Structural transitions in methane plus ethane gas hydrates - Part I: uppertransition 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 hy drates 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 + C2 H6 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. Predicti ons suggest that the hydrate structure above the incipient hydrate formatio n conditions may be different than that at the incipient conditions. The pr actical applications of these structural transitions in the CH4 + C2H6 hydr ates are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.