Equilibrium distribution and diffusion of mixed hydrogen-methane gas in gravity field

TL;DR

This study investigates how gravity affects the distribution and diffusion of hydrogen-methane gas mixtures in pipelines, revealing that stratification is negligible under typical conditions and providing quantitative diffusion times.

Contribution

It combines molecular dynamics with thermodynamic and diffusion theories to systematically analyze gas stratification and diffusion times in pipelines under gravity.

Findings

Hydrogen partial pressure decreases slower with altitude due to smaller molar mass.

Gas stratification is negligible at typical pipeline altitudes and temperatures.

Diffusion time to reach equilibrium can be up to 300 years for large pipelines.

Abstract

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.