(293f) On the Selection of Optimal Configuration for Ternary Distillation

Ternary distillation has recently drawn much attention, owing to the recent extensive investigations on dividing-wall column (DWC), or its equivalence the Petlyuk column, and thermally coupled distillation configurations with their significant potential in energy saving. However, except the DWC and thermally coupled distillation, a number of alternatives for tenary distillation may be atractive in terms of Total Annual Cost (TAC). The cost of a ternary distillation configuration depends on the composition, relative voatilities, and mass fractions of the components in the feed, allocation of the components among the product streams and so on. In addition, a ternary separation can also be used for initial splits of multicomponent mixture into three streams of more than one component for further processing. In this case, the specifications for the key components may be rather flexible, or the effective ease of separetion can be different from those for three-component mixture separation. In the precent work, rigorous simulation and optimization were performed to analyze and compare, in terms of TAC, different promising distillation configurations, including simple columns sequences with or without heat-integration, complex column with side-rectifier or side-stripper, and fully thermally coupled, or DWC for ternary separation. The triangle diagrams of the feed composition were employed to show the partition of the diagram by the different distillation configurations that have the lowest TAC in the corresponding composition areas by the partition on the triangle diagram. We demonstrated that the feed composition, the relative volatilities between adjacent components have strong influnce on the partition and, thus, the choice of the distillation configuration of the lowest TAC for a specific feed composition. We also found in the present work that the purity specifications of the products also affect significantly the partition. We showed that, even with the same Ease of Separation Idex (ESI, the ratio the relative volatility between the light and middle components to that between the moddle and heavy components, traditionally used to indicate the suitibility for a DWC), the partition is quite different if the specified products’ purities are different. Based on this finding, a new index was introduced to represent the influence of the product purities on the selection of the optimal distillation configuration for a given feed composition. Rules were proposed, and systematic approach, based on the rules, for selecting optimal distillation configuration of ternary distillation was also proposed for some simplified cases.