Phase Equilibrium of Methane Hydrate Encapsulated Isotopologue ¹³CH₄ and Singly Deuterated Methane

Abstract

Hydrate equilibrium conditions for 13CH4 were measured for temperatures ranging from 270 to 278 K and for pressures ranging from 2.3 to 4.2 MPa. The equilibrium pressure values in a 13CH4–H2O system were nearly the same as those in a 12CH4–H2O system. The similarity between the equilibrium pressures for these two systems can be used to explain why an existing study found nearly no fractionation of CH4 carbon isotopes during the formation of synthetic CH4 hydrates. We also measured the hydrate equilibrium conditions for singly deuterated CH4 (CH3D) in a temperature range of 270–273 K and pressure range of 2.3–2.6 MPa. The quadruple points (the coexistence of ice, liquid water, solid hydrate, and gaseous methane) for the 13CH4–H2O, CH3D–H2O, and CH4–H2O systems were determined using phase equilibrium (p–T) data and the Clausius–Clapeyron equation. The difference between the equilibrium pressures for CH3D and CH4 hydrates below the quadruple point (∼0.02 MPa) was slightly smaller than above the quadruple point (∼0.04 MPa) as reported by an existing study. The results of the phase equilibrium p–T conditions for hydrates encapsulating CH4 isotopologues can be used to explain the difference in fractionation behaviors for carbon and hydrogen isotopes of CH4 during the formation of the CH4 hydrate. Moreover, we characterized the crystal structure and the cage occupancies of the 13CH4 hydrate using powder X-ray diffraction and Raman spectroscopy. Our results suggest that the effect of encapsulating 13CH4 molecules in hydrate crystals was nearly the same as for CH4.

Stable Isotope Fractionation of Nitrogen between Gas and Hydrate Phases

“…The experimental results of the hydrogen stable isotope fractionation of methane during methane hydrate formation suggested that the equilibrium pressure of the CH 3 D hydrate is greater than that of the CH 4 hydrate, which was confirmed by measurements of their equilibrium pressures . In contrast, the stable isotope fractionation of carbon in methane during methane hydrate formation was negligible and below the detection limits, ,, which is consistent with the observation that the equilibrium pressures of the 13 CH 4 and 12 CH 4 hydrates are similar . This type of stable isotope fractionation of guest gases has been reported for ethane, carbon dioxide, , and hydrogen sulfide, as well as methane …”

“…14 In contrast, the stable isotope fractionation of carbon in methane during methane hydrate formation was negligible and below the detection limits, 12,15,16 which is consistent with the observation that the equilibrium pressures of the 13 CH 4 and 12 CH 4 hydrates are similar. 17 This type of stable isotope fractionation of guest gases has been reported for ethane, 12 carbon dioxide, 15,18 and hydrogen sulfide, 19 as well as methane. 12 The objective of this study is to determine the stable isotope fractionation factor of nitrogen during the formation of gas hydrate-encapsulated nitrogen.…”

Effect of Pressure on Hydrogen Isotope Fractionation in Methane during Methane Hydrate Formation at Temperatures Below the Freezing Point of Water

“…Similarly, the stable isotope fractionation of guest molecules during the formation of gas hydrates may be explained by the differences in the inclusion properties of isotopologues: CH 3 2 H is less included than CH 4 , with a difference of 4.8 ± 0.4‰ in their δ 2 H values at 274.2 K [9] (δ values are defined in the Section 3), which is consistent with the slightly higher equilibrium pressures of CH 3 2 H hydrates than those of CH 4 hydrates [17]. Additionally, the negligible carbon isotope fractionation of methane during the formation of methane hydrates is consistent with the almost equal equilibrium pressures of the 13 CH 4 and 12 CH 4 hydrates [18].…”

Effect of Pressure on the Hydration Number of Argon Hydrate

“…In the phase equilibrium measurement, a porous sample of argon hydrate was formed in the pressure cell and an equilibrium condition was achieved without agitating the device. The method is the same as the previous work. , Approximately 10 g of fine ice powder was placed in the pressure cell and vacuumed at a temperature of 77 K. Argon gas prepared with the amount needed to achieve equilibrium conditions of the liquid water, hydrate, and vapor at 273.2 K was trapped in the cell at 77 K. Argon hydrate was formed by melting the ice powder at 273.2 K under high pressure of argon. The equilibrium pressure at each temperature was determined using a step-by-step isochoric method of heating and cooling.…”

“…The quadruple point is the four-phase (vapor + hydrate + ice + water) equilibrium condition of the argon–H 2 O system. The quadruple point of the argon–H 2 O system was determined via the same method as in previous studies. , To estimate the hydration number, the difference in dissociation enthalpy between phases of vapor + hydrate + ice and vapor + hydrate + water at the quadruple point of the argon–H 2 O system was used. This method of estimating the hydration number has been used in earlier studies. − …”