Energy-efficient Extractive Distillation System:Design And Optimization

Extractive distillation is a widely used alternative which can be exploited to separate azeotropic mixtures. In general, for a two-component mixture the extractive distillation process is made up of two columns, i.e. an extractive diatillation column and a solvent recovery column or three columns, i.e. a preconcentration column?an extractive diatillation column and a solvent recovery column. To improve the energy efficiency of the extractive diatillation, energy-saving method is studied in this paper for both two-column sequence and three-column sequence. Rigorous steady state simulations were implemented on chemical process simulation software Aspen Plus. A global economic optimization is carried out using the total annual cost(TAC) as objective function.Based on a three-column conventional extractive column process, an energy efficiency process is explored by combining pre-concentration column and entrainer recovery column which provides potential energy saving and capital investment reduction. Two case studies are demonstrated to verify the above-mentioned energy and economic advantages: the separation of isopropyl alcohol-water using DMSO as entrainer and the separation of acetonitrile-water using ethylene glycol as entrainer. In both cases, based on the global economic optimization, a design with optimized operation conditions for the conventional and the improved process is developed. It is revealed that the new process offers energy savings of 13.7 % and 16.3 % respectively, 12.1 % and 14.4% of the total annual costs is reduced respectively as compared to those of the conventional process. In fact, a very special aspect of extractive distillation is that the component with the higher-boiling point maybe removed as the distillate from the extractive column. In this paper, two important mixtures are investigated to demonstrate this phenomenon: the separation of n-propanol-water using N-methyl-2-pyrrolidone(NMP) as entrainer and the separation of ethyl acetate-ethanol using furfural as entrainer. In order to separate these two mixtures, a three-column extractive distillation sequence including preconcentration column is applied to diluted fresh feedstock. Based on this three-column conventional extractive distillation system, an innovative energy-saving distillation process with lower capital investment is developed. For the two separated mixtures, based on global economic optimization, a design with optimum design variables are developed for both of the conventional distillation process and the new proposed distillation process. Results reveal that the new process can offer 22.8% and 17.3% energy-savings respectively. 17.6 % and 15.2% of reduction in total annual costs(TAC) can be obtained as well when compared to the conventional distillation process. Based on two-column conventional extractive column process, an energy efficiency process is explored by feed splitting and combining pre-concentration column and entrainer recovery column which provides potential energy saving and capital investment reduction. Two case studies are demonstrated to verify the above-mentioned energy and economic advantages: the separation of isopropyl alcohol-water using DMSO as entrainer and the separation of acetonitrile-water using ethylene glycol as entrainer. In both cases, based on the global economic optimization, a design with optimized operation conditions for the conventional and the improved process is developed. It is revealed that the new process offers energy savings of 14.0% and 14.8% respectively, 11.5% and 13.1% of the total annual costs is reduced respectively as compared to those of the two-column process.