Improved design of an extractive distillation system with an intermediate-boiling solvent

Abstract

Most extractive distillation systems use a solvent (entrainer) that is higher-boiling than the two key components. A two-column sequence is used in which the bottoms of the second column is the solvent stream and is recycled back to the first extractive column. There are some chemical systems in which the entrainer is an intermediate-boiling component. In this situation there are two alternative separation sequences. The direct sequence takes the light key component overhead in the first column, and the solvent recycle is the distillate from the second column. The indirect sequence takes the heavy key component out the bottom of the first column and the solvent is the bottoms from the second column. This paper explores a design proposed in the literature for separating methanol and toluene using the intermediate-boiling solvent triethylamine. The indirect sequence is shown to be more energy efficient. More importantly the solvent flowrate given in the published paper is shown to be much larger than required. Designs with greatly reduced solvent recycle flowrates reduce energy requirements by 50%.