Water Content of Methane Gas in Equilibrium with Hydrates

Abstract

Experimental measurements of water content of methane gas in equilibrium with hydrate are presented at 1000 and 1500 psia for temperatures greater than -10 OF. A new chromatographic technique for determining water concentration of gas in equilibrium with hydrate was used to eliminate some of the errors inherent in previous investigations. A method is suggested for calculating the water content of methane in equilibrium with its hydrate above 32 O F at any pressure up to 1500 psia. The experimental data indicate that methane should be significantly drier than indicated by existing water content charts in order to prevent hydrate formation. The data reported herein should be strictly interpreted as applying only to methane, rather than natural gases, due to the fact that methane forms a different hydrate structure from that normally formed by natural gases.Empirical relationships from coexistence data form the basis for a description of the vapor-liquid critical region for pure fluids which is independent of but generally consistent with the scaling hypothesis. These relationships are: rectilinearity with temperature for mean density and for mean isochoric slope (dPld nP. issuing from the coexistence curve; and power law behavior for the vapor-liquid differences of density, enthalpy, and (d f / d 7&.The present description displays excellent agreement with data mapping divergences for various thermodynamic properties at the critical point. With one notable exception, this description also agrees with the theoretical predictions of the scaling hypothesis. The exception is: the present description produces 8 = 1 -2 p as a lower bound which is somewhat larger than the scaling hypothesis assertion that 8 = a. Unfortunately, the data cannot distinguish between these results. Another interesting result is that the present description correctly predicts maxima with temperature for both mean enthalpy and mean entropy near the critical point.

The fate of CO2 hydrate released in the ocean

“…In a submersible study of hydrothermal field in the Okinawa trough, Sakai et al observed that a CO -rich fluid (containing approximately 86% CO ) was secreted from the sea floor at 1335-1550 m depth; upon contact with seawater at 3)8°C, a thin hydrate layer formed rapidly at the interface of seawater and the effluent. The conditions under which hydrates formed were consistent with the CO -water-hydrate phase diagram developed by Song and Kobayashi (1987), according to which hydrates would form in CO -water systems at pressures greater than 4)5 MPa and temperature less than 283 K. The evidence of formation of CO hydrate in the ocean stimulated the study of CO hydrate and many investigations of formation of CO hydrate in water simulating the deep ocean environment have been conducted (e.g., Aya et al, 1992;Austvik and L+ken, 1992;Golomb, 1993;Golomb et al, 1992;North et al, 1993;Fujioka et al, 1994;Saji et al, 1995). Based on these studies, ocean disposal of anthropogenic CO in the hydrate form at depths (500 m (the pressure and temperature required for hydrate formation can be satisfied at 500 m in many oceans) is believed to be an optimum method to sequester CO in the ocean and it is predicted that the disposed CO hydrate may be sequestered in the deep ocean for a long period of time (Austvik and L+ken, 1992;Golomb, 1993;Golomb et al, 1993;Saji et al, 1992;Noda et al, 1994).…”

“…Since the van der Waals diameter of the CO molecule is larger than the free diameters of the pentagonal-dodecahedral cavities (Sloan, 1990), these small cavities are unoccupied statistically in CO hydrate . Thus, in practice, the chemical formula for CO hydrate is 6CO…”

“…In CO hydrate, CO and water are associated without ordinary chemical bonding but through complete enclosure of a set of CO molecules in the cavities of the clathrate formed by water molecules. Since the van der Waals diameter of the CO molecule is smaller than the free diameters of most cavities in the clathrate, and since interactions between CO molecules and water lattice are subject largely to van der Waals forces (Sloan, 1990), CO molecules in the hydrate have a diffusive nature (Teng, 1996). Hydrogen bonding of the hydrate framework is formed with geometric distortions and the framework is unstable without filling a large percentage of its cavities with CO molecules (Berecz and Balla-Achs, 1983;Sloan, 1990;Teng, 1996).…”

“…Since the van der Waals diameter of the CO molecule is smaller than the free diameters of most cavities in the clathrate, and since interactions between CO molecules and water lattice are subject largely to van der Waals forces (Sloan, 1990), CO molecules in the hydrate have a diffusive nature (Teng, 1996). Hydrogen bonding of the hydrate framework is formed with geometric distortions and the framework is unstable without filling a large percentage of its cavities with CO molecules (Berecz and Balla-Achs, 1983;Sloan, 1990;Teng, 1996). Based on the CO -water-hydrate phase diagram (Song and Kobayashi, 1987) and the characteristics of CO hydrate (Teng, 1996), CO hydrate is stable only when (1) p*4)5 MPa (required by mechanical stability), (2) ¹)283 K (required by thermal stability), and (3) the occupancy of CO in the clathrate is larger than the critical value required by the framework stability (i.e., physicochemical stability).…”

Experimental low temperature water content in gaseous methane, liquid ethane, and liquid propane in equilibrium with hydrate at cryogenic conditions

“…Sloan et al [18] presented experimental water content of methane gas in equilibrium with hydrate for 1000 and 1500 psia for temperatures greater than −10 • F. Aoyagi et al [19] reported experimental measurements of water content in methane gas in equilibrium with hydrate at 500-1500 psia for temperatures ranging from −38.5 to 26 • F. Song and Kobayashi [20][21][22][23][24] also reported the water content of methane-propane-water, carbon dioxide-water, and carbon dioxide-methane-water systems in the non-aqueous phases in equilibrium with hydrate or liquid water. Subsequently, Song and Kobayashi [25,26] presented the water content of pure ethane, propane, and their mixtures in equilibrium with liquid water or hydrates.…”