Modeling and simulation of phase change and non-ideality in multistream heat exchangers

Abstract

Abstract Accurate simulation and design of cryogenic processes is challenging largely due to the presence of multistream heat exchanger (MHEX) unit operations. In important processes such as liquefied natural gas production, the streams in these operations are multicomponent mixtures and undergo phase changes which significantly alter their physical properties. However, the locations of the transitions between phases are complex functions of the process variables and not necessarily known a priori . These challenges are addressed in this article by extending a newly developed modeling strategy for MHEXs to include the detection of phase changes and rigorous flash calculations when the thermodynamic state of the fluids is described by a cubic equation of state (CEOS). The resulting model is nonsmooth and is solved by an extension of the vector forward mode of automatic differentiation in conjunction with a linear programming based Newton method. A simulation of the Poly Refrigerant Integrated Cycle Operations (PRICO) process is presented to demonstrate the model and solution algorithm.