A retrofit approach for heat exchanger networks

Abstract

This paper presents a two-stage procedure for the optimal redesign problem of existing heat exchanger networks. In the first stage, a mixed-integer linear programming (MILP) model is proposed for the retrofit at the level of matches that is based upon a classification of the possible structural modifications. The objective function of this optimization model seeks to minimize: (a) the cost of purchasing new heat exchangers; (b) the cost of additional area; and (c) the piping cost, and is subject to a set of constraints that describe: (a) the heat flow model; (b) the area estimation; (c) the calculation of additional area; and (d) the match—exchanger assignments. The solution of the retrofit model at the level of matches provides information about the process stream matches and their heat loads, the placement/reassignment of new and existing heat exchangers, estimates of the required area of each match and the required increase or decrease of area in each heat exchanger, and estimates of the repiping cost associated with introducing new matches, installing new heat exchangers, moving existing exchangers and repiping streams. In the second stage, the information generated in the first stage is used to postulate a superstructure containing all possible network configurations. The solution of a nonlinear programming problem (NLP) based upon this superstructure gives a retrofitted heat exchanger network. The proposed procedure is demonstrated with three example problems.