A SCREENING AND OPTIMIZATION APPROACH FOR THE RETROFIT OF HEAT-EXCHANGER NETWORKS

This paper presents a systematic procedure for the retrofit design of heat-exchanger networks (HEN's). The approach is divided into a prescreening stage and an optimization stage. In the prescreening stage, the economic feasibility of the project is analyzed with lower bounds on cost for utility, additional area, and structural modifications. The bounds are used to construct a prescreening cost plot to estimate the maximum savings that can be achieved. In the optimization stage, some information from the prescreening stage is used to construct a novel superstructure that has embedded alternative retrofit designs for reassigning existing exchangers to different streams, introducing new exchangers to the network, and adding piping and area modifications. The superstructure is optimized with a mixed integer nonlinear programming (MINLP) formulation to determine the retrofit network requiring the least total annual cost. Heat loads, minimum approach temperature (EMAT), and hot stream/cold stream matches are not fixed but are optimized in order to accurately account for the trade-offs between capital and energy cost. Several examples are presented to illustrate this method.