Experimental and Simulation Study for the Dissociation Behavior of Gas Hydrates – Part I: CH₄ Hydrates

A study of methane hydrate dissociation under different temperature and pressure conditions combining in situ and ex situ Raman spectroscopic measurements, confocal microscopic characterizations, powder X-ray diffraction measurements, and molecular dynamics simulations was conducted. Both the experimental and the simulated data show that a distinction must be made between the dissociation behavior above and below the freezing point of water. During the dissociation process at temperatures near or below the freezing point of water, the simple CH4 hydrates showed well-known self-preservation behaviors. The formation of a quasi-liquid or amorphous phase due to the decompositions of the hydrate cavities at the outer layers of the hydrate crystal terminated the further decomposition of the hydrate phase. For a CH4 hydrate above the ice point, the dissociation appeared to be initiated at the surface of the hydrate phase. While significant amounts of the hydrate phase were present, the ratio of methane guests in the large and small cages remained constant. After large amounts of the hydrate phase decomposed, potential fragmentation of the remaining hydrate phase into collections of hydrate cages, which resulted in the preferred breakup of the tetrakaidecahedra (51262), was over the pentagonal dodecahedra (512).