SYNTHESIS OF HEAT INTEGRATED DISTILLATION SEQUENCES

The synthesis of optimal heat integrated distillation sequences is difficult because of the many alternative solutions which exist. This work presents two methods for solving this synthesis problem. The first method involves generating a superstructure of feasible solutions and searching this superstructure for the optimal solution using a mixed integer linear program. Two algorithms for generating superstructures and two formulations of the mixed integer linear program have been developed. This method requires much less CPU time than previously developed methods.; The second method is developed by representing a distillation column as an area on a T-Q diagram. This area is characterized by a width equal to the heat duty of the distillation column and a height equal to the difference between the bottoms and distillate temperatures. A very easily calculated lower bound on the minimum utility use of single distillation tasks and of distillation sequences is developed using this representation and the principles of multieffect distillation for the case where the heat duty and temperature differences across a column are not functions of pressure. A method for calculating this bound when the heat duty and temperature difference are pressure dependent is also presented. The representation of a distillation column as an area on a T-Q diagram and the lower bounds on the utility use are combined to produce methods for synthesizing distillation systems which use only minimum utilities and for synthesizing minimum cost distillation systems.; Both synthesis techniques are demonstrated using three component and five component example problems.