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Synthesis of Multicomponent Azeotropic Distillation Sequences
Guilian Liu
[Thesis].University of Manchester;2003.
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Abstract
A large number of distillation sequences can be generated to separate amulticomponent azeotropic mixture. However, there is no systematic andefficient method for synthesising promising sequences, which also considerrecycle connections and flowrates. In this work, a systematic procedure isdeveloped for synthesising economically promising distillation sequencesseparating multicomponent homogeneous azeotropic mixtures. The procedureuses spherically approximated distillation boundaries, a shortcut column designmethod, and allows recycle and sequence alternatives to be screened. Bothfeasibility and design are addressed.Approximation of a distillation boundary as a spherical surface is a simple nonlinear,yet more accurate representation of the actual boundary than a linearapproximation. For shortcut column design, azeotropes are treated as pseudocomponents and the relative volatilities of all singular points of the system arecharacterised, based on the transformation of vapour-liquid equilibriumbehaviour in terms of pure components into that in terms of singular points.Once the relative volatilities of singular points are obtained, the classicalFenske-Underwood-Gilliland method can be used to design columns separatingazeotropic mixtures. This method is extremely computationally efficient and canbe applied to homogeneous azeotropic mixtures with any number ofcomponents; the results are useful for initialising rigorous simulations usingcommercial software and for assessing feasibility of proposed splits. Togetherwith the spherical approximation of distillation boundaries, this shortcut method provides a basis for evaluating distillation sequences with recycles.Analysis of feasibility requirements of splits, component recovery requirementsand the effects of recycles on the performance of proposed splits allows rulesand procedures for selecting recycles to be proposed. Recycles withcompositions of either singular points or mixtures of singular points areidentified that are beneficial to the feasibility of sequences and the recovery ofcomponents. The principles are applicable to azeotropic mixtures with any number of components; using these procedures, recycle structures can begenerated and are much simpler than the superstructures of recyclealternatives.The sequence synthesis procedure of Thong and Jobson (2001c) allows allpotentially feasible sequences to be generated. To screen among thesesequences, a split feasibility test and a two-step screening procedure areproposed. In the first step, feasibility of splits is tested efficiently and sequencescontaining either infeasible or sloppy splits are eliminated. In the second step,sequences containing sloppy splits are generated, based on the evaluation ofsequences containing only feasible sharp splits. Using this procedure, thenumber of distillation sequences identified using the procedure of Thong andJobson (2001c) can be significantly reduced.A systematic methodology is proposed for the synthesis and evaluation ofmulticomponent homogeneous azeotropic distillation sequences. Themethodology is computationally efficient. It is demonstrated through a casestudy, the synthesis of distillation sequences separating a five-componentmixture, in which two homogeneous azeotropes are formed, and for which over5000 sequences producing pure component products can be generated. Usingthis methodology, only ten sequences are evaluated to identify three promisingsequences. The evaluation of each sequence using the shortcut column designmethod is extremely efficient compared with that using the boundary valuemethod.