Heat exchanger network synthesis for batch processes by involving heat storages with cost targets

Based on time slice model (TSM), two methods, in both of which heat storages are employed to realize the indirect heat exchange operation between different time intervals, are proposed for the synthesis of heat exchanger networks (HENs) for batch processes. Firstly in the first method, a non-linear programming (NLP) model aiming at the minimum total annual cost (TAC) of networks is formulated, with which the operating cost for utilities as well as the capital cost for heat exchanger units are minimized simultaneously. Direct and indirect heat exchange operations are both considered, so this method is suitable for the two time intervals in which cold and hot streams coexist. However, when there are time intervals in which only either cold or hot streams involved, the first method is not feasible to fix the problem. So the second method is proposed in which only indirect heat exchange operation is considered. A graphical approach is introduced for the sub-networks synthesis of concerned intervals. The proposed two methods are complementary as they are applicable to different interval cases. Temperatures of heat storages in both methods are all determined through building mathematical models with the target of minimum TAC. Heat storages are added between the different time intervals and more heat can be recovered with the TAC reducing in some extent. The better HEN structure is gotten and the problem on matching streams of different time intervals is solved through the proposed methods. At last, the two methods are combined to solve an example to demonstrate the application and effectiveness of the proposed methods.