An improved Helmholtz energy model for non-polar fluids and their mixtures. Part 3: Application to natural gases and related systems

• A Helmholtz energy model is presented for natural gas systems.
• The Helmholtz energy is obtained from an extended corresponding states model plus a function that depends on temperature, density and composition.
• For pρT data of natural gases, the overall AAD was 0.06 percent.
• Percentage AADs were 0.220 for speeds of sound and 1.115 for isobaric heat capacities.
• Results compared favourably with the GERG-2004 equation of state.

In this work a Helmholtz energy mixture model that had been previously published by the author was applied to natural gases and the related multicomponent systems. The Helmholtz energy of the mixture is the sum of two terms: one is an extended corresponding states model and the other is a correction term that is mixing rule, in terms of local compositions, of a temperature- and density-dependent function. Local compositions are calculated with a coordination number model for square-well fluids.The systems of interest were ternary, quaternary, multicomponent systems and natural gases. Deviations were calculated for pρT data, speeds of sound, isobaric heat capacities, isobaric and isenthalpic enthalpy differences, saturated-liquid densities of liquefied natural gases and vapour pressures of ternary systems. Representative percentage average absolute deviations were: 0.060 in pρT data of pipeline-quality natural gases; 0.099 in pρT data of unusual-composition natural gases; 0.249 in pρT data of rich natural gases; 0.220 in speeds of sound; 1.150 in isobaric heat capacities; 0.146 in saturated-liquid densities of liquefied natural gases and 1.267 in vapour pressures of ternary systems. Those results met the international standards of accuracy in the prediction of natural gas thermodynamic properties and were also comparable with those obtained with the GERG-2004 equation of state.