Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Phase Equilibrium of Isotopologue Methane Hydrates Enclathrated CH3D and CD4 Takahiro Ozeki,† Yuki Kikuchi,† Satoshi Takeya,‡ and Akihiro Hachikubo*,† †
Kitami Institute of Technology, 165 Koen-cho, Kitami 090-8507, Japan National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba 305-8565, Japan
‡
ABSTRACT: Hydrate equilibrium conditions formed from deuterated methane isotopologues (CH3D and CD4) were measured over a temperature range of (273 to 278) K and a pressure range of (2.6 to 4.2) MPa. The phase boundary of CH3D and CD4 hydrates was shifted to higher pressures and lower temperatures than that of the CH4 hydrate. Crystal structure and cage occupancies of gas hydrate crystals formed from CH3D and CD4 were characterized by Raman spectroscopy. The isotope effect of methane hydrates revealed in this study can be used to explain the isotopic fractionation of hydrogen in methane during the formation of clathrate hydrates observed in natural settings, as reported by previous studies.
1. INTRODUCTION Gas hydrates are crystalline clathrate compounds that have guest gas molecules trapped inside hydrogen-bonded water cages. Natural gas hydrates containing methane as the principle guest gas component occur at low temperatures and high partial pressures, such as those observed in the sea and lake bottom sediments and permafrost layers. These methane reserves are a potential global source of energy1−3 and are a concern for adding to global warming4−6 by their dissociation. Because methane molecule comprises carbon and hydrogen, three types of methane isotopologues are found to naturally occur: 12 CH4, 13CH4, and 12CH3D. The difference in their molecular weight causes an “isotope effect”, resulting in different physicochemical properties. Pure methane hydrates are a “mixed-gas hydrate” of 12CH4, 13CH4, and CH3D. The equilibrium pressure of pure methane (mainly 12CH4) hydrate has been previously reported;7−10 however, the equilibrium pressures of 13CH4 and CH3D hydrates have not been reported thus far. Moreover, the fractionation of hydrogen isotopes observed during the formation of methane hydrates has been reported, and it was observed that the δD of hydrate-bound methane was 4.8 ± 0.4‰ lower than the residual molecules.11 Such isotopic differences between the hydrate-bound and sediment gases provide information about the formation process of naturally occurring methane hydrates.12,13 The isotopic fractionation also suggests that CH3D molecules are excluded from the hydrate phase because the equilibrium pressure of the CH3D hydrate is more than that of the CH4 hydrate. Therefore, understanding the phase equilibrium of hydrate encaged methane isotopologues is important for further understanding naturally occurring methane hydrates. In this study, we measured the equilibrium pressures of clathratehydrate-encaged methane isotopologues (CH4, CH3D, and CD4) © XXXX American Chemical Society
at temperatures between (273 and 278) K. Moreover, we obtained cage occupation of the methane isotopologues in the hydrate lattice using Raman spectroscopy.
2. EXPERIMENTAL SECTION 2.1. Materials and Experimental Apparatus. CH3D (purity: 98%, Taiyo Nippon Sanso), CD4 (purity: 99%, Taiyo Nippon Sanso), and CH4 (purity: 99.99% for methane, including 1.1% of 13CH4, Takachiho Chemical Industrial) were used as guest gases in this study. Distilled and deionized water was used as host molecules. The experimental apparatus for phase equilibrium measurements is depicted in Figure 1. Methane hydrate samples were synthesized in a stainless-steel high-pressure cell (TVS-1-5, Taiatsu Techno, volume: 5 mL) equipped with a pressure gauge (AP-14S, Keyence). The uncertainty in the pressure measurements was 0.01 MPa. The pressure gauge was sealed with waterproof silicon sealant. The whole system comprised the pressure cell, pressure gauge, and a ball valve connected to 1/16 in. stainless-steel tubing was immersed in a temperature-controlled liquid bath (TRL-N11L, Thomas Kagaku). The temperature of the system was measured using a calibrated platinum resistance thermometer (testo 735, Testo). The liquid bath controlled the temperature within ±0.05 K, and the uncertainty of the temperature measurement was 0.05 K. 2.2. Experimental Procedure. We formed fine powder samples of methane hydrate in the pressure cell and achieved an equilibrium condition without agitating the system above 273.2 K. Approximately 1 g of fine ice powder (specific surface Received: March 15, 2018 Accepted: May 18, 2018
A
DOI: 10.1021/acs.jced.8b00203 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
area ca. 360 m2 kg−1, corresponding to 18 μm of diameter of equal volume-to-surface-area sphere) was placed in the pressure cell, and vacuum was applied at a temperature of 77 K. The methane isotopologue 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. Methane hydrates were formed by melting the ice powder at a temperature of 273.2 K under a high pressure of methane. The equilibrium hydrate dissociation and formation conditions were determined using a step-by-step isochoric method of heating and cooling. Three-phase liquid water (Lw)−hydrate (H)−vapor (V) equilibrium conditions were achieved by controlling the temperature by increasing it by 0.4 K and then decreasing it by 0.2 K. In this process, the methane hydrate was dissociated and formed and the pressure became constant over a few hours to several days, whereas the temperature of the liquid bath reached the specified temperature within 10 min. Because two values of equilibrium pressure at each temperature by heating and cooling were obtained, we determined the phase equilibrium points as their average temperatures and pressures. The temperatures and pressures were continuously recorded, and the pressure−temperature (P−T) curve of the equilibrium data was plotted. Equilibrium pressures of the deuterated methane (CH3D and CD4) and CH4 hydrates were simultaneously measured using twin high-pressure cells (Figure 1) to observe
were obtained at 83 K under atmospheric pressure of N2 gas. The peak position was analyzed by Voigt fitting functions to calculate the peak ratio of methane in large to small cages. The cage occupancies and the hydration numbers of the crystals were estimated from the peak ratios using a statistical thermodynamic model.14,15 Because the area of the peaks represents the amount of methane in each cage, the cage occupancies of large cages (θL) and small cages (θS) are expressed as
θS 3I = S θL IL
(1)
where IL and IS are the areas of the peaks for large and small cages, respectively. A statistical thermodynamics expression for the chemical potential of water molecules in the cubic structure I hydrate is shown as −Δμ W ° =
RT [6 ln(1 − θL) + 2 ln(1 − θS)] 46
(2)
where ΔμW° is the difference in chemical potentials between ice and a hypothetical empty lattice of the structure I. The value of ΔμW° used in this work was 1297 J mol−1.16 R and T are the gas constant and the temperature for hydrate formation.
Figure 1. Schematic of the experimental apparatus.
the differences between them. The phase equilibrium data were obtained in the temperature range of (273 to 278) K. To verify the crystal structure and cage occupancy ratios of large (51262) and small (512) cages of the methane hydrate, Raman spectroscopic measurements were performed. The Raman spectra of methane hydrates with encaged CH3D, CD4, and CH4 were measured using a Raman spectrometer (RMP210, JASCO Corporation) equipped with a 532 nm excitation source (100 mW), a single holographic diffraction grating (1800 grooves mm−1), and a CCD detector. The spectrum pixel resolution was 0.8 to 1.0 cm−1/pixel in the range of 1900− 3100 cm−1. A polypropylene peak at 1460 cm−1 was used for routine calibration of the monochromator, and the wavenumber was also calibrated using atomic emission lines from a neon lamp. The Raman spectra for the C−H and C−D symmetric stretch regions of enclathrated methane isotopologues
Figure 2. Three-phase equilibrium p−T conditions for CH3D, CD4, and CH4 hydrates: red circles, CH3D−H2O (this study); blue squares, CD4−H2O (this study); green diamonds, CH4−H2O (this study); open triangles, CH4−H2O (ref 7); open squares, CH4−H2O (ref 8); open diamonds, CH4−H2O (ref 9); open circles, CH4−H2O (ref 10). B
DOI: 10.1021/acs.jced.8b00203 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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Accordingly, θL and θS were determined by combining eqs 1 and 2. The hydration number n is expressed as n=
46 6θL + 2θS
Table 2. Three-Phase Equilibrium p−T Conditions in CD4−H2O and CH4−H2O Systemsa
(3)
3. RESULTS AND DISCUSSION To ensure the accuracy of the experimental apparatus described in the Experimental Section, we measured the p−T curve of equilibrium data of CH4 hydrate. As depicted in Figure 2, the equilibrium data for the CH4 hydrate obtained in this study are in good agreement with previous literature data,7−10 indicating that p−T curves can be obtained accurately using the abovementioned experimental apparatus. The p−T data for the Lw−H−V three-phase equilibrium for the CH3D−H2O and CD4−H2O systems are listed in Tables 1 Table 1. Three-Phase Equilibrium p−T Conditions in CH3D−H2O and CH4−H2O Systemsa
a
T/K
PCH3D−H2O/MPa
PCH4−H2O/MPa
273.23 273.43 273.62 273.82 274.01 274.22 274.41 274.63 274.82 275.02 275.22 275.42 275.61 275.81 276.00 276.21 276.42 276.62 276.83 277.04 277.24 277.44 277.64
2.69 2.74 2.79 2.84 2.90 2.96 3.02 3.08 3.14 3.20 3.26 3.33 3.39 3.46 3.53 3.60 3.67 3.75 3.82 3.90 3.99 4.06 4.15
2.82 2.87 2.93 2.98 3.04 3.10 3.16 3.23 3.29 3.35 3.41 3.49 3.56 3.63 3.71 3.78 3.86 3.94 4.02 4.10
a
T/K
PCD4−H2O/MPa
PCH4−H2O/MPa
273.35 273.57 273.77 273.98 274.17 274.36 274.57 274.77 274.97 275.17 275.37 275.55 275.75 275.95 276.15 276.34 276.55 276.76 276.94 277.14 277.35
2.80 2.86 2.91 2.97 3.03 3.09 3.15 3.21 3.28 3.34 3.41 3.47 3.54 3.61 3.68 3.75 3.83 3.90 3.98 4.06 4.14
2.68 2.74 2.79 2.85 2.90 2.96 3.02 3.08 3.14 3.20 3.27 3.33 3.39 3.46 3.53 3.60 3.67 3.74 3.81 3.89 3.97
Uncertainties of T and P are 0.05 K and 0.01 MPa, respectively.
Uncertainties of T and P are 0.05 K and 0.01 MPa, respectively.
and 2, respectively. The data are plotted in Figure 2 with the p−T data of the Lw−H−V three-phase equilibrium conditions of the CH4 hydrate. The equilibrium pressures in the CH3D− H2O system are higher by ∼0.04 MPa compared with the corresponding values in the CH4−H2O system in the temperature range between (273 and 278) K. Moreover, the equilibrium pressures observed in the CD4−H2O system are ∼0.14 MPa higher than the corresponding values in the CH4−H2O system in the same temperature range. These differences in equilibrium pressures are not large, but the deviations are larger than the measurement errors in this study. Thus our experimental results indicate a change of the equilibrium pressures in the methane− water system due to the isotope effect of CH3D and CD4. For example, the equilibrium pressure of C2H6 hydrate was found to be lower than that of the CH4 hydrate,17 resulting in more C2H6 concentration than CH4 in the hydrate phase during the formation process of CH4 and C2H6 mixed-gas hydrates.17,18
Figure 3. Raman spectra of isotopologue methane hydrates in the C−H stretching vibration mode region of methane. The spectra were recorded at 0.1 MPa and 83 K. a.u., Arbitrary units. C
DOI: 10.1021/acs.jced.8b00203 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
pressure of the CH3D hydrate is more than that of CH4 hydrates. δD of hydrate-bound methane is 4.8 ± 0.4‰ lower than that of ambient gaseous methane at 274.2 K during the formation process.11 Therefore, the difference in equilibrium pressures between CH3D and CH4 hydrates obtained in this study agrees well with the previous report.11 Figure 3 depicts the Raman spectra of the C−H stretching vibration mode of methane hydrate samples obtained from the CH3D−H2O and CH4−H2O systems. In CH4 hydrate, the symmetric stretching peak (ν1) is separated into two peaks at 2904 and 2916 cm−1, corresponding to CH4 molecules in large and small cages of the cubic structure I.15 In CH3D hydrate, the doublet at 2900 and 2960 cm−1 is attributed to a Fermi resonance due to an interaction between the C−H symmetric stretching (ν1) and C−H degenerate deformation (2ν5).19 These peaks can be separated further into two peaks at 2897 and 2902 cm−1 and 2955 and 2964 cm−1, respectively, corresponding to CH3D molecules in large and small cages of the cubic structure I. The broad peaks around 3100 cm−1 correspond to the O−H stretching vibrations of H2O. Figure 4 depicts the Raman spectra of the C−D stretching vibration mode of methane hydrate samples obtained from the Table 4. Cage Occupancies for Large (θL) and Small (θS) Cages and Hydration Number (n) of CH3D and CH4 Hydrates fitting peaks CH3D hydrate 2955 cm−1 and 2964 cm−1 2190 cm−1 and 2199 cm−1 CH4 hydrate 2904 cm−1 and 2916 cm−1
Figure 4. Raman spectra of isotopologue methane hydrates in the C−D stretching vibration mode region of methane. The spectra were recorded at 0.1 MPa and 83 K. a.u., Arbitrary units.
θL
θS
n
0.97 ± 0.00 0.97 ± 0.00
0.94 ± 0.02 0.90 ± 0.02
5.99 ± 0.01 6.02 ± 0.02
0.96 ± 0.02
0.95 ± 0.06
6.01 ± 0.04
CH3D−H2O and CD4−H2O systems. Similar to the C−H symmetric stretching peaks, the two peaks at 2190 and 2199 cm−1 are assigned as the C−D stretching of CH3D molecules in large and small cages; however, the broad peak of CD4 (2092− 2102 cm−1) could not be separated. The observed Raman peaks between 1900 and 3100 cm−1 and their assignments are
Similarly, CH3D molecules are excluded from the hydrate phase, resulting in isotopic fractionation of hydrogen in methane during the formation of methane hydrates because the equilibrium
Table 3. Observed Raman Frequencies of Hydrate-Bound Methane and Assignments to the Vibrational Modesa guest molecule/cage
hydrate
CH3D large cage small cage large cage small cage large cage small cage
CD4 large and small cages
2897 2902 2955 2964 2190 2199 2582 2305 2092−2102 2179 1956
CH4 large cage small cage
a
2904 2916 3051 2574
gas19
assign
vibrational mode
2914
ν1
CH3 s-stretchb
2973
ν1
CH3 s-stretchb
2200
ν2
CD stretch
2600 2310
2ν3 2ν6
CH3 s-deform CH3 rock
2109 2184 1992
ν1 2ν2 2ν4
sym stretch deg deform deg deform
2917
ν1
sym stretch
3067 2612
2ν2 2ν4
deg deform deg deform
Uncertainty is
