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Feasibility and column sequencing for the distillation of homogeneous multicomponent azeotropic mixtures
Geometric methods have been developed for the design and sequencing of distillation columns in mixtures that exhibit nonideal behavior, especially for mixtures which have azeotropes and a large number of components. The work in this thesis will cover three main topic areas: (1) product feasibility, (2) sidestream column design methods, and (3) distillation column sequencing.^ Many of the geometric design and sequencing methods developed in the past for azeotropic mixtures are limited to mixtures with three component and to two product columns. For three component mixtures, it is possible, by an analysis of the residue curve map, to determine which products are feasible. For four component mixtures, this is still possible, although visualizing these systems is much more complex. For mixtures with more than four components, this is no longer possible, and an algebraic method is needed.^ Detailed information about the composition and stability of the singular points in the residue curve equations along with the way in which singular points are connected is used to determine the feasible products for mixtures with any number of components. This information about the structure of the mixture is stored in two types of matrices, the directed adjacency and the closely related reachability matrix. From these two matrices, it is possible to determine the total number of regions, and to identify the nodes and saddles.^ An extension of the methods developed for two product columns has been developed for sidestream distillation columns, which have three products. These columns, unlike two product columns, can remove a saddle as a product. This work allows for the explicit and efficient calculation of the number of stages, the feed stage location, and the minimum flows and energy requirements, given the feed specifications and purity requirements.^ The starting point for the design of sidestream columns is an analysis of the fixed points in the residue curve map and their stabilities. We show that azeotropes or pure components whose geometric character in the residue curve map is a saddle can be removed as a single product for feeds in the same distillation region. Consequently, a sidestream column can be used in place of a sequence of two simple columns to separate ternary mixtures. The analysis is also used to develop some understanding of the limitations of sidestream columns, both in their cost versus simple columns, and in their feasibility. This geometric design method for sidestream columns will be described.^ The ultimate goal of research conducted in the past two decades is to develop a systematic method to sequence distillation columns in complex mixtures. A method has been developed to design sequences of distillation columns which exploit information on which products are feasible. This method assumes that splits that completely recover a component in a product stream are desired. ^
Raymond Edwin Rooks,
"Feasibility and column sequencing for the distillation of homogeneous multicomponent azeotropic mixtures"
(January 1, 1997).
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