Accurate density measurements for a 91% methane natural gas-like mixture

Accurate density data are necessary to validate equations of state (EOS) used in custody transfer of natural gas. The American Gas Association (AGA) AGA8-DC92 EOS, currently the industry standard, has been validated against a databank of gas mixtures composed mostly of binary data. Accurate density data for multicomponent mixtures are necessary to check the ability of AGA8-DC92 to cover the entire range of pressure, temperature, and compositions encountered in custody transfer. This paper uses a magnetic suspension densimeter to measure the densities of mixtures containing nine components with n-pentane being the heaviest. We have used a high pressure, high temperature, compact single-sinker magnetic suspension densimeter to measure densities of a simulated natural gas mixture having 91% methane composition after validating its operation by measuring densities of pure argon, nitrogen, and methane in the range (270 to 340) K and (3.447 to 34.474) MPa. The agreement with the reference EOS for the pure components is about 0.03%.The estimated 2σ uncertainty of the densities after compensating for apparatus and fluid effects is 0.12% with 93% of that value coming from the composition uncertainty. The measured data indicate the advisability of performing an extensive study of the AGA8-DC92 EOS for natural gas mixtures.