Pseudo-retrograde hydrate phenomena at low pressures

A Gibbs free energy minimization technique has been applied to calculating phase equilibria of solid hydrates. The van der Waals and Platteeuw theory was used to predict the fugacity of water in each of the hydrate phases. Model parameters for ethane and propane were optimized to pure and binary incipient hydrate equilibrium data as well as to structural transition points for the ethane+propane+water system.It is usually assumed that hydrates never dissociate with an increase in pressure. Predictions show, however, that for a wide water-free composition range, slight increases in pressure will result in the dissociation of sII hydrates (pseudo-retrograde dissociation). Pressure versus temperature and pressure versus composition phase diagrams for the ethane+propane+water system showed that pseudo-retrograde phenomena exist at low pressures (approximately 10-15atm) near a temperature of 278K. Pseudo-retrograde hydrate behavior was predicted in the ethane+i-butane+water and ethane+propane+decane+water systems as well. Two different experimental methods were used to verify the model predictions for the ethane+propane+water system.