A correlation of densities of liquid natural gas mixtures from surface tension

In this work, we apply an equation of state based on statistical–mechanical perturbation theory to liquid natural gas mixtures. Three temperature-dependent parameters are needed to use the equation of state: the second virial coefficient, B2(T), an effective van der Waals covolume, b(T), and a scaling factor, α(T). The second virial coefficients are calculated from a correlation based on the surface tension, γtr, and the liquid density at the triple-point, ρtr α(T), and b(T) can also be calculated from second virial coefficients by a scaling rule. Based on the theory, these two temperature-dependent parameters depend only on the repulsive branch of the potential function, and therefore, by our procedure, can be found from γtr and ρtr. The theory has a considerable predictive power, since it permits the construction of the p–V–T surface from the surface tension plus the triple-point density. The equation of state is tested on 27 liquid mixtures. The results indicate that the liquid density can be predicted within about 5%, over a range of temperature.