The Studies On Synthesis Of Heat-integrated Water Allocation Network Considering Stream Mixing

With the increasing scarcity of water and energy resource, heat-integrated waterallocation network (WAHEN) synthesis, which aims to improve the utilizationefficiency of water and energy, has come into vogue in Process System Engineering.However, WAHEN synthesis becomes rather complex due to the coupling of waterallocation network (WAN) and heat exchanger network (HEN) and the existence ofnon-isothermal mixing. In order to reduce the complexity of WAHEN synthesis, aneffective strategy to constrain the number of streams for heat integration is proposedby analyzing the energy transfer of process streams during non-isothermal mixing.Then a superstructure that involves non-isothermal mixing is developed for WAHEN.Hence, only fresh water streams, mixed streams and waste water streams areconsidered for heat integration. In this case, the number of streams that involve inHEN design will not exceed3N (N is number of water-using process) and the modelsize is well controlled. When determining utility target of WAN, the operationtemperature, freshwater source temperature, discharged temperature and minimumapproach temperature ΔTminare employed to partition the entire temperature rangeinto temperature intervals in linear programming (LP) transshipment model in orderto capture the relative positions of streams to each temperature interval. Throughdefining logical variables that indicate the relative positions of streams, WAHENsynthesis is formulated as Generalized Disjunctive Programming (GDP) problemsbased on LP transshipment model. The GDP is reformulated as Mixed IntegerNonlinear Programming (MINLP) problem with the aid of using binary variables andBig-M Constraint Method. After obtaining an optimal WAN that features theminimum fresh water and utility consumption while maximizing non-isothermalmixing, a detailed HEN to satisfy the targeted utility consumption can be designedadequately by the principles of pinch design methods.Finally, three examples from literature are applied to test the feasibility andeffectiveness of the proposed WAHEN model. Our results show that the proposedapproach is able to simultaneously minimize fresh water usage and utilityconsumption for WAHEN. Moreover, the resulting WAN maximizes the direct heattransfer by non-isothermal mixing to reduce the number of streams for heat integration and the number of heat exchangers in HEN. That reduces the complexityof HEN design greatly.