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in Wiley InterScience (www.interscience.wiley.com).Multistream heat exchangers (MSHE) enable the simultaneous exchange of heat among multiple streams, and are preferred in cryogenic processes such as air separation and LNG. Most MSHEs are complex; proprietary and involve phase changes of mixtures. Although modeling MSHE is crucial for process optimization, no such work exists to our knowledge. We present a novel approach for deriving an approximate operational (vs. design) model from historic data for an MSHE. Using a superstructure of simple 2-stream exchangers, we propose a mixed-integer nonlinear programming (MINLP) formulation to obtain a HE network that best represents the MSHE operation. We also develop an iterative algorithm to solve the large and nonconvex MINLP model in reasonable time, as existing commercial solvers fail to do so. Finally, we demonstrate the application of our work on an MSHE from an existing LNG plant, and successfully predict its performance over a variety of seasons and feed conditions.

Most literature on the synthesis of heat exchanger networks via mathematical programming methods has dealt with phase changes by assuming nearly isothermal conditions. Many multicomponent phase changes of practical interest (e.g., those in sub‐ambient processes) occur over ranges of temperatures and exhibit nonlinear temperature‐enthalpy relations (T‐H curve). In such cases, isothermal approximations may lead to inferior or unacceptable networks. In this article, we propose a mixed‐integer nonlinear programming formulation and a solution algorithm to incorporate nonisothermal phase changes in heat exchanger network synthesis. We approximate the nonlinear T‐H curves via empirical cubic correlations, and propose a procedure to ensure minimum temperature approach at all points in the exchangers. Our approach successfully solves two industry examples and shows promise for significant cost reductions when compared with existing processes. © 2009 American Institute of Chemical Engineers AIChE J, 2009

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