Replacement mechanism of methane hydrate with carbon dioxide from microsecond molecular dynamics simulations

Replacement of CH4 in hydrate form with CO2 is a candidate for recovering CH4 gas from its hydrates and storing CO2. In this work, microsecond molecular dynamics simulations were performed to study the replacement mechanism of CH4 hydrate by CO2 molecules. The replacement process is found to be controlled cooperatively by the chemical potentials of guest molecules, "memory effect'', and mass transfer. The replacement pathway includes the melting of CH4 hydrate near the hydrate surface and the subsequent formation of an amorphous CO2 hydrate layer. A large number of hydrate residual rings left after the melting of CH4 hydrate facilitate the nucleation of CO2 hydrate and enhance the dynamic process, indicating the existence of so-called "memory effect''. In the dynamic aspect, the replacement process takes place near the surface of CH4 hydrate rather easily. However, as the replacement process proceeds, the formation of the amorphous layer of the CO2 hydrate provides a significant barrier to the mass transfer of the guest CH4 and CO2 molecules, which prevents the CH4 hydrate from further dissociation and slows down the replacement rate.