(401a) Unique Design Considerations for Maximum-Boiling Azeotrope Via Extractive Distillation System: Acetic Acid/N,N-Dimethylacetamide Separation

Extractive distillation system is commonly used in industry to separate azeotropic or close boiling mixtures. Adding a suitable heavy entrainer into the system can enhance the relative volatility of the original two components so that the lighter component can be obtained from the distillate of an extractive distillation column, while another component can be obtained from the distillate of a second entrainer recovery column. The bottoms of the second column containing mostly the entrainer can be recycled back to the extractive distillation column. For minimum-boiling azeotropic mixture, the enhancement capability of relative volatility is most important for the selection of suitable entrainer. As for maximum-boiling azeotrope, another important feature of the ternary system needs to be taken into consideration also for the determination of suitable entrainer. This is because the type of ternary diagram/RCM for maximum-boiling azeotrope is different than that of the minimum-boiling azeotrope. A distillation boundary will be formed for the ternary diagram/RCM of maximum-boiling azeotrope with a heavy entrainer. Depending on the degree of curvature of the distillation boundary, the feasible minimum entranier-to-feed ratio can be estimated with given product purity specifications. Thus together with the enhancement of relative volatility, the estimated minimum value of entranier-to-feed ratio can be used as another screening tool for selecting a suitable entrainer of maximum-boiling azeotrope systems.

An extractive distillation system for separating acetic acid and N,N-Dimethylacetamide is used as an illustrative example to elaborate the new findings for determining the most suitable entrainer for maximum-boiling azeotorpe systems. Optimal design flowsheets of two candidate entrainers, triethylene glycol (TEG) and tetraethylene glycol (TTEG), are obtained to compare the total annual cost and the steam cost of the two candidate extractive distillation systems. It is demonstrated that the estimated minimum value of entranier-to-feed ratio is crucially important in the determination of the most suitable entrainer. Using TTEG as entrainer, although having greater enhancement of the relative volatility of acetic acid over N,N-Dimethylacetamide, the feasible minimum entranier-to-feed ratio is much larger than that of the TEG system. Thus, the economics of the overall system using TTEG as entrainer is inferior to that of the system using TEG as entrainer. This minimum value of each candidate entrainer can easily be estimated by the ternary diagram/RCM of the candidate system without needing of time-consuming procedure in obtaining the optimal design flowsheet of the extractive distillation system.