Exploring the effect of important parameters on decomposition of gas hydrate structure I: A molecular dynamics simulation study

The aim of this study was to investigate the stability of methane hydrate in the NPT ensemble using MD simulation. Methane hydrate at 6 different cage occupancies ranging from 75 to 100% was placed arbitrarily in a cubic cell. Methane hydrate decomposition was explored at the equilibrium pressure for varied temperatures (T = 290, 300, and 310 K). The effect of cage occupancy of the hydrate structure - ranging from fully hydrate to empty small cage structures - on the hydrate stability and decomposition rate was examined. Simulation results showed that at constant temperatures, the decomposing process was accelerated at lower cage occupancy. Decomposition process increased the total energy of the system and enhanced the diffusion coefficient. When hydrate structure decomposed, the coulombic energy term was increase about 18% while LJ potential energy decreased about 33%. In stable hydrate crystal, diffusion coefficient of water oxygen in hydrate had an order of magnitude of 10−15 m2/s while in decomposed hydrate structure, the diffusion coefficient order of magnitude was changed to 10−9 m2/s. This changed the RDF curve of the system from solid to liquid-like structure by reducing the number of peaks versus distance. In addition, the value of coordination number of water-water molecules at a distance of 1 nm in the system during the decomposing process enhanced about 20% which confirmed changing the structure from solid state to liquid. Also, after dissociation of hydrate structure, the number of hydrogen bonding were decreased. Finally, results suggested that each of these parameters could be successfully applied to predict the decomposition of gas hydrate structure from ice-like stable phase to liquid-like structure.