Molecular dynamics and thermodynamic modeling of gas hydrates

Natural gas combines with water at low temperature and high pressures to form gas hydrates. Gas hydrates are crystalline compounds in which the water molecules form the host lattice and gas molecules occupy the interstices as guests. Constant pressure Molecular Dynamic simulations of gas hydrates indicate that a large guest causes the host lattice to stretch. The exact influence of the guest size parameter on the stability of hydrate structure was studied. The thermodynamic model which is generally used to predict the hydrate equilibrium conditions assumes the host structure to be independent of the guest. With the help of MD simulations, it was shown that this assumption is not valid. The thermodynamic model for predicting phase behavior of hydrates was modified to incorporate the effect of guest size and lattice stretching. With the help of the new guest-dependent model the phase equilibrium conditions of the hydrates can be successfully calculated without the drastic parameter adjustment required previously. Based on these studies a method is developed to estimate the lattice stretching effect in hydrates formed with gas mixtures. In mixed hydrates the effect of lattice stretching is obtained as a function of the hydrate phase composition. The most important advantage of the new model is that the same Kihara parameters can be used to predict the hydrate phase and gas phase properties.