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Exploiting pressure effects in the distillation of homogeneous azeotropic mixtures

Jeffrey P Knapp, University of Massachusetts - Amherst

Abstract

While it has been known for about 130 years that azeotropes are influenced by changing pressure, this fact has never been fully exploited. In this dissertation it is shown how pressure effects can be used to advantage or can foil attempts to distill homogeneous azeotropic mixtures and how an analysis of the fixed points of the distillation model leads to new important insights into extractive distillations.^ As the pressure varies, not only the composition but also the number of azeotropes can change. A change in the number of azeotropes corresponds to a transcritical bifurcation. A bifurcation-theoretic treatment provides a model-independent method for locating global changes in the residue-curve map of a mixture as the pressure changes and for predicting the presence of severe tangent pinches.^ Existing methods for synthesizing thermally-integrated distillation sequences frequently fail when applied to nonideal and azeotropic systems because changing column pressures often introduces new azeotropes and distillation boundaries into the mixture, which make some separation tasks infeasible. A new, systematic thermal-integration procedure is presented that combines a bifurcation and residue-curve map analysis with the method of Andrecovich and Westerberg and results in substantial energy savings.^ A new pressure-swing distillation process is presented that makes it possible to separate mixtures in which the desired products lie in different distillation regions when one end of the distillation boundary is a pressure-sensitive azeotrope. Thus pressure-insensitive binary azeotropes can be separated using novel entrainers that form pressure-sensitive distillation boundaries.^ In addition to a minimum reflux, every extractive distillation also exhibits a maximum reflux, above which the desired separation is impossible, and a minimum entrainer flow rate, below which the separation is also impossible. Both of these quantities are shown to correspond to bifurcations of the finite-difference equations describing the middle section of the column and, given a VLE model, both can be easily calculated knowing only information about the column feed and the desired product compositions--i.e., prior to any column design calculations.^ Both the minimum entrainer flow and the maximum reflux ratio have important implications for extractive distillations. A comparison of minimum entrainer flows provides a simple method to discriminate between entrainers with the same feasible residue-curve map. A heuristic is presented for estimating a near optimal design value of the entrainer flow. Maximum reflux results in the separation becoming more difficult at both low and high reflux ratios, requiring nonstandard control action in certain reflux ranges. ^

Subject Area

Engineering, Chemical

Recommended Citation

Jeffrey P Knapp, "Exploiting pressure effects in the distillation of homogeneous azeotropic mixtures" (January 1, 1991). Doctoral Dissertations Available from Proquest. Paper AAI9207419.
http://scholarworks.umass.edu/dissertations/AAI9207419

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