The Role of Recycle in Countercurrent Recycle Distillation Cascades. II. Extractive Distillation and Hybrid Cascades

Abstract

Stage recycle requirements for a specified separation in a counter current recycle cascade vary with local (stage) composition and local α in a complex manner. The amount of stage wise recycle necessary to maintain stage separation is lower for higher values of α, and also decreases as the local stage compositions in the separation cascade approach desired product compositions. In Part I of this paper it was shown how this latter behavior can be used to design ideal and/or ‘squared-off’ cascades which minimize total inter stage flow and equipment size. In addition, α can be significantly increased for some systems, by use of a suitable extractive agent, that is, by using extractive distillation. Part II of this paper discusses some of the possible benefits of using extractive distillation with respect to reducing the number of theoretical stages required and also greatly reducing the required recycle ratio (and thus energy) requirements for a specified separation relative to ordinary distillation. It is also shown that it is theoretically possible to design an extractive-ordinary hybrid distillation cascade which could offer some advantages over extractive distillation for some systems. The proposed hybrid cascade would use the same (low) reflux ratio necessary in the center of the extractive distillation section (where α is high) throughout the cascade, including the two ordinary distillation sections (where α is low without agent). This is possible because, in the ordinary distillation sections, stage compositions are closer to the required product compositions. Depending on the specific system and equipment design, the hybrid cascade could reduce the number of extractive distillation stages required for a specified separation, require less inventory of extractive agent, and require less total inter stage flow and/or fewer actual stages than that required for straight extractive distillation. Results of some preliminary calculations are presented for some hybrid cascades for systems with α = 1.1/1.2 and α = 1.4/2.1, for ordinary and extractive distillation, respectively. Calculations for a system with α = 1.5/ 3.5 suggest that a hybrid cascade may not be suitable for that system, but shows the possible benefits of using extractive distillation for such a system. These applications further illustrate the consequences of the stage wise dependence of minimum reflux ratio in counter current recycle cascades.

Notes

Personal communication, Fu-Ming Lee, Glitch Technology Corp., 2020 Dairy Ashford, Houston, TX 77077.