Non-equilibrium molecular dynamics simulation of gas flow in organic nanochannels

• Adsorption and transport of helium, argon, and methane in graphite channels is studied by performing MD simulations.
• For all gases, whether the adsorption affinity of the gas is high or low, plug-shaped velocity profiles are established.
• Contribution of the adsorbed molecules can be more than 50% of the total mass flux of the channel.
• This study shows that the transport through organic nano-scale conduits is essentially diffusive.
• For Argon and Methane, the diffusion coefficients decrease as the Knudsen number increases.

The objective for this work is to investigate the contribution of the adsorbed phase to the mass flux and comparing transport of gases with different adsorption affinities in organic nano-scale channels. In this work, force-driven Non-Equilibrium molecular dynamics (NEMD) simulations are used to compare the transport of gases with high adsorption affinity (Methane and Argon) with the ones with low adsorption affinity (Helium), for channel heights of 2, 4, 6, and 8 nm at two Knudsen numbers of 0.1 and 0.2. Velocity and mass flux profiles across the channel for Argon, Methane, and Helium are compared. Transport diffusion coefficients and molecular flux of these gas are also calculated. Furthermore, adsorption properties are analysed using Grand Canonical Monte Carlo simulations.For all the gases studied, plug-shaped velocity profiles are observed irrespective of the channel size and Knudsen number. Mass flux profiles of Argon and Methane across the channels demonstrate a significant contribution of adsorbed molecules to the total mass flux. Furthermore, as Knudsen number increases, the contribution of the adsorbed phase to the total mass flux becomes higher. Molecular flux of Helium is smaller than that of Argon and Methane for all channel sizes. The calculated diffusion coefficients of Methane are higher than those for Argon for all the channel sizes and they decrease as the channel size increases. For Argon and Methane, the diffusion coefficients become smaller as Knudsen number increases. For Helium, the diffusion coefficients are weak functions of the channel size and Knudsen number. Based on the results, contribution of the adsorbed molecules can be more than 50% of the total mass flux of the channel. For the pressure ranges studies, transport diffusivity of Helium is less sensitive to pressure and Knudsen number compared to Argon and Methane.This study shows that the transport through organic nano-scale conduits is essentially diffusive. Therefore, to have a realistic model for predicting the recovery of fluids from unconventional resources, the transport equations in organic nanopores should be replaced by the diffusive transport equations.