Regular Article

Application of near critical behavior of equilibrium ratios to phase equilibrium calculations

¹ CNRS UMR 5150, Laboratoire des Fluides Complexes et leurs Réservoirs, Université de Pau et des Pays de l’Adour, BP 1155, 64013 Pau Cedex, France
² Centre Scientifique et Technique Jean Feger, Total, avenue Larribau, 64018 Pau Cedex, France

^* Corresponding author: dnichita@univ-pau.fr

Received: 14 June 2019
Accepted: 18 September 2019

Abstract

We examine the asymptotic behavior of the equilibrium ratios (K_i) near the convergence locus in the pressure-temperature plane. When the Equation of State (EoS) is analytical, which is the case of most EoS of engineering purpose, K_i tends towards unity or, equivalently, its logarithm lnK_i tends to zero, according to a power ½ of the distance to this locus. As a consequence, if lnK_i is expressed as a linear combination of pure component parameters with coefficients only depending on mixture phase properties (i.e., reduction parameters), these coefficients obey a similar power law. Deviations from the ½ power law are thus fairly limited for lnK_i and for the reduction parameters (at least in the negative flash window between the convergence locus and the phase boundaries), which can be exploited to speed up flash calculations and for quickly determining approximate saturation points and convergence pressures and temperatures. The chosen examples are representative synthetic and natural hydrocarbon mixtures, as well as various injection gas-hydrocarbon systems.