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Molecular dynamics simulation of replacement of CH₄ in hydrate with CO₂

Qi, Yingxia, Ota, Masahiro, Zhang, Hua
Energy conversion and management 2011 v.52 no.7 pp. 2682-2687
cages, carbon dioxide, crystals, dissociation, gases, hydrogen bonding, melting, methane, models, molecular dynamics, spectroscopy
Molecular dynamics simulations are used to study the mechanism of guest replacement of CH₄ hydrate with CO₂. The well-known OPLS-AA and TIP4P potential models are used for the interactions between guest–guest and water–water species, respectively. The simulations are performed on a combination of 4×4×4 unit cell replica of fully-occupied structure I hydrate and CO₂ gas phase. The simulation results confirm that CH₄ is released from its hydrate and enter into the gas phase by the replacement with CO₂, which has been testified by in situ Raman spectroscopic experiments. Without the hydrate dissociation, CH₄ molecules are difficult to run out of while CO₂ to penetrate into the interior cages of the hydrate block because of the barrier of the cage walls constructed by hydrogen-bonding network of water. It is conjectured that the replacements of CH₄ hydrate with CO₂ will spend long time without the dissociation, or at least slightly melting of the hydrate crystals once time. The replacement process may be divided into three steps: the cages are broken firstly, then CH₄ molecules run out of the cages and meanwhile CO₂ molecules enter into the void cages and occupy them in stead.