This equation of state is based on the Benedict-Webb-Rubin equation, and on Pitzer's extended theory of corresponding states. Thermodynamic data are correlated as a function of critical temperature and pressure and the acentric factor as follows:
Z = Zo + (w / wr ) ( Zr - Zo )
where: Z = compressibility factor
w = acentric factor
and:
o and r subscripts denote Simple and Reference fluids, respectively.
The work of Plocker, et. al., was to introduce new empirical mixing rules which are purported to better handle mixtures of asymmetric molecules. The mixing rules use molar critical volumes to determine the mixture critical temperature, with adjustable binary parameters used in the cross expressions. The critical pressure is calculated with an empirical expression, as a function of the mixture acentric factor, critical temperature and critical volume.
The adjustable binary parameters are determined from experimental data. Because the mixture critical constants are determined from empirical expressions, the method is not accurate around the critical point. The authors recommend not using the method above a reduced temperature of 0.96.
See "Lee-Kesler-Plocker (LKP)" under "Equations of State" in PRO/II Reference Manual, Chapter 2, Volume 1 for more information.