H₂, CO₂, and CH₄ Adsorption Potential of Graphite: Implications for Gas Separation Membrane Technology

Graphite has the potential to act as membrane for gas separation in the petroleum industry. Unconventional sources such as petroleum reservoirs that have in situ hydrogen (H₂) production or depleted gas reservoirs where H₂ is stored after extraction are often contaminated with CH₄ and CO₂. A green downhole wellbore membrane for H₂ separation would eliminate CO₂ and other gases hence resulting in better economics. Currently, not much attention has been given to how various sizes of pores affect gas (CO₂, H_2, and CH₄) adsorption on graphite. To investigate adsorption behavior of hydrogen (H₂), carbon dioxide (CO₂), and methane (CH₄) on graphite with different pore sizes at different pressures ranging from 2.75 to 41.3 MPa as well as temperatures between 373 and 423 K, we conducted molecular dynamics simulation. It was found that there exists a direct relationship between adsorption capacity with pressure and pore size and indirectly with temperature. The maximum adsorption capacity for CO₂, CH₄, and H₂ in 8 nm graphite at 41.3 MPa and 373 K was 95 mol/Kg, 40 mol/kg, and 25.7 mol/kg, respectively. In all graphite pore sizes, the adsorption followed the trend: CO₂ > CH₄ > H₂. With respect to pore size, regardless of pressure and temperature, adsorption was ranked as 8 nm > 4 nm > 2 nm. This is attributed to the larger pore and molecular size of CO₂, which increases its affinity toward graphite. The Langmuir model has a good fit for predicting hydrogen and methane adsorption capacity and was found to be less accurate, however, for CO₂ for any temperature. These findings provide a preliminary dataset on the gas adsorption affinity of graphite and have potential implications for gas separation membrane technology.