Novel Control Strategy for Maximum Boiling Extractive Distillation Systems: Acetone/Chloroform Separation

There are quite a few papers already published in open literature addressing the overall control strategy of minimum-boiling extractive distillation system, although the two-column extractive distillation systems for maximum-boiling and minimum-boiling separations are exactly the same in terms of design flowsheet. However, the control of maximum-boiling extractive distillation system should be considered as categorically different from that of minimum-boiling extractive distillation system. The main reason is because of the difference in types in ternary residual curve maps. For maximum-boiling extractive distillation system, an extra distillation boundary is present to divide the whole ternary diagram into two distillation regions. Thus, to ensure two products at high purity, entrainer-to-feed ratio has to be set greater than a minimum ratio. This minimum ratio can be estimated by the location of feed composition in the ternary diagram. For control consideration in rejecting unmeasured feed composition disturbances, a commonly used strategy of holding the entrainer-to-feed at a constant ratio may not work for maximum-boiling system. The purpose of this paper is to show a novel strategy of fixing the entrainer-to-D2 ratio in the overall control structure for two-column maximum-boiling extractive distillation systems. A systematic procedure is developed in determining the control structure to be a dual-point or single-point in a particular column, and further to select the proper temperature control point(s) in the two columns. The demonstrating example is acetone-chloroform separation by using N-methyl-2-pyrrolidone as the heavy entrainer. The devised overall control strategy only relies on tray temperature measurements without the need of any online composition analyzer.