(661b) Molecular Dynamics Simulations of WATER Cavity Distortion for Determining Clathrate Hydrate Equilibria

Gas hydrate is a non-stoichimetric crystalline compound with hydrogen bonded water molecules and gas molecules connected with van der Waals force. The research on gas hydrate became emphasized because of its potential to be used as a future energy resource (Natural gas hydrate contains over 90% of methane and can be used as the source of natural gas) and as an energy storage medium. An accurate thermodynamic model is needed to predict the dissociation condition of natural gas hydrate to produce the natural gas from hydrate deposit or to predict the storage condition of hydrogen or natural gas in hydrate. In the present work, equilibrium conditions of structure I and structure II gas hydrates are predicted using Molecular Dynamic (MD) Simulation. The simulations are worked on a highly portable C-program package called ?Moldy®' and conducted at the High Performance Cluster Computer at Texas A&M University-Kingsville. The equilibrium conditions were calculated based on the distortion theory. Instead of using a rigid lattice assumption, lattice expansion or stretching assumption is employed for the prediction of hydrate equilibria and the unit cell size of each gas hydrate was adjusted to fit the experimental equilibrium conditions at various temperature conditions. A constant temperature and volume ensemble (canonical ensemble) is used to obtain equilibrium pressures of hydrates at any given temperature through MD simulation. As the size of gas molecules in water cavities increase, the unit cell size of gas hydrate is increased. The unit cell volume expansion on temperature was also observed. Based on the optimized unit cell of gas hydrate at 273K, chemical potentials for theoretical empty cavity for each single gas hydrate was calculated. The calculation results were compared with empirical values. The error between calculated value and experimental values was around 20 to 30%.