Occupation and Release Behavior of Guest Molecules in CH4

Mar 17, 2014 - occupation behavior of specific components of gas mixtures within them, ... Further evaluation of the temperature-dependent occupation ...
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Occupation and Release Behavior of Guest Molecules in CH4, CO2, N2, and Acetone Mixture Hydrates: An In Situ Study by Raman Spectroscopy Youngrok Seo,† Sulki An,† Jeong-Woo Park,† Byeong-Soo Kim,† Takeshi Komai,‡ and Ji-Ho Yoon*,† †

Department of Energy and Resources Engineering, Korea Maritime and Ocean University, Busan 606-791, Korea Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan

‡

ABSTRACT: For practical applications of gas hydration (formation of gas hydrates) in environmental and technological processes, considerable knowledge regarding the thermodynamic stability and structural features of these hydrates, as well as the occupation behavior of specific components of gas mixtures within them, is essential. Herein, the hydrate phase equilibria of a system comprising CH4/CO2/N2 (55/40/5) + aqueous acetone solutions (1, 3, and 5.56 mol %) were determined in the temperature range 273−285 K and under pressures up to 4.5 MPa. Gas compositions in the hydrate phase were also obtained by evaluating the following variables: (1) hydrate-formation temperature and pressure, (2) concentration of acetone, and (3) type of hydrate structure: (a) structure I or (b) structure II. The crystal structures of the gas hydrates formed from the acetone and CH4 + CO2 + N2 mixture gas were also evaluated by both X-ray diffraction and Raman spectroscopy. In addition, structural identification of the CH4 + CO2 + N2 + acetone hydrates formed by varying the concentration of acetone (0, 1, 3, and 5.56 mol %) was performed. Further evaluation of the temperature-dependent occupation behavior of CH4 and CO2 in structure II hydrate cages in the temperature range 150−290 K indicates that CH4 and CO2 gradually escaped from the hydrate frameworks with increasing temperature, up to 255 K, at which point the CH4 + CO2 + N2 + acetone hydrate completely decomposed.

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Lee6 suggested a technique for recovery of CO2 from flue gas by hydrate formation. One of the unique characteristics of gas hydrates is the ability to concentrate high levels of guest molecules such as natural gas and hydrogen. At the standard temperature and pressure conditions, 1 m3 of CH4 hydrate can in theory contain almost 170 m3 of CH4 gas.11,12 In addition, gas hydrates are characterized by the self-preservation effect, whereby gas hydrates are able to maintain their temperature a few degrees above the dissociation temperature but below the ice point of water.13 Accordingly, extensive studies have been performed to determine the feasibility of gas hydration as a storage and transportation medium in a wide range of industries. Lee et al.14 verified the applicability of hydrate formation for the storage and transportation of landfill gas, which typically consists of CH4, CO2, and N2. Gudmunsson and Borrehaug15 and Kanda16 further suggested that storage and transportation of natural gas using hydrate technology would be beneficial to oil and gas companies, especially in the case of moderate conveyance distances and small volumes. For practical applications of gas hydrates to the aforementioned environmental and technical processes, considerable knowledge pertaining to the thermodynamic stability, occupation behavior of specific components in gas mixtures, and structural analysis of gas hydrates is needed. However, there is a dearth of available experimental data on CH4 + CO2 + N2 mixture gases,17 and there is almost no experimental knowledge

INTRODUCTION A gas hydrate is an inclusion compound formed by the interactions between water and relatively low molecular weight gases under appropriate conditions, i.e., low temperatures and high pressures. On the basis of differences in the sizes and shapes of the cavities of gas hydrates, they have been classified into three common families: structures I, II, and H. Each structure gives rise to different lattice dimensions for optimal guest occupancy. The lattice unit of structure I hydrates contains two small 512 and six large 51262 cages; whereas, structure II is composed of sixteen 512 and eight 51264 cages.1 The structure H hydrate, which has three types of cages, 512, 435663, and 51268, was discovered by Ripmeester et al.2 On the basis of the size ratio of the guest relative to the cavity, occupancy of the hydrate cage by guest molecules may be selective.3,4 For example, CH4 and CO2 can form the structure I hydrate by occupying both the 512 and 51262 cages. In contrast, it is well-known that C3H8, acetone, and tetrahydrofuran (THF) stabilize the 51264 cages of structure II.1 Over the past 60 years, there has been a shift in focus toward the study of natural gas hydrates. Gas hydrate has been considered as a troublemaker in petroleum processes, owing to plugging of gas pipelines,1 which is mainly due to gas hydrate formation between water and light hydrocarbons such as methane, ethane, and propane. However, gas hydrates are also recognized as a prospective energy source, given that large deposits of methane are trapped in the form of the gas hydrate in the permafrost zone and subsea sediment.5 From an environmental perspective, certain studies have been concerned with the formation, decomposition, and separation of specific constituents from gas mixtures.6−10 For example, Kang and © 2014 American Chemical Society

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September 10, 2013 March 13, 2014 March 17, 2014 March 17, 2014 dx.doi.org/10.1021/ie500057f | Ind. Eng. Chem. Res. 2014, 53, 6179−6184

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immediately immersed in liquid nitrogen contained in a stainless-steel container. Gas composition in the hydrate was checked after complete decomposition of the gas hydrate for at least 5 h at 353 K using a circulator (JEIO TECH, HTRC-10). Powdered samples of the hydrate, of diameter ∼200 μm, were then prepared for Raman spectroscopic analysis. A customized Raman spectrometer using the second harmonic (532 nm) of an Nd:YAG laser (Excelsior, EXLSR-532-150-CDRH) with a maximum power of 150 mW was used in this work. The size of the laser spot incident to the samples was ∼20 μm, and the spectral resolution achieved using a spectrograph (SpectraPro, 2500i) and a multichannel air-cooled CCD detector (Princeton Instruments, PIXIS 100B) was about 1.5 cm−1. The acquisition time for a single spectrum was about 120 s on average. Control of the sample temperature during Raman measurement was achieved using a temperature-controlled microscope stage (Linkam, THMS 600); the temperature was varied from 150 to 290 K in increments of 5 K with a temperature stability of 99.9% was supplied by Sigma-Aldrich Chemical Co. The initial concentrations of the gas mixtures in the cylinder and the hydrate components were determined using gas chromatography (Younglin, ACME 6100), equipped with a thermal conductivity detector and Porapak-Q column. Procedures. The evacuated equilibrium cell was charged with 60 cm3 of aqueous acetone solutions (1, 3, and 5.56 mol %) and then pressurized to the desired pressure with the mixture gas. Once the cell conditions were stabilized, hydrate nucleation was induced by cooling the system to about 5 K below the expected hydrate-formation temperature. After hydrate formation was complete and the system reached a stable state, the cell temperature was slowly elevated by means of a digital temperature controller to decompose the formed hydrate at a rate of 1 K per hour. When a minute hydrate crystal remained and the system temperature remained stable for at least 8 h after stabilization of the system pressure, the temperature and pressure were recognized as the H−Lw−V equilibrium dissociation condition. A more detailed description of the experimental procedure and apparatus is presented elsewhere.14,18 Sample Analysis. The initial mixture gas was reacted with aqueous acetone solutions (0, 1, 3, and 5.56 mol %) at a desired isobaric and isothermal condition for hydrate formation. During the hydrate formation, the gas phase was refreshed using a solenoid valve and metering valves to minimize changes in the system pressure and the composition in the gas phase. Once the hydrate was formed, the sample was kept in contact with the mixture gas for at least 2 days to facilitate complete hydrate formation. When the hydrate formation was complete, the stabilized system pressure and temperature was recognized as an equilibrium H−V condition. After completion of the reaction, the vessel was then removed from the bath and

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RESULTS AND DISCUSSION The phase (H−Lw−V) equilibria of the CH4 + CO2 + N2 + acetone hydrate were acquired in the temperature range 273− 285 K and pressure range up to 4.5 MPa. The measured equilibrium data were tabulated in Table 1 and plotted in Table 1. Experimental Dissociation Pressures of CH4 + CO2 + N2 + Acetone Hydrates for Various Concentrations of Acetone acetone concentration (mol %) 1

3

5.56

temperature (K)

pressure (MPa)

273.2 274.8 277.6 280.0 275.5 278.6 281.1 283.2 274.8 276.1 279.1 279.9 281.3 282.8 284.1

1.61 2.09 3.02 4.20 1.41 2.26 3.23 4.40 1.07 1.33 1.87 2.17 2.86 3.47 4.08

Figure 1. Additionally, Figure 1 presents a comparison of the present measurements with the phase boundaries for pure CO2, pure CH4, and CH4 + CO2 + N2 hydrates, calculated using the predictive Soave−Redlich−Kwong model.19 Comparison of our measurements with the phase boundaries for pure CO2 and pure CH4 hydrates reveals that the equilibrium points of the CH4 + CO2 + N2 + acetone hydrate system were shifted to lower pressure and higher temperature conditions than those of pure CH4 hydrate. However, for the 1 mol % acetone hydrate, the phase equilibria are located at higher pressure and lower temperature conditions than those of pure CO2 hydrate, as shown in Figure 1. 6180

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Figure 2. X-ray diffraction patterns of CH4 + CO2 + N2 + acetone hydrates for various concentrations of acetone. Asterisks correspond to peaks: (black) ice Ih; (red) structure I hydrate; (blue) structure II hydrate.

Figure 1. Dissociation conditions of CH4 + CO2 + N2 + acetone hydrates for various concentrations of acetone. Black solid and dashed lines are the dissociation boundary of pure CO2 and pure CH4 hydrates, respectively, calculated using the predictive Soave−Redlich−Kwong model.19 The red solid line is the dissociation boundary of CH4 + CO2 + N2 hydrate, calculated using the predictive Soave− Redlich−Kwong model.19

the CH4 + CO2 + N2 hydrate can be indexed using a cubic unit cell (space group Pm3n) with a unit cell parameter of a = 11.95 Å. Apart from some peaks which correspond to hexagonal ice (Ih), all XRD peaks of the CH4 + CO2 + N2 hydrate are attributed to structure I. In contrast, the XRD pattern of CH4 + CO2 + N2 + acetone hydrate (5.56 mol %) should be assigned to structure II, which is indexed using a cubic unit cell (space group Fd3m) with a unit cell parameter of a = 17.3 Å. We note that the unit cell parameters of CH4 + CO2 + N2 and CH4 + CO2 + N2 + acetone (5.56 mol %) hydrates agree well with those of CH4 + CO2 (structure I) and CH4 + CO2 + THF (structure II) hydrates,20 respectively. Further, the Miller indices for the CH4 + CO2 + N2 + acetone (1 and 3 mol %) hydrates can be indicative of the coexistence of structure I and II hydrates. Raman spectroscopy was used to elucidate the crystal structures of the gas hydrates formed from acetone and the CH4 + CO2 + N2 mixture gas. Figure 3 shows the respective Raman spectra of the CH4 + CO2 + N2 hydrate and pure acetone hydrate formed from aqueous 5.56 mol % acetone solution without gas. The arrows indicate the Raman peaks of guest molecules in the structure I hydrate. For the pure acetone hydrate, the prominent Raman peaks at 2925 cm−1 and 1700 cm−1 corresponding to the C−H and C−O stretching vibrations, respectively, of the acetone molecules were monitored.

Gas compositions in the hydrate phase were experimentally determined at various two-phase (H−V) equilibrium conditions, as shown in Table 2. The composition in the hydrate phase as a function of the hydrate-formation temperature was evaluated by varying the temperature as follows: 270.2, 274.2, 278.2, and 280.0 K. As shown in Table 2, the CH 4 concentrations of the hydrate phase were higher than those of the vapor phase at all conditions, indicating that the CH4 molecules occupy the small cavities of the structure II hydrate more readily than CO2 and N2 molecules. We note that, for the CH4 + CO2 + N2 mixture structure I hydrate, the CO2 concentrations of the hydrate phase were higher than those of the vapor phase.14 Gas composition in the hydrate phase was evaluated while varying the concentration of acetone (0, 1, 3, and 5.56 mol %). In general, the CH4 + CO2 + N2 + acetone hydrates are considered to form structure II hydrates; whereas, the CH4 + CO2 + N2 hydrates are structure I hydrates.14,18 Table 2 presents gas compositions in the hydrate phase based on the acetone- and water-free concentration, revealing a higher CH4 concentration in the structure II hydrate than that in the structure I hydrate. The XRD measurements were used to identify the crystal structure of hydrate samples (Figure 2). The XRD pattern of

Table 2. Hydrate−Vapor (H−V) Two-Phase Equilibria of CH4 + CO2 + N2 + Acetone Hydrates gas composition in vapor phase (mol %)a

condition

a

gas composition in hydrate phase (mol %)a

run no.

acetone composition (mol %)

T (K)

P (MPa)

CH4

CO2

N2

CH4

CO2

N2

1 2 3 4 5 6 7

5.56 5.56 5.56 0 1 3 5.56

270.2 274.2 278.2 280.0 280.0 280.0 280.0

3.45 3.90 4.65 5.00 5.00 5.00 5.00

55 55 55 55 55 55 55

40 40 40 40 40 40 40

5 5 5 5 5 5 5

57.9 58.9 61.9 53.0 56.0 60.0 63.4

38.6 36.7 34.7 44.2 42.1 37.3 33.8

3.5 4.4 3.4 2.8 1.9 2.7 2.8

Water- and acetone-free concentration. 6181

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Figure 3. Raman spectra of CH4 + CO2 + N2 hydrate and pure acetone hydrate.

Figure 5. Raman spectra of CO2 molecules encaged in hydrate framework for various concentrations of acetone.

Figure 4 shows variations in the Raman peaks of CH4 in the hydrate, based on the acetone concentration. The Raman peaks

molecules were incorporated into the cages of the structure I and II hydrate frameworks, respectively. In addition, the Raman peaks at 1275 and 1380 cm−1 correspond to the Fermi resonance effect in CO2.21,23 On the basis of the data in Figure 5, where Raman peaks are observed at 1278 cm−1 for the sample produced in the presence of 0 mol % acetone and at 1273 cm−1 in the case of the sample produced using 5.56 mol % acetone, it can be deduced that CO2 was incorporated into the structure I and II hydrates, respectively. Because of the lack of splitting of the Raman peaks, the cage type into which the CO2 molecules were incorporated could not be conclusively defined.21 However, Fleyfel and Delvin suggests that CO2 is present in both cage types of the structure I hydrate,24 i.e., in both the small and large cavities. For the stoichiometric molar concentration of acetone of 5.56 mol %, it is confirmed that CO2 molecules were preferentially encaged in the small cavities of the structure II hydrates rather than in the large cavities. The other samples, produced with 1 and 3 mol % acetone, exhibit a shift in the Raman peak toward that of the 5.56 mol % acetone sample, indicative of the change in the volume ratio between structure I and II hydrates with increasing acetone concentration. As shown in Figure 6, the Raman peaks of N2 occurred at ca. 2324 and 2330 cm−1. The Raman peak at 2324 cm−1 is recognized as a characteristic peak of the N−N vibration of N2 incorporated into the hydrate framework. Because of the absence of defining features for the Raman peaks to delineate the type of hydrate structure and cavity size occupied by N2, Figure 6 does not provide conclusive information regarding the hydrate structure and the cavities occupied by N2. However, it should be noted that pure N2 hydrates form structure II hydrates.25,26 The Raman peak at 2330 cm−1 is consistent with that of free N2 gas, supplied to prevent adhesion of air to the glass. Structural identification and investigation of the occupancy behavior of guest molecules in hydrates are requirements for fully understanding the gas compositions in the hydrates formed using various concentrations of acetone. The change of volume ratio between the structure I and II hydrate was observed with increasing acetone concentration, indicating a lower CO2 but higher CH4 occupancy. Therefore, it was clear that acetone occupied the large cages of the structure II

Figure 4. Raman spectra of CH4 molecules encaged in hydrate framework for various concentrations of acetone.

of CH4 encapsulated in the small cavities of the structure I and II hydrates are known to occur at 2915 and 2913 cm−1,21 respectively. The Raman peak of CH4 in the large cavities of structure I hydrate occurs at 2903 cm−1.22 In addition, the Raman peak at 2915 cm−1 shifted to 2913 cm−1 as the acetone concentration increased. These results imply a change in the volume ratio between structure I and II hydrate with increasing acetone concentration. In addition, the disappearance of a Raman peak at near 2903 cm−1 indicates that CH4 does not occupy the large cavities of the structure II hydrates when an acetone concentration of 5.56 mol % is used.21 Furthermore, the coexistence of structure I and II hydrates with the use of 1 and 3 mol % acetone is indicated by the Raman peaks intermediate between those of the structure I and II hydrates, which is consistent with our XRD results. Figure 5 shows the Raman peaks of CO2 in the hydrates for various acetone concentrations. The characteristic Raman peaks of CO2 molecules at 1278 and 1273 cm−1 indicate that the CO2 6182

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Figure 8. Normalized relative intensities of Raman peaks for CH4 and CO2 molecules incorporated into CH4 + CO2 + N2 + acetone hydrate, as a function of temperature.

Figure 6. Raman spectra of N2 molecules encaged in hydrate framework for various concentrations of acetone.

hydrate, resulting in competition between CH4 and CO2 for the small 512 cages of the structure II hydrate. However, because of the higher preference of CH4 for the 512 cages of the hydrates, its concentration increases relative to that of CO2.27,28 The influence of temperature on the release behavior of CH4 and CO2 from the CH4 + CO2 + N2 + acetone hydrate framework is shown in Figures 7 and 8. Figure 7 shows the

+ N2 + acetone hydrate at atmospheric pressure. Furthermore, it is observed that CH4 and CO2 gradually escaped from the hydrate framework as the temperature was increased to 255 K, with a corresponding reduction in intensity in the Raman spectrum.

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CONCLUSIONS In this study, phase equilibrium data were acquired for the CH4/CO2/N2 (55/40/5) + acetone solutions (1, 3, and 5.56 mol %) system, indicating that each guest molecule competes with each other for optimum occupancy of the hydrate lattices at the corresponding equilibrium conditions. Variations in the gas compositions in the hydrate phase were evaluated under different conditions: (1) hydrate-formation temperature and pressure, (2) acetone concentration, and (3) the type of hydrate structure: (a) structure I or (b) structure II. The CH4 molecules were found to occupy the small cavities of structure II hydrate more easily, resulting in a lower concentration of CO2 but higher concentration of CH4 in the hydrate framework. Structural identification and evaluation of the occupation behavior of guest molecules in the hydrates measured by both XRD and Raman spectroscopy demonstrated that when 0 mol % acetone was present in the system, the structure I hydrate was formed, with a higher concentration of CO2 than CH4. In contrast, for the 5.56 mol % acetone, structure II hydrates were observed with a higher concentration of CH4 than CO2. A shift in the Raman peaks of CH4 and CO2 for 1 and 3 mol % acetone and XRD measurements indicates the coexistence of structure I and II hydrates, resulting in simultaneously higher CH4 but lower CO2 compositions in the hydrates. Temperature-dependent occupation of CH4 and CO2 in the structure II hydrates was observed in the temperature range 150−290 K. CH4 and CO2 were confirmed to gradually escape from the hydrate lattice with increasing temperature up to 255 K, which is the decomposition temperature of the CH4 + CO2 + N2 + acetone hydrate, with a corresponding reduction in the intensity of the peaks in the Raman spectrum. Overall, the experimental data presented herein provide insights into the thermodynamic stability of the CH4 + CO2 + N2 + acetone system. Because the results also provide basic knowledge pertaining to gas compositions in the hydrate phase under different conditions, the results can be adopted for practical

Figure 7. Temperature-dependent Raman spectra of CH4 + CO2 + N2 + acetone hydrate, representing occupation behavior of guest molecules.

Raman peaks of CH4 and CO2 normalized relative to the Raman peak of water molecules in the hydrate phase that appeared at 3150 cm−1. Figure 8 shows the temperature dependence of the relative intensity of the CH4 and CO2 Raman peaks. As shown in Figure 8, the CH4 + CO2 + N2 + acetone system completely decomposed at about 255 K, as a result of a complex combination of factors, including the melting point of the acetone hydrate and the dissociation temperature of the CH4 + CO2 + N2 + acetone hydrate at atmospheric pressure. According to Wilson and Davidson,29 pure 5.56 mol % acetone hydrates decompose at 252 K, which agrees well with the dissociation temperature of the CH4 + CO2 6183

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(16) Kanda, H. Economic Study on Natural Gas Transportation with Natural Gas Hydrate (NGH) Pellets. Presented at the 23rd World Gas Conference, Amsterdam, June 5−9, 2006. (17) Murphy, P. J.; Roberts, S. Laser Raman spectroscopy of differential partitioning in mixed-gas clathrates in H2O−CO2−N2− CH4 fluid inclusions: Implications for microthermometry. Geochim. Cosmochim. Acta 1995, 59, 4809−4824. (18) Du, J. W.; Liang, D. Q.; Li, D. L.; Li, X. J. Phase Equilibrium Data of Binary Hydrate in the System Hydrogen + Acetone + Water. J. Chem. Eng. Data 2010, 55, 4532−4535. (19) Yoon, J. H.; Yamamoto, Y.; Komai, T.; Kawamura, T. PSRK method for gas hydrate equilibria: I. Simple and mixed hydrates. AIChE J. 2004, 50, 203−214. (20) Lee, Y. J.; Kawamura, T.; Yamamoto, Y.; Yoon, J. H. Phase Equilibrium Studies of Tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2 + CH4 Hydrates. J. Chem. Eng. Data 2012, 57, 3543− 3548. (21) Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of clathrate hydrates via Raman spectroscopy. J. Phys. Chem. B 1997, 101, 7371−7377. (22) Schicks, J. M.; Lu, H.; Ripmeester, J. A.; Ziemann, M. The characteristics of gas hydrates formed from H2S and CH4 under various conditions. Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, British Columbia, Canada, July 6−10, 2008. (23) Wright, R. B.; Wang, C. H. Density Effect on the Fermi Resonance in Gaseous CO2 by Raman Scattering. J. Chem. Phys. 1973, 58, 2893−2895. (24) Fleyfel, F.; Delvin, J. P. FT-IR spectra of 90 K films of simple, mixed, and double clathrate hydrates of trimethylene oxide, methyl chloride, carbon dioxide, tetrahydrofuran, and ethylene oxide containing decoupled water-d2. J. Phys. Chem. 1988, 92, 631−635. (25) Kang, S. P.; Lee, H.; Lee, C. S.; Sung, W. M. Hydrate phase equilibria of the guest mixtures containing CO2, N2 and tetrahydrofuran. Fluid Phase Equilib. 2001, 185, 101−109. (26) Davidson, D. W.; Handa, Y. P.; Ratcliffe, C. I.; Ripmeester, J. A.; Tse, J. S.; Dahn, J. R.; Lee, F.; Calvert, L. D. Crystallographic Studies of Clathrate Hydrates. Part I. Mol. Cryst. Liq. Cryst. 1986, 141, 141− 149. (27) Uchida, T.; Ikeda, I. Y.; Takeya, S.; Kamata, Y.; Ohmura, R.; Nagao, J.; Zatsepina, O. Y.; Buffett, B. A. Kinetics and Stability of CH4−CO2 Mixed Gas Hydrates during Formation and Long-Term Storage. ChemPhysChem 2005, 6, 646−654. (28) Lee, H.; Seo, Y. W.; Seo, Y. T.; Moudrakovski, I. L.; Ripmeester, J. A. Recovering Methane from Solid Methane Hydrate with Carbon Dioxide. Angew. Chem., Int. Ed. 2003, 42, 5048−5051. (29) Wilson, G. J.; Davidson, D. W. Dielectric evidence for acetone hydrate. Can. J. Chem. 1963, 41, 264−273.

applications geared toward gas storage and/or separation of specific components from gas mixtures. Furthermore, by verifying the temperature−dependent occupation behavior of guest molecules in the hydrates, the information presented in this study may be applied to optimizing the temperature for transportation of gas hydrates.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program (2010-0007026) through the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning, the LINC program (OPEnS) funded by the Ministry of Education, and the Specialized University in Offshore Plant funded by the Ministry of Trade, Industry and Energy.

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REFERENCES

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