Pipeline Operations with Hydrogen - Part II: Equation of State Investigation

To evaluate the feasibility of hydrogen utilization a high-accuracy EoS is crucial. Perhaps the most impactful fluid property from a hydraulic perspective is density. To better understand the accuracy of various EoS, we will compare density predictions to measured data. In this blog post, we will examine said problems and discuss methodologies to arrive at solutions. A full technical description of this work is available in the paper?PSIG 2216.

The Hernandez-Gomez?[1]?et al data set from 2018 measured the density of methane-hydrogen binary mixtures over a range of compositions, pressures, and temperatures relevant to transport of H2?in natural gas pipelines.

The best known and most widely used EoS is that of Peng-Robinson (PR) from 1973. PR is a cubic EoS, which makes the job of coding up this EoS in simulators easy for software developers but does not provide the best physical description of fluids. More modern takes on EoS are complex models such as Groupe Européen de Recherches Gazières (GERG) 2008 and American Gas Association (AGA) 8. It should be noted that AGA8D is a composition-based method for predicting fluid properties, while AGA8P utilizes bulk properties of the gas to predict fluid properties. AGA8P utilized CO2?content, specific gravity, and heat content as inputs, while AGA8D uses a more detailed approach.

Gas densities predictions from GERG, AGA8, and PR EoS are compared to the measured data from Hernandez-Gomez 2018 in Figures 1-3.

Figure1: Comparison for 5/95 H2?CH4?@ 80.3 °F over a range of pressures; Left) Density measured and predicted; Right) Density error percentage (predicted – measured)/measured

Figure2: Comparison for 10/90 H2?CH4?@ 80.3 °F over a range of pressures; Left) Density measured and predicted; Right) Density error percentage (predicted – measured)/measured

Figure3: Comparison for 50/50 H2?CH4?@ 125.3 °F over a range of pressures; Left) Density measured and predicted; Right) Density error percentage (predicted – measured)/measured

From the figures above we noted that while PR may be acceptable for quick screenings, the error is larger than should be acceptable for a realistic analysis of natural gas systems. GERG or AGA8 are recommended for modelling hydrogen blends in transport and processing. Furthermore, the error or PR increases with increased H2?concentration. It should be noted that the figures above were generated using the Multiflash thermodynamic simulation software, which does not support the AGA8P EoS. Thus, AGA8D was used for this analysis. Results for AGA8P would be expected to be worse than those of AGA8D.??A summary of the findings from Figure 1, Figure 2 and Figure 3 are provided in Table 1.

Table1:?Summary of Results for Comparison of EoS Models to Measured H2/CH4?Densities

For those that want to jump ahead, this entire post is available now on our?blog.

1. Hernandez-Gomez, R.; Tuma, D.; Perez, E.; and Chamorro, C. R. “Accurate Experimental (P, rho, and T) Data for the Introduction of Hydrogen into the Natural Gas Grid (II): Thermodynamic Characterization of the Methane-Hydrogen Binary System from 240 to 350 K and Pressures up to 20 MPa.” J. Chem. Eng. Data 2018, 63, 1613-1630