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

Abstract

Literature data for the saturation pressure P(hl1g) 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(h1h2l1g) 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 h1 is stable below 26.7°C and the structure II type methane hydrate phase h2 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(h1s1g) 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 ΔHot(h1[Formula: see text] s1g) = 18058 J mol–1 with SE 608 J mol–1 at -8.28°C. The quadruple point Q(h1s1l1g) 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(h1[Formula: see text]l1g) between –0.29 and 25.54°C for dissociation of structure I hydrate h1 into liquid water l1 and methane gas. Temperature dependence of ln Kp(h1[Formula: see text]l1g) is well-represented by a three-parameter thermodynamic equation that gives both estimates and their standard errors for (i) ΔHot(h1[Formula: see text]l1g) and ΔCpot(h1[Formula: see text]l1g), the standard enthalpy and heat capacity changes, respectively, for hydrate h1 dissociation, and for (ii) n = r, the approximate formula number of the hydrate CH4·nH2O at each experimental temperature. The formula CH4·6.205H2O with SE 0.066H2O is found for the structure I methane hydrate h1 with water at quadruple point Q(h1s1l1g) –0.29°C; an approximate formula CH4·5.759H2O with SE 0.077H2O is found at quadruple point Q(h1h2l1g) 26.7°C. Between 26.98 and 46.87°C, the 17-equilibrium constants Kp(h2[Formula: see text]l1g) for dissociation of structure II methane hydrate h2 into liquid water l1 and methane gas are represented by a constrained three-parameter thermodynamic equation. For structure II methane hydrate the formula CH4·5.822H2O with SE 0.064H2O is found at quadruple point Q(h1h2l1g) 26.7°C and the formula CH4·5.699H2O with SE 0.064H2O at 46.87°C. Molar volumes and cohesive energy densities of the methane hydrates are compared with equilibrium compressed water.Key words: clathrate hydrates of methane, two methane gas hydrates, formula of structure I methane hydrate, thermodynamics of clathrate hydrate dissociation, dissociation equilibrium constants of structure I methane hydrate, standard enthalpy and heat capacity changes for dissociation of structure I methane hydrate, methane hydrates' transition temperature, formula of structure II methane hydrate, dissociation equilibrium constants of structure II methane hydrate, standard enthalpy change for dissociation of structure II methane hydrate, methane hydrates' cohesive energy density.