This paper reports P-F-T data (P is pressure, F is mass density, T is temperature) for a ternary mixture that resembles a residual natural gas. The measurements utilized a high-pressure, single-sinker magnetic-suspension densimeter (MSD) from (200 to 450) K up to 170 MPa and an automated isochoric apparatus to determine densities and phase boundary data up to 20 MPa. The MSD technique yielded accurate data, with better than a 0.05 % estimated error at two standard deviations up to 200 MPa. The relative uncertainties for phase-boundary temperatures and pressures were 0.45 % and 0.04 %, respectively. The isochoric densities had essentially the same estimated error as the MSD data. The GERG-2004 and AGA8-DC92 equations of state compared well to the density data. A new method provided saturation densities from isochoric data with a relative uncertainty of 0.12 %, compensating the isochoric densities for cell and transfer-line volume changes and for mass interchange between the cell and the combination of the transfer line and pressure transducer.

This paper presents residual values of Helmholtz energy (A r/(RT)), entropy (S r/R), and internal energy (U r/(RT)) for a ternary mixture that resembles a distribution natural gas between 223.15 K and 303.15 K up to 20 MPa. The methodology uses isochoric and isothermal density measurements to apply corrections to the residuals for Helmholtz energy (δA r/(RT)), entropy (δS r/R), and internal energy (δU r/(RT)) calculated using an equation of state. The method is demonstrated for three representative equations of state: REFPROP, Peng–Robinson, and Redlich–Kwong. Accurate (p−ρ–T) isochoric and isothermal data provide experimental values of the compression factor (Z) and the derivative of pressure with respect to temperature at constant density (∂p/∂T )ρ which are used to calculate residual entropies and energies. The results obtained by using different equations of state at the same conditions have slight differences in the residual values. It is possible to represent those differences by equivalent changes in temperature for entropy (δT S) and internal energy (δT U) that are ≤ 0.5 K for temperatures above 225 K. For this mixture, the values of δA r/(RT), δS r/R, and δU r/(RT) determined from REFPROP are sufficiently small that no corrections are required. For the Peng–Robinson and Redlich–Kwong equations of state, a global fit describing the residual corrections presents a practical application. The residual deviations of the plots lie within ± 0.0005, ± 0.001, and ± 0.002 in the residuals for δA r/(RT), δS r/(RT), and δU r/(RT), respectively.

This paper reports p–ρ–T (pressure, mass density, temperature) data measured for a mixture with molar composition (0.95039 methane + 0.03961 ethane + 0.01000 propane) using a high-pressure, single-sinker, magnetic-suspension densimeter (MSD) and high- and low-pressure automated isochoric apparatus at temperatures from (135 to 500) K and pressures to 200 MPa. The composition is representative of a residual natural gas in pipelines, but the range of conditions covers conditions possibly encountered in production and processing. The k = 2 relative uncertainty for the density measurements using the MSD is 5 × 10–4 ·ρ based upon an uncertainty analysis for the instrument. The isothermal densities measured in the MSD in combination with the low- and high-pressure isochoric data determine additional density data with essentially the same estimated relative uncertainty as the MSD in the high temperature range (above 300 K) and a relative uncertainty of 3 × 10–3 ·ρ at lower temperatures. The measured densities range from (433.170 to 27.493) kg·m–3. The GERG-2008 equation of state compares well to the density data. Although the data behave in an expected manner, they cover ranges of temperature and pressure beyond those previously examined. Additionally, the paper describes a new, high-pressure isochoric apparatus as well as a methodology for compensating volume changes and the mass interchanges in the high-pressure cell. The latter technique allows experimental determination of the cell distortion coefficients.