Simultaneous integrated design for heat exchanger network and cooling water system

Abstract

Heat exchanger network and cooling water system are two major elements of energy systems in processing plants. Such two subjects have a very close interaction with each other. However, most of current researches firstly synthesize heat exchanger network and then design cooling water system. This sequential methodology probably misses the optimum solutions, and results in some suboptimal designs from an overall perspective. To overcome this limitation of traditional methods, in present paper a simultaneous methodology is introduced to integrate heat exchanger network and cooling water system as a whole system. Unlike conventional approaches, the methodology treats cooling water asa special cold stream whose mass flow rate, initial and final temperatures are all unknown variables and require to be optimized. The methodology mainly makes use of a modified stage-wise superstructure that covers most possible configurations for integrating heat exchanger network and cooling water system. The mathematical optimization model corresponding to the superstructure is a mixed integer nonlinear programming (MINLP) problem. The total annual cost (TAC) is set as the objective function composed by utility cost, pumping cost, and capital cost of cooling tower and heat exchanger. An industrial case study is used to demonstrate the capabilities of the proposed methodology.