Multistream heat exchangers: Equation-oriented modeling and flowsheet optimization

Multistream heat exchangers (MHEXs), typically of the plate-fin or spiral-wound type, are a key enabler of heat integration in cryogenic processes. Equation-oriented modeling of MHEXs for flowsheet optimization purposes is challenging, especially when streams undergo phase transformations. Boolean variables are typically used to capture the effect of phase changes, adding considerable difficulty to solving the flowsheet optimization problem. A novel optimization-oriented MHEX modeling approach that uses a pseudo-transient approach to rapidly compute stream temperatures without requiring Boolean variables is presented. The model also computes an approximate required heat exchange area to determine the optimal tradeoff between operating and capital expenses. Subsequently, this model is seamlessly integrated in a previously-introduced pseudo-transient process modeling and flowsheet optimization framework. Our developments are illustrated with two optimal design case studies, an MHEX representative of air separation operation and a natural gas liquefaction process. (c) 2015 American Institute of Chemical Engineers AIChE J, 61: 1856-1866, 2015