Adsorption-desorption hysteresis of gas mixtures/graphite system through molecular simulation: Implications for gas separation membrane technology

Graphite could potentially serve as a membrane for enhanced gas separation in the petroleum sector. The extraction of in-situ hydrogen (H₂) in natural gas reservoirs will be challenging due to the presence of methane (CH₄) and carbon dioxide (CO₂). A downhole wellbore membrane to selectively produce hydrogen while prohibiting the flow of CO₂ and other gases would improve economic viability. However, investigations on graphite for gas transport of CO₂, H₂, and CH₄ in terms of hysteresis have received little attention. For this paper, we used molecular dynamics simulation to investigate H₂, CO₂ and CH₄ adsorption/desorption behavior on graphite under various pressures (2.75–41.37 MPa) and temperatures (350 K). By combining Grand Canonical Monte Carlo with molecular dynamics, the simulation not only investigates sorption but also examines the interaction between graphite and gases. Our results indicate that the adsorption capacity is directly correlated to pressure and the adsorption curve reaches the maximum at a high pressure of 41.37 MPa. At extreme pressures, the rate of adsorption increases, but at a lower pressure indicating limited availability of adsorbent pores. The sorption capacity non-linearly with pressure limits both the adsorption and desorption process with a maximum sorption capacity of about 11.3 mmol/g for H₂, 17 mmol/g for CH₄ and 43 mmol/g for CO₂ at a pressure of about 40 MPa. The desorption curve slightly overlaps the adsorption curve and marks the hysteresis effect between the two processes of adsorption and desorption. The effect of hydrogen on the graphite is less than carbon dioxide or methane. The discrepancy among adsorbent preferences for carbon dioxide arises from size, polarizability or interaction energy. In terms of diffusion coefficients, hydrogen can be said to have a higher diffusion coefficient than that of carbon dioxide while carbon dioxide has a higher one when compared to methane, probably due to its lighter molecular weight and weaker adsorption.