Thermodynamics Research Center / ThermoML | Journal of Chemical and Engineering Data

ABSTRACT

High-accuracy equations of state (EOS) are crucial to industrial applications such as natural gas custody transfer and the development of gas processing facilities. The GERG-2008 EOS, which is the ISO standard for calculating the equilibrium properties of natural gas mixtures, has a stated relative uncertainty in density of 0.1% over the temperature range from (250 to 450) K at pressures up to 35 MPa. However, for the crucial (methane + propane) binary system, most of the density data sets used in the development of the GERG EOS that extend to pressures above 20 MPa have root-mean-square relative density deviations of (0.4 to 0.8)% and maximum relative deviations of up to approximately 3%. In this work, a single-sinker magnetic-suspension densimeter adapted from a commercial sorption apparatus was used to measure the density of two (methane + propane) mixtures along four isotherms at T = (256, 310, 366, and 422) K over the pressure range from (24 to 35) MPa. The binary mixtures were prepared gravimetrically with methane mole fractions of 0.9472 and 0.8924, respectively. The titanium sinker s volume (V 3.17 cm3) was determined at each temperature as a function of pressure using pure methane and the reference equation of state for this pure fluid, which has a relative expanded uncertainty of 0.03%. The relative combined expanded uncertainty (k = 2) in mixture density measurements ranged from (0.11 to 0.24)%, including the uncertainty in composition. The measured binary mixture densities had relative deviations from those calculated with the GERG-2008 EOS of less than 0.14%. When considered together with other recently measured high-accuracy density data for this binary system, these results help prioritize the nature of future improvements needed for the EOS.