Evidence of structure II hydrate formation from methane plus ethane mixtures

Methane and ethane are each known to form structure I (sI) hydrates. Howeve r, Raman and NMR spectroscopic measurements made on clathrate hydrates form ed from binary gas mixtures of methane (CH4) and ethane (C2H6) indicate tha t structure II (sII) hydrate forms in this system for certain compositions. Raman band frequencies for ethane and C-13 NMR chemical shifts for ethane were used to identify the hydrate structure in these studies. Raman spectra obtained from six different CH4 + C2H6 hydrates at L-w-H-V equilibrium con ditions and 274.2 K indicate a change in hydrate structure from sI to sII b etween 72.2 and 75 mol% of methane in the vapor. Areas under peaks in the R aman spectra suggest that this change in hydrate structure is associated wi th a 20% change in hydrate guest composition. Absolute cage occupancies and hydration numbers are obtained from the Raman data. Analysis of the sI to sII transition and a phase diagram for the CH4 + C2H6 hydrate system are di scussed. High-resolution, solid-state C-13 NMR measurements at 253 K confir m that CH4 + C2H6 gas mixtures can form either sI or sII hydrates, dependin g on the vapor composition. (C) 2000 Elsevier Science Ltd. All rights reser ved.