An efficient, robust and reliable initialization of phase fractions in Rachford-Rice system of equations for three-phase split calculation

Abstract

Multiphase flash calculations are crucial in compositional and thermal simulations for complex fluid systems in which three phases may co-exist. Solution of the Richford-Rice system of equations is an important operation in the multiphase flash. Within the literature, solution methods have been proposed, based on optimization methods and on nonlinear equation solution methods. Many methods in these two approaches may fail if the initial estimate is too inaccurate, and may require a substantial amount of computation only to arrive at a trivial solution in cases where the assumed number of phases is too high. The present work deals with an initialization method for the solution of the Rachford-Rice system of equations based on approximating each of the functions by means of tangent lines. It is proposed to solve the system of equations by applying a two-step Newton-type method. Through extensive numerical experimentation, the efficiency, robustness and reliability of the proposed initialization method for the solution of the Rachford-Rice system of equations by means of a Newton-type method is shown.