A new combining rule for mixture equations of state: higher order composition dependencies reduce to quadratic composition dependence. Application to natural gas mixtures

A combining rule that reduces third and higher order virial coefficient type mixing rules to quadratic is used to simplify the form of a very accurate equation of state and is applied to various natural gas component binary mixtures. The accuracy obtained is comparable to that reported for the original equation of state and yet it reduces significantly the number of parameters necessary to describe the mixture. The systems tested are binary mixtures of the following gases: CH4, N2, CO2, C2H6, H2, C3H8 and CO. This study proves that the representation of the third virial coefficient as quadratic in the mole fractions is highly accurate for these binary systems. This representation, therefore should be accurate for systems which are non-associating and non-highly polar fluids.