Stage-wise Superstructure Based Synthesis And Optimization Of Heat Exchanger Network And Heat-integrated Water Allocation Network

Water and energy are two essential resources in process industries.Chemical process industry values the usage of these two resources not only because of the process configuration but also because of the increasing environmental regulation since the wastewater of chemical plants contains both organic and inorganic contaminants and most of the energy comes from fossil fuels consumption.Therefore,the design of heat exchanger network(HEN)and heat-integrated water allocation network(WAHEN)has been considered seriously.At first,by replacing the conventional binary variable in heat exchanger network synthesis(HENS)model,a nonlinear approximation term is developed to indicate the existence of heat exchanger so as to reduce the amount of variables and constraint as well as enhance the model’s computational efficiency.With this approximation,a new HENS model was formulated considering simultaneously nonisothermal mixing and temperature-dependent heat capacity flow rate.All the variables of the model are stated with clear upper and lower bounds.A 10-stream(5 hot process streams and 5 cold process streams)benchmark HENS problem is solved by the developed model.The results show that the proposed model can generate a better cost-optimal heat exchanger network(HEN)with more accurate configuration in relatively short solution time compared with literatures.Then a new stage-wise superstructure for heat integrated water allocation networks(WAHEN)synthesis is proposed in this paper.Streams outside the water allocation network(WAN)are introduced to increase heat exchange opportunities and the level of heat integration.Given these outside streams can be any liquid or gas streams other than only water streams,the superstructure is designed to be compatible for multiple utilities.The mathematical model based on the proposed superstructure is formulated with no discrete variables.One modified WAHEN case is used for the application of our WAHEN synthesis approach.Since the proposed superstructure describes WAHEN from a different perspective,ε-constraint method based multi-objective optimization(MOO)instead of total annual cost(TAC)minimization has been conducted in the solution process to provide more optimal solutions for evaluation and selection.