The comparison of experimental and theoretical hydrate numbers
Galloway, Travis J
Master of Science
Experimental apparatus and an effective technique for a more precise determination of the hydrate numbers of gas hydrates were developed. Hydrate numbers (the ratio of the moles of water to moles of gas in the gas hydrate) were determined for pure component ethane and methane hydrates at several equilibrium decomposition conditions and compared with hydrate numbers calculated from the solid solution theory of van der Waals and Platteeuw as extended by Saito et al. and Nagata et al. Two methods were used for each experimental determination. Method I involved calculation of the difference in the number of moles of free gas in a closed system (as determined by pressure-volume-temperature changes) before and after complete conversion of gas and liquid water to gas hydrate. In Method II the number of moles of gas in the hydrate were determined by direct measurement of the volume of gas evolved during hydrate decomposition at constant pressure. The experimental hydrate numbers for ethane hydrate were determined at equilibrium decomposition conditions of 118.0 PSIA, 40.0°F; 119.4 PSIA, 40.2°F; and 225.0 PSIA, 48.9°F. The results essentially confirmed the hydrate numbers calculated from the solid solution theory. The maximum discrepancy from the theoretical values for ethane hydrate was a hydrate number of 8.01 (Method II at 119.4 PSIA, 40.2°F) compared to a theoretical hydrate number of 8.25. The experimental results supported the contention that only the larger cavities of Structure I are occupied for ethane hydrate, since the lowest hydrate number obtained experimentally was 7.89, whereas the limiting hydrate number assuming complete occupancy of only the larger cavities of Structure I is 7-2/3. The experimental hydrate numbers for methane hydrate were determined at equilibrium decomposition conditions of 1030 PSIA, 50.0°F; 1032 PSIA, 50.1°F; 1902 PSIA, 60.0°F; and 1901 PSIA, 59.9°F. The experimental hydrate numbers for methane hydrate were generally lower than the theoretical values. The maximum discrepancy was an experimental hydrate number of 5.93 (Method II at 1030 PSIA, 50.0°F) compared to a theoretical hydrate number of 6.30. No experimental hydrate number for methane hydrate, however, was less than the limiting value of 5-3/4, which represents complete occupancy of large and small cavities in Structure I.