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

• Published in: Oil & gas science and technology Vol. 74; p. 77
• Main Authors: Nichita, Dan Vladimir, Broseta, Daniel, Montel, François
• Format: Journal Article
• Language: English
• Published: Paris EDP Sciences 2019; Institut Français du Pétrole (IFP)
• Subjects: asymptotic behavior; Asymptotic properties; Binary Interaction Parameters (BIPs); Coefficients; Convergence; convergence pressure; critical point; equation of state; Equations of state; Equilibrium; Hydrocarbons; Loci; Mathematical analysis; negative flash; Parameters; Phase equilibria; Physics; Power law; Ratios; Reduction; reduction method; Saturation; Binary Interaction Parameters (BIPs); critical point; equation of state; asymptotic behavior; negative flash; reduction method; convergence pressure
• ISSN: 1294-4475; 1953-8189; 2804-7699
• DOI: 10.2516/ogst/2019049

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 ln
K
i
tends to zero, according to a power ½ of the distance to this locus. As a consequence, if ln
K
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 ln
K
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.