THE ROLE OF RECYCLE IN COUNTERCURRENT RECYCLE DISTILLATION CASCADES .2. EXTRACTIVE DISTILLATION AND HYBRID CASCADES

Stage recycle requirements for a specified separation in a countercurr ent recycle cascade vary with local (stage) composition and local alph a in a complex manner. The amount of stagewise recycle necessary to ma intain stage separation is lower for higher values of alpha, and also decreases as the local stage compositions in the separation cascade ap proach desired product compositions. In Part I of this paper it was sh own how this latter behavior can be used to design ideal and/or ''squa red-off' cascades which minimize total interstage flow and equipment s ize. In addition, alpha can be significantly increased for some system s, by use of a suitable extractive agent, that is, by using extractive distillation. Part II of this paper discusses some of the possible be nefits of using extractive distillation with respect to reducing the n umber of theoretical stages required and also greatly reducing the req uired recycle ratio (and thus energy) requirements for a specified sep aration relative to ordinary distillation. It is also shown that it is theoretically possible to design an extractive-ordinary hybrid distil lation cascade which could offer some advantages over extractive disti llation for some systems. The proposed hybrid cascade would use the sa me (low) reflux ratio necessary in the center of the extractive distil lation section (where alpha is high) throughout the cascade, including the two ordinary distillation sections (where alpha is low without ag ent). This is possible because, in the ordinary distillation sections, stage compositions are closer to the required product compositions. D epending on the specific system and equipment design, the hybrid casca de could reduce the number of extractive distillation stages required for a specified separation, require less inventory of extractive agent , and require less total interstage flow and/or fewer actual stages th an that required for straight extractive distillation. Results of some preliminary calculations are presented for some hybrid cascades for s ystems with alpha = 1.1/1.2 and alpha = 1.4/2.1, for ordinary and extr active distillation, respectively. Calculations for a system with alph a = 1.5/3.5 suggest that a hybrid cascade may not be suitable for that system, but shows the possible benefits of using extractive distillat ion for such a system. These applications further illustrate the conse quences of the stagewise dependence of minimum reflux ratio in counter current recycle cascades.