Analysis and Comparison of the Alpha Functions of SRK Equation of State

Alpha functions of Soave-Redlich-Kwong (SRK) equation of state proposed by Soave, Twu, and Luo were different in mathematic tendency. They were compared in modeling methane-alkanes equilibria with van der Waals mixing rule and Modified Huron-Vidal (MHV1) mixing rule, respectively. Results showed that Luo's alpha function was a little more accurate than Soave's, and Twu's alpha function lacked accuracy in modeling methane-alkanes equilibrium. SRK equation of state was expanded as virial form, and then the equivalent terms were contrasted with terms of virial equation of state. Results showed that Soave's and Luo's alpha functions matched the tendency of virial coefficient better than Twu's, and Luo's alpha function matched better than Soave's in wide temperature range, which sustained the conclusions of phase equilibria calculation. Luo's alpha function keeps decreasing when Tr>1 and becomes negative at sufficient high temperature, thus the conventional cubic equation of state expressed pressure as the sum of repulsion pressure pR (>0), and attraction pressure pA (<0) could be improved to be the sum of hard-sphere repulsion pressure pH (>0) and intermolecular force pressure pI (pI<0 at low temperature and pI>0 at sufficient high temperature).