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Heat-integrated water network synthesis is an important water and energy saving strategy that could lead to immediate practical benefits,and thus has gained much attention in both industrial and academic areas.In water network,water streams are usually allowed to mix as long as the mixing stream meets the requirements of water-using units.In this case,stream mixing (direct heat transfer) may have better energy saving effect when compared with indirect heat transfer since mixing can transfer heat without heat exchanger (indirect heat transfer) and overcome the associated limitation on the minimum temperature driving force.However,uncontrolled mixing,especially improper mixing sequence,may weaken energy saving effect or even result in energy penalty on utility target due to energy degradation caused by mixing.In order to cope with this problem,the effect of mixing sequence among multi-stream on system’s utility target is first explored based on the heuristic rules of mixing between two streams coupled with thermodynamics analysis method,and then some innovative mixing rules for mixing sequence among multi-stream are presented.For the sake of clear elaboration,mixing sequence is divided into 2 categories: multi-stream mixing sequence and multi-unit operational sequence.Multi-stream mixing sequence refers to mixing sequence of streams at the inlet of a process unit whereas multi-unit operational sequence is operational sequence of different process units that involve stream mixing.Multi-stream mixing sequence is first studied to understand its mechanism,followed by multi-unit operational sequence.The analysis from energy composite curve shows that: For multi-stream mixing sequence,(1) as long as simultaneous mixing of multi-stream does not cause ΔT contradiction,whatever mixing sequence is does not matter; (2) if simultaneous mixing of multi-stream causes ΔT contradiction,mixing that cause energy penalty should be identified by applying heuristic rules of two stream mixing and then go back to step (1).The mixing sequence of multi-stream can be obtained until all the possible mixings that cause energy penalty have entirely been excluded.For multi-unit operational sequence,the operational procedure is implemented as the same as that of multi-stream mixing sequence.Our target is to formulate some pellucid and thorough rules of stream mixing sequence by investigating its effect on utility target.These rules can be used to identify the most appropriate mixing sequence and improve the energy performance of a water network with several mixing scenarios,benefiting not only the system’s utility target but also heat integration of heat exchanger network.