UC Irvine

Methane hydrates are crystalline solids of water that contain methane molecules trapped inside their molecular cavities. Gas hydrates with methane as a guest molecule form Structure I hydrates with a unit cell containing 46 water molecules arranged on 2 small dodecahedral cages and 6 tetra decahedral large cages. An ideal Structure I methane hydrate unit cell contains 8 methane molecules, with one in each cage. Methane molecules are classified according to whether they occupy the large tetra decahedral or the small dodecahedra cells. The influence of occupation and the difference between the behavior of methane release during the dissociation process for the different cage types is the major interest of this work.To assess and analyze the structure evolution during the dissociation of methane hydrates, a series of molecular dynamics simulations using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) is conducted. The dissociation conditions examined include different heating rates at 0.8 TK/s, 4TK/s, 40 TK/s and 400 TK/s, and different temperature increments, ∆T, as steps of 80 K, 85 K, 90 K, 95 K and 100 K above hydrate equilibrium stability conditions for 5 ns. Both simulated systems were first equilibrated at 270 K and 5 MPA. The potential energy of the system, mean-squared displacement (MSD), and the radial distribution function were analyzed to determine the full process of dissociation, temperature changes, molecular diffusive behavior, and structure evolution. Temperature step results showed the earliest dissociation starting 50 ps into the simulation at a ∆T of 100 K, while at a ∆T of 80 K, dissociation was not observed. There was not a clear dissociation preference observed between large and small cages, so it appears that the dissociation affects the entire structure uniformly when temperature increases are applied throughout the system rather than transported from a boundary. Temperature ramping simulations showed that the dissociation temperature increased with an increased heating rate. The mean-squared displacement results for the oxygen atoms in the water molecules at a high heating rate of 400 TK/s showed a similar behavior to that for methane gas. This behavior may be an indicator of fast evaporation for water molecules, while at slower heating rates methane molecules showed much higher MSD values, indicating diffusive behavior. As in the temperature step simulation there were not clear differences in dissociation between large and small cages, which suggests homogeneous dissociation in all cases. While this study showed that a total occupied hydrate will experience homogeneous dissociation independent of the heating method, (i.e., either gradual temperature increases or constant high temperature exposure), future studies can examine the stability of the hydrate at different occupancies. Additionally, since hydrate is exposed to liquid water in natural settings, it is important for future studies to analyze the impacts of a liquid water interface in the stability and dissociation dynamics of methane hydrate.