Simultaneous optimization strategy for synthesizing heat exchanger networks with multi-stream mixers

A systematic procedure is proposed in this paper to incorporate the options of merging and/or splitting process streams from multiple origins in heat exchanger network (HEN) design. The utility and capital costs of a traditional HEN may both be reduced significantly with this practice since: (1) the direct heat-exchange operations are more efficient thermodynamically, (2) the mixers are in general less expensive than the indirect heat-transfer units, and (3) the matches between hot and cold streams can be more appropriately placed by taking advantage of the added structural flexibility. A state-space concept is adopted in this work to construct a superstructure for capturing the characteristics of network configuration. More specifically, any HEN (with or without multi-stream mixers) is viewed as a collection of two interconnected blocks, i.e., the process operator and the distribution network. A mixed integer nonlinear program (MINLP) is then formulated accordingly for one-step minimization of the total annualized cost. Based upon the proposed stochastic initiation strategy and solution clustering method, an efficient algorithm is developed to obtain the global optimum of this MINLP model with high creditability. Several examples are also presented to demonstrate the feasibility and benefits of the proposed approach.