Evidence of structure II hydrate formation from methane plus ethane mixtures

Methane and ethane are each known to form structure I (sI) hydrates. However, Raman and NMR spectroscopic measurements made on clathrate hydrates formed from binary gas mixtures of methane (CH4) and ethane (C2H6) indicate that 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 conditions and 274.2 K indicate a change in hydrate structure from sI to sII between 72.2 and 75 mol% of methane in the vapor. Areas under peaks in the Raman spectra suggest that this change in hydrate structure is associated with 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 discussed. High-resolution, solid-state C-13 NMR measurements at 253 K confirm that CH4 + C2H6 gas mixtures can form either sI or sII hydrates, depending on the vapor composition. (C) 2000 Elsevier Science Ltd. All rights reserved.