Isothermal Phase Equilibria for Methane + Ethane + Water Ternary System Containing Gas Hydrates

Abstract

Abstract:It is well known that methane + ethane mixed-gas hydrate exhibits the structural phase transition between structure-I and -II although both of pure guest species form the structure-I hydrate at certain pressures and temperatures (Subramanian, Kini, Dec and Sloan, Chem. Eng. Sci., vol. 55, pp. 1981-1999. In the present study, isothermal phase equilibrium (pressure -composition) relations for the methane + ethane mixed-gas hydrate system were measured accurately at 279.1 K -288.1 K. In addition, the equilibrium composition in the mixed-gas hydrate phase was investigated by mass and volume balances analysis. At all temperature conditions, the equilibrium curves exhibit discontinuity around given equilibrium compositions (water free), which supports that the structural phase transition occurs. The pressure of structural transition point increases as temperature rises, and the mole fractions of both hydrate phases at the structural transition point are almost independent of temperature.

Structural phase transitions of methane+ethane mixed-gas hydrate induced by 1,1-dimethylcyclohexane

“…The peaks detected at 2887 and 2942 cm À 1 are Fermi resonance peaks originated from the C 2 H 6 molecule. The Raman shifts are similar to those obtained from the CH 4 þC 2 H 6 s-II hydrate (Subramanian et al, 2000a;Hashimoto et al, 2008). The Raman spectra indicate that the mixed-gas hydrates are the s-II hydrate in the mole fraction range of y C 2 H 6 ¼0.20-0.25.…”

“…The Raman peak observed at 992 cm À 1 (Fig. 1) corresponds to that of the C 2 H 6 encaged in the L-cage of s-II (Subramanian et al, 2000a;Hashimoto et al, 2008). The peaks detected at 2903 and 2914 cm À 1 (Fig.…”

“…With further increase in y C 2 H 6 , the single peak derived from the C 2 H 6 molecule encaged in the M-cage of s-I (Subramanian et al, 2000a;Morita et al, 2000;Hashimoto et al, 2008) was detected at 1000 cm À 1 (Fig. 1) and the crystal structure of the mixed-gas hydrate transformed from s-II to s-I around y C 2 H 6 ¼0.30.…”

“…2. The Raman peaks detected at 2904 and 2915 cm À 1 are derived from the CH 4 molecules in the M-and S-cages of s-I (Sum et al, 1997;Nakano et al, 1999;Subramanian et al, 2000a, b;Susilo et al, 2007;Hashimoto et al, 2008). The peaks detected at 2891 and 2947 cm À 1 are the Fermi resonance peaks of the C 2 H 6 molecule in the M-cage (Subramanian et al, 2000a, b;Hashimoto et al, 2008).…”

“…In addition, the crystal structure depends on the guest concentration as reported in the CH 4 þethane (C 2 H 6 ) (Subramanian et al, 2000a, b;Ballard and Sloan, 2000;Hashimoto et al, 2008), CH 4 þcyclopropane (Makino et al, 2005), CH 4 þtetrafluoro-methane (Kunita et al, 2007) mixed-gas hydrate systems. Although each simple hydrate constructs s-I, the crystal structure of the mixed-gas hydrates changes between s-I and s-II depending on the mole fraction of guest species (water free).…”