Molecular-dynamics-informed peng-robinson equation of state for cryogenic H2/Air and He/Air mixtures

Thermodynamic property data of hydrogen mixtures are essential for accurate modeling of mixed refrigerants and hydrogen-air systems. However, such properties are largely unavailable for these systems at cryogenic conditions, introducing uncertainties in the design of cost-effective liquefaction cycles and the reliable development of safety tools for predicting accidental hydrogen release in the environment. Here, atomistic Molecular Dynamics (MD) simulations are employed to generate extensive unsaturated density-pressure-temperature (ρPT) datasets for H 2 -N 2 -O 2 and He-N 2 -O 2 mixtures across a wide range of pressures and mole fractions at cryogenic temperatures (30 – 175 K), where experimental data is lacking. The MD method is validated by the attainment of ρPT of pure components that find excellent agreement with experiments. A modified Peng-Robinson equation of state (EOS) is proposed with reparametrized mixing rules by fitting the MD-obtained density data for H 2 -air and He-air mixtures. The proposed MD-fitted EOS is benchmarked against available density experiments of binary O 2 -N 2 , H 2 -N 2 and He-N 2 mixtures, exhibiting comparable performance to RefProp and the classic Peng Robinson and PC-SAFT EOS. The MD-fitted EOS is implemented in a Computational Fluid Dynamics simulation of cryogenic H 2 release in air. In such ternary mixtures, significant differences in temperature, density, and flow velocity are observed near the orifice from those predicted by Peng-Robinson EOS with classical mixing rules. The MD-fitted EOS can be readily employed in continuum models for the prediction of ternary mixture densities, offering a computationally efficient validated alternative to RefProp, which relies on extrapolations of thermodynamic properties from pure components in cryogenic conditions.