Multi-objective optimization of heat exchanger networks based on analysis of minimum temperature difference and accumulated CO2 emissions

Abstract

A multi-objective optimization method of heat exchanger network (HEN) on the basis of partition of minimum approach temperature (ΔTmin) intervals is proposed to minimize the total annual cost and accumulated CO2 emissions in construction and operation phases. In this method, both dominant and recessive CO2 emissions are considered, and the effects of ΔTmin on multi-objective optimization problems of HEN are investigated. We find that a multi-objective optimization problem may reduce to a single objective optimization problem as ΔTmin changes. The critical ΔTmin that distinguishes the intervals for the single objective optimization and the multi-objective optimization problem can be identified by combining the pinch analysis and mathematical programming method. The optimal structure and CO2 emissions of HEN are obtained in the single objective optimization problem, whereas the Pareto front of two objectives and a trade-off strategy are obtained in the multi-objective optimization problem. The HEN in a vacuum gas oil hydro-treating unit in a refinery is taken as a case study. Results indicate that the partition of ΔTmin intervals helps to identify the nature of the multi-objective optimization problems and reduce the computation loads of the multi-objective optimization problem of HENs. In addition, the optimal solutions of HEN obtained by the ε-constraint method distribute uniformly, and the dominant objective changes gently within the intervals of ΔTmin for the multi-objective optimization.