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. Mo del parameters for ethane and propane were optimized to pure and binary inc ipient 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 pr essure. Predictions show, however, that for a wide water-free composition r ange, slight increases in pressure will result in the dissociation of sII h ydrates (pseudo-retrograde dissociation). Pressure versus temperature and p ressure versus composition phase diagrams for the ethane+propane+water syst em showed that pseudo-retrograde phenomena exist at low pressures (approxim ately 10-15 atm.) near a temperature of 278 K. Pseudo-retrograde hydrate be havior was predicted in the ethane+i-butane+water and ethane+propane+decane +water systems as well. Two different experimental methods were used to ver ify the model predictions for the ethane+propane+water system. (C) 2001 Els evier Science B.V. All rights reserved.