Diffusive nature of different gases in graphite: Implications for gas separation membrane technology

Abstract

Graphite membranes have gained attention in membrane technology for gas separation due to their high stiffness, strength, and stability in corrosive and high-temperature environments. Graphite exists naturally in geologic media and can also be prepared by synthetic processes. In- situ hydrogen (H₂) production in petroleum reservoirs or H₂ storage in depleted gas reservoirs results in contamination with CH₄ and CO₂. To address the separation challenge at the surface, a sustainable downhole wellbore membrane must be available to separate H₂, leaving behind CO₂ and other gases to improve operational economics. Currently, there are no studies in the literature on the self-diffusivity of gases (CO₂, H₂, and CH₄) in graphite as a function of pore size. To overcome time constraints in diffusivity experiments, this work utilized mathematical models and molecular simulations to delineate the self-diffusivity of gases in graphite of different pore sizes.

To acknowledge subsurface operational conditions during in situ hydrogen production, we considered a temperature of 360 K and a wide pressure spectrum from 2 MPa to 21 MPa. In this study, we explored the diffusive nature of H₂, CH₄, and CO₂ gases in different nanopore-sized graphite using analytical and molecular simulation approaches. We validated the results by presenting unrestricted case density calculations. First, effective diffusivity was calculated using the mean free pore path, followed by gas adsorption at high pressures (10–21 MPa) and a temperature of 350 K. The study utilized theoretical models and molecular dynamics (MD) simulations to determine the self-diffusivity of gases in graphite systems with various structures.