CALCULATION OF PHASE-DIAGRAMS OF GAS-HYDRATES

The phase diagrams of gas-hydrates are calculated using van der Waals and Platteeuw's model with a Kihara spherical-core potential for the interaction between water and the guest molecule. The Kihara parameters are obtained independent of the mutual solubility of gas and water by constraining the model to the experimental dissociation pressures along the hydrate-ice-gas equilibrium line. Further, the fugacity coefficients in that region are close to unity wherefor the Kihara parameters are rather insensitive to the choice of equation of state. We use the Redlich-Kwong-Soave equation of state to calculate the fugacity the guest molecule. The gas solubility in the water rich phase is likewise calculated from the Redlich-Kwong-Soave equation of state combined with the DDLC (Density Dependent Local Composition ) mixture model. The calculated phase diagrams comprise the hydrate - ice - gas, the hydrate -water - gas/condensate, the hydrate-gas-condensate, and the hydrate structure I - structure II - gas/condensate lines. Further we have investigated the influence of the uncertainty of the lattice cavity size on the location of the hydrate structure I - structure II - gas/ condensate equilibrium curve which turns out to be very sensitive to even minor changes in the cavity size. The thermodynamically stable phase is determined by a total Gibbs energy minimization of the system.