Methane Hydrate Structure I Dissociation Process and Free Surface Analysis

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 two small dodecahedral cages and six tetra decahedral large cages. This study assesses the influence of occupation and the behavior of methane release from the molecular perspective during the dissociation process, particularly for the purpose of testing a series of molecular dynamics simulations. The dissociation cases conducted include an ideal 4 × 4 × 4 and 2 × 2 × 2 supercell methane hydrate system while inducing dissociation with two different types of temperature-rising functions for understanding the limitation and capability. These temperature-rising functions are temperature ramping and a single temperature step simulating in 5-7 various conditions. Temperature step results showed the earliest dissociation starting 50 ps into the simulation at an ΔT of 100 K, while at an ΔT of 80 K, dissociation was not observed. There was not a distinct 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 transport from a boundary. Temperature ramping simulations showed that the dissociation temperature increased with a higher 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 behavior similar to that for methane gas. As in the temperature step simulation, there were no clear differences in dissociation between large and small cages, which suggested homogeneous dissociation in all cases. Finally, a coordination analysis was performed on a 3 × 4 × 4 structure I methane hydrate with two free surfaces to demonstrate clear free surface boundaries and its location.