Synthesising an optimal heat exchange system for the separation of multicomponent mixtures

Abstract

The article considers the issues of the optimal organisation of a heat exchange system for a CDU/VDU primary oil distillation unit. In order to determine the economic feasibility of the heat exchange system, a pinch analysis was performed. As a result of the pinch analysis we obtained composite curves of hot and cold flows and a large compound curve, which helped us to determine the maximum possible amount of energy recovered by the process flows, the type of energy carriers according to their energy level, and the optimal driving energy that ensures the minimum optimality criterion, i.e., the total of the operating and capital costs. The results of the pinch analysis showed that about 30% of the energy can be recovered by the process flows of the distillation unit. Analysis of methods for the synthesis of optimal heat exchange systems demonstrated that the decomposition method is one of the most effective for solving large-scale problems. It implies dividing the synthesis problem into several subproblems of lower dimension: linear programming problems to minimize energy costs, discrete-continuous problems of linear programming to minimize the amount of heat exchange equipment, and nonlinear programming problems to determine the minimum total reduced costs. Using the decomposition-based synthesis algorithm we determined the optimal structures of the heat exchange system and the modes of operation of the heat exchangers and obtained the economic estimates for these structures. The results of the study demonstrated that the total reduced cost can be reduced by 40%, if the functioning heat exchange system is reorganised. Heat exchange systems designed with the function of thermal energy recovery can also save up to 6377 thousand cu/year, which is possible due to the use of surplus energy for heating process flows of nearby units.