Approximate design of multiple-feed/side-stream distillation systems

Abstract

An approximate, equation-based design method is presented to describe the separation of multicomponent mixtures in multiple-feed towers with or without side streams. Simple expressions for the selection of the controlling feed and the determination of feasible side-stream compositions are found. Several counterintuitive results are found. It is shown that the minimum reflux, rather than the boiling point, is a better criterion for ordering feeds to the column. For example, vapor rates are sometimes reduced by placing lower boiling point feeds below higher boiling point feeds in multiple-feed columns. Complex configurations, using multiple-feed and side-stream columns, can reduce the operating costs for distillation without large increases in capital costs. Examples are shown with vapor-rate savings as large as 30%. These complex column arrangements may be useful in process retrofits.