Optimization Of Heat Exchanger Network Retrofit Considering Multiple Factors

Heat exchanger networks synthesis and retrofit have been the most important subjects during process integration field. The impact of network retrofit on improving energy saving has obvious effect. During the last over50years, various kinds of methods have been developed for heat exchanger network synthesis andretrofit. There has been made rapid process in this subject. The research results have been extensively to actual process industry for obtaining large energy-saving benefit. After1990s, due to rising energy prices, these subjects have attracted much more attention from researchers. The main focus of heat exchanger network retrofit is to develop a more effective and practical retrofit method, which can remove impractical assumptions and avoid unfavourable computational complexity. Research advances in retrofit approaches have an importent influence on improving energy recovery of the whole process industry’.This paper mainly contributes to propose a novel simultaneous approach for heat exchanger network retrofit considering multiple factors-topology modification, heat transfer enhancement and pressure drop limitation. Our main work includes four aspects as follows:1、Mathematical model of heat exchanger network retrofit. We proposed an improved stage-wise superstructure for heat exchanger network retrofit problems, which consists of exsiting exchangers, new heat exchangers, additional area and exchangers reassignments. Based on the improved superstructure, a simultaneous optimization mathematical model without the assumption of isothermal mixing is formulated to solve retrofit problems, which is a mixed integer non-linear programming model.2、Heat exchanger network retrofit with considering pressure drop. Based on the stage-wise superstructure model, the stream pressure drop calculation formulations are incorporated into the retrofit mathematical model for considering pumping device and pumping power costs during the total annual cost. In this study, a two-level optimization strategy is used for solving the above-mentioned complex model. In the outer level, the genetic algorithm has been used for generating new network structures and finding the optimal one. In the inner level, the PSO/GA hybrid algorithm has been used for optimizing operating parameters and sending the best cost of the achieved to the upper level for determination of fitness of existing network structure. The research results prove that the retrofit approach considering pressure drop enables the designer to study tradeoff among pumping cost, additional area cost and operating cost of the network, and is therefore to be reliable and applicable in engineering design.3、Heat exchanger network retrofit with considering heat-transfer enhancement. Heat transfer enhancements are very attractive options for heat exchanger network retrofit. To consider the influence of heat transfer enhancement on the retrofit option, combined with practical heat transfer enhancement techniques, an extended retrofit model is proposed to obtain a trade-off among enhancement cost, topology adjustment cost and operating cost. The case study indicates that the extended model can reasonably find the enhancement position in the network. During the optimal network topology, reasonable heat transfer enhancement may decrease the additional area demanding, and therefore a more optimal solution can be obtained with lower investment costs and investment recovery period.4、Heat exchanger network retrofit with considering heat-transfer enhancement and pressure drop limitation. Based on the researche results of heat exchanger network retrofit approaches with considering heat-transfer enhancement and pressure drop respectively, a more practical retrofit model considering the relationship between the heat transfer enhancement and increasing pressure drop is proposed to avoid the unreasonable pressure drop. A project case is studied to analysis the pressure drop control method through designing reasonable geometry structure of heat exchangers. The research results show that several exchanger structure modification methods are effective in tackling the problem of increased pressure drop. Among those exchanger structure modification methodologies, changing the shell arrangement and using helical baffle are effective in mitigation increases in the shell sidel pressure drop. Changing the shell arrangement and reducing the number of tube passes can be used to reduce the tube side pressure drop. Through the research results of heat exchanger network retrofit with considering multiple factors-network topology modifications, heat transfer enhancement and pressure drop, the proposed approach and the conclusions have important value of not only in theory but also practicality on the research field of heat exchanger network retrofit.