Geometry of Separation Boundaries: Systems with Reaction

Residue curves and residue curve maps have been extensively studied for over 100 years. Much of the literature in this field deals with systems that do not react. Chemical reactions can influence residue curve maps in some important ways. For example, it is known that reactions can lead to both the appearance and disappearance of stationary points (azeotropes), and that reactive azeotropes can exist even in systems that otherwise would be considered thermodynamically ideal. It follows that chemical reactions can influence the very existence of separation boundaries and, therefore, the design and synthesis of reactive separation processes. Lucia and Taylor (AIChE J. 2006, 52, 582) have shown that distillation boundaries in nonreacting ternary liquid mixtures can be defined as local maxima in the line integral from any unstable node to all reachable stable nodes. This paper shows how this geometric approach can be adapted for the rigorous determination of separation boundaries in reacting mixtures. Evidence is provided to show that boundaries in systems with two degrees of freedom are maximum line integrals. Examples of such systems include four-component systems with a single equilibrium reaction and ternary systems with a kinetically controlled reaction.