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

Abstract

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.