Distillation pinch points and more

Abstract Rising energy costs have spawned renewed interest in improving methodologies for the synthesis, design and/or retrofitting of separation processes. It is well known that energy use in many process industries is dominated by separation tasks—particularly distillation. In this work, the shortest stripping line approach recently proposed by Lucia, Amale, & Taylor (2006) is used to find minimum energy requirements in distillation. The new aspects of this work show that this shortest stripping line approach can find minimum energy requirements for (1) Distillations with feed pinch, saddle pinch, and tangent pinch points. (2) Distillations for which the minimum energy solutions do not correspond to a pinch point. (3) Processes with multiple units (e.g., reactive distillation, extraction/distillation, etc.). Other novel features of this work also shows that the shortest stripping line approach (4) Can be used to identify correct processing targets in multi-unit processes. (5) Encompasses longstanding methods for finding minimum energy requirements including the McCabe-Thiele method and boundary value methods. A back-to-front design approach based on shortest stripping lines is used so that correct processing targets can be identified so that all tasks can be synthesized simultaneously in such a way that the most energy efficient designs are achieved. New problem formulations that take the general form of nonlinear programming (NLP) and mixed integer nonlinear programming (MINLP) problems are given and a novel global optimization algorithm is presented for obtaining energy efficient process designs. A variety of ideal and nonideal distillations, including examples with four or more components, are used to demonstrate the efficacy of the shortest stripping line approach. The examples with more than three components are particularly significant because they clearly illustrate that the proposed approach can be readily used to find minimum energy requirements for distillation problems involving any number of components. Many geometric illustrations are used to highlight the key ideas of the method where appropriate.

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