Invariant rectifying-stripping curves for targeting minimum energy and feed location in distillation

Abstract

Invariant rectifying-stripping (IRS) curves are proposed that are independent of the feed location and operating reflux of the distillation column for a given separation problem. IRS curves represent the enthalpy surpluses and deficits in the rectifying and stripping sections, respectively, as a function of temperature for all possible values of reflux and reboil. The IRS curves provide a new representation on the temperature-enthalpy diagram to set distillation column targets prior to detailed design for minimum energy requirement, feed location, feed preconditioning, and side-exchanger loads. The application of the proposed concepts to two binary distillation examples (one featuring a tangent pinch) and a multicomponent distillation example illustrates the usefulness of the IRS curves in properly locating the feed, determining the minimum utility requirements, and reducing the tedium of repeated simulations. The IRS curves are rigorously invariant and provide the absolute minimum utility requirements for binary systems (ideal as well as non-ideal); however, they are near-invariant and predict the near-minimum utility requirements for multicomponent systems (where the pseudo-binary concept of a light and heavy key is employed).