Robust Flash Calculations through Nonsmooth Inside-Out Algorithms

Abstract The ability to solve many important problems in process systems engineering is dependent on being able to perform vapor-liquid equilibrium (flash) calculations rapidly and consistently. The flash equations are particularly challenging to solve for non-ideal systems with many components, and many process simulators use some version of the inside-out algorithms ( Boston and Britt, 1978 ) for performing such calculations. However, these methods assume that the calculation result is always a two-phase mixture, which is not guaranteed outside of a range of input parameters that is not generally known a priori . This article continues the development of nonsmooth inside-out algorithms ( Watson et al., 2017 ) which retain the benefits of the original algorithms without the possibility of loss of reliability or performance when only a single phase is present. This article shows how this can be accomplished for the fixed pressure-entropy flash through the use of nonsmooth equations that relax the equilibrium constraints when necessary, automatically allowing convergence to either single-phase or two-phase solutions.