Simulation And Optimal Selection Of Distillation Schemes For Separating Ternary Mixtures

Distillation is the most widely used industrial separation technique, but, at the same time, is the biggest energy consumer in process industries. By using heat integrated distillation sequences or thermally coupled distillation configurations including side-stripper and side-rectifier, significant energy savings can be achieved. Literatures have shown that, in some circumstances, the fully thermally coupled distillation column(or Dividing wall column, DWC) configuration can reach more than 30% of energy savings compared with conventional schemes of simple column sequences. However, lack of reliable design method and potential control problems have greatly limited the application of DWC in industries. Some studies have shown that the energy efficiency of any distillation configuration depends greatly on the mixtures and feed compositions. To select the best distillation scheme for separating a given mixture with a specific composition is thus an important problem of optimal decision in the design..In the present study, rigorous simulation is used to investigate and compare the total annual costs(TACs) of various distillation schemes for the separation of ternary mixtures, in order to identify the optimal one for a particular mixture with its feed condition. The distillation schemes studied in the present work include simple distillation sequences with and without energy-integrations, partially thermally coupled schemes(side-rectifier and side-stripper) and fully thermally coupled scheme(DWC). Six different ternary mixtures with various feed conditions are selected in the investigation of the present work, and the best distillation scheme is identified for each particular mixture and composition, so that the triangle map of feed composition for any mixture is divided into different areas, in each of which the best distillation scheme is identical. The results show that the mixture properties and its composition have significant influence on the total annual cost, and each of the traditional distillation sequence with energy-integration, partially thermally coupled distillation schemes and the fully thermally coupled scheme can have advantage over the others at a particular mixture and a composition condition. Based on the analysis, a number of rules are proposed for selecting the best distillation scheme for separating ternary mixtures, and as a result, a heuristic algorithm is established to optimize the selection of distillation schemes for ternary mixture separation. And then the proposed algorithm is validated by an illustrating example problem.Three improved dividing wall columns configurations with reduced number of intercolumn vapor transfers proposed by Agrawal are rigorously simulated to show their equivalencies to the original scheme, and by the column diameter designs, their feasibilities for industrial applications are validated.