Determination of Diffusion Constants and Partial Pressures in the Adsorption of Methane-Propane and Methane-Ethane Mixtures

Natural gas (NG) has advantages vs. gasoline/diesel: lower CO₂ emissions per unit energy, less emission of NOx’s, particulate matter and unburned hydrocarbons. However, storing NG is problematic due to its low density, requiring very high pressures, costly compression and storage systems. A solution is storage by adsorption in porous media such as activated carbon (AC). NG is comprised mainly of methane, thus most studies have focused on adsorption of pure methane. Here we investigate the influence of heavier alkanes commonly found in NG (propane, ethane) on the adsorption process.

We performed extensive molecular dynamics simulations of mixtures of methane-propane and methane-ethane at T=300-400K and P=0-600atm. The system considered were AC’s comprised of slit-shaped pores with interlayer spacings H=8-20Å. We observed that heavier hydrocarbons adsorb preferentially, but remain mobile, promising for the intended application. We also solved a common problem with simulations of molecules with high adsorption affinity: the difficulty to determine their partial pressure. We developed an Arrhenius-type relationship allowing the calculation of these partial pressures from relationships between energy distributions of the different molecules in the simulations.