In physics and thermodynamics, the Redlich–Kwong equation of state is an empirical algebraic equation that relates temperature, pressure, and volume of gases. It is generally more accurate than the van der Waals equation and the ideal gas equation at temperatures above the critical temperature. It was formulated by Otto Redlich and Joseph Neng Shun Kwong in 1949. It showed that a two-parameter, cubic equation of state could well reflect reality in many situations, standing alongside the much more complicated Beattie–Bridgeman model and Benedict–Webb–Rubin equation that were used at the time. Although it was initially developed for gases, the Redlich–Kwong equation has been considered the most modified equation of state since those modifications have been aimed to generalize the predictive results obtained from it. Although this equation is not currently employed in practical applications, modifications derived from this mathematical model like the Soave Redlich-Kwong (SRK), and Peng–Robinson have been improved and currently used in simulation and research of vapor–liquid equilibria.
Equation The Redlich–Kwong equation is formulated as:
p = R T V m − b − a T V m ( V m + b ) , {\displaystyle p={\frac {R\,T}{V_{m}-b}}-{\frac {a}{{\sqrt {T}}\;V_{m}\,(V_{m}+b)}},}
where:
p is the gas pressure R is the gas constant, T is temperature, Vm is the molar volume (V/n), a is a constant that corrects for attractive potential of molecules, and b is a constant that corrects for volume. The constants are different depending on which gas is being analyzed. The constants can be calculated from the critical point data of the gas:
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