The isothermal flash problem: New methods for phase split calculations

Abstract

Various numerical approaches have been adopted to solve the single‐stage isothermal flash problem. These approaches result in two classes of methods. The first includes equation‐solving methods that try to solve a nonlinear equation system; the second is based on a minimization of the total Gibbs free energy. Most of these methods may fail to find a solution or may lead to erroneous solutions near critical conditions when an equation of state is applied to both the vapor and liquid phases. New methods for solving the problem are proposed. Combining the simplicity in structure of the conventional successive‐substitution method and the efficiency of some unconstrained minimization algorithms, they all ensure convergence to local minima of the Gibbs free energy. The new methods are compared from the standpoints of computer storage and calculational effort requirements. The performance of these methods is tested on four multicomponent systems taken from literature, and a comparison is made with other published methods. Copyright © 1987 American Institute of Chemical Engineers