  Repurposing existing natural gas pipelines is a promising solution for
large-scale transportation of mixed hydrogen-methane gas. However, it remains
debatable whether gravitational stratification can notably affect hydrogen
partial pressure in the gas mixture. To address this issue, we combined
molecular dynamics simulation with thermodynamic and diffusion theories. Our
study systematically examined the equilibrium distribution of hydrogen-methane
mixtures in gravity fields. We demonstrated that partial pressures of both
gases decrease with altitude, with hydrogen showing slower decrease due to its
smaller molar mass. As a result, the volume fraction of hydrogen is maximized
at the top end of pipes. The stratification is more favorable at low
temperature and large altitude drops, with notable gas stratification only
occurring at extremely large drops in altitude, being generally negligible even
at a drop of 1500 m. Furthermore, we showed that the diffusion time required to
achieve the equilibrium distribution is proportional to gas pressure and the
square of pipeline height. This requires approximately 300 years for a 1500 m
pipeline at 1 bar. Therefore, temporary interruptions in pipeline gas
transportation will not cause visible stratification. Our work clarifies the
effect of gravity on hydrogen-methane gas mixtures and provides quantitative
insights into assessing the stratification of gas mixtures in pipelines.
