Structural considerations and modeling in the synthesis of heat integrated – thermally coupled distillation sequences

This paper deals with the design of mixed thermally coupled-heat integrated distillation sequences. The approach considers from conventional columns (each distillation column with a condenser and a reboiler) to fully thermally coupled systems (only one reboiler and one condenser in the entire system). A discussion about superstructure generation and the convenience of using a representation based on separation tasks instead of equipment is presented, as well as a set of logical rules in terms of boolean variables which allow to systematically generate all the feasible structures. Based on the logical rules an algorithm is developed for generating valid sequences (even by hand) without explicitly solving the logical equations. All the specific aspects related with inter-column heat integration when partially thermally coupled systems are considered. A disjunctive programming formulation for extracting the optimal solution is presented. The model is based on the Fenske, Underwood Gilliland equations. However, the disjunctive formulation allows easily the use of any other shortcut, aggregated or even rigorous model without almost modifying the structure of the formulation. Finally three examples are presented.