SoaveRedlichKohn

Soave-Redlich-Kwong (SRK) equation of state, sometimes misspelled as Soave-Redlich-Kohn, is a cubic equation of state used to model the pressure-volume-temperature behavior of fluids, particularly hydrocarbons and natural-gas mixtures. It was introduced by Soave in 1972 as a modification of the Redlich-Kwong equation, replacing the attraction term with a temperature-dependent function α(T). The formulation provides improved vapour-liquid equilibrium predictions for many nonpolar and lightly polar substances.

In molar form, the equation is P = RT/(V - b) - a(T) / [V (V + b)], where P is pressure, T is temperature, V is molar volume, and R is the gas constant. The attractive term is a(T) = a0 α(T), with a0 = 0.42747 R^2 Tc^2 / Pc and b = 0.08664 R Tc / Pc. The temperature dependence is contained in α(T) = [1 + m (1 - sqrt(T/Tc))]^2, where m = 0.480 + 1.574 ω - 0.176 ω^2. Here Tc, Pc, and ω are the critical temperature, critical pressure, and acentric factor of the pure component, respectively.

For mixtures, SRK uses standard mixing rules with binary interaction parameters k_ij to fit experimental data: a_mix and b_mix are computed from the pure-component parameters and interaction terms. The equation is cubic in the molar volume or compressibility factor Z, yielding up to three real roots; the physically meaningful root is selected based on phase state.

SRK remains widely used due to its balance of simplicity and accuracy, but its performance can vary for polar, associating, or strongly nonideal fluids, and empirical parameters may be required for mixtures.