A Theory of Lattice Distortion in Gas Hydrates

Abstract

Water and some nonpolar gases combine at low temperatures and high pressures to form nonstoichiometric clathrates called gas hydrates. The water molecules in gas hydrates are linked to each other through hydrogen bonds to form a host lattice with polyhedral cavities that are large enough for small gas molecules such as methane, ethane, and propane to occupy. Common gas hydrates are known to occur in three structural forms, structure 1 and structure 2 with cubic symmetry and structure H with hexagonal symmetry, although the present study is based only on structure 1 and structure 2 hydrates. Commonly cited structural details are given. The present work will show that a fixed set of structural parameters is not appropriate. Constant pressure molecular dynamic simulations of gas hydrates with guests of different size indicate that the equilibrium cell volume increases with guest size. The effect of lattice stretching due to guest size on the reference chemical potential difference between the empty lattice and water is calculated, and a new thermodynamic model with a guest dependent reference chemical potential difference, {Delta}{mu}{degree}, is used for predicting the equilibrium conditions for single component (one gas + water) gas hydrates and multicomponent gas hydrates. This model avoidsmore » potential parameter adjustment (fitting) for prediction of equilibrium conditions. A relationship for obtaining the reference chemical potential difference for multicomponent gas hydrates (mixture gas hydrates) from the hydrate phase composition is proposed.« less