An improved Helmholtz energy model for air and the related systems

In this work a Helmholtz energy model is applied to the prediction of thermodynamic properties of air, the related binary mixtures and the intervening pure components. The Helmholtz energy of the mixture is represented as two contributions: one from a proven accurate extended corresponding states model and the other is a correction term. The corresponding states model relies on pure-component shape factors relative to nitrogen and extension to mixtures with the van der Waals one-fluid mixture model with ordinary combining rules. The correction term is temperature-, density- and composition-dependent with the use of a theoretically consistent local composition model with a coordination number model derived from lattice gas theory. For air the obtained average absolute deviations in densities were 0.090 per cent, 0.15 per cent in speeds of sound, 0.28 per cent in bubble-point pressures and 0.30 per cent for dew-point pressures. For the three associated binary mixtures, the absolute average deviations in densities were within 0.14 per cent and 0.63 per cent for bubble-point pressures. For oxygen and argon, the absolute average deviations were within 0.07 per cent in densities, 0.45 per cent in VLE properties and 0.012 per cent in speeds of sound.