Energy Requirements of Distillation: Exergy, Pinch Points, and the Reversible Column

Abstract

This paper considers the problem of finding minimum energy requirements of a single-feed adiabatic distillation column for a given separation task. Over the years, many attempts to understand this problem have been made. This paper presents an approach which is at the intersection of the thermodynamic and geometric points of view. In it, an analysis of multicomponent distillation is carried out via the notion of power of separation, which is akin to compositional exergy. It is shown mathematically that this concept, although taking its roots in thermodynamics, has a strong link with the geometry of liquid composition trajectories in multicomponent distillation and is also related to the traditional McCabe−Thiele diagram for binary distillation. By considering pinch-point curves for adiabatic column sections and their link with reversible column profiles, a characterization of minimal energy requirements of the single-feed adiabatic distillation process is proposed. As a consequence, a shortcut method to determine minimal energy requirements of multicomponent distillation is introduced. Examples validating this approach for multicomponent distillation of ideal, nonideal, zeotropic, and azeotropic mixtures with up to six components are presented. These examples indicate that the new method can, in principle, treat any number of mixture components. All types of pinch behavior are covered, that is, the method can find minimal energy designs associated to feed, saddle, or tangent pinch points. Case studies where pinched minimum energy solutions exist, where pinched solutions can be reduced to nonpinched solutions, and where no pinched solutions exist are presented. In all these cases, the new method can find, if they exist, pinched and nonpinched solutions.