Different Mechanism Effect between Gas-Solid and Liquid-Solid Interface on the Three-Phase Coexistence Hydrate System Dissociation in Seawater: A Molecular Dynamics Simulation Study

Almost 98% of methane hydrate is stored in the seawater environment, the study of microscopic mechanism for methane hydrate dissociation on the sea floor is of great significance to the development of hydrate production, involving a three-phase coexistence system of seawater (3.5% NaCl) + hydrate + methane gas. The molecular dynamics method is used to simulate the hydrate dissociation process. The dissociation of hydrate system depends on diffusion of methane molecules from partially open cages and a layer by layer breakdown of the closed cages. The presence of liquid or gas phases adjacent to the hydrate has an effect on the rate of hydrate dissociation. At the beginning of dissociation process, hydrate layers that are in contact with liquid phase dissociated faster than layers adjacent to the gas phase. As the dissociation continues, the thickness of water film near the hydrate-liquid interface became larger than the hydrate-gas interface giving more resistance to the hydrate dissociation. Dissociation rate of hydrate layers adjacent to gas phase gradually exceeds the dissociation rate of layers adjacent to the liquid phase. The difficulty of methane diffusion in the hydrate-liquid side also brings about change in dissociation rate.