A fugacity model for gas hydrate phase equilibria

A classical thermodynamic model for the equilibrium pressures of different guests of structure I and II gas hydrates is proposed that removes the need for reference energy parameters used in the van der Waals and Platteeuw (v dWP) type models. The assumption of a constant crystal lattice for differen t guests within a structure, which is not in agreement with quantum chemist ry calculations, is removed. This model uses published Kihara cell potentia l parameters determined from viscosity and second virial coefficient data, unlike previous models that fit these parameters to hydrate data. Quantum m echanical calculations were used to reduce the number of fitted parameters in the new model. This model greatly improves upon the accuracy of previous models and is applicable over a wide temperature range. Percent absolute a verage deviations in the predicted equilibrium pressures is 3.27% with two or three guest-hydrate specific parameters compared to more than 11% with t hree adjustable parameters in the vdWP type models and 8.55% using a recent thermodynamic model that uses five guest-hydrate parameters.