PSRK

The equation of state of Soave is defined as follows:

${\displaystyle P={\frac {RT}{v-b}}-{\frac {a\alpha (T)}{v(v+b)}}.}$

The original α-function has been replaced by the function of Mathias–Copeman:[2]

${\displaystyle \alpha (T_{r})=\left[1+c_{1}\left(1-{\sqrt {T_{r}}}\right)+c_{2}\left(1-{\sqrt {T_{r}}}\right)^{2}+c_{3}\left(1-{\sqrt {T_{r}}}\right)^{3}\right]^{2}.}$

The parameters of the Mathias–Copeman equation are fitted to experimental vapor-pressure data of pure components and provide a better description of the vapor pressure than the original relation. The form of the equation is chosen as it can be reduced to the original Soave form by setting the parameters c₂ and c₃ to zero. Additionally, the parameter c₁ can be obtained from the acentric factor, using the relation

${\displaystyle c_{1}=0.48+1.574\,\omega -0.176\,\omega ^{2}.}$

This may be performed if no fitted Mathias–Copeman parameter is available.

The PSRK mixing rule calculates the parameters a and b of the equation of state by

${\displaystyle {\frac {a}{bRT}}=\sum _{i}x_{i}{\frac {a_{i}}{b_{i}RT}}-{\frac {{\frac {g_{0}^{E}}{RT}}+\sum x_{i}\ln {\frac {b}{b_{i}}}}{0.64663}}}$

and

${\displaystyle b=\sum _{i}x_{i}b_{i},}$

where the parameters a_i and b_i are those of the pure substances, their mole fractions are given by x_i, and the excess Gibbs energy by g^E. The excess Gibbs energy is calculated by a slightly modified UNIFAC model.