THERMODYNAMIC PROPERTIES AND DISSOCIATION CHARACTERISTICS OF METHANE AND PROPANE HYDRATES IN 70-ANGSTROM-RADIUS SILICA-GEL PORES

The pressure-temperature profiles for the hydrate-ice-gas and hydrate-liquid water-gas equilibria were measured for methane and propane hydrates in 70-angstrom-radius silica gel pores. In both cases, the equilibrium pressures were 20-100% higher than those for the bulk hydrates. The dissociation characteristics of the gas hydrates in pores were also studied calorimetrically by heating the hydrates under about zero pressure from 100 K to room temperature. It was found that after the initial dissociation into ice and gas the hydrate became totally encapsulated among the pore walls and the ice caps formed at the pore openings. The hydrate thus trapped in the interior of the pore remained stable up to the melting point of pore ice. These results are similar to those obtained in our previous studies on the bulk hydrates which are also stabilized by a shielding layer of ice. However, the apparent increase in the stability of the pore hydrates was found to be much larger than that of the bulk hydrates. The composition of methane hydrate in 70-angstrom pores was determined to be CH4.5.94H2O, and its heat of dissociation into pore water and gas, obtained calorimetrically, was 45.92 kJ mol-1; the corresponding values in the bulk phase are 6.00 and 54.19 kJ mol-1, respectively.