Synthesis of multi-stream heat exchanger network for multi-period operation with genetic/simulated annealing algorithms

The multi-period synthesis of multi-stream heat exchanger network (MSHEN) can be formulated as a mixed integer nonlinear programming (MINLP) model and the obtained MSHEN is over-synthesized for all operational period. In this paper a novel strategy for synthesizing flexible MSHEN under multi-period operation that involves specified changes in stream temperatures and flow rates is presented. In the first stage, the multi-period MSHEN is over-synthesized by the temperature–enthalpy (T–H) diagram method based on the stream pseudo-temperature. Compared with the superstructure-based multi-period MINLP model, the decision variables of the simultaneous multi-period NLP model proposed are only the stream heat transfer temperature difference contributions. The model features that the size and the complexity of the problem are reduced significantly and the feasibility of the initial feasible solutions are guaranteed, which will enhance the calculation efficiency of the problem. In the second stage, the optimal MSHEN obtained in the first stage is improved to overcome the problem of over-synthesis. The structure of the over-synthesis MSHEN is retained and each heat exchanger area is optimized considering the multi-period operation in order to make the MSHEN less costly. The objective in both stages is the total annual cost of MSHEN and the global solution can be guaranteed by genetic/simulated annealing algorithm (GA/SA) at high probability. Finally, two examples are illustrated to demonstrate the performance of the strategy for multi-period MSHEN synthesis.