11468
J. Phys. Chem. B 2006, 110, 11468-11474
Phase Transitions in Mixed Gas Hydrates: Experimental Observations versus Calculated Data Judith M. Schicks,* Rudolf Naumann, and Jo1 rg Erzinger GeoForschungsZentrum Potsdam, Section 4.2, Telegrafenberg, 14473 Potsdam, Germany
Keith C. Hester, Carolyn A. Koh, and E. Dendy Sloan, Jr. Center for Hydrate Research, Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401 ReceiVed: February 28, 2006; In Final Form: April 6, 2006
This paper presents the phase behavior of multicomponent gas hydrate systems formed from primarily methane with small amounts of ethane and propane. Experimental conditions were typically in a pressure range between 1 and 6 MPa, and the temperature range was between 260 and 290 K. These multicomponent systems have been investigated using a variety of techniques including microscopic observations, Raman spectroscopy, and X-ray diffraction. These techniques, used in combination, allowed for measurement of the hydrate structure and composition, while observing the morphology of the hydrate crystals measured. The hydrate formed immediately below the three-phase line (V-L f V-L-H) and contained crystals that were both light and dark in appearance. The light crystals, which visually were a single solid phase, showed a spectroscopic indication for the presence of occluded free gas in the hydrate. In contrast, the dark crystals were measured to be structure II (sII) without the presence of these occluded phases. Along with hydrate measurements near the decomposition line, an unexpected transformation process was visually observed at P-T-conditions in the stability field of the hydrates. Larger crystallites transformed into a foamy solid upon cooling over this transition line (between 5 and 10 K below the decomposition temperature). Below the transition line, a mixture of sI and sII was detected. This is the first time that these multicomponent systems have been investigated at these pressure and temperature conditions using both visual and spectroscopic techniques. These techniques enabled us to observe and measure the unexpected transformation process showing coexistence of different gas hydrate phases.
1. Introduction Natural gas hydrates are ice-like crystalline solids composed of hydrocarbon gases, or “guests”, such as methane, ethane, propane, carbon dioxide, or hydrogen sulfide.1 A threedimensional framework consisting of water cavities is formed by hydrogen bonding of the water molecules; the water cavities are stabilized due to the inclusion of guest molecules. There are three typical gas hydrate structures capable of holding light hydrocarbons: structure I, structure II, and structure H.1 In general, guests such as methane and ethane form structure I hydrate, with partial filling of the small cavity, and larger molecules such as propane and isobutane form structure II hydrate with the small cavities empty. Even larger molecules such as neohexane are known to form structure H hydrate in the presence of a help gas such as methane. However, the relationship between structure and size is not straightforward in either case. Investigations on pure methane hydrates showed that methane formed structure H and other nonclassical hydrate structures at very high pressures (>100 MPa).2-4 Another study showed that structure II and structure I methane hydrates can coexist for short periods under moderate pressure and temperature conditions.5 Particularly when structure I and structure II * Corresponding author. E-mail:
[email protected].
hydrate formers are combined in a mixture, it can be difficult to predict which hydrate structure will form. The preferred hydrate structure ultimately depends on the temperature, pressure, and composition of the system, which translates to the lowest Gibbs free energy. Unexpected transitions from structure I to structure II were observed in investigations of a methane + ethane system (both form sI as pure components) by Subramanian et al.6 Also, structural transitions of clathrate hydrates formed from a natural gas have been reported by Jager and Sloan,7 who observed that hydrates formed at lower pressures (
