Performance of Twelve Different Equations of State for Natural Gas and Hydrogen Blends

Abstract

The aspiration for blending hydrogen (H2) into natural gas (NG) in gas transmission systems is high and is happening globally. However, the principal properties of the blended mixtures and their thermodynamic derivatives can significantly vary depending on the Equation of State (EOS) employed. There is a need to arrive at the best performing EOS for the prediction of the blended mixtures from low to high concentration of H2 in the blend with NG. Twelve different EOS were evaluated against measured data found in the open literature of pure H2, binary mixtures with alkanes and mixtures with NG. Three measured properties were found, namely density, speed of sound and isobaric heat capacity (Cp) in the range of pressures up to 50 MPa and temperatures in the range of −20°C to +80°C and H2 concentration up to 88%(mole). The total number of measured data points are 629 for pure H2 and 1788 for Binaries and NG mixtures with H2. Performance of each EOS is based on the average of the absolute error (deviation%) between predicted vs. measured parameters. These were: density, which represents the principal performance of the EOS with respect the basic formulation of P, ρ and T, the speed of sound, which represents a thermodynamic derivative with respect to entropy, and isobaric heat capacity which represents a thermodynamic derivative with respect to enthalpy. All other thermodynamic derivatives can be related to these three parameters (e.g., J.T coefficient, isochoric heat capacity, enthalpy, internal energy,, compressibility factor and Helmholtz and Gibbs free energies, etc.). Transport properties predictions are based on other empirical and semi-empirical correlations that are independent of the EOS and hence were not considered. It was found that, for the most part and for pure H2, GERG2008 EOS performed best in predicting the above three principal parameters followed by AGA8. For Binary and NG mixtures with H2, again GERG2008 was found to be the best performing EOS for all ranges of P and T, while the second-best performer is BWRS in the range of 0.1–3 MPa and AGA8 in the range of 3–25 MPa, respectively.