Energy-Saving Designs for Separation of a Close-Boiling 1,2-Propanediol and Ethylene Glycol Mixture

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Abstract

Separation of 1,2-propanediol and ethylene glycol is an important task for coproduction of these two compounds via hydrogenolysis of glycerol for the purpose of utilizing this biodiesel byproduct. These two compounds exhibit close-boiling behavior which requires many stages of a regular column and also large energy consumption to meet stringent product purity specifications. In this paper, several alternative designs are investigated in order to save steam cost and also total annual cost of this separation task. Alternative designs considered include multieffect distillation, heterogeneous azeotropic distillation, and extractive distillation. Significant reductions of 38.3% in steam cost and 30.6% in total annual cost as compared to the regular column can be obtained by a design flowsheet via an extractive distillation system using triethylene glycol as entrainer. Methods for further improving the economics of this extractive distillation system have also been investigated. A simple worthwhile improved design is to preserve the energy from the hot entrainer to preheat the fresh feed via a feed-effluent heat exchanger. With this simple improvement, a further 12.8% reduction in steam cost can be made as compared to the original extractive distillation system.

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