| Gas hydrates are non-stoichiometric crystalline molecular complexes formed by the physical combination of water and low molecular weight gases such as methane, propane and isobutane. In this study, dissociation pressures for single or multi component gas hydrates are calculated with a distortion model. For the prediction of dissociation pressures of single component gas hydrates, different Δμ° (chemical potential difference between water and the theoretical empty cavity), and different Δh° (enthalpy at 0°C and 0 Kpa) for each gas hydrates are used. Dissociation pressures for multi-component gas hydrates can be accurately calculated by analyzing compositionally non-linear excess Gibbs potential changes of empty gas hydrate at various compositions. For obtaining Δμ°, the assumption that lattices can be distorted due to the size of guest molecules is used with Kihara parameters obtained from virial coefficient data without adjustment. Thus, this model is less parameter dependent. For calculating the dissociation pressure of multi-component gas hydrates, we use the assumption that Δμ°mix is changed according to the composition of gas in hydrate phase. By defining the ideal gas hydrate mixture as one where Δμ°mix is linear in composition, the excess Gibbs potential of the empty lattice was calculated from the difference between Δμ°ideal and Δμ°calculated. We applied a new model similar to regular solution theory to predict the excess Gibbs potential of empty gas hydrate for binary mixtures. With two adjustable parameters the model calculates accurate Δμ°mix. The adjustable parameters for binary mixtures are used for calculating the dissociation pressure of multi-component gas hydrates with a newly defined mixing rule.; The average error (difference between predicted dissociation pressure and experimental dissociation pressure) obtaining from this new model for simple gas hydrates was less than 10%. The prediction of dissociation pressure for multi-component gas hydrates was also done. |
