Aqueous nonelectrolyte solutions. Part XVIII. Equilibrium pressures of two methane hydrates with water. Formulae and dissociation thermo -dynamic functions for the structures I and II methane hydrates

• Published in: Canadian Journal of Chemistry
• Main Author: Glew, David N
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2002
• Subjects: Methane hydrate
• Online Access: http://dx.doi.org/10.1139/v02-017; http://www.nrcresearchpress.com/doi/pdf/10.1139/v02-017

Literature data for the saturation pressure P(hl 1 g) of methane hydrate with water, at 102 temperatures between –0.29 and 46.87°C, are properly represented by two distinct equations, with a quadruple point Q(h 1 h 2 l 1 g) transition temperature at 26.7°C with standard error (SE) 0.9°C and 55.5 MPa with SE 5.3 MPa. The structure I type methane hydrate phase h 1 is stable below 26.7°C and the structure II type methane hydrate phase h 2 is stable above 26.7°C. Between –0.29 and 25.54°C, 85 equilibrium pressures of methane hydrate with water are best represented, with SE 1.33% on a single pressure measurement, by a four-parameter thermodynamic equation. The corresponding equilibrium methane fugacities are represented, with SE 0.94% on a fugacity determination, by a five-parameter equation. Between 26.98 and 46.87°C, 17 equilibrium methane hydrate pressures with water are best represented, with SE 2.22% on a pressure measurement, by a three-parameter equation. Composition of the equilibrium aqueous phase is evaluated using methane fugacity with the solubility equation including a Poynting correction. Literature data between –2.22 and –14.10°C, for the saturation pressure P(h 1 s 1 g) of structure I methane hydrate with ice, are properly represented by a two-parameter equation, with SE 1.1% on a single pressure measurement. Standard enthalpy change for structure I methane hydrate dissociation into ice and methane gas is found to be ΔH ot (h 1 [Formula: see text] s 1 g) = 18058 J mol –1 with SE 608 J mol –1 at -8.28°C. The quadruple point Q(h 1 s 1 l 1 g) is estimated at –0.290°C with SE 0.0064°C and at 2.527 MPa with SE 0.053 MPa. Using the classical thermodynamic method, as described for deuterium sulfide D-hydrate, methane hydrate equilibrium fugacities define 85 equilibrium constants Kp(h 1 [Formula: see text]l 1 g) between –0.29 and 25.54°C for dissociation of structure I hydrate h 1 into liquid water l 1 and methane gas. Temperature dependence of ln Kp(h 1 [Formula: see text]l 1 g) is well-represented by a ...