Section: 6 | Properties of Gas Clathrate Hydrates |

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One or more tables in this document differ to those in the book. This is due to space restrictions in the book.

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The table 'TABLE 4a. Phase Equilibria of Ternary CH₄+C₃H₈+Water Hydrate ' has one or more different columns to those in the book version.
The table 'TABLE 2b. Phase Equilibria of Ethane Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 2c. Phase Equilibria of Propane Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 2d. Phase Equilibria of Carbon Dioxide Hydrate (Ref. 1)' has one or more different columns to those in the book version.
The table 'TABLE 2e. Phase Equilibria of Nitrogen (N₂) Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 2f. Phase Equilibria of Xenon Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 2g. Phase Equilibria of Hydrogen Sulfide Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 3. Phase Equilibria of Ternary Methane+MCH (Methylcyclohexane)+Water Structure H Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 4b. Phase Equilibria of Ternary CH₄+CO₂+Water Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 4c. Phase Equilibria of Ternary CH₄+H₂S+Water Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 4d. Phase Equilibria of Ternary CH₄+C₂H₆+Water Hydrate' has one or more different columns to those in the book version.
The table 'TABLE 5a. Methane Hydrate Phase Equilibria in the Presence of Sodium Chloride' has one or more different columns to those in the book version.
The table 'TABLE 5b. Methane Hydrate Phase Equilibria in the Presence of Methanol (MeOH)' has one or more different columns to those in the book version.
The table 'TABLE 5c. Methane Hydrate Phase Equilibria in the Presence of 1,2-Ethanediol (MEG)' has one or more different columns to those in the book version.

How to Cite this Reference

PROPERTIES OF GAS CLATHRATE HYDRATES

Carolyn A. Koh, M. Naveed Khan, and E. Dendy Sloan

Gas clathrate hydrates (also known as gas hydrates) are crystalline inclusion compounds composed of hydrogen-bonded water cavities (host) that encage small gas (guest) molecules in cavities. Generally, a maximum of one guest molecule occupies each water cavity. Typical guest molecules that form gas hydrates are methane, ethane, carbon dioxide, and propane (see gas hydrate phase equilibria data in Table 2). The three important structures of gas hydrates are: cubic structure I (sI) with two small and six large cavities (cages); cubic structure II (sII) with sixteen small and eight large cavities; and hexagonal structure (sH) with three small.,two medium, and one large cavities in the unit cell. The structural and physical properties of each type are given in Tables 1a and 1b. Data have been taken from the references indicated.

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TABLE 1a. Gas Hydrate Structural Properties*

Structure	sI Small	sI Large	sII Small	sII Large	sH Small	sH Medium	sH Large
Crystal system	Cubic	Cubic	Cubic	Cubic	Hexagonal	Hexagonal	Hexagonal
Space group	Pm3n (No. 223)^b	Pm3n (No. 223)^b	Fd3m (No. 227)^b	Fd3m (No. 227)^b	P6/mmm (No. 191)^b	P6/mmm (No. 191)^b	P6/mmm (No. 191)^b
Lattice description	Primitive	Primitive	Face centered	Face centered	Hexagonal	Hexagonal	Hexagonal
Lattice parameters^a	a = 12 Å α = β = γ = 90^o	a = 12 Å α = β = γ = 90^o	a = 17.3 Å α = β = γ = 90^o	a = 17.3 Å α = β = γ = 90^o	a = 12.2 Å, c = 10.1 Å α = β = 90^ο, γ = 120^o	a = 12.2 Å, c = 10.1 Å α = β = 90^ο, γ = 120^o	a = 12.2 Å, c = 10.1 Å α = β = 90^ο, γ = 120^o
Ideal unit cell formula	6(5¹²6²)·2(5¹²)^.46H₂O	6(5¹²6²)·2(5¹²)^.46H₂O	8(5¹²6⁴)·16(5¹²)·136H₂O	8(5¹²6⁴)·16(5¹²)·136H₂O	1(5¹²6⁸)·3(5¹²)·2(4³5⁶6³)·34H₂O	1(5¹²6⁸)·3(5¹²)·2(4³5⁶6³)·34H₂O	1(5¹²6⁸)·3(5¹²)·2(4³5⁶6³)·34H₂O
Cavity	Small	Large	Small	Large	Small	Medium	Large
Description	5¹²	5¹²6²	5¹²	5¹²6⁴	5¹²	4³5⁶6³	5¹²6⁸
Number of cavities/unit cell	2	6	16	8	3	2	1
Average cavity radius (Å)^c	3.95	4.33	3.91	4.73	3.94^d	4.04^d	5.79^d
H₂O molecules/cavity^e	20	24	20	28	20	20	36

^*All values are from Ref. 1 unless noted otherwise.
^a Lattice parameters are a function of temperature, pressure, and guest composition. Typical average values given.
^b Space group reference numbers from the International Tables of Crystallography.
^c The average cavity radius will vary with temperature, pressure, and guest composition.
^d From the atomic coordinates measured using single crystal x-ray diffraction on 2,2-dimethylpentane·5(Xe,H₂S)·34H₂O at 173 K (Ref. 2). The Rietveld refinement package, GSAS was used to determine the atomic distances for each cage oxygen to the cage center.
^e Number of oxygen atoms at the periphery of each cavity.

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TABLE 1b. Physical Properties of sI, sII Hydrates Compared to Ice Ih

Property	Ice Ih	sI	sII
Dielectric constant at 273 K	94	~58	~58
H₂O reorientation time at 273 K (µs)	21	~10	~10
H₂O diffusion jump time (µs)	2.7	>200	>200
Isothermal Young’s modulus at 268 K (10⁹Pa)	9.5	8.4^est	8.2^est
Poisson's ratio	0.3301^f	0.31403^f	0.31119^f
Bulk modulus (GPa)	9.097^f	8.762^f	8.482^f
Shear modulus (GPa)	3.488^f	3.574^f	3.6663^f
Compressional velocity, V_p (m/s)	3870.1^f	3778^f	3821.8^f
Shear velocity, V_s (m/s)	1949^f	1963.6	2001.14^g
Linear thermal expansion at 200 K (K^–1)	56 x 10^–6	77 x 10^–6	52 x 10^–6
Thermal conductivity (W m^–1 K^–1) at 263 K	2.18±0.01^h	0.51±0.01^h	0.50±0.01^h
Adiabatic bulk compression at 273 K (GPa)	12	14^est	14^est
Heat capacity (J kg^–1 K^–1)	1700±200^h	2080	2130±40^h
Refractive index (632.8 nm, –3 °C)	1.3082 (Ref. 9)	1.346 (Ref. 9)	1.350 (Ref. 9)
Density (g/cm³)	0.91^j	0.94	1.291^k

^**All values are from Refs. 1, 3, 4, and 5 unless noted otherwise.
^f At 253–268 K, 22.4–32.8 MPa (ice, Ih), 258–288 K, 27.1–62.1 MPa (CH₄, sI), 258–288 K, 30.5–91.6 MPa (CH₄–C₂H₆, sII), Ref. 6.
^g At 258–288 K, 26.6–62.1 MPa, Ref. 7.
^h At 248–268 K (ice, Ih), 253–288 K (CH₄, sI), 248–265.5 K (THF, sII), Ref. 8.
^j Fractional occupancy (calculated from a theoretical model) in small (S) and large (L) cavities: sI = CH₄: 0.87 (S) and CH₄: 0.973 (L); sII = CH₄: 0.672 (S), 0.057 (L); C₂H₆: 0.096 (L) only; C₃H₈: 0.84 (L) only.
^k Calculated for 2,2-dimethylpentane^.5(Xe,H₂S)^.34H₂O, Ref. 2; est = estimated.

References for Table 1

Sloan, E .D. and Koh, C. A., Clathrate Hydrates of Natural Gases, Third Edition, CRC Press, 2008.
Udachin, K. A., Ratcliffe, C. I., Enright, G. D., and Ripmeester, J. A., Supramol. Chem., 8, 173, 1997. [https://doi.org/10.1080/10610279708034933]
Davidson, D. W., Natural Gas Hydrates (Cox, J. L., Ed.) Butterworths, Boston, 1, 1983.
Davidson, D. W., Handa, Y. P., and Ripmeester, J. A., J. Phys. Chem., 90, 6549, 1986. [https://doi.org/10.1021/j100282a026]
Ripmeester, J. A., Ratcliffe, C. I., Klug, D. D., and Tse, J. S., in Proc. First International Conference on Natural Gas Hydrates, (Sloan, E.D., Happel, J., and Hnatow, M.A., Eds.) Annals of the New York Academy of Sciences, 715, 161, 1994. [https://doi.org/10.1111/j.1749-6632.1994.tb38832.x]
Helgerud, M. B., Circone, S., Stern, L., Kirby, S., and Lorenson, T. D., in Proc. Fourth International Conference on Gas Hydrates, p. 716, Yokohama, May 19–23, 2002.
Helgerud, M .B., Waite, W. F., Kirby, S. H., and Nur, A., Can. J. Phys., 81, 47, 2003. [https://doi.org/10.1139/p03-016]
Waite, W. F., Gilbert, L. Y., Winters, W. J., and Mason, D. H., in Proc. Fifth International Conference on Gas Hydrates, Trondheim, Norway, June 13–16, Paper 5042, 2005.
Bylov, M. and Rasmussen, P., Chem. Eng. Sci., 52, 3295, 1997. [https://doi.org/10.1016/S0009-2509(97)00144-9]

Table 2: Phase Equilibria Data of Gas Clathrate Hydrates

This table gives measured phase equilibria data of pure sI and sII gas clathrate hydrates (see Table 1 for gas hydrate structure and physical property data). The temperature and pressure conditions at which gas hydrates are stable are listed here for typical guest molecules (Tables 2a–2g). For example, data for methane hydrate (Table 2a) show that at 277.1 K, methane hydrate will dissociate at pressures below 3.81 MPa. In addition to small hydrocarbons (methane, ethane, and propane) and carbon dioxide, the nitrogen (N₂), xenon, and hydrogen sulfide hydrate systems have been measured extensively, and the hydrate phase equilibria data are available at temperatures and pressures as low as about 211.2 K and 6.5 kPa to as high as about 340.15 K and 1500 kPa.

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TABLE 2a. Phase Equilibria of Methane Hydrate (Ref. 1)

T/K	P/MPa
Continued on next page...
Ice-Hydrate-Vapor (Ref. 2)
262.4	1.79
264.2	1.9
266.5	2.08
268.6	2.22
270.9	2.39
Liquid Water-Hydrate-Vapor (Ref. 2)
273.7	2.77
274.3	2.9
275.4	3.24
275.9	3.42
275.9	3.43
277.1	3.81
279.3	4.77
280.4	5.35
280.9	5.71
281.5	6.06
282.6	6.77
284.3	8.12
285.9	9.78
Liquid Water-Hydrate-Vapor (Ref. 3)
295.7	33.99

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PROPERTIES OF GAS CLATHRATE HYDRATES

TABLE 1a. Gas Hydrate Structural Properties*

TABLE 1b. Physical Properties of sI, sII Hydrates Compared to Ice Ih

References for Table 1

Table 2: Phase Equilibria Data of Gas Clathrate Hydrates

TABLE 2a. Phase Equilibria of Methane Hydrate (Ref. 1)

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