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Dissociation Behavior of Methane-Ethane Mixed Gas Hydrate Coexisting Structures I and II Masato Kida,† Yusuke Jin,† Nobuo Takahashi,‡ Jiro Nagao,*,† and Hideo Narita† Methane Hydrate Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), 2-17-2-1 Tsukisamu-Higashi, Toyohiraku, Sapporo, Hokkaido 062-8517, Japan, and Department of Materials Science and Engineering, Kitami Institute of Technology (KIT), 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan ReceiVed: June 16, 2010; ReVised Manuscript ReceiVed: August 5, 2010
Dissociation behavior of methane-ethane mixed gas hydrate coexisting structures I and II at constant temperatures less than 223 K was studied with use of powder X-ray diffraction and solid-state 13C NMR techniques. The diffraction patterns at temperatures less than 203 K showed both structures I and II simultaneously convert to Ih during the dissociation, but the diffraction pattern at temperatures greater than 208 K showed different dissociation behavior between structures I and II. Although the diffraction peaks from structure II decreased during measurement at constant temperatures greater than 208 K, those from structure I increased at the initial step of dissociation and then disappeared. This anomalous behavior of the methane-ethane mixed gas hydrate coexisting structures I and II was examined by using the 13C NMR technique. The 13C NMR spectra revealed that the anomalous behavior results from the formation of ethanerich structure I. The structure I hydrate formation was associated with the dissociation rate of the initial methane-ethane mixed gas hydrate. 1. Introduction Gas hydrates are crystalline clathrate compounds that can encage gas molecules in their polyhedral cages of hydrogenbonded water molecules. Three main types of crystal structure of gas hydrate are known, designated as structure I (sI), structure (sII), and structure (sH). The crystal structure, which depends on the guest molecule size,1 is stabilized by the guest-host interaction. Natural gas hydrate, which exists in deep marine/ lacustrine and permafrost environments, is expected to provide natural gas resources. On the other hand, it has been suggested that gas hydrate formation be used for the storage and transport of natural gas.2 In this system, the self-preservation of gas hydrate is a key phenomenon for storage and transportation under a metastable condition, for which it is assumed that the ice formed on the surface during hydrate dissociation restricts further dissociation of the hydrate.2 It is important to clarify the dissociation behavior of gas hydrate for understanding the guest molecular stability. The relation between dissociation behavior of gas hydrates to ice and the self-preservation effects has been investigated via visual observation3 and crystallographic observation with use of powder X-ray diffraction (PXRD).4–6 Methane and ethane are principal components of natural gas. Both methane hydrate and ethane hydrate exhibit sI structure, in which the unit cell consists of two 12-hedral cages and six 14-hedral cages of water molecules.1 On the other hand, in a methane-ethane mixed gas system, a mixed gas with appropriate gas compositions forms sII hydrate, in which the unit cell comprises 16 12-hedral cages and 8 16-hedral cages of water molecules.7–11 Furthermore, in this mixed gas system, the coexistence of sI and sII has been reported not only in laboratory * To whom correspondence should be addressed. E-mail: jiro.nagao@ aist.go.jp. † National Institute of Advanced Industrial Science and Technology. ‡ Kitami Institute of Technology.
conditions8–10 but also in natural conditions.12 The coexistence tends to be observed in the structural transition zone between sI and sII in this mixed gas system.8–10 Recently, an even more complex coexistence of sII and sH hydrates containing C1-C7 hydrocarbons was observed in the natural gas hydrate obtained from the Cascadia margin.13 In this case, previous reports show that the dissociation condition of the complex hydrates differed greatly from those of the simple sI or sII hydrates.13 That is, knowledge of dissociation behavior of gas hydrate containing multiple crystal structures is important to elucidate the gas hydrate’s stability. Gupta et al. reported details of the dissociation behavior of simple methane hydrate at the molecular level obtained using NMR examination.14 They confirmed a lack of preferential decomposition of an sI methane hydrate cage.14 In contrast, Dec et al. reported behavior of thermally activated decomposition of methane-ethane mixed gas hydrate with sI.15 In this case, previous reports show that the 14-hedral cage encaging ethane decomposes more rapidly than the 12-hedral cage encaging methane.15 This study examined the dissociation behavior of methaneethane mixed gas hydrate coexisting structures I and II at constant temperatures less than 223 K and atmospheric pressure. The dissociation behaviors on the crystal lattice level and the guest molecular level were investigated with use of PXRD and solid-state 13C NMR techniques. 2. Materials and Experimental Methods The hydrate sample was synthesized from mixed gas containing 58.9% methane and 41.1% ethane with pressures of 4.7 MPa at 277 K, using a high-pressure vessel with volume of about 1.5 × 10-4 m3. The gas composition was chosen for a sample coexisting as sI and sII based on the reported relation between hydrate structure and gas composition in this mixed gas system.8–10 The methane-ethane mixed gas hydrates were formed by contact reactions between 30 g of liquid water and the mixed gas, whereas the sample was stirred at 500 rpm.
10.1021/jp1055667 2010 American Chemical Society Published on Web 08/16/2010
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Figure 1. PXRD profile of the methane-ethane mixed gas hydrate at 146 K.
The PXRD profiles were acquired with an X-ray diffraction apparatus with Cu KR radiation (40 kV, 249 mA, Rint-2500; Rigaku Corp.). The powdered samples were introduced into an opened quartz glass capillary cell (2.0 mm diameter, 0.01 mm thickness, 20 mm length; Hilgenberg GmbH) for PXRD measurements at temperatures of around 123 K with liquid nitrogen. After PXRD measurements were conducted at 146 K without the hydrate dissociation, the time-resolved PXRD measurements were performed at constant temperatures of 198-223 K. The measurement temperature was controlled by flowing cooled dry nitrogen gas. The variation of temperature during the measurements was (1.5 K. The time-resolved PXRD profiles were collected with use of a step width of 0.06 deg with a counting time of 0.6 s/step. The 13C single-pulse NMR and Cross-Polarization and Magic Angle Spinning (CP-MAS) 13C NMR spectra were measured with an NMR spectrometer (100 MHz, JNM-AL400; JEOL) equipped with a probe for solid samples (SH40T6; JEOL). The powdered samples were introduced into a zirconia sample tube (6 mm diameter, 22 mm length; JEOL) at liquid nitrogen temperature. First, the 13C single-pulse NMR and CP-MAS 13C NMR spectra were conducted at 171 K without the hydrate dissociation. The measurement temperature was controlled by using cooled dry nitrogen gas. The time-resolved CP-MAS 13C NMR spectra were acquired at constant temperatures of 198, 213, and 218 K. The spinning rate of the sample tube was 3.6 kHz. The other experimental conditions were described in detail in a previous paper.11
Figure 2. Solid-state 13C NMR spectra of the methane-ethane mixed gas hydrate at 171 K: (a) single-pulse 13C NMR spectra and (b) CPMAS 13C NMR spectra.
3. Results and Discussion The PXRD profile of the methane-ethane mixed gas hydrate sample at 146 K is presented in Figure 1. The PXRD peaks are assigned based on those already reported.10 The PXRD pattern obtained showed that the sample contained sI and sII hydrate crystals, and hexagonal ice (Ih). The dissociated gas composition was 44.4% methane and 55.6% ethane. The single-pulse 13C NMR spectrum at 171 K is presented in Figure 2a. The sample yielded four 13C single-pulse NMR signals at 7.61, 6.11, -4.38, and -6.62 ppm, which are attributed to ethane molecules in 14-hedral cages of sI, ethane molecules in 16-hedral cages of sII, methane molecules in 12hedral cages of sI and sII, and methane molecules in 14-hedral cages of sI, compared with the 13C NMR spectra already reported.11 The CP-MAS 13C NMR spectrum is portrayed in Figure 2b. Signal enhancement with the CP technique provided an additional 13C NMR signal from methane molecules in 16hedral cages of sII at a chemical shift of -8.32 ppm. The 3D plots of time-resolved PXRD patterns collected during the hydrate dissociation at 198 and 213 K are depicted in Figure
Figure 3. 3D plot of the time-resolved PXRD pattern: (a) at 198 K and (b) at 213 K. Labeled numbers are Miller indices of sI hydrate (sI), sII hydrate (sII), and ice Ih (Ih).
3. At 198 K, the intensities of PXRD peaks from sI and sII hydrate crystals were decreased with elapsed time, whereas those from Ih were increased, indicating the formation of ice during the hydrate dissociation. Figure 3a shows that both sI and sII hydrate crystals were dissociated at a constant temperature of 198 K and were converted completely to Ih crystals after about 7 h. At 213 K, although the intensities of PXRD peaks from sII hydrate crystals were decreased, those from sI hydrate crystals were increased at about 1 h elapsed time; they then decreased (Figure 3b).
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Figure 4. Change in relative PXRD intensity ratios of I/Iinitial and Ifinal/I: (a) at 198 K, (b) at 213 K, and (c) at 218 K. Open squares and triangle symbols respectively show I/Iinitial ratios of the (321) plane of sI hydrate and the (440) plane of sII hydrate. The open diamond symbol is the Ifinal/I ratio of Ice Ih.
The detailed changes in the PXRD peak intensities during the hydrate dissociation at representative temperatures are portrayed in Figure 4, as manifested through the relative intensity ratios of the (321) plane of sI hydrate, (440) plane of sII hydrate, and (10-10) plane of Ih. The relative intensity ratio is defined as the ratio of (I/Iinitial) for the (321) plane of sI hydrate and (440) plane of sII hydrate and that of (Ifinal/I) for the (10-10) plane of Ih. Here, I, Iinitial, and Ifinal are the baseline-corrected peak intensity, the baseline-corrected peak intensity at initial profile, and the baseline-corrected peak intensity at final profile after complete hydrate dissociation, respectively. On the one hand, at 198 K, both the relative intensity ratios between those of the (321) plane of sI hydrate and the (440) plane of sII hydrate decreased linearly with time and reached zero after 6.68 h elapsed time, whereas that of the (10-10) plane of Ih increased and reached 1.0. The slope of the I/Iinitial ratios of the (321) plane of sI hydrate during the hydrate dissociation was almost identical with that of the (440) plane of sII hydrate. This fact indicates that both structures were dissociated simultaneously. The slope of the Ifinal/I ratio corresponding to the Ih formation was symmetrical about those of the I/Iinitial ratio corresponding to the hydrate dissociation, which suggested that the hydrate dissociation transforms the hydrate crystals with both structures directly into Ih. On the other hand, at 213 K, the changes in the I/Iinitial ratios with elapsed time differed greatly between the (321) plane of sI hydrate and the (440) plane of sII hydrate. As portrayed in Figure 4b, only the I/Iinitial ratio of the (440) plane of sII hydrate decreased with time and became zero after about 2 h elapsed time. The I/Iinitial ratio of the (321) plane of sI hydrate increased and reached about 1.6 at 1.04 h elapsed time; then it decreased toward zero, where the variation of the Ifinal/I ratio of the (10-10) plane of Ih was nonlinear. The small variation of the Ifinal/I ratio of Ih during the sI formation until the elapsed time of 1.6 h suggests that Ih attributable to the sII hydrate dissociation was consumed for the sI hydrate formation. The sI hydrate formation from Ih attributable to the sII hydrate dissociation was also observed at 218 K (Figure 4c) in the same manner as the case at 213 K. The highest I/Iinitial ratio of the (321) plane of sI hydrate and the slope of variation in the (440) plane of sII hydrate is shown for each temperature in Figure 5. The highest values were obtained by using a function approximation for the change in the ratio during the hydrate dissociation. Remarkable sI hydrate formation during the hydrate dissociation was observed at temperatures ranging from 208 to 223 K (represented by solid symbols). At this temperature range, the slopes of change in the (440) plane of sII hydrate tended to be smaller than those
Figure 5. Changes in the highest relative PXRD intensity ratio of the (321) plane of sI hydrate and the slope of change in the (440) plane of sII hydrate as a function of temperature. Square and diamond symbols are the highest relative PXRD intensity ratios of the (321) plane of sI hydrate and the slope of change in the (440) plane of sII hydrate, respectively. The sI formation is presented as solid symbols.
at temperatures less than 203 K, suggesting that the sI hydrate formation tended to occur when the dissociation rate of the sII hydrate is higher. The maximum I/Iinitial ratio of the (321) plane of sI hydrate was obtained at 213 K; it is expected to depend on the balance between the dissociation rate of initial sII hydrate and the formation rate of sI hydrate. The time-resolved CP-MAS 13C NMR spectra at 198, 213, and 218 K are depicted in Figure 6. The following discussion specifically addresses the three high-intensity NMR signals from ethane molecules in 14-hedral cages and 16-hedral cages, and methane molecules in 12-hedral cages. At 198 K, the intensities of all three NMR signals decreased at the same rate with elapsed time, indicating that all cages are decomposed simultaneously. Although Dec et al. reported that the sI 14-hedral cage encaging ethane decomposes more rapidly than the sI 12-hedral cage encaging methane of methane-ethane mixed gas hydrate with simple sI,15 no remarkable preferential decomposition was found under the experimental conditions used for this study. The difference between our observation and theirs might result from the differences in dissociation rate or time resolution between them. As another possibility, the delay of decomposition of the sI 12-hedral cage can be masked by coexisting sI and sII. On the other hand, at 213 and 218 K, the relative intensity ratios of the three NMR signals varied dramatically over time. Relative integrated intensity ratios, A/A0, are shown against the time
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Figure 6. Time-resolved CP-MAS 13C NMR spectra: (a) at 198 K, (b) at 213 K, and (c) at 218 K.
Figure 7. Change in the relative intensity ratio of NMR signal from ethane in 14-hedral cages (sI), ethane in 16-hedral cages (sII), and methane in 12-hedral cages (sI, sII): (a) at 198 K, (b) at 213 K, and (c) at 218 K. Open circles, triangles, and square symbols respectively denote A/A0 ratios of ethane in 14-hedral cages of sI, ethane in 16-hedral cages of sII, and methane in 12-hedral cages of sI and sII.
elapsed at each temperature in Figure 7. Here A and A0 are the integrated intensity of the 13C NMR signal and that in the initial spectrum at each temperature, respectively. At 198 K, the A/A0 ratios of all three signals decreased linearly with almost identical
slope, which demonstrates that the methane and ethane molecules were released from each cage simultaneously during the dissociation. The slopes of the NMR data obtained at 198 K were -0.17 for all cages, which shows good agreement with
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Figure 8. Phase diagrams of sII hydrate containing 60% methane and 40% ethane in the hydrate phase and sI hydrate containing 37% methane and 63% ethane in the hydrate phase. Open square and diamond symbols are respectively the experimental phase equilibrium data on methane hydrate17 and ethane hydrate.18
the slopes of the PXRD data (slope: -0.15 for sI, -0.14 for sII) collected at 198 K. This fact indicates that the release behaviors of methane and ethane guest molecules correspond to the decomposition of the two hydrate structures. At 213 K, the A/A0 ratios of the NMR signals attributable to methane in 12-hedral cages in common between sI and sII and ethane in 16-hedral cages of sII decreased with almost identical slope with elapsed time. The A/A0 ratios of signal from ethane in 14-hedral cages of sI decreased only slightly with time at the initial step, then started to decrease with about 0.5 h elapsed time, indicating that the release of ethane molecules from 14-hedral cages of sI is delayed during the dissociation. This anomalous behavior is expected to correspond to the sI hydrate reformation observed with PXRD measurements. In a similar fashion, at 218 K, only the decrease in the A/A0 ratio of ethane in 14-hedral cages was delayed. The ethane fractions in the sI and sII hydrates were estimated respectively as 63% and 40% from the 13C NMR data based on the reported relation between NMR signal intensity ratio and ethane fraction in the hydrate phase in the methane-ethane mixed system.11 The three-phase equilibrium of ice-hydratevapor (I-H-V) for sI hydrate containing 37% methane and 63% ethane in the hydrate phase and for sII hydrate containing 60% methane and 40% ethane in the hydrate phase was calculated with CSMHYD,16 a phase-equilibrium calculation program, as presented in Figure 8 together with those that were calculated16 and measured experimentally17,18 for pure methane hydrate and ethane hydrate. The three-phase equilibrium curves of I-H-V for the sI and sII hydrates suggested that the sI hydrate containing 37% methane and 63% ethane can be stable at milder pressure-temperature condition than the sII hydrate containing 60% methane and 40% ethane. As shown in Figure 8, at temperatures greater than around 215 K, the equilibrium pressures of the initial sI and sII hydrates are above 0.1 MPa, indicating that the initial sI and sII hydrates are unstable under atmospheric pressure. Namely, the sI hydrate formation during the hydrate dissociation should be due to sI hydrate formed from different guest gas composition with the initial sI hydrate. As the obtained NMR data show, the release behaviors of methane in 12-hedral cages which are common to sI and sII show good
Kida et al. agreement with that of ethane in 16-hedral cages of sII, suggesting that both structures of the initial hydrate were decomposed simultaneously by assuming that the dissociation rate of the cage is equivalent for cages of all types in the dissociation condition studied. That is, the delay of decrease in NMR signal from ethane in 14-hedral cages of sI results from the formation of ethane-rich sI hydrate. The dashed line in Figure 8 depicts the maximal value of partial pressure of ethane in the gas released from the initial sI and sII hydrates in the experimental system under atmospheric pressure. The colored area bounded by the dashed line and the I-H-V equilibrium curve of ethane hydrate shows the possible condition for the ethane-rich sI hydrate formation attributable to dissociation of the initial methane-ethane mixed gas hydrate. In this study, the PXRD data suggest that the formation of ethane-rich sI hydrate depends on the dissociation rate of the initial methaneethane mixed gas hydrate. When the dissociation rate of the initial hydrate is lower than its diffusion rate to the air, the sI hydrate would not occur. The PXRD measurements showed that the ethane-rich sI hydrate formation did not occur at temperatures below 203 K. This is attributable to the low ethane partial pressure caused by the slow dissociation rate of the initial hydrate. On the other hand, the shift from sI hydrate reformation to dissociation at temperatures greater than 208 K engenders the decrease in ethane fraction in the vapor phase because of the ethane-rich sI hydrate formation. 4. Conclusion The dissociation behavior of the methane-ethane mixed gas hydrate (containing 44.4% methane and 55.6% ethane in the hydrate crystals) coexisting structures I and II at constant temperature and atmospheric pressure was observed using timeresolved PXRD and 13C NMR measurements. At 198 and 203 K, both sI and sII decomposed with no preference. All cages released methane and ethane molecules simultaneously. At temperatures higher than 208 K, dissociation of the initial hydrate caused formation of ethane-rich sI hydrate, where the methane 12-hedral cages of sI and sII and the ethane 16-hedral cage were decomposed almost simultaneously. Acknowledgment. We thank Drs. T. Ebinuma, H. Oyama, H. Ohno (AIST), H. Sakagami (KIT), and T. Uchida (Hokkaido University) for their fruitful discussion. References and Notes (1) Sloan, E. D., Jr. Nature 2003, 426, 353. (2) Gudmundson, J.; Borrehaug, A. In Proceedings of the 2nd International Conference on Natural Gas Hydrates, Toulouse, France, 1996; Monfort, J. P., Ed.; p 415. (3) Shimada, W.; Takeya, S.; Kamata, Y.; Uchida, T.; Nagao, J.; Ebinuma, T.; Narita, H. J. Phys. Chem. B 2005, 109, 5802. (4) Takeya, S.; Shimada, W.; Kamata, Y.; Ebinuma, T.; Uchida, T.; Nagao, J.; Narita, H. J. Phys. Chem. A 2001, 105, 9756. (5) Takeya, S.; Ebinuma, T.; Uchida, T.; Nagao, J.; Narita, H. J. Cryst. Growth 2002, 237-239, 379. (6) Takeya, S.; Ripmeester, J. A. Angew. Chem., Int. Ed. 2008, 47, 1276. (7) Subramanian, S.; Kini, R. A.; Dec, S. F.; Sloan, E. D., Jr. Chem. Eng. Sci. 2000, 55, 1981. (8) Subramanian, S.; Ballard, A. L.; Kini, R. A.; Dec, S. F.; Sloan, E. D., Jr. Chem. Eng. Sci. 2000, 55, 5763. (9) Uchida, T.; Takeya, S.; Kamata, Y.; Ikeda, Y. I.; Nagao, J.; Ebinuma, T.; Narita, H.; Zatsepina, O.; Buffett, B. A. J. Phys. Chem. B 2002, 106, 12426. (10) Takeya, S.; Kamata, Y.; Uchida, T.; Nagao, J.; Ebinuma, T.; Narita, H.; Hori, A.; Hondoh, T. Can. J. Phys. 2003, 81, 479. (11) Kida, M.; Sakagami, H.; Takahashi, N.; Hachikubo, A.; Shoji, H.; Kamata, Y.; Ebinuma, T.; Narita, H.; Takeya, S. J. Jpn. Pet. Inst. 2007, 50, 132.
Dissociation of Methane-Ethane Mixed Gas Hydrates (12) Kida, M.; Khlystov, O.; Zemskaya, T.; Takahashi, N.; Minami, H.; Sakagami, H.; Krylov, A.; Hachikubo, A.; Yamashita, S.; Shoji, H.; Poort, J.; Naudts, L. Geophys. Res. Lett. 2006, 33, L24603. (13) Lu, H.; Seo, Y.-T.; Lee, J.-W.; Moudrakovski, I.; Ripmeester, J. A.; Chapman, N. R.; Coffin, R. B.; Gardner, G.; Pohlman, J. Nature 2007, 445 (7125), 303. (14) Gupta, A.; Dec, S. F.; Koh, C. A.; Sloan, E. D., Jr. J. Phys. Chem. C 2007, 111, 2341. (15) Dec, S. F.; Bowler, K. E.; Stadterman, L. L.; Koh, C. A.; Sloan, E. D., Jr. J. Phys. Chem. A 2007, 111, 4297.
J. Phys. Chem. A, Vol. 114, No. 35, 2010 9461 (16) Sloan, E. D., Jr. CSMHYD, a phase-equilibrium calculation program package accompanying the following book: Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998. (17) Makogon, T. Y.; Sloan, E. D., Jr. J. Chem. Eng. Data 1994, 41, 315. (18) Falabella, B. J.; Vanpee, M. Ind. Eng. Chem. Fundam. 1974, 13, 228.
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