Study On Flexible Synthesis Of Multi-Stream Heat Exchanger Network In The Low Temperature Processes

Multi-stream Heat Exchanger is widely used in heat exchanger network (HEN) of the low temperature processes, which is more easily to be affected by the uncertainty parameters in the application of engineering because of its small heat transfer temperature difference. So the flexible HEN problem in the low temperature processes is more evident. Most papers focus on the flexible synthesis of two-stream HEN. The research on the flexible synthesis of multi-stream HEN is limited because of the complexity of the problem and the difficulty of finding the global optimum solution. In this paper the study on the flexible synthesis of multi-stream HEN (MSHEN) in the low temperature processes is carried out with the support of German Research Council Foundation (DFG) and main research works are as follows:(1) With the thermodynamic analysis of the heat transfer processes system, an effective NLP formulation for the flexible synthesis of MSHEN is presented based on the stream pseudo T-H diagram method and the strategy of topology union set for HEN of multiple operating points. The decision variables are stream heat transfer temperature difference contribution values, which will help to select the reasonable temperature difference values in the low temperature processes. Moreover, no binary variables exist in the formulation, vertical match approach on the T-H diagram can deal with thel complex constraints in heat transfer feasibility and all solutions in evolution procedure are feasible. So the complexity of the flexible synthesis of MSHEN is remarkably reduced, and an effective method is proposed for the flexible synthesis of the large scale MSHEN.(2) A simultaneous method of heat exchanger area optimization of flexible MSHEN is proposed. The authors point out that the "over-synthesis" problem exists in the flexible synthesis of MSHEN, which results in that the capital cost of the flexible MSHEN is more expensive and the heat exchanger areas are relatively overplus under operation. So in this paper heat exchanger areas that obtained by the strategy of topology union set for HEN of multiple operating points are taken as the decision variables to distribute the heat exchanger areas, in which the simultaneous optimization between the heat exchanger areas and the total annual cost of the flexible MSHEN is achieved. The method can overcome the "over-synthesis" problem of the flexible MSHEN.(3) With the higher probability to find the global optimum solution of the flexible MSHEN synthesis problem. The flexible MSHEN synthesis problem can be formulated as an MINLP problem. Many local optima of this kind of problem may be existed due to its non-linearity, non-differentiable and non-convexity. In the paper, the flexible MSHEN synthesis problem is optimized by genetic/simulated annealing algorithm (GA/SA). The generation approaches for initial feasible solution are propsed, which will enhance the calculation efficiency of GA/SA. Moreover, some operators such as crossover operator, mutation operator, OCX and EC operators are appropriately designed for the flexible MSHEN synthesis problem. The performance of the strategy for flexible MSHEN synthesis based on GA/SA is demonstrated successfully by the results of two examples.(4) As engineering pratice, the strategy for flexible MSHEN synthesis is applied to an industrial case, a MSHEN of demethanizer system in ethylene plant. According to the characteristic of MSHEN in the low temperature processes, a MSHEN synthesis method is presented; at the same time the flexible MSHEN of demethanizer system in ethylene plant is synthesized based on the proposed strategy. The results show that the strategy is efficient and dependable for the flexible synthesis of large scale MSHEN, wich can be taken as the guidance for the application of engineering.