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. M. N. Ammar and H. RenonCentre Reacteurs & Processus Ecole Nationale Superieure des Mines de New methods for solving the problem are proposed. Combining the Paris 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.

Liquid–liquid equilibria for the ternary system water + tetradecane + 2-butyloxyethanol

“…where x expl ijk and x calc ijk are the experimental and calculated mole fraction, respectively, of component i in phase j along a tie line k. The liquid-liquid equilibrium flash calculation [21] was applied to evaluate the compositions of both liquid phases along each tie line by using the experimental total compositions as input data. As one can see in Figs.…”

A diverse approach for the solution of the isothermal multiphase flash problem. Application to vapor‐liquid‐liquid systems

“…The first step, performed by the new approach, is a stability analysis applied only once and on the initial system only. The test, based on the well-known tangent-plane criterion (Baker et al, 1982;Michelsen, 1982a), uses a new objective function (Stateva and Tsvetkov, 1991).…”

General Strategy for Stability Testing and Phase-Split Calculation in Two and Three Phases

“…However, stability testing based on local minimization requires multiple initial estimates to avoid missing the instability. In phase-split calculation, the popular methods include the successive substitution iteration (SSI), quasi-Newton, Newton, steepest-descent, and their various modifications and combinations (Michelsen 1982b(Michelsen , 1993Trangenstein 1987;Lucia et al 1985;Ammar and Renon 1987). Among them, the SSI combined with Newton method is probably the best option.…”