Computation of multiphase equilibrium phenomena with an equation of state

This paper presents a general approach to multiphase equilibrium computations using an equation of state. Various types of calculations are described: flash calculations, saturation-pressure and saturation-temperature calculations, and calculations of the feed composition corresponding to a given saturation pressure and temperature.A stagewise procedure for flash calculations using the QNSS method allows efficient computation of phase equilibria for systems with any number of phases. The initial guess of the composition of an extra phase which is required to start the iteration is obtained from an analysis of the Gibbs-energy surface.Newton's method is used to construct multiphase boundaries on various types of diagrams (pressure-temperature, pressure-composition and temperature-composition). Multiphase critical points on these boundaries are also estimated by interpolation.The algorithms developed are tested for a typical reservoir-oil-CO₂ mixture which exhibits three-phase liquid 1-liquid 2-vapor (L₁L₂V) separation. The predictions obtained using the Peng-Robinson of state are satisfactory.