Molecular dynamics simulation of replacement of CH4 in hydrate with CO2

Abstract Molecular dynamics simulations are used to study the mechanism of guest replacement of CH 4 hydrate with CO 2 . 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 CO 2 gas phase. The simulation results confirm that CH 4 is released from its hydrate and enter into the gas phase by the replacement with CO 2 , which has been testified by in situ Raman spectroscopic experiments. Without the hydrate dissociation, CH 4 molecules are difficult to run out of while CO 2 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 CH 4 hydrate with CO 2 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 CH 4 molecules run out of the cages and meanwhile CO 2 molecules enter into the void cages and occupy them in stead.