(33f) Necessity of Curvature in Distillation Boundary for Feasible Extractive Distillation Systems in Separating Maximum-Boiling Azeotropes

Extractive distillation is commonly used in industry for separation of azeotropic mixture. A heavy entrainer is added into an extractive distillation column to greatly enhance the relative volatility of one component so that it can go to the distillate of this column while the entrainer and the other component go to the column bottoms. These two components can easily be separated in an entrainer recovery column with heavy entrainer at bottom stream for recycling to the first extractive distillation column. For the separation of maximum-boiling azeotropes, design configuration of the two-column system is exactly the same as the ones for minimum-boiling azeotropic systems. However, because of different topology of ternary diagram for the maximum-boiling system, unique design consideration should be taken to properly design such separation system. Unlike minimum-boiling system having no distillation boundary in the ternary system, there will be a distillation boundary for the maximum-boiling system with a heavy entrainer. Depending on the degree of curvature of distillation boundary, feasible minimum entranier-to-feed ratio can be estimated with given product purity specifications. This minimum ratio can be considered as another screening tool for selecting suitable entrainer of maximum-boiling azeotropic systems. Two examples are presented to demonstrate the important findings for the separation of maximum-boiling azeotrope via extractive distillation system. In the first example of separating acetone and chloroform, over 20% savings in total annual cost and energy can be realized by using a newly proposed entrainer of N-methyl-2-pyrrolidone for this system as opposed to using literature suggested dimethyl sulfoxide or ethylene glycol as entrainer. In the second example of separating phenol and cyclohexanone, the savings in total annual cost and energy are even more significant by using a newly proposed entrainer for this system as opposed to using literature suggested acetophenone adding into this separation system.