Phase Equilibrium for Gas Hydrates Formed from Deuterium Oxide

Publication Date (Web): May 20, 2015 ... The deuterium isotopic effect of host water molecules on hydrate equilibrium temperatures for the three phase...
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Phase Equilibrium for Gas Hydrates Formed from Deuterium Oxide Masato Kida,* Mizuho Watanabe, Yusuke Jin, and Jiro Nagao* Methane Hydrate Production Technology Research Group, Research Institute of Energy Frontier, Department of Energy and Environment, National Institute of Advanced Industrial Science and Technology (AIST), Sapporo 062-8517, Japan ABSTRACT: In this study, we measured the hydrate equilibrium conditions for simple methane, ethane, propane, and krypton hydrates formed from liquid/solid deuterium oxide in order to understand the effect of deuterium replacement in the host framework on the gas hydrate equilibrium conditions. We obtained the equilibrium conditions by recording pressure and temperature for gas hydrate formation and dissociation under deuterium oxide-rich conditions in the pressure range of (0.142 to 5.49) MPa and the temperature range of (263.4 to 282.3) K. The hydrate equilibrium pressure for deuterium oxide systems at a fixed temperature decreases in the order of methane, krypton, ethane, and propane. The hydrate equilibrium temperatures for all liquid deuterium oxide systems increase compared with those for its usual liquid water systems at fixed pressure. The deuterium isotopic effect of host water molecules on hydrate equilibrium temperatures for the three phases including liquid water at a fixed pressure becomes greater in the order of methane, ethane, krypton, and propane. The hydrate equilibrium conditions for all solid deuterium oxide systems are approximately consistent with those for its usual liquid water systems. The gas hydrate crystals formed from deuterium oxide were characterized by 13C NMR spectroscopy.

1. INTRODUCTION Gas hydrates are crystalline guest−host compounds comprising hydrogen-bonded water molecules (host molecules) and gas species (guest molecules). Gas molecules such as small hydrocarbons, carbon dioxide, nitrogen, hydrogen, or novel gases are trapped in the cage structures in hydrate frameworks.1 When a single gas component forms a hydrate crystal under normal pressure (∼several tens of MPa), cubic crystal structures I and II (respectively denoted by sI and sII) are formed depending on the gas species.1 Natural gas hydrates containing methane as a principle guest component are widely distributed under submarine or permafrost environments and are expected to be a new natural gas resource.2,3 The establishment of a method for recovering gas from natural gas hydrate deposits by hydrate dissociation is a current engineering challenge. The high storage capacity of gas, the selectivity of trapped gas, the hydrate formation/dissociation heat, and the phase changes from hydrate formation and dissociation are key properties for the applications of natural gas/hydrogen storage and transportation,4,5 molecular separation/condensation,6 and thermal storage.7 Gas hydrate frameworks stabilize at appropriate pressure and temperature conditions depending on the gas species or additive substances, such as salts or alcohols.8 For example, in the hydrocarbon family of simple sI and sII hydrate formers, the hydrate equilibrium temperature increases with the number of carbon atoms in a molecule at fixed pressure.9 The addition of salts or alcohols into the liquid water phase lowers the hydrate equilibrium temperature and increases the hydrate equilibrium pressure.8 A better understanding of the factors affecting the phase equilibrium of gas hydrates is important for the engineering of hydrate systems. Chun et al. reported that the © XXXX American Chemical Society

replacement of hydrogen atoms in the host water molecules of simple carbon dioxide and chlorodifluoromethane hydrates with deuterium atoms has the effect of shifting the hydrate equilibrium temperature of liquid water−hydrate−vapor phases under fixed pressures to higher regions.10 Although the gas hydrates formed from deuterium oxide (D2O) have been widely studied to better understand the crystallographic structure of hydrate frameworks using a neutron diffraction technique,11−13 there is no data on the thermodynamic hydrate stability of various guest molecules. In the present study, we measured the phase equilibriums for simple methane (CH4), ethane (C2H6), propane (C3H8), and krypton (Kr) hydrates formed from D2O for the three phases including liquid or solid water phases in order to understand the effect of the host framework on gas hydrate equilibrium conditions. The cage occupation of the guest hydrocarbons in the D2O hydrate frameworks were identified using solid-state 13C NMR spectroscopy.

2. EXPERIMENTAL SECTION 2.1. Materials. In this study, deuterium oxide with 99.9 atom % D, supplied by Aldrich Chemical Co. Inc. was used. Methane (Sumitomo Seika Chemicals Co. Ltd.), ethane, propane, and krypton (Takachiho Chemical Industrial Co. Ltd.) with purities of > 0.999 mole fraction, > 0.9995 mole fraction, > 0.999 mole fraction, and > 0.9995 mole fraction, respectively, were used as guest gases. Distilled water (H2O) was used to evaluate the reliability of the hydrate equilibrium Received: March 23, 2015 Accepted: May 8, 2015

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hydrate dissociation condition. The reliability of the experimental apparatus and procedure was confirmed by measuring the three-phase equilibrium conditions for ethane hydrate formation from H2O. Simple CH4, C2H6, and C3H8 hydrates formed from D2O ice were recovered from the vessel by quenching using liquid nitrogen. The 13C single-pulse NMR spectra of the recovered D2 O hydrate samples were measured using an NMR spectrometer (100 MHz, Avance III 400; Bruker BioSpin, Bruker Corp.), equipped with a probe for solid samples, at approximately 150 K. The 13C single-pulse NMR spectra were obtained under the following conditions: a 5.5 μs 13C pulse length (90°), a 50 s pulse delay time, 12 or 40 acquisitions, and a 3.0 kHz spinning rate at the magic angle. The 13C chemical shift values were determined at 298 K using adamantane as an external reference material with a methine carbon peak set at 29.472 ppm.14

conditions, as determined by the study’s experimental apparatus and procedure. 2.2. Experimental Apparatus. Two different experimental setups were used for the three-phase equilibriums of liquid deuterium oxide (LWD)−hydrate (H)−vapor (V) and deuterium oxide ice (ID)−hydrate (H)−vapor (V). The phase equilibrium conditions of LWD−H−V were obtained using a stainless steel high-pressure vessel (approximately 1.2·10−4 m3), equipped with an impeller for stirring, and a constanttemperature bath (Thermo Haake C41P). The phase equilibrium conditions of ID−H−V were measured using a stainless steel high-pressure vessel (approximately 1·10−5 m3) and a constant-temperature bath (Thermo Electron Corp. Neslab RT740). The pressure (p) in the systems was measured with two pressure sensors: AP-13S (Keyence Co.) for the pressures below 1.0 MPa and AP-14S (Keyence Co.) for pressures above 1.0 MPa. The uncertainty of the pressure measurements was ± 0.005 MPa at the pressures below 1.0 MPa and ± 0.05 MPa at the pressures above 1.0 MPa. Temperature (T) of solid or solid−liquid phases in the systems was detected using calibrated type-T thermocouples. The expanded uncertainty of temperature was ± 0.1 K with a confidence level of approximately 95 %. 2.3. Experimental Procedure. After thoroughly replacing the air in the vessel with a gas used as a guest component, we brought the introduced gas in the vessel up to the desired pressure. We introduced into the vessels approximately 50 g of liquid D2O to achieve equilibrium conditions of LWD−H−V and 2 g of D2O ice sieved below 212 μm to achieve the equilibrium conditions of ID−H−V. We obtained the three-phase equilibrium conditions by recording the p−T values for gas hydrate formation and dissociation under D2O-rich conditions. We continued to record the p−T conditions, which became constant over ∼several days, after hydrate formation and dissociation in the three phase equilibrium condition. The phase equilibrium p−T conditions of LWD−H−V were measured by employing the following procedure. First, the gas hydrates were formed at temperatures above the ice point of D2O (temperature range of (277.1 to 282.3) K) with a 300 rpm impeller speed. When the variation in the initial pressure and temperature resulting from gas hydrate formation became stable, it was ascertained that the systems have reached the equilibrium state. Next, the system was repressurized up to the desired pressure. When the p−T values returned to those of the initial equilibrium condition by additional hydrate formation, the system was regarded as being in a D2O-rich condition. Thereafter, the set value of the bath temperature was increased in steps of 1 K (heating rate: approximately 1 K·min−1), the dissociation p−T condition was measured at each constant bath temperature. After measuring the dissociation p-T conditions, the formation p−T conditions were measured by decreasing the temperature in steps of 1 K (cooling rate: approximately 0.5 K· min−1). The phase equilibrium conditions of ID−H−V were obtained by employing the following procedure. First, the gas hydrates were formed at temperatures below the ice point of D2O (temperature range of (263.4 to 276.3) K) with a gas−solid contact reaction. After pressure stabilization, the system pressure was increased by approximately 0.1 MPa. Once the p−T values were observed to return to those of the initial condition, a D2O-rich condition in the systems was said to be established. Finally, the system was depressurized to obtain the

3. RESULTS AND DISCUSSION The hydrate formation pressures for the C2H6−H2O system were measured to be 0.380 MPa at 268.5 K and 0.484 MPa at 273.9 K. Its dissociation pressures were 0.379 MPa at 268.4 K and 0.484 MPa at 273.9 K using the apparatus and procedures described above. The resulting formation and dissociation conditions are in good agreement with the hydrate equilibrium condition for C2H6−H2O systems in the literature,15−17 as shown in Figure 1. This indicates that the experimental

Figure 1. Hydrate formation and dissociation p−T conditions and the hydrate equilibrium conditions for the C2H6−H2O system: □, formation p−T conditions (this study); ◇, dissociation p−T conditions (this study); +, hydrate equilibrium conditions (refs 15 16, and 17).

apparatus and procedure in the present study is reliable for the phase equilibrium condition measurements of gas hydrates. Hydrate dissociation p−T conditions measured in this study are used as hydrate equilibrium conditions in the following part of this paper. Table 1 shows the hydrate equilibrium p−T conditions for CH4−D2O, C2H6−D2O, C3H8−D2O, and Kr−D2O systems. Furthermore, these conditions are plotted in Figure 2 together with those of CH4−H2O, C2H6−H2O, C3H8−H2O, and Kr− H2O systems in the literature.15−23 As shown in Figure 2, the equilibrium pressures at a fixed temperature for gas hydrates formed from D2O decrease in the order of CH4, Kr, C2H6, and C3H8. This tendency conforms to that exhibited by gas hydrates formed from H2O. For a given guest gas system including liquid water, the hydrate equilibrium temperature at a fixed pressure for a D2O system shifts to a higher region than that for its H2O system. In contrast, for a given guest gas system including solid ice, the equilibrium conditions for the D2O system are nearly consistent with those for its H2O system. This fact suggests that the deuterium isotopic effect of host water molecules on the B

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Table 1. Hydrate Equilibrium p−T Conditions for CH4− D2O, C2H6−D2O, C3H8−D2O, and Kr−D2O Systems guest molecule

Ta/K

pb/MPa

methane

263.4 265.6 267.6 269.5 271.6 272.5 273.5 275.5 277.4 278.3 279.3 280.3 281.2 282.3 264.0 266.0 267.9 270.0 271.9 273.2 274.1 276.1 277.1 278.1 279.1 280.1 264.5 265.0 267.3 270.3 273.5 276.3 277.5 278.8 279.7 280.5 264.3 267.2 270.1 273.4 277.4 278.9 279.7 280.7 281.7

1.97 2.10 2.23 2.38 2.52 2.59 2.67 2.84 3.29 3.62 4.00 4.46 4.92 5.49 0.347 0.377 0.407 0.443 0.480 0.501 0.523 0.563 0.613 0.693 0.780 0.882 0.142 0.145 0.155 0.173 0.199 0.220 0.260 0.323 0.402 0.504 1.04 1.16 1.25 1.38 1.77 1.95 2.14 2.36 2.61

ethane

propane

krypton

Figure 2. Hydrate equilibrium p−T conditions for simple CH4, C2H6, C3H8, and Kr hydrates: ■, CH4−D2O (this work); □, CH4−H2O (refs 15, 17, and 18); ◆, C2H6−D2O (this work); ◇, C2H6−H2O (refs 15, 16, and 17); ▲, C3H8−D2O (this work), △, C3H8−H2O (refs 15, 17, 19, 20, and 21); ●, Kr −D2O (this work), ○, Kr −H2O (refs 22 and 23).

three-phases hydrate equilibrium pressure and temperature on the basis of the Clausius−Clapeyron equation,9 d ln p ΔH =− d(1/T ) zR

(1)

where z and R are the gas compressibility and universal constant, respectively. Figure 3 shows semilogarithmic plots of

Figure 3. Semilogarithmic plots of pressure versus reciprocal temperature for the hydrate equilibrium conditions for CH4, C2H6, C3H8, and Kr with liquid water: ■, CH4−D2O (this work); □, CH4− H2O (refs 15 and 17); ◆, C2H6−D2O (this work); ◇, C2H6−H2O (refs 15, 16, and17); ▲, C3H8−D2O (this work), △, C3H8−H2O (refs 15, 17, 19, 20, and 21); ●, Kr−D2O (this work), ○, Kr−H2O (ref 23).

pressure versus reciprocal temperature (ln p versus 1/T) for the hydrate equilibrium conditions of CH4, C2H6, C3H8, and Kr at temperatures above the ice point of D2O. The solid and open symbols indicate the hydrate equilibrium conditions for the LWD−H−V phases in the present study and the LWH−H−V phases in the literature, respectively. The plots of ln p versus 1/ T for all D2O systems exhibited good linearity. The straight-line slope of the plots, ΔH values at the equilibrium pressure at the ice point of D2O (276.97 K), and the equilibrium temperature shift from the LwH−H−V to LWD−H−V lines for a given guest molecule at a fixed pressure (ΔT) are shown in Table 2. The ΔH and ΔT values were calculated at the ice point of D2O by using the linear fit in Figure 3. The z values determined by the Lee and Kesler correlation25 were used for the estimation of ΔH values. The plot of LWD−H−V phases in the CH4−D2O

The expanded uncertainty of T value was ± 0.1 K with a confidence level of approximately 95 %. bUncertainty of p values was ± 0.005 MPa at the pressures below 1.0 MPa and ± 0.05 MPa at the pressures above 1.0 MPa. a

hydrate equilibrium conditions for the three phases including the liquid water phase is greater than that for the three phases including the ice phase for all guest gas species examined in the present study. The hydrate dissociation enthalpy reflects the hydrogen bonds in the crystal and the cage occupations.24 The molar dissociation enthalpy of gas hydrates (ΔH) can be approximated by a straight-line slope of the semilogarithmic plots of C

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Table 2. Straight-Line Slope of Semilogarithmic Plots of Pressure (ln p) versus Reciprocal Temperature (1/T) for the Hydrate Equilibrium Conditions, Linear Correlation Coefficient of the Fitting Line, The Molar Dissociation Enthalpy of Gas Hydrates (ΔH), the Equilibrium Temperature Shift from the LwH−H−V to LWD−H−V Lines for a Given Guest Molecule at a Fixed Pressure (ΔT) guest

system

CH4

Lw −H−V LwH−H−V15,17 LwD−H−V LwH−H−V15−17 LwD−H−V LwH−H−V15,17,19−21 LwD−H−V LwH−H−V23

C2H6 C3H8 Kr

D

temperature range T/K (277.4 (273.4 (277.1 (273.4 (277.5 (273.7 (277.4 (277.4

to to to to to to to to

282.3) 282.6) 280.1) 282.8) 280.5) 278.2) 281.7) 282.21)

system is expressed by the following equation: ln p = −8194(1/ T) + 30.73 (linear correlation coefficient, r = 0.9999). Further, this plot is shifted to a higher temperature region along that of LwH−H−V phases in the CH4−H2O system at a fixed pressure. From the extrapolation of the linear plot for the CH4−D2O system, the temperature shift at a fixed equilibrium pressure at the ice point of D2O (1/T = 0.00361) is estimated to be +1.9 K. Wang et al. predicted that the deuterium isotopic effect of host water molecules on the three-phase equilibrium line of CH4 hydrate is +2.5 K.26 The deuterium effect on CH4 hydrate in the present study is smaller than that predicted for CH4 hydrates and smaller than the experimental result for CO2 hydrate. For the three phases including liquid water, the difference in the hydrate equilibrium conditions between the experimental data and theoretical predictions suggests that the deuterium isotopic effect of the host water molecules on the hydrate equilibrium condition depends on the species of the guest gas. The straight line for the C2H6−D2O system including the liquid water phase is fitted by the equation ln p = −9329(1/ T) + 33.18 (r = 0.9998). From this linear relationship, the temperature shift at a fixed equilibrium pressure is estimated to be +2.1 K at the ice point of D2O. The plot of ln p versus 1/T for the C3H8−D2O system including liquid water phase is expressed by the equation ln p = −16828(1/T) + 59.27 (r = 0.9927). The three-phase equilibrium line of LWD−H−V for C3H8 is calculated to shift by +2.5 K at the ice point of D2O relative to that of the LwH−H−V for C3H8. The straight line for the Kr−D2O system including the liquid water phase is expressed by the equation ln p = −7163(1/T) + 26.37 (r = 0.9947). The three-phase equilibrium line of LWD−H−V for Kr is calculated to shift by +2.3 K in comparison with that of LwH− H−V for Kr. The deuterium effect of host water molecules on hydrate equilibrium temperatures for the three phases including liquid water at a fixed pressure becomes greater in order of CH4, C2H6, Kr, and C3H8. In the family of linear hydrocarbons, the temperature shift values increase with the diameters of guest molecules. We consider that the difference in the deuterium effect is caused by the differences in guest−host interactions or caused by cage occupation depending on the guest molecules. The ΔH values for LWD−H−V phases were estimated to be 63.4 (kJ·mol−1) for CH4, 72.9 (kJ·mol−1) for C2H6, 133 (kJ·mol−1) for C3H8, and 57.0 (kJ·mol−1) for Kr, which approximately agree with those for LWH−H−V phases. This agreement suggests that the hydrate dissociation enthalpy is not greatly affected by the deuterium replacement in the hydrate host frameworks. We studied the cage occupation of the guest hydrocarbons to identify the D2O hydrate frameworks using solid-state 13C

d ln p/d(1/T)

r

ΔH /(kJ·mol−1 gas)

ΔT/K

−8194 −7876 −9329 −9621 −16828 −17080 −7163 −7607

0.9999 0.9996 0.9998 0.9960 0.9927 0.9963 0.9947 0.9997

63.4 59.9 72.9 73.7 133 130 57.0 55.8

+1.9 +2.1 +2.5 +2.3

NMR spectroscopy. Figure 4 shows the single-pulse 13C NMR spectra of the CH4−D2O, C2H6−D2O, and C3H8−D2O hydrate

Figure 4. Single-pulse 13C MAS NMR spectra of simple CH4, C2H6, and C3H8 hydrates formed from D2O.

samples. The CH4−D2O hydrate sample shows two 13C NMR signals at −4.1 and −6.5 ppm, which are attributed to the CH4 molecules trapped in the small and large cages of the sI hydrate framework on the basis of the comparison with the 13C NMR spectra of CH4−H2O hydrate in the literature.27 On the basis of the ratio of the number of small and large cages in the sI unit cell, the CH4 occupancy ratio θS/θL of the small cages and the large cages in the sI hydrate framework can be estimated by the integrated intensities ratio of the 13C NMR signals from the guest CH4 molecules.28 Thus, θS/θL for CH4 hydrate formed from D2O is estimated to be 0.913, which is in good agreement with the literature value for CH4 hydrate formed from H2O (0.916).28 The C2H6−D2O hydrate sample shows a single 13C NMR signal at 7.7 ppm, attributed to methyl carbons of ethane trapped in the large cages of the sI hydrate framework, as compared with the 13C NMR spectra of C2H6−H2O hydrate in the literature.27 This observation shows that C2H6 molecules are incorporated into only the large cages of sI that are constructed by the D2O host framework. The cage occupation of C2H6 in the sI framework is consistent with the case of C2H6−H2O hydrate. The C3H8−D2O hydrate sample exhibits two 13C NMR signals at 16.9 and 17.6 ppm, which we assigned to methylene and the methyl carbons of the propane trapped in the large cages of the sII hydrate framework, respectively, as compared with the 13C NMR spectra of C3H8−H2O hydrate in the literature.29 Our detection of only 13C NMR signals from the C3H8 molecules in the large cages of the sII framework suggests that C3H8 molecules occupy only the large sII cages in the D2O framework, which supports the neutron diffraction data in the literature.12 The 13C NMR spectra of the hydrocarbon−D2O hydrate samples suggest that the cage occupation by guest hydrocarbons in the hydrate frameworks of D

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(7) Bi, Y.; Guo, T.; Zhu, T.; Zhang, L.; Chen, L. Influences of additives on the gas hydrate cool storage process in a new gas hydrate cool storage system. Energy Convers. Manage. 2006, 47, 2974−2982. (8) CSMHYD, a phase-equilibrium calculation program package accompanying the following book: Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998. (9) Sloan, E. D.; Fleyfel, F. Hydrate dissociation enthalpy and guest size. Fluid Phase Equilib. 1992, 76, 123−140. (10) Chun, M.-K.; Yoon, J.-H.; Lee, H. Clathrate Phase Equilibria for the Water + Deuterium Oxide + Carbon Dioxide and Water + Deuterium Oxide + Chlorodifluoromethane (R22) systems. J. Chem. Eng. Data 1996, 41, 1114−1116. (11) Halpern, Y.; Thieu, V.; Henning, R. W.; Wang, X.; Schultz, A. J. Time-Resolved in Situ Neutron Diffraction Studies of Gas Hydrate: Transformation of Structure II (sII) to Structure I (sI). J. Am. Chem. Soc. 2001, 123, 12826−12831. (12) Rawn, C. J.; Rondinone, A. J.; Chakoumakos, B. C.; Circone, S.; Stern, L. A.; Kirby, S. H.; Ishii, Y. Neutron powder diffraction studies as a function of temperature of structure II hydrate formed from propane. Can. J. Phys. 2003, 81, 431−438. (13) Thompson, H.; Soper, A. K.; Buchanan, P.; Aldiwan, N.; Creek, J. L.; Koh, C. A. Methane hydrate formation and decomposition: Structural studies via neutron diffraction and empirical potential structure refinement. J. Chem. Phys. 2006, 124, 164508−1−12. (14) Hayashi, S.; Hayamizu, K. Chemical Shift Standards in HighResolution Solid-State NMR (1) 13C, 29Si, and 1H Nuclei. Bull. Chem. Soc. Jpn. 1991, 64, 685−687. (15) Deaton, W. M.; Frost, E. M. Gas hydrates and their relation to the operation of natural gas pipe lines. U.S. Bur. Mines Monogr. 1946, Mongraph 8. (16) Roberts, O. L.; Brownscombe, E. R.; Howe, L. S. Constitution diagrams and composition of methane and ethane hydrates. Oil Gas J. 1940, 39, 37−43. (17) Yasuda, K.; Ohmura, R. Phase equilibrium for clathrate hydrates formed with methane, ethane, propane, or carbon dioxide at temperatures below the freezing point of water. J. Chem. Eng. Data 2008, 53, 2182−2188. (18) Adisasmito, S.; Frank, R. J.; Sloan, E. D. Hydrates of carbon dioxide and methane mixtures. J. Chem. Eng. Data 1991, 36, 68−71. (19) Robinson, D. B.; Metha, B. R. Hydrates in the propane carbon dioxide- water system. J. Can. Petrol. Technol. 1971, 10, 33−35. (20) Kubota, H.; Shimizu, K.; Tanaka, Y.; Makita, T. Thermodynamic. Properties of R13 (CClF3), R23 (CHF3), R152a (C2H4F2) and propane hydrates for desalination of seawater. J. Chem. Eng. Jpn. 1984, 17, 423−429. (21) Thakore, J. L.; Holder, G. D. Solid vapor azeotropes in hydrateforming systems. Ind. Eng. Chem. Res. 1987, 26, 462−469. (22) Jin, Y.; Matsumoto, K.; Nagao, J.; Shimada, W. Phase equilibrium conditions for krypton clathrate hydrate below the freezing point of water. J. Chem. Eng. Data 2011, 56, 58−61. (23) Sugahara, K.; Sugahara, T.; Ohgaki, K. Thermodynamic and Raman spectroscopic studies of Xe an Kr hydraets. J. Chem. Eng. Data 2005, 50, 274−277. (24) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2007. (25) Lee, B. I.; Kesler, M. G. A generalized thermodynamic correlation based on three-parameter corresponding states. AIChE J. 1975, 21, 510−527. (26) Wang, X.; Schultz, A. J.; Halpern, Y. Kinetics of methane hydrate formation from polycrystalline deuterated ice. J. Phys. Chem. A 2002, 106, 7304−7309. (27) Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D., Jr. Evidence of structure II hydrate formation from methane + ethane mixtures. Chem. Eng. Sci. 2000, 55, 1981−1999. (28) Ripmeester, J. A.; Ratcliffe, C. I. Low-temperature crosspolarization/magic angle spinning carbon-13 NMR of solid methane hydrates: structure, cage occupancy, and hydration number. J. Phys. Chem. 1988, 92, 337−339.

D2O are consistent with those of H2O. The same cage occupations contribute to the hydrate dissociation enthalpy, which remains unchanged by the deuterium replacement in the hydrate host frameworks.

4. CONCLUSION We measured the hydrate equilibrium conditions for simple CH4, C2H6, C3H8, and Kr hydrates formed from D2O for the three phases including liquid or solid water phases to better understand the deuterium isotopic effect of hydrate host frameworks on the thermodynamic stability of gas hydrates. We reported the equilibrium conditions in the pressure range of (0.142 to 5.49) MPa and the temperature range of (263.4 to 282.3) K by recording p−T values for gas hydrate formation and dissociation under deuterium oxide-rich conditions. The equilibrium pressure at a fixed temperature for the gas hydrates formed from D2O decreases in the order of CH4, Kr, C2H6, and C3H8. The deuterium isotopic replacement of host water molecules functions mainly in the hydrate equilibrium conditions for the liquid water systems. The hydrate equilibrium temperatures for all liquid D2O systems increase as compared with those for its liquid H2O systems at fixed pressure. The hydrate equilibrium conditions for all D2O ice systems are approximately consistent with those for its H2O ice systems. The deuterium effect of host water molecules on hydrate equilibrium temperatures for the three phases including liquid water at a fixed pressure becomes greater in the order of CH4, C2H6, Kr, and C3H8. The ΔH values for LWD−H−V phases were estimated to be 63.4 (kJ·mol−1) for CH4, 72.9 (kJ· mol−1) for C2H6, 133 (kJ·mol−1) for C3H8, and 57.0 (kJ·mol−1) for Kr. The 13C NMR spectra of the hydrocarbon hydrates formed from D2O suggest that the cage occupation by guest hydrocarbons in the hydrate frameworks of D2O is consistent with that of H2O.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS We thank Drs. M. Oshima and S. Muromachi of AIST for their fruitful discussions. REFERENCES

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