Spectroscopic Measurements on Binary, Ternary, and Quaternary

Jun 13, 2007 - Isostructural and cage-specific replacement occurring in sII hydrate with external CO2/N2 gas and its implications for natural gas prod...
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Ind. Eng. Chem. Res. 2007, 46, 5080-5087

Spectroscopic Measurements on Binary, Ternary, and Quaternary Mixed-Gas Molecules in Clathrate Structures Tsutomu Uchida,*,†,‡ Satoshi Takeya,‡ Yasushi Kamata,‡,§ Ryo Ohmura,‡,| and Hideo Narita‡ DiVision of Applied Physics, Graduate School of Engineering, Hokkaido UniVersity, N13 W8 Kita-ku, Sapporo 060-8628, Japan, and National Institute of AdVanced Industrial Science and Technology (AIST), 2-18 Tsukisamu-Higashi, Toyohira-ku, Sapporo 062-8517, Japan

To reveal the preferential enclathration of various guest molecules of natural gas into the hydrate structure, systematically prepared hydrate samples from mixed gases such as ethane-propane (C2H6-C3H8), methaneethane-propane (CH4-C2H6-C3H8), and methane-ethane-propane-iso-butane (CH4-C2H6-C3H8-i-C4H10) with powdered ice were analyzed by spectroscopic techniques. Powder X-ray diffraction analysis at approximately 150 K was used to determine the crystal structure of the sample. Microscopic Raman spectroscopy measured at approximately 120 K revealed the enclathration of guest molecules into various cages. Gas chromatographic analysis on feed gases and the gases retrieved from hydrate samples supported the mass balance estimations derived from the spectroscopic measurements. These results, together with previous studies, revealed the preferential cage-occupation rule of hydrocarbon molecules. The collection of the Raman spectra of the guest molecules in these hydrate samples and in various phases of pure systems are useful for natural sample analysis by this technique. Introduction Gas hydrates are nonstoichiometric inclusion compounds with a host framework composed of water molecules that are stabilized through the inclusion of gas molecules (guest molecules) within cagelike cavities. Phase equilibrium studies have been conducted on various single- and mixed-gas hydrates. However, comparatively fewer studies have been conducted on the physicochemical properties of gas hydrates. The crystallographic structures of single-guest hydrates are determined by the relation between the guest-molecule size and the volume of the vacant space in the cages (e.g., the work of Sloan1). However, mixed-gas hydrates are more complicated. For example, methane (CH4) and ethane (C2H6) are both structure I (sI) hydrate formers, but when they are mixed in a suitable composition, CH4-C2H6 mixed-gas and H2O form the structure II (sII) hydrate.2 Little information about guest molecule distributions in the hydrate structure has been available. It is also noted that the synthesis of homogeneous mixedgas hydrates is very difficult because the preferential cage occupancy of the guest molecules causes the fractionation of the vapor phase, which results in the formation of mixed-gas hydrates with different compositions. Uchida et al.3 clearly showed the inhomogeneous distribution of sI and sII hydrates from the methane-propane-water (CH4-C3H8-H2O) system. They also indicated that the combination of some physicochemical property measurements such as those of the crystallographic structure, gas composition, and gas content are useful in understanding the complicated fractionation process in the mixed-gas hydrate formation. Such physicochemical property measurements are promising methods that provide the basic knowledge required for the * To whom correspondence should be addressed. Tel.: +81-11-7066635. Fax: +81-11-706-6635. E-mail: [email protected]. † Hokkaido University. ‡ National Institute of Advanced Industrial Science and Technology. § Present address: Railway Technology Research Institute. | Present address: Department of Mechanical Engineering, Faculty of Science and Engineering, Keio University.

evaluation of the quantity of hydrates or to understand the role of encaged gases as the natural-gas resources, as functional materials, as a sink or source of the global carbon cycle, etc. The crystal structures of gas hydrates are determined by diffraction techniques such as X-ray and neutron diffraction. The gas compositions, contents, and the volume of the guest gas contained in a unit mass of sample are measured by dissociating a portion of the sample. Both processes are generally carried out in the laboratory. Nuclear magnetic resonance (NMR) measurements have been applied to measure the cage occupancy ratio of the guest molecules in the hydrate directly and quantitatively.4 In recent times, Raman spectroscopy has been developed as a useful tool for physicochemical property measurements. Sum et al.5 and Uchida et al.6 reported that the Raman spectra of CH4 hydrates provide the cage occupancy ratio of CH4 in small and large cages in sI hydrates. Later, Subramanian et al.2,7 and Uchida et al.8 indicated that Raman spectroscopic measurements can characterize both the cage occupancy ratio and the difference between the crystallographic structures of CH4-C2H6 mixed-gas hydrates. On the basis of these researches, Raman spectroscopy has been used for in situ and nondestructive methods for the hydrate characterizations. For example, Uchida et al.9 found that the encaged C3H8 signals were observed in natural-gas hydrates obtained from the Mallik 5L-38 research well even though C3H8 could not be detected by gas chromatographic measurements on the retrieved gas from the same sample. These results indicated the local concentration of C3H8 molecules in the hydrate structure and suggested naturally occurring fractionation during the hydrate formation in natural conditions. Recently, Brewer et al.10-12 have developed an in situ Raman spectroscopic system for the remotely operated vehicle (ROV). Wilson et al.13 compared the Raman spectra with those measured via NMR for the same sample; they indicated that the quantitative measurements of the mixed-gas hydrates obtained from the Raman spectra have some uncertainties due to guest-guest interactions. Uchida et al.8 also indicated that Raman spectroscopy cannot be used for the identification of crystal structures without any other crystallographic evidence.

10.1021/ie070153w CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 5081 Table 1. Gas Compositions of the Various Phases and Gas Concentration of Hydrates in the C2H6-C3H8-H2O System run no.

D1

D2

D3

initial C2H6 concentration in vapor [%] initial pressure [MPa] final C2H6 concentration in vapor [%] final pressure [MPa] gas content [mL g-sample-1] C2H6 concentration in hydrate [%]

28 0.28

47 0.14

0.22 41 2

0.11 NA 15

73 0.3 75 0.28 29 18

The quantitative comparisons are required on the laboratorysynthesized samples in order to avoid wrong evaluations. In our previous papers, we have measured the physicochemical properties on various mixed-gas hydrates such as CH4-C2H6,8 CH4-C3H8,3 CH4-carbon dioxide (CO2),14 and CH4-CO2neohexane15 using Raman spectroscopy, X-ray diffraction, and gas chromatography. In this study, we provide the systematic measurements of multicomponent gas systems such as C2H6-C3H8, CH4-C2H6C3H8, and CH4-C2H6-C3H8-iso-butane (i-C4H10) using Raman spectroscopy, X-ray diffraction, and gas chromatography. These mixed-gas systems had wide gas-composition ranges compared with those observed in natural conditions since we aimed not only to simulate the natural hydrate occurrence but also to obtain the systematic understanding of mixed-gas hydrate formation. Experimental Section We prepared the hydrate samples by an ice-gas interaction procedure. The details of the experimental setup and procedures are explained elsewhere;8 therefore, we explain the procedure in brief here. In our 258 K cold room, approximately 30 g of ice particles, each with a diameter of 1-2 mm, were loaded into a pressure vessel with an approximate volume of 1 × 10-3 m3. The mixed gases with known compositions were then initially pressurized in the vessel at the following experimental conditions: 0.14-0.30 MPa for binary, 1.35 MPa for ternary, and 1.90 MPa for quaternary mixed gases. These conditions were selected to prevent the condensation of the feed gas; further, they were sufficient to prevent the large concentration change during the hydrate formation and sufficiently low to prevent the formation of simple guest hydrates. Pure ice that was formed from deionized and distilled water (resistivity of 18.2 MΩ cm) and systematically prepared high-quality mixed gases (Hokkaido Air Water Co., Ltd.) were used in our experiments. To increase the extent of conversion of ice to hydrate, the vessel with a stirring rod was rotated at approximately 50 rpm. The sample was crushed into a fine powder (size less than 100 µm) by the crushing process. The hydrate sample was cooled following the completion of the reaction prior to depressurization and was stored in a bottle at liquid-nitrogen temperature. The gas composition of the initial feed gas and the final one after the complete reaction were measured by on-line gas chromatography (GC; Shimadzu model GC-14B). The gas composition in the hydrate was measured on the retrieved gas with part dissociation of the sample by an off-line GC (AREA model MC-200). The uncertainty in the GC measurement was estimated to be within 5% of each measured composition value. The gas content of the sample was simultaneously measured using a wet gas meter (Shinagawa model W-NKDa-1A) at room temperature. The crystal structures were determined using a Cu KR X-ray diffractometer (40 kV, 250 mA; Rigaku, model RINT-2000). The hydrate samples were loaded into a quartz-glass capillary cell (Hilgenberg; diameter 2.0 mm, thickness 0.01 mm) that

Figure 1. X-ray diffraction profiles obtained from D1, D2, and D3 hydrate samples. The crystallographic structures of the hydrates were found to be structure II. Asterisks indicate the diffraction peaks of ice.

was installed at the top of the goniometer. These measurements were taken at 113.0 ( 1.0 K by blowing cold, dry nitrogen gas at atmospheric pressure around the sample. Another piece of the specimen was placed under a 40× objective lens for the microscopic Raman spectroscopy. The scattered radiation was collected through a slit with 180° geometry at 200 µm. The diameter of the incident laser beam focused on the specimen was maintained at approximately 5 µm. Therefore, we collected the Raman spectra at more than five positions for each sample and averaged them. The temperature of the specimen was maintained at approximately 150 K during the measurements by controlling the flow rate of the gas that vaporized from the liquid nitrogen. The spectrum of the neon-emission was used for the correction of the wavenumber measurement, which allowed an accuracy of approximately (1 cm-1. Each Raman spectrum was then analyzed by deconvolution following which the peaks were fitted to Voigt curves to estimate their intensities (area of the fitting curve) and positions (peak wavenumber). Results and Discussions (1) C2H6-C3H8 Mixed-Gas Hydrates. We analyzed three binary mixed-gas hydrate samples with different compositions. The details of the vapor compositions and gas chromatographic analysis data are shown in Table 1. The initial C2H6 concentration ranged from 28 to 73%. As shown in Table 1, the final pressure was less than the initial, which indicated that a certain amount of gas hydrate was formed. Since the volume ratio of ice to vapor was small, both the pressure drop and the composition change of the vapor phase were negligible. The formation of the gas hydrate was also confirmed by the X-ray diffraction measurements. Figure 1 shows the powder diffraction patterns of the obtained samples. These figures show that all the samples included sII hydrate crystals and ice crystals. Comparing the peak intensities of each crystal and the gas chromatographic measurements, we estimated that the extents of conversion of ice to mixed-gas hydrates were low. It would

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Figure 2. C-H stretching mode of Raman spectra on a D3 sample. Both of the peaks from C2H6 and C3H8 molecules are observed. Each point indicates the raw spectrum data, and the thick solid line is the average. Dashed lines denote the deconvoluted peaks by fitting the Voigt model.

be mainly because the driving force of hydrate formation was low at the experimental conditions in the present study. The Raman spectroscopic observations were performed on both the C-C stretching mode and the C-H stretching mode. Figure 2 shows the C-H stretching mode of the sample. Several peaks are observed of which the 2003 and 2940 cm-1 peaks are assigned as the C2H6 molecules encaged in the sII hydrate.8 On the other hand, the Raman peaks for the C3H8 molecules are observed at approximately 2868, 2878, 2899, and 2916 cm-1 in these systems. These peaks are very similar to those observed in pure C3H8 hydrate.3 Figure 3 shows the typical C-C stretching mode of (a) C2H6 molecules and (b) C3H8 molecules. A smaller signal/noise ratio would be caused by the small amount of guest molecules in the sample. Figure 3a suggests that the C2H6 molecules observed in the sample are encaged in 51264 cages. This result is consistent with those obtained by X-ray diffraction where the formed crystals were all sII hydrates. On the other hand, Figure 3b shows the C3H8 molecules in the sample; two peaks are observed for itsone at 870 cm-1 and the other at 876 cm-1. On comparison with the previous work,3 these two peaks are assigned to the C3H8 molecules in the liquid phase and to the 51264 cages of the sII hydrate, respectively. The formation of liquid propane could be attributed to the condensation of the remaining gas during the cooling of the high-pressure vessel prior to the sample recovery. Figure 3a and b indicated that both C2H6 and C3H8 molecules were encaged into 51264 cages. The relative proportions of these two peak intensities changed during the enclathration of the guest molecules into 51264 cages; however, the GC measurements suggested that C3H8 molecules are remarkably enriched in hydrates. It is reasonable to consider the higher stability of the pure C3H8 hydrate compared to that of the pure C2H6 hydrate. In previous studies, C2H6 or C3H8 molecules were considered more preferable to be encaged into 51264 cages than CH4 molecules. Thus, the experimental results suggest that the preferential enclathration in the 51264 cage is CH4 < C2H6 < C3H8. This order may also determine the preferential crystal structure formed under conditions such as those in the present study. (2) CH4 + C2H6 + C3H8 Mixed-Gas Hydrates. We then analyzed three samples of CH4-C2H6-C3H8 ternary mixedgas hydrates formed from different compositions. The details

Figure 3. C-C stretching mode of the Raman spectra of (a) the C2H6 molecule and (b) the C3H8 molecule in the D3 sample. Each point indicates the raw spectrum data, and the thick solid line is the average. Dashed lines denote the deconvoluted peaks by fitting the Voigt model. Table 2. Gas Compositions of the Various Phases and Gas Concentration of Hydrates in the CH4-C2H6-C3H8-H2O System run no.

T1

T2

T3

initial composition in vapor [%] initial pressure [MPa] final composition in vapor [%] final pressure [MPa] gas content [mL g-sample-1] composition in hydrate [%]

90/5/5 1.75 95/4/1 1.4 120.2 69/8/23

92/4/4 1.75 96/3/1 1.35 115.9 65/8/27

98/1/1 1.75 99/1/0 1.45 87.9 84/6/10

of the vapor compositions and GC analysis data are shown in Table 2. The initial CH4 concentration ranged from 90 to 98%. As shown in Table 2, all the final vapor compositions in these systems were CH4-enriched, and both the C2H6 and C3H8 concentrations decreased. Thus, these higher hydrocarbons were enriched in the hydrate phases. In comparison with these gases, we found that the C3H8 molecule was the most enriched in the hydrate phase. This result is consistent with the results obtained in the C2H6-C3H8 mixed-gas experiments mentioned in the previous section and agrees qualitatively with thermodynamic calculations.1 The gas-content data in Table 2 shows that a certain amount of hydrates was formed in these formation conditions. Since the experimental P-T conditions were carefully selected such that neither pure CH4 hydrate (sI) nor CH4-C2H6 hydrate (sI) was formed, we expected that all the hydrate samples included every component gas in the hydrate phase, although their compositions would change slightly due to the fractionation and the resulting change in vapor composition with the progress of the hydrate formation. The X-ray diffraction patterns of the powdered samples are shown in Figure 4. These patterns indicate that all three samples

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Figure 4. X-ray diffraction profiles obtained from T1, T2, and T3 hydrate samples. The crystallographic structures of the hydrates were found to be structure II. Asterisks indicate the diffraction peaks of ice.

consisted of sII hydrate with a unit cell of size 17.22 ( 0.19 and ice crystals. This result is interesting since only CH4 and C2H6 were detected in the final vapor phase for the T3 sample, which might form the sI hydrate. The sI hydrate formation in this system is not observed mainly because the pressure was less than the equilibrium pressure of the sI hydrate (neither pure CH4 hydrate nor CH4-C2H6 mixed-gas hydrate), as expected prior to the experiments. Raman spectra were observed on both the C-C stretching mode and C-H stretching mode. Figure 5 shows the C-H stretching mode of samples (a) T1 and (b) T3. The largest peaks at (a) 2912 ( 3.5 cm-1 and (b) 2913 ( 4.4 cm-1 arise from the CH4 molecules encaged in 512 cages; the second largest peak in Figure 5b at 2902 ( 2.4 cm-1 corresponds to the CH4 molecules in the 51264 cage.3,8 Here, each uncertainty indicates the variation of measurements from the mean value. The remaining small peaks are related to the C-H stretching mode of both C2H6 and C3H8 molecules. All these peaks are co-incident with those observed in Figure 2. Therefore, it is considered that both C2H6 and C3H8 molecules were encaged in 51264 cages in sII hydrates. Since the double peaks of the CH4 molecules were observed only in sample T3, we considered that the CH4 molecules could not be included in the 51264 cages in samples T1 and T2. This is verified when we compare the vapor compositions of these hydrate samples. If the C2H6 and C3H8 molecules together occupied less than 33% of the cages, some vacant 51264 cages that can include CH4 molecules are expected to exist in the sII structure which is constructed by 33% of 51264 cages and 67% of 512 cages. The enclathration process of the guest molecules is therefore explained as follows: C2H6 and C3H8 molecules preferentially occupy 51264 cages with some competition (although the C3H8 molecule is considered to be more feasible), and the CH4 molecules would fit into vacant cavities, which would be mainly 512 cages. The result that both C2H6 and C3H8 molecules are encaged in 51264 cages in sII hydrate is also confirmed by the Raman spectra of the C-C stretching mode of C2H6 and C3H8 molecules shown in Figure 6a and b, respectively. The relative peak intensity ratios of the C-C stretching mode of the C2H6 and C3H8 molecules (IC2/IC3) for the three ternary mixed-gas

Figure 5. C-H stretching mode of Raman spectra in (a) the T1 sample and (b) the T3 sample. Each point indicates the raw spectrum data, and the thick solid line is the average. Dashed lines denote the deconvoluted peaks by fitting the Voigt model. The largest peaks are the spectra of CH4 in hydrate samples, which indicate that most of the CH4 molecules were included in 512 cages only in the T1 sample, whereas they were included in both 512 and 51262 cages in the T3 sample.

hydrates would be proportional to a composition ratio of CC2/ CC3 in the hydrate phase. Since the scatter of each peak intensity ratio was large due to the small number of measurements, it is difficult to predict the composition ratio between C2H6 and C3H8 in the hydrate phase quantitatively from the peak intensity ratio IC2/IC3, although the peak intensity ratio was found to be increased qualitatively with the increase of the composition ratio CC2/CC3 in the hydrate phase. The quantitative estimation is left for future studies to obtain a larger amount of experimental data. We attempted to estimate the cage occupancy of each guest molecule from the intensity ratio of the Raman spectra. We observed the double peaks in the C-H stretching mode of the CH4 molecules in the hydrate phase, including a relatively large amount of CH4 (approximately larger than 67%). And the X-ray diffraction measurements revealed that all the samples were sII clathrate structures. So, it was suggested that CH4 molecules in these samples were encaged in both 512 and 51264 cages. If we estimate the occupancy ratio of CH4 molecules in 512 and 51264 cages, we can understand the distribution of all the guest molecules in the hydrate cages. However, as pointed out in a previous study,3 it is very difficult to estimate the contribution of CH4 molecules to the intensity of the Raman peak at 2901 cm-1 because the C3H8 molecule spectrum is superimposed on it. To estimate the CH4 contribution to this peak, as the first approximation, we subtracted the peak intensities of the C-H stretching mode of the C3H8 molecules obtained in the pure C3H8 hydrate (shown in Figure 4 in the work of Uchida et al.3) by fitting the peak intensities of the largest double peak in this range at 2868 and 2877 cm-1. We then estimated the modified

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Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 Table 3. Cage Occupancy Estimations of CH4-C2H6-C3H8 Mixed-Gas Hydratesa run no.

T1

T2

T3

CH4 in 512 cage θC1 S [%] CH4 in 51264 cage θC1 L [%] C2H6 in 51264 cage θC2 L [%] C3H8 in 51264 cage θC3 L [%] hydration number n calculated hydration numberb molar ratio of hydrate in sample X [%]

100 1.9 23 66 5.8 (0.5) 10.5 65

92 1.3 21 78 6.0 (0.5) 7.8 64

100 17 17 26 6.6 (0.5) 9.0 50

a The numbers in brackets are estimated uncertainties of n. b By using the commercially provided software, CMSHYD1.

On the basis of the condition θC1 S e 1, we estimated the cage occupancies of guest molecules, as shown in Table 3. Here, we assumed that θC1 S ) 1 when R exceeded 1, whereas R ) 1 was estimated less than 1. when θC1 S Using the cage occupancies, we can estimate the hydration number of the crystal, n, as follows: C1 C2 C3 n ) 136/(16θC1 S + 8(θL + θL + θL ))

(4)

Since we measured the gas content in this sample, we can also estimate the molar ratio of the hydrate to the total amount of sample (conversion ratio from ice to hydrate), X, as considering the molar ratio of water between in the hydrate and initial value, which can be obtained by the following equation:

X ) wn(V0V-1 - M)-1 Figure 6. C-C stretching mode of Raman spectra of (a) the C2H6 molecule and (b) the C3H8 molecule in the T2 sample. Each point indicates the raw spectrum data. The thick solid line is the average, which coincides with the deconvoluted peaks by fitting the Voigt model.

peak intensity ratio [IL/IS]C1 between the resulting double CH4 peaks and denoted it as parameter A, where Raman peak intensities at 2901 and 2912 cm-1 are denoted as IL and IS, respectively. The parameter A is related to the cage occupancy ratio with a factor of 2 since the number of 512 cages is double of that of 51264 cages in sII structure (16:8 in an unit cell): C1 A ) [IL/IS]C1 ) θC1 L /2θS

(1)

where θkj is the cage occupancy of j cage (either j ) 512 (S) or 51264 (L)) with molecule k. On the other hand, we obtained the composition ratio of CH4 on C2H6 + C3H8 in the hydrate CC1/ (CC2 + CC3), where Ck is the concentration of molecule k obtained by GC measurements. In this system, C2H6 and C3H8 molecules are found to occupy only 51264 cages, whereas CH4 molecules may be included in both 512 and 51264 cages, if available. Then, the 512 cage is assumed to be occupied only by CH4. Here, we related the composition ratio of CH4 to other hydrocarbons, denoted by parameter B, to the cage occupancy ratio as follows: C1 C1 B ) CC1/(CC2 + CC3) ) (2θC1 S + θL )/(R - θL )

(2)

If we introduce the parameter R as the total occupancy of C2 C3 the 51264 cage by guest molecules, here R ) θC1 L + θL + θL e 1, we obtained the following equation from the above eqs 1 and 2:

θC1 S ) 2RB(4 + A + AB)

(3)

(5)

where w ) 18.0 g mol-1 represents the molar mass of H2O, V0 ) 22.4 × 103 cm3 mol-1 is the volume of ideal gas at standard temperature and pressure conditions, V [cm3 g-sample-1] is the gas content in the sample, and M [g mol-1] is the average mass of the mixed gas in the hydrate. The estimated n and X are also shown in Table 3. These results indicated that the cage occupation was determined by the competition between the three guest molecules. The preferential enclathration in the 51264 cage is CH4 < C2H6 < C3H8, which coincides with the prediction in the previous section. Therefore, we verified their order in the ternary mixedgas system experimentally. Furthermore, if a certain number of larger hydrocarbon molecules exist, nearly all the 51264 cages tend to be completely occupied. However, when the concentration of CH4 is high, a certain number of large (51264) cages remain vacant. Table 3 also suggests that n tends to decrease and X tends to increase with a decrease in the CH4 concentration in the initial vapor. On comparing these varieties, it is clear that the difference in the gas contents of the sample mainly depended on the variation of the hydrate concentration. This variation might have resulted from the driving force of the crystal growth of the gas hydrates, which is proportional to the difference between the experimental and equilibrium pressure, P - Pe. Since Pe is higher when the initial vapor contains a larger amount of CH4, the driving force decreases in the mixed gas with high CH4 concentration. In such a case, the reaction period would be small because the driving force instantly reduces due to the experimental pressure drop and CH4-concentration rise related to the hydrate formation. Wilson et al.13 suggested that the quantitative estimations of the cage occupancies from the Raman spectra had some uncertainties because the guest-guest interaction enclathrating in neighbor cages affects the Raman scattering cross sections. This would also be the case in this study; therefore, we

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 5085 Table 5. Comparison of the Raman Peak Positions of the i-C4H10 Molecule in Various Phasesa phase Raman peak position (FWHM) [cm-1]

measured temperature [K] measured pressure [MPa]

gas

liquid

hydrate

798 (3) 2871 (8) 2890 (15) 2909 (8) 2935 (12) 2960 (21) 272 0.4

810 (3) 2872 (7) 2890 (18) 2910 (5) 2938 (5) 2969 (13) 200 0.1

751 (8) 775 (13) 797 (4) 2881 (4) 2895 (7) 2918 (3) 2945 (16) 2968 (5) 297 0.3

a The number in parentheses is the full width at half-maximum (FWHM) for each raman peak.

Figure 7. X-ray diffraction profile obtained from the Q1 hydrate sample. The crystallographic structure of the hydrate was found to be structure II. Asterisks indicate the diffraction peaks of ice.

Table 4. Gas Compositions of the Various Phases and Gas Concentration of Hydrates in the CH4-C2H6-C3H8-i-C4H10-H2O Systema guests

initial conc in vapor [%]

final conc in vapor [%]

conc in hydrates [%]

CH4 C2H6 C3H8 i-C4H10

88 7 2 3

90 10 0.1 0.2

94 (1.1) 1.1 (0.2) 1.4 (0.3) 3 (0.6)

run no.

initial pressure [MPa]

final pressure [MPa]

gas content [mL g-hydrate-1]

Q1

1.9

1.6

141.4

a

The number in parentheses is the uncertainty of each gas composition.

combined the composition data measured by gas chromatography to estimate the cage occupancies. If one wishes to estimate the cage occupancies quantitatively from the Raman spectra alone, further experimental and theoretical corrections would be required. (3) CH4 + C2H6 + C3H8 + i-C4H10 Mixed-Gas Hydrates. To investigate the hydrate formation from natural gases, we carried out an experiment similar to that described in the previous sections using a CH4 + C2H6 + C4H8 + i-C4H10 quaternary mixed gas as a simulated natural gas. The details of the vapor compositions and GC analysis data are shown in Table 4. As shown in this table, the larger molecules such as C3H8 and i-C4H10 are preferentially encaged, as expected from previous experiments. However, it is interesting that the smallest molecule CH4 is also enriched in the hydrate phase, whereas C2H6 remains in the vapor phase. This is not expected from the thermodynamic calculation. Table 4 also shows that the gas content was the largest among all the samples examined in this study. This seems to have resulted from the large amount of CH4 (approximately 95%) included in the hydrate cages. However, as identified by X-ray diffraction analysis (see Figure 7), this sample includes only the sII type hydrate and some quantity of ice. Then, we considered that all the CH4 molecules were encaged in both 512 and 51264 cages, as discussed in the previous section. We observed the C-C stretching mode of the Raman spectra of this sample. The Raman spectra of the C-C and C-H stretching modes of the i-C4H10 molecules in the various phases were investigated prior to the measurements of the mixed-gas hydrates. The measured spectra are summarized in Table 5. The

Figure 8. C-C stretching mode of the Raman spectra of C2H6, C3H8, and i-C4H10 molecules in the Q1 sample. Each point indicates the raw spectrum data. The thick solid line is the average, which coincides with the deconvoluted peaks by fitting the Voigt model.

Raman peaks obtained in the C-C stretching mode of the pure i-C4H10 hydrate were found to be in good agreement with those in a previous study.7 Figure 8 shows the C-C stretching mode range of the Raman spectra of the Q1 sample. In comparison to the previous Raman spectroscopic observations and the Raman spectra of the pure i-C4H10 hydrate, we assigned the obtained peaks as follows:

811 cm-1 (∆: 4 cm-1): i-C4H10 molecules in the 51264 cages of the sII hydrate 877 cm-1 (∆: 4 cm-1): C3H8 molecules in the 51264 cages of the sII hydrate 975 cm-1 (∆: 5 cm-1): unknown 992 cm-1 (∆: 3 cm-1): C2H6 molecules in the 51264 cages of the sII hydrate where ∆ is the full width at half-maximum of each peak. These results confirmed that most of the C2H6, C3H8, and i-C4H10 molecules were encaged only into 51264 cages. The ratio of peak intensities of the lines corresponding to i-C4H10, C3H8, and C2H6 was consistent with the relative proportions of the three species in the gas from the dissociated hydrate measured with the GC. Therefore, it is concluded that all the C2H6, C3H8, and i-C4H10

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Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 Table 6. Cage Occupancy Estimations of the Q1 Samplea cage occupancy[%] guest molecule

512

51264

total

CH4 C2H6 C3H8 i-C4H10 hydration number n molar ratio of hydrate in sample X [%]

100 0 0 0

0.1∼26 2.4∼2.7 3.0∼3.4 6.5∼7.4

71 (4) 0.9 (0.1) 1.1 (0.1) 2.3 (0.2) 7.5 (0.5) 59 (4)

a

The number in parentheses is the uncertainty of each value.

the Raman intensity ratio and cage occupancy ratio of the CH4 molecules in sII cages is then explained as follows: C1 RC1 × IL/IS ) θC1 L /2θS

Figure 9. C-H stretching mode of the Raman spectra of the Q1 sample. All of the peaks from CH4, C2H6, C3H8, and i-C4H10 molecules are observed in this region. Each point indicates the raw spectrum data, and the thick solid line is the average. Dashed lines denote the deconvoluted peaks by fitting the Voigt model.

molecules are included in the 51264 cages in the sII hydrate and their ratio is approximately C2H6:C3H8:i-C4H10 ) 2:3:5, which suitably coincides with that obtained by the GC measurement on the dissolved gas. The unknown peak (975 cm-1) has not yet been assigned. All the other phases (gas or liquid) of pure hydrocarbon molecules have Raman shifts that are different from this wavenumber. Since these spectra are large enough to be distinguished from the noise components, they will be investigated in the future. The existence of the CH4 molecules in both cages of the sII hydrate is also confirmed by the C-H stretching mode (Figure 9). In this figure, we can observe the CH4 molecules in 512 cages at 2912 cm-1 and those in 51264 cages at 2901 cm-1. The other small peaks are probably those of the C-H stretching mode of either C2H6, C3H8, or i-C4H10. In order to estimate the cage occupancy ratio of CH4 molecules, it is necessary to ignore the effect of the Raman spectra of the other guest molecules. Here, we ignore the contributions of the other guest molecules for the peak of CH4 in 512 cages because this peak intensity is significantly strong. However, for the 2901 cm-1 peak, we must consider several contributions from the other molecules. In the previous section, the contribution of the C3H8 molecules at around 2901 cm-1 could be removed by extracting their Raman spectra using that of pure C3H8 hydrate. For the Q1 sample, however, it is impossible to apply such a process because the Raman spectra of the C-H stretching mode may not be assumed as a simple overlap of each contribution. Then, we define the fitting parameter RC1 as the contribution ratio of CH4 molecules in 51264 cages on the intensity of the 2901 cm-1 peak, that is, (1 - RC1) indicates the contributions of other hydrocarbons on the 2901 cm-1 peak intensity. Here, we estimate the CH4-contribution ratio RC1 for the 2901 cm-1 peak in order to obtain the cage occupancy ratio of CH4 molecules. The measured Raman peak intensities at 2901 and 2912 cm-1 are denoted as IL and IS, respectively. All the guest molecules are assumed to be included in 51264 cages, whereas only CH4 molecules are considered to be encaged in 512 cages. Here, we notice the cage occupancies of CH4 molecules in 51264 and 512 C1 cages as θC1 L and θS , respectively. The relationship between

(6)

On the other hand, we applied the GC data to estimate the distribution of gases using the gas-concentration ratio of CH4 to other hydrocarbons B, here B ) CC1/(CC2 + CC3 + CC4) and Ck represents the gas content of a gas molecule k (where C1, C2, C3, and C4 stand for CH4, C2H6, C3H8, and i-C4H10, respectively). Parameter B is related to the cage occupancy ratio as mentioned by eq 2, where R is the total cage occupancy of 51264 cages by the guest molecules. Here, we assume θC1 S to be unity because the CH4 molecule concentration is sufficiently larger than the other guest molecules. By applying the measured experimental data to both equations, we obtained the relation between R and RC1 as follows:

R ) 0.28RC1 + 0.12

(7)

Since 0 < RC1 < 1, R is in the range of 0.12 < R < 0.40. Then, we can obtain each guest-molecule distribution with a slight variation, as shown in Table 6. In the quaternary mixedgas system, it is indicated that the larger molecules favorably encaged 51264 cages in the sII hydrate, as expected in previous studies. Then, we confirm that the preferential enclathrationorder rule of hydrocarbon species in 51264 cages can be extrapolated to the molecule as large as i-C4H10 molecules. Since the initial CH4 concentration was relatively high for the Q1 sample, however, CH4 occupation in 51264 cages was also high. In addition, the hydration number of this hydrate is found to be approximately 7.5. This result allows us to estimate the molar ratio of the gas hydrate in the sample to be approximately 60%. This is also in agreement with the estimation based on the total gas consumption or gas-content measurements. Therefore, we concluded that spectroscopic analyses are sufficiently feasible for both the estimations of microscale measurements which provide the guest-molecule distributions, and those of macroscale measurements which are important for bulk-sample treatments, even for the multicomponent systems. Conclusions Raman spectroscopic analyses with several physicochemical property measurements on systematically prepared hydrate samples containing various hydrocarbon gases were performed in order to investigate the crystal structures and the distribution of the guest molecules in cages. We proposed estimation procedures for the cage occupancy of the guest molecules by Raman spectroscopy together with powder X-ray diffraction and gas chromatography. The procedure was based on the relative intensity ratio of the Raman spectra of the CH4 molecules encaged in two different cages. The results indicated that the larger (at least as large as i-C4H10) molecules favorably encaged

Ind. Eng. Chem. Res., Vol. 46, No. 14, 2007 5087

51264 cages in the sII hydrate with competition, whereas the CH4 molecules mainly occupied 512 cages. The larger molecules, with lower decomposition pressure at the same temperature, are more concentrated in the hydrate phase. We conclude, therefore, that spectroscopic analyses can be employed to estimate the distribution and proportions of different gas species in the hydrate, cage occupancies, and hydration numbers. Acknowledgment This work was partially supported by the Research Consortium for Methane Hydrate Resources in Japan (MH21 Research Consortium). We also acknowledge the members of Methane Hydrate Research Laboratory, AIST, for their support for the experiment. Literature Cited (1) Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker: New York, 1998. (2) 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. (3) Uchida, T.; Moriwaki, M.; Takeya, S.; Ikeda, I. Y.; Ohmura, R.; Nagao, J.; Minagawa, H.; Ebinuma, T.; Narita, H.; Gohara, K.; Mae, S. Two-Step Formation of Methane-Propane Mixed Gas Hydrates in a BatchType Reactor. AIChE J. 2004, 50, 518. (4) Ripmeester, J. A.; Ratcliffe, C. I. Low-Temperature Cross-Polarization/Magic Angle Spinning 13C NMR of Solid Methane Hydrates: Structure, Cage Occupancy, and Hydration Number. J. Phys. Chem. 1988, 92, 337. (5) Sum, A. K.; Burruss, R. C.; Sloan, E. D., Jr. Measurement of clathrate hydrates via Raman spectroscopy. J. Phys. Chem. B 1997, 101, 7371. (6) Uchida, T.; Hirano, T.; Ebinuma, T.; Narita, H.; Gohara, K.; Mae, S.; Matsumoto, R. Raman Spectroscopic Determination of Hydration Number of Methane Hydrates. AIChE J. 1999, 45, 2641.

(7) Subramanian, S.; Sloan, E. D., Jr. Trends in vibrational frequencies of guests trapped in clathrate hydrate cages. J. Phys. Chem. B 2002, 106, 4348. (8) Uchida, T.; Takeya, S.; Kamata, Y.; Ikeda, I. Y.; Nagao, J.; Ebinuma, T.; Narita, H.; Zatsepina, O.; Buffett, B. A. Spectroscopic observations and thermodynamic calculations on clathrate hydrates of mixed gas containing methane and ethane: determination of structure, composition and cage occupancy. J. Phys. Chem. B 2002, 106, 12426. (9) Uchida, T.; Uchida, T.; Kato, A.; Sasaki, H.; Kono, F.; Takeya, S. Physical properties of natural gas hydrate and associated gas-hydrate-baring sediments in the JAPEX/JNOC/GSC et al. Mallik 5L-38 gas hydrate production research well. Geol. SurV. Can. GSC Bull. 2005, 585, 105. (10) Brewer, P.; Malby, G.; Pasteris, J.; White, S.; Peltzer, E.; Wopenka, B.; Freeman, J.; Brown, M. Development of a laser Raman spectrometer for deep-ocean science. Deep-Sea Res. 2004, 51, 739. (11) Pasteris, J. D.; Wopenka, B.; Freeman, J.; Brewer, P.; White, S.; Peltzer, E.; Malby, G. Spectroscopic success and challenges; Raman spectroscopy at 3.6 km depth in the ocean. Appl. Spectrosc. 2004, 58, 195A. (12) White, S. N.; Dunk, R. M.; Peltzer, E. T.; Freeman, J. J.; Brewer, P. G. In situ Raman analyses of deep-sea hydrothermal and cold seep systems (Gorda Ridge and Hydrate Ridge). Geochim. Geophys. Geosyst. 2006, 7, Q05023. (13) Wilson, L. D.; Tulk, C. A.; Ripmeester, J. A. Instrumental techniques for the investigation of methane hydrates: cross-calibrating NMR and Raman spectroscopic data. Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, May 19-23, 2002; Vol. 2, p 614. (14) Uchida, T.; Ikeda, I. Y.; Takeya, S.; Kamata, Y.; Ohmura, R.; Nagao, J.; Zatsepina, O. Y.; Buffett, B. A. Kinetics and Stability of CH4CO2 Mixed Gas Hydrates During Formation and Long-Term Storage. ChemPhysChem 2005, 6, 646. (15) Uchida, T.; Ohmura, R.; Ikeda, I. Y.; Nagao, J.; Takeya, S.; Hori, A. Phase equilibrium measurements and crystallographic analyses on structure-H type gas hydrate formed from the CH4-CO2-neohexane-water system. J. Phys. Chem. B 2006, 110, 4583.

ReceiVed for reView January 26, 2007 ReVised manuscript receiVed March 30, 2007 Accepted May 8, 2007 IE070153W