of infinite reflux and an infinite number of trays has relevant implications for columns operating at finite reflux and with a finite number of trays. Using numerically constructed bifurcation diagrams for specific examples, we show that these multiplicities tend to vanish for small columns and/or for low reflux flows. Finally, we comment on the effect of multiplicities on column design and operation for some specific examples.

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