Heat Exchanger Networks Retrofit Based On Modified Superstructure

Heat exchanger network is a widely used technology system in the fields of energy and power,petrochemical industry,and so on.It is an important part of the process system which affects the overall energy consumption and economical efficiency.The problems of resources and environment are becoming more and more prominent,but the most of the existing heat exchanger networks have unreasonable design and poor performance,bringing serious energy waste.Therefore,the retrofit of the existing heat exchanger networks is a topic worthy of study.The stage-wise superstructure model is the commonly used model in the study of heat exchanger network.But the model had assumed that only one heat exchanger on every split stream and the isothermal mixing,which were unreasonable.However,in the superstream model,it is allowed a series of heat exchangers on the same split stream.In this work,the stage-wise nodes superstructure model was presented after improving the stage-wise superstructure model.Several nodes were set on each split stream and every node could setup a heat exchanger.Furthermore,nonisothermal mixing was used in mixers.The case study showed that the stage-wise nodes superstructure model proposed in this paper is suitable for the simultaneous synthesis and retrofit of heat exchanger networks with the better results than the stage-wise superstructure model.In the retrofit of heat exchanger networks,the reassignment of existing heat exchangers and the arrangement of new heat exchangers are achieved by the exchanger reassignment strategies generally.However,the range of changing heat transfer area of the existing exchanger reassignment strategies is improper when recycling the existing heat exchangers.In this paper,the exchanger reassignment strategies were improved.The retrofitting of existing heat exchangers becomes more reasonable when fully utilized.Based on the established model and the proposed method,a mixed integer nonlinear programming(MINLP)model for the retrofit of heat exchanger networks was established with the objective of annual total cost.The genetic algorithm with improved exchanger reassignment strategy was used to solve the problem and programmed in MATLAB environment.Furthermore,the human-computer interactive software was developed to control the solver program.Several cases were solved with the results better than the literatures,which verified the feasibility and rationality of the superstructure model and solving method presented in this work.At present,there are some unreasonable assumptions in the retrofit of heat exchanger networks,which would be solved with the development of technology and methods.