Structural transitions in methane plus ethane gas hydrates - Part I: upper transition point and applications

Although methane and ethane form sI hydrates by themselves, the binary gas mixtures of these components form sII hydrates at certain compositions. In our earlier work, it was reported that the structure of methane + ethane hydrates changes from sI to sII over a methane vapor composition range (y(CH4)) of 0.72-0.75. In this work, Raman spectroscopic measurements on CH4 + C2H6 system suggest that the hydrate structure reverts from sII to sI over a y(CH4) range of 0.992 (+/- 0.005)-0.994 (+/- 0.007). C-13 NMR measurements on this system suggest that the methane hydrate composition increases by at least 0.2 mole fraction at the lower structural transition point. Predictions suggest that the hydrate structure above the incipient hydrate formation conditions may be different than that at the incipient conditions. The practical applications of these structural transitions in the CH4 + C2H6 hydrates are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.