Article pubs.acs.org/jced
Phase Equilibrium Studies of Tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 Hydrates Yun-Je Lee,†,§ Taro Kawamura,‡,∥ Yoshitaka Yamamoto,‡ and Ji-Ho Yoon*,† †
Department of Energy and Resources Engineering, Korea Maritime University, Busan 606-791, Korea Methane Hydrate Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki 305-8569, Japan
‡
ABSTRACT: The hydrate phase equilibrium behaviors of tetrahydrofuran (THF) + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 were investigated over wide ranges of temperature, pressure, and concentration. The dissociation conditions of THF + CH4 and THF + CO2 hydrates were shifted to lower pressures and higher temperatures from the dissociation boundaries of pure CH4 and pure CO2 hydrates. X-ray diffraction results revealed that the CH4 + CO2 and THF + CO2 + CH4 hydrates prepared from a CH4/CO2 (50:50) gas mixture formed structure I and II clathrate hydrates, respectively. Raman measurements provided detailed information regarding the cage occupancy of CH4 and CO2 molecules encaged in the hydrate frameworks. For the CH4 + CO2 hydrates, the concentrations of CO2 in the hydrate phase were higher than those in the vapor phase. In contrast, for the THF + CO2 + CH4 hydrates, the concentrations of CO2 in the hydrate phase were lower than those in the vapor phase.
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INTRODUCTION Gas hydrates are crystalline inclusion compounds formed by the interactions between hydrogen-bonded water hosts and guest molecules. Based on the differences in cavity shape and size of the gas hydrates, there are three distinct structures: structure I (sI), structure II (sII), and structure H (sH). The cubic sI hydrate consists of two small 512 and six large 51262 cages in the lattice unit, and the cubic sII hydrate has sixteen small 512 and eight large 51264 cages.1 The hexagonal sH hydrate consists of three types of cages: small 512, medium 435663, and large 51268. For thermodynamic stability, the sH hydrate requires a large guest molecule such as adamantane and methylcyclohexane with smaller help gas for cage stability.2,3 It is also known that, at high pressures over 2 GPa, a new crystalline structure of methane hydrate can stably form.4 Udachin and Ripmeester5 also reported a new clathrate hydrate structure showing bimodal guest hydration based on the stacking of structure cage layers. Over the past 60 years, significant attention has been paid to research on gas hydrates, since the realization that the plugging problem in gas pipelines during natural gas transport is mainly due to gas hydrate formation between water and light hydrocarbons such as methane, ethane, and propane. Therefore, numerous studies on the phase equilibrium, kinetics, and spectroscopic investigation of gas hydrates have been reported. In addition, gas hydrates are recognized as a potential energy source because a substantial amount of the methane hydrate naturally exists in the permafrost region and under subsea sediment.6 In the case of carbon dioxide hydrate, the disposal of carbon dioxide on the ocean floor in the form of the gas hydrate has been carefully studied.7 Natural gas hydrates are also © 2012 American Chemical Society
recognized as potential media for gas storage and transportation applications.8,9 For practical applications of gas hydrates in these technologies and processes, considerable knowledge regarding the thermodynamic stability, structural identification, and cage occupancy of guests of gas hydrates would be of particular importance. In general, the methane hydrate is stable under high pressures over 2.5 MPa at 273 K, whereas the carbon dioxide hydrate exists stably even at 1.3 MPa at 273 K. Therefore, the formation condition of gas hydrates with methane and carbon dioxide mixture depends on the composition of the gas mixture. CH4 molecules can occupy both the 512 and 51262 cages of the sI hydrate framework. The occupancy of CH4 molecules in the small and large cages slightly depends on with the hydrate equilibrium conditions. In general, the cage occupancy ratio of CH4 hydrate, θL/θS, is ∼1.26, where θL and θS are the cage occupancies of CH4 molecules in the large and small cages of the sI hydrate, respectively.10,11 This indicates that the small cages are less occupied than the large cages. For pure CO2 hydrate, the CO2 molecules predominantly occupy the large cages of the sI hydrate rather than the small cages, resulting in the approximate ratio θL/θS ∼ 3.12 and a hydrate number n = 7.0. These different characteristics are attributed to the effect on the cage occupancy of the molecular size of the guests and the interaction between hosts and guests. Therefore, when the gas hydrate forms from CH4 and CO2 gas mixtures, the CH4 and Received: July 4, 2012 Accepted: November 2, 2012 Published: November 9, 2012 3543
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initiate hydrate nucleation. When the formation of gas hydrates was completed and the pressure remained constant, the temperature of the equilibrium cell was elevated to a desired temperature. At isothermal conditions, the system pressure was slowly decreased using a metering valve. When the initial dissociation of gas hydrates was detected by visual observation through the sight windows, the metering valve was quickly closed to stop further dissociation. The large amount of hydrate solid was in equilibrium with the liquid and vapor phases for at least 8 h, which indicated the final state of the three-phase H− L−V (hydrate−liquid−vapor) equilibrium condition. The vapor phase was circulated through a sampling valve (Rheodyne, 7010) and analyzed at least five times with the online gas chromatograph. The sampling volume was 100 μL, and the sampling speed was less than 1 s, indicating that there was no effect of the sampling step on the equilibrium condition. To analyze the composition of the hydrate phase, the vapor phase was evacuated for 5 s, and then the sampling valve was quickly closed to isolate the hydrate phase. The temperature in the cell was elevated to 298 K for complete dissociation of the hydrate phase. The evolving gases from the hydrate crystals were analyzed at least three times via the sampling valve and gas chromatograph. The compositions of the vapor and hydrate phases were reproducible within a mole fraction of 0.003. The reproducibility and reliability of our measurements with the equilibrium cell have been confirmed through the comparison with the literature data, as shown in the previous work.12 4. Synchrotron X-ray Diffraction. The crystalline structure of the gas hydrate samples was determined by highresolution synchrotron XRD with a wavelength of 1.5496(1) Å at beamline 8C2 at the Pohang Accelerator Laboratory (PAL). The diffraction pattern was scanned in the range of 8° to 149° with a step length of 0.05°. The phase identification was conducted using the CMPR program,13 and the unit cell constants were determined by a full profile fitting method using the EXPGUI package.14 5. Raman Spectroscopy. A customized Raman system with a Nd:YAG laser (Excelsior, EXLSR-532-150-CDRH) that emits at 532 nm with a maximum power of 150 mW was used. The size of the laser spot on the samples was ∼20 μm, and the spectral resolution was ∼1.5 cm−1, using a spectrograph (SpectraPro, 2500i) and a CCD detector (Princeton Instruments, PIXIS:100B). The average acquisition time for a single spectrum was ∼120 s. The temperature of the samples during Raman measurements was controlled to 77 K using a microscope stage (Linkam, THMS 600).
CO2 molecules compete with each other for the best occupancy of the cages in the sI hydrate framework. Tetrahydrofuran (THF) alone can form sII hydrate with water host molecules. In this structure, THF molecules only occupy the large 51264 cages because of their large molecular size. It is also known that the THF hydrate can form the sII structure with CH4 or CO2 as a small guest molecule. Thus, we could easily expect that, for the THF hydrate with CH4 and/or CO2 guests, there should be a thermodynamic shift of the dissociation boundary to higher temperatures and pressures. In this study, we investigated the thermodynamic stability of binary and ternary clathrate hydrates formed with CH4, CO2, and THF guests. The dissociation behaviors of THF + CH4 and THF + CO2 hydrates were observed over a wide range of temperatures and pressures. Isothermal pressure−composition (P−x) measurements of CH4 + CO2 and THF + CO2 + CH4 hydrates were performed at specific temperatures. In addition, synchrotron X-ray diffraction (XRD) and Raman spectroscopy were used to identify the crystal structures and guest distributions in the cages of the gas hydrates.
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EXPERIMENTAL SECTION 1. Materials and Apparatus. A high-pressure equilibrium cell was used to determine the phase equilibria of the gas hydrates. The equilibrium viewing cell had a volume of ∼200 cm3 and was made of SUS 316 and equipped with two reinforced sight glasses at both sides. These materials allowed visual observation of the phase transformations in the hydrate and liquid phases under high-pressure conditions, up to 20 MPa. The cell contents were vigorously agitated by a magnetic driver attached to the top of the cell. A thermocouple thermometer (K-type) with a resolution of 0.1 K and a highpressure transmitter (Keller) with a resolution of 0.01 MPa were used to determine the temperature and pressure in the cell. The pressure and temperature in the cell were measured with the accuracy of ∼0.1 K and ∼0.02 MPa, respectively. CO2 with a purity of 99.999 % and CH4 with a purity of 99.95 % were supplied by Deok-Yang Energen Co., and THF of 99.9 % purity was supplied by Sigma-Aldrich Chemical Co. 2. Dissociation Pressure Measurement. The equilibrium cell was charged with liquid samples (∼100 cm3) and pressurized to the desired pressure with gases. To initiate hydrate nucleation, the liquid mixtures under the high-pressure conditions were cooled to ∼5 K below the anticipated hydrateforming temperature. Hydrate nucleation could be induced by agitating the magnetic driver in the cell. When the gas hydrates were formed and the system pressure was kept constant, the cell temperature was then slowly (with a rate of 0.1 K for 30 min) elevated to dissociate the formed hydrates into a condition where the hydrate phase coexisted with the liquid and vapor phases. The nucleation and dissociation steps were repeated at least twice to diminish a hysteresis phenomenon. When minute hydrate crystals remained and the system temperature was constant for at least 8 h after the system pressure was stabilized, the resulting temperature and pressure were considered at an equilibrium condition. A more detailed description of the experimental method is given elsewhere.12 3. Isothermal P−x Behavior. The equilibrium cell containing the liquid samples was pressurized with CO2 and CH4 gas mixtures. The gas mixtures were prepared from pure CO2 and CH4 gases by controlling the loading pressure, and their compositions were checked with a gas chromatograph (Younglin, ACME 6100). The cell temperature was lowered to
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RESULTS AND DISCUSSION The equilibrium dissociation conditions of the THF + CH4 and THF + CO2 hydrates are shown in Figure 1 and listed in Table 1. In addition, the figure shows the comparison of our measurements with previously reported values.15−18 In this study, a stoichiometric concentration of 5.56 mol % THF was used for liquid solutions. It was interesting to note that there were no large differences in the dissociation boundaries in the range of (3 to 6) mol % THF for both THF + CH4 and THF + CO2 hydrates. Upon decreasing the THF concentration to (1 to 1.5) mol %, we found a relatively large shift of the dissociation boundary to higher pressures and lower temperatures from the dissociation conditions of the 5.56 mol % THF hydrates. However, we note that, for all THF concentrations, the dissociation conditions of the THF + CH4 and THF + CO2 3544
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Table 1. Equilibrium Dissociation Pressure (P) and Temperature (T) of THF (1) + CH4 (2) and THF (1) + CO2 (2) Hydrates system
T/K
P/MPa
THF (1) + CH4 (2)
277.9 282.1 286.0 290.2 293.1 295.1 297.0 299.2 300.1 301.1 302.3 303.2 304.1 305.0 279.7 281.1 282.7 283.8 285.0 286.1 287.2 287.8 288.2 288.7 289.3 289.4 289.8 290.5 291.1
0.33 0.44 0.68 1.37 2.12 2.82 3.73 5.26 6.02 6.90 8.13 9.46 10.9 12.5 0.18 0.29 0.47 0.62 0.81 1.03 1.30 1.48 1.62 1.78 2.00 2.03 2.20 2.60 3.17
THF (1) + CO2 (2)
Figure 1. Equilibrium dissociation pressures. (a) THF + CH4 hydrate. □, 1.07 mol % THF;15 ○, 3 mol % THF;16 △, 5 mol % THF;15 ▽, 6 mol % THF;17 ●, 5.56 mol % THF (this work). (b) THF + CO2 hydrate. ○, 1.56 mol % THF;18 △, 2.75 mol % THF;18 ▽, 2.99 mol % THF;18 ●, 5.56 mol % THF (this work).
+ CO2 + CH4 hydrates. A CH4/CO2 (50:50) gas mixture was used for formation of both hydrates. For the CH4 + CO2 hydrate, prominent Raman peaks for the C−H stretching vibration of CH4 molecules were observed at (2904 and 2915) cm−1, indicating that the CH4 molecules were encaged in the large 51262 and small 512 cages of the sI hydrate framework.11,20 The characteristic Raman peaks for CO2 molecules at (1278 and 1381) cm−1 also indicated that the CO2 molecules were encaged over the cages of the sI hydrate framework. We note that the Raman peak of CO2 in the gas phase was observed at (1284 and 1388) cm−1. These split peaks were due to the Fermi resonance effect, representing the C−O symmetric stretching (ν1) and O−C−O bending (2ν2) modes of the CO2 molecules. From the Raman results for CO2 and CH4 molecules, we confirmed that the CH4 + CO2 mixture hydrates formed the sI clathrate hydrate, which was consistent with the XRD measurements. For the THF + CO2 + CH4 hydrate, a single Raman peak for the C−H stretching vibration of CH4 molecules was observed at 2914 cm−1. This indicated that the CH4 molecules were encaged only in the small 512 cages of the sII hydrate framework, whereas the large 51264 cages were fully occupied by THF molecules at the stoichiometric concentration of 5.56 mol %. Therefore, it was clear that the Raman peaks at (1275 and 1381) cm−1 should be recognized as characteristic peaks representing the C−O symmetric and O− C−O bending vibrations of CO2 molecules in the small 512 cages of the sII hydrate. The Raman peak position was very similar to that of CO2 molecules trapped in the small 512 cages
hydrates were shifted to lower pressures and higher temperatures from those of pure CH4 and pure CO2 hydrates. The binary THF hydrates formed from CH4 or CO2 guests have been identified as sII hydrates. To elucidate the crystal structure of gas hydrates formed from THF, CH4, and CO2 guest mixtures, we performed synchrotron XRD measurements. Figure 2a shows the XRD pattern of the CH4 + CO2 hydrate formed from the CH4/CO2 (50:50) gas mixture. Apart from several peaks corresponding to hexagonal ice (Ih), all of the distinctive Bragg peaks of the CH4 + CO2 mixture hydrate could be attributed to the sI hydrate.19 The crystal structure was indexed using a cubic unit cell (space group Pm3n) with a unit cell parameter of a = 11.869(2) Å. When the THF + CO2 + CH4 hydrate was formed from the 5.56 mol % THF solution and the CH4/CO2 (50:50) gas mixture, the resulting gas hydrate was confirmed to be sII hydrate, indexed using a cubic unit cell (space group Fd3m) with a unit cell parameter of a = 17.224(2) Å (Figure 2b). The Miller indices assigned at each diffraction angle were similar to those of sII binary THF hydrates.12 Raman spectroscopy provided detailed information on the cage occupancy of guest molecules in the cages and the crystal structure of gas hydrates. Figure 3 shows the Raman spectra of CH4 and CO2 molecules trapped in the CH4 + CO2 and THF 3545
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Figure 2. XRD patterns of (a) CH4 + CO2 and (b) THF + CO2 + CH4 hydrates. Both hydrates were formed from a CH4/CO2 (50:50) gas mixture; the THF concentration in the THF + CO2 + CH4 hydrate was 5.56 mol %. Asterisks indicate peaks for ice, Ih.
Figure 3. Raman spectra of CH4 + CO2 (blue) and THF + CO2 + CH4 (red) hydrates. (a) Raman spectra of CH4 molecules trapped in the cages of the hydrates. (b) Raman spectra of CO2 molecules trapped in the cages of the hydrates. Both hydrates were formed from a CH4/CO2 (50:50) gas mixture; the THF concentration in the THF + CO2 + CH4 hydrate was 5.56 mol %.
of THF + CO2 and 1,4-dioxane + CO2 binary sII hydrates.12 For pure CH4 hydrate, the cage occupancy ratio was θL/θS ≈ 1.26, where θL and θS are the cage occupancies of CH4 molecules in the large and small cages of the sI hydrate, respectively.10,11 It is known that CO2 molecules preferentially occupy the large cages of the sI hydrate rather than the small cages. The peak area ratio of the Raman spectra provides indicative information on the cage occupancy ratio of CH4 molecules in the small and large cages of the hydrate framework. The Raman results revealed θL/θS (CH4) ≈ 0.85 when the CH4 + CO2 mixture hydrates were produced from the CO2/CH4 (50:50) gas mixture. This indicated that the population of CH4 molecules in the small cages was higher than that in the large cages, and the CO2 molecules preferentially occupied the large cages. In Figure 4, the isothermal P−x diagram of the CH4 + CO2 mixture hydrate is plotted as a function of pressure at (274 and 278) K. The experimental data are listed in Table 2. As also shown in Figure 1, the experimental data obtained in the present study agreed well with the results calculated using CSMHYD software.1 Based on the water-free concentration, the concentrations of CO2 in the hydrate phase were higher than those in the vapor phase under all conditions. From the experimental and theoretical results, it was expected that more than 60 % of the cages in the sI hydrate were occupied by CO2 molecules, when the CH4 + CO2 mixture hydrate formed from the CO2/CH4 (50:50) gas mixture. This was due to the
Figure 4. Isothermal P−x diagram of the CH4 (1) + CO2 (2) hydrate at 274 K (○) and 278 K (□). The data are depicted on the basis of water-free concentration. The lines represent theoretical predictions using the CSMHYD software.
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described above, the THF + CO2 + CH4 hydrates form the sII hydrate, and the large cages are fully occupied by THF molecules in the stoichiometric concentration. Therefore, the CH4 and CO2 molecules compete with each other for the best occupancy of the small cages in the sII hydrate framework. Unlike the CH4 + CO2 hydrates, the concentrations of CO2 in the hydrate phase were lower than those in the vapor phase. This was caused by the preferential occupation of CH4 molecules in the small 512 cages of the sII hydrate, which was similar to the occupation behavior in the small 512 cages of the sI CH4 + CO2 hydrate. In particular, it is interesting to note that the dissociation pressure of the THF + CO2 hydrate at 283 K was almost identical to that of the THF + CH4 hydrate, as shown in Figures 5 and 6. Thus, at temperatures higher than
Table 2. Pressure (P), Composition of Gas Phase (y2), and Composition of Hydrate Phase (h2) at Temperature (T) of CH4 (1) + CO2 (2) and THF (1) + CO2 (2) + CH4 (3) Hydrates on the Basis of Water-Free Concentration system
T/K
P/MPa
y2
h2
CH4 (1) + CO2 (2)
274
1.41 1.53 1.71 1.95 2.13 2.41 2.54 2.90 2.23 2.40 2.56 2.72 3.18 3.42 3.65 4.24 0.50 0.50 0.51 0.52 0.51 0.52 1.37 1.43 1.48 1.57 1.75 2.02 2.36
1 0.787 0.566 0.404 0.260 0.149 0.109 0 1 0.808 0.648 0.541 0.305 0.202 0.125 0 0 0.253 0.429 0.661 0.763 1 0 0.164 0.283 0.419 0.620 0.818 1
1 0.882 0.765 0.626 0.446 0.304 0.203 0 1 0.891 0.778 0.720 0.455 0.388 0.257 0 0 0.181 0.363 0.629 0.720 1 0 0.139 0.227 0.352 0.590 0.769 1
278
THF (1) + CO2 (2) + CH4 (3)
283
290
Figure 6. Comparison of dissociation pressures of THF + CH4 (●, red) and THF + CO2 (■, black) hydrates. The lines suggest the trend of the data, and the THF concentration was 5.56 mol %.
283 K, the dissociation pressures of the THF + CO2 hydrate were higher than those of the THF + CH4 hydrate. This may be caused by the different stabilization effects for CH4 and CO2 molecules occupying the small cages of the sII hydrates, depending on temperature and pressure.
preferential occupation by CO2 molecules in the hydrate cages, especially the large cages, which was consistent with the Raman results. The isothermal P−x equilibria of the THF + CO2 + CH4 hydrates are shown in Figure 5 and listed in Table 2. As
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CONCLUSIONS In this study, we investigated the phase equilibrium behavior of the THF + CH4, THF + CO2, CH4 + CO2, and THF + CO2 + CH4 hydrate systems. The dissociation behavior of THF + CH4 and THF + CO2 hydrates was observed using a high-pressure viewing cell. The dissociation boundary of both hydrates was shifted to lower pressures and higher temperatures from those of pure CH4 and pure CO2 hydrates. From the XRD measurements, we confirmed that the CH4 + CO2 hydrate prepared from the CH4/CO2 (50:50) gas mixture revealed the sI hydrate, whereas the THF + CO2 + CH4 hydrate formed the sII hydrate. The Raman measurements of the CH4 + CO2 hydrate indicated that the population of CH4 molecules in the small 512 cages of the sI hydrate framework was higher than that in the large 51262 cages, and the CO2 molecules preferentially occupy the large 51262 cages. For the THF + CO2 + CH4 hydrate, the CH4 and CO2 molecules were encaged only in the small 512 cages of the sII hydrate framework, whereas the large 51264 cages were fully occupied by THF molecules at the stoichiometric concentration of 5.56 mol %. The isothermal P− x behavior of the CH4 + CO2 and THF + CO2 + CH4 hydrates was investigated at several temperatures. Based on the water-
Figure 5. Isothermal P−x diagram of the THF (1) + CO2 (2) + CH4 (3) hydrate at 283 K (○) and 290 K (□). The data are depicted on the basis of water- and THF-free concentration. The lines suggest the trend of the data, and the THF concentration was 5.56 mol %. 3547
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presence of THF, propylene oxide, 1,4-dioxane and acetone. Fluid Phase Equilib. 2001, 189, 99. (17) Zhang, Q.; Chen, G. J.; Huang, Q.; Sun, C. Y.; Guo, Z. Q.; Ma, Q. L. Hydrate Formation Conditions of a Hydrogen + Methane Gas Mixture in Tetrahydrofuran + Water. J. Chem. Eng. Data 2005, 50, 234. (18) Delahaye, A.; Fournaison, L.; Marinhas, S.; Chatti, I. Effect of THF on Equilibrium Pressure and Dissociation Enthalpy of CO2 Hydrates Applied to Secondary Refrigeration. Ind. Eng. Chem. Res. 2006, 45, 391. (19) McMullan, R. K.; Jeffrey, G. A. Polyhedral Clathrate Hydrates. IX. Structure of Ethylene Oxide Hydrate. J. Chem. Phys. 1965, 42, 2725. (20) 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.
free concentration, the concentrations of CO2 in the hydrate phase for the CH4 + CO2 hydrates were higher than those in the vapor phase. In contrast, for the THF + CO2 + CH4 hydrates, the concentrations of CO2 in the hydrate phase were lower than those in the vapor phase.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Korea CCS R&D Center, Korea Institute of Energy Research, Daejeon 305-343, Korea. ∥ Tsukuba Bio-Frontier Center, Chugai Technos Corp., Ibaraki 305-0047, Japan. Funding
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (2008-313-D01285; 2008-0061974; 2010-0007026) and the Nuclear R&D Program (M2AM062008-03931). Notes
The authors declare no competing financial interest.
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