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Soave-Redlich-Kwong Equation of State

The Soave-Redlich-Kwong1 equation of state is a modification of the original EOS by Redlich and Kwong (RK) from 19492. The SRK EOS is a common cubic EOS used in many areas of petroleum and chemical engineering.

SRK EOS Structure

The SRK EOS model is defined by

where \(a\) and \(b\) are the EOS model parameters. The pure component parameters (\(a\) and \(b\)) can be calculated by

In equations \eqref{eq:a_srk}, \eqref{eq:b_srk} and \eqref{eq:alpha} the coefficients \(\Omega_a\) and \(\Omega_b\) , and the value of \(\alpha(T)\) can be calculated by

Component Properties

The component properties are composed of the critical pressure (\(p_c\)), critical temperature (\(T_c\)), accentric factor (\(\omega\)) and molecular weight (\(MW\)). Another important component property for cubic EOS models are the volume shift factors descried by Peneloux in 19823. The volume shifts were introduced as a correction term to the phase behaviour predictions of the molar volume predictions.

Mixing Rules


The main article for cubic EOS models describes the mixing rules with some more detail.

The original mixing rule developed by van der Waals is a quadratic mixing rule. The set of equations describing the mixing are given by

where \(a_{ij}=\sqrt{a_ia_j}\), \(u_i\) can be the liquid composition (\(x_i\)), vapor composition (\(y_i\)) or the total composition (\(z_i\)) and \(a_i\) and \(b_i\) are the component EOS parameters for component \(i\).

Modifications to the \(a\) and \(b\) mixing rules were developed to enhance the performance of the EOS models. The most common modified mixing rules are given by

where \(k_{ij}\) is often referred to as the binary interaction parameter (BIP) or sometimes referred to as the binary interaction coefficient (BIC).

  1. G. Soave. Equilibrium constants from a modified redlich-kwong equation of state. Chemical engineering science, 27:1197–1203, 1972. doi:

  2. O. Redlich and J. N. S. Kwong. On the thermodynamics of solutions. v. an equation of state. fugacities of gaseous solutions. Chemical reviews, 44:233–244, 1949. 

  3. A. Péneloux, E. Rauzy, and R. Fréze. A consistent correction for redlich-kwong-soave volumes. Fluid phase equilibria, 8:7–23, 1982. doi: