(190ao) Prediction of the Chemical Potential of Theoretical Empty Cavity Using MD Simulation

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 and as an energy storage medium. In the present work, equilibrium conditions of structure II gas hydrate of propane or isobutane are predicted using Molecular Dynamic (MD) Simulation. The simulations are worked on a highly portable C-program package called ?Moldy®'. The basic thermodynamic model to predict gas hydrate equilibrium was first presented by van der Waals and Platteeuw using a rigid lattice assumption. In this research, lattice expansion or stretching assumption is employed for the prediction of hydrate equilibria. An attempt to prove the distortion model using MD simulations results is also done. A constant temperature and volume ensemble (canonical ensemble) is used to obtain equilibrium pressures of hydrates at any given temperature through MD simulation. The size of the unit cell of each gas hydrate, which is not a constant value in distortion theory, was adjusted to fit the experimental equilibrium conditions. The unit cell size at 273K for propane hydrate was 17.5797 Å and that for isobutene hydrate was 17.70169 Å. The unit cell volume expansion on temperature was also observed. Based on the calculated unit cell size, chemical potentials for empty hydrate structure at 273K was calculated for single gas hydrates (propane, isobutene) were calculated using Moldy® and Dmo values for the distortion model were calculated and compared with empirical values. The maximum error between calculated value and experimental values was 24% for isobutene hydrate.