Gas-Hydrate Phase Equilibrium for Mixtures of Sulfur Hexafluoride

The three-phase equilibrium lines of the SF6 + H2 mixed gas hydrate system were measured by the method of continuous measurement using a quartz crysta...
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Gas-Hydrate Phase Equilibrium for Mixtures of Sulfur Hexafluoride and Hydrogen Da-Hye Park, Bo Ram Lee, Jeong-Hoon Sa, and Kun-Hong Lee* Department of Chemical Engineering, Pohang University of Science & Technology, San 31, Hyoja-Dong, Pohang-Si, Gyeongbuk 790-784, Korea ABSTRACT: The three-phase equilibrium lines of the SF6 + H2 mixed gas hydrate system were measured by the method of continuous measurement using a quartz crystal microbalance. The gas-hydrate equilibrium data were obtained at pressures ranging from (0.3 to 4.5) MPa and at temperatures from (279.15 to 283.15) K. The nuclear magnetic resonance (NMR) spectra of xH2 = 0.75 mixed gas hydrate at pressures of (4.5 and 5.5) MPa and at a temperature of 279.65 K showed the presence of encaged hydrogen in small cages. The hydrogen content in the mixed feed gas varies from xH2 = 0 to 0.90.

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INTRODUCTION Hydrogen is viewed as a promising clean fuel for the transportation systems of the future. In establishing a hydrogen-based economy, one of the major issues is the development of effective storage and delivery technologies. A conventional storage system for liquid hydrogen has inherent drawbacks, safety concerns, and technical problems.1 However, materials for hydrogen storage, such as metal hydrides and highly porous solids, are the subject of recent active research. These candidates have attracted attention for their capability to improve thermodynamic or kinetic performance.2 Clathrate hydrates, which are nonstoichiometric crystalline compounds with a host framework consisting of water molecules and guest molecules trapped inside ice-like cages, are promising materials that can solve the problems associated with hydrogen. Simple hydrogen hydrates are usually formed at high pressures and low temperatures, with an assistant component required to enable gas hydrates formation at moderate conditions. Tetrahydrofuran, or THF, is well-known as an assistant additive that forms a structure II hydrate with small molecules such as hydrogen.3−7 A number of studies have focused on assessments of the ability of THF to act as a thermodynamic promoter, which reduces the hydrate formation pressure.3,5,7 However, even with promoters, there is a trade-off between the thermodynamic advantages and the hydrogen storage capacity. For this reason, finding suitable promoter molecules is necessary to satisfy industrial criteria with respect to economic issues. Furthermore, due to concern for the probable ecological problems that would follow environmental contamination, an easily recoverable promoter should be considered. Such environmental concerns led us to investigate gaseous promoters instead of water-soluble promoters. Both SF6 hydrate and THF hydrate belong to structure II in the atmospheric pressure region. The thermodynamic © 2012 American Chemical Society

boundaries of SF6 hydrate have been reported by several researchers.8−15 Sugahara et al.13 presented a three-phase coexistence curve and Raman spectrum in the temperature range of (274 to 313) K and at pressures up to 155 MPa. Using X-ray diffraction analysis, Manakov et al.10 observed the phase transition of SF6 hydrate at pressures above 50 MPa, and Alkado et al.11 also reported phase transitions at higher pressures based on Raman spectra analysis. In addition, Lee et al.14 and Cha et al.15 reported the hydrate phase equilibrium of a gas mixture including SF6. In this study, we attempted to determine the thermodynamic feasibility of using SF6 as a promoting agent for hydrogen storage. Herein, we report the equilibrium of hydrogen and sulfur hexafluoride mixture gas hydrate formation measured by novel equipment developed using the quartz crystal microbalance (QCM) technique for the quick screening of promoters.16

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EXPERIMENTAL SECTION Materials. Research grade SF6 and H2 gases with mole fraction purities of 99.99 % were used for his study. We prepared gas mixtures with hydrogen mole fractions of xH2 = 0, 0.50, 0.75, 0.85, and 0.90. Apparatus. The experimental apparatus for the hydrate equilibrium measurements is described in detail in our previous work.16 A schematic of the experimental apparatus is presented in Figure 1. The pressure cell connected to the QCM and the supply vessel are immersed in a temperature-controlled bath. The cell has a volume of 11 cc and can withstand pressures of Received: December 4, 2011 Accepted: March 27, 2012 Published: April 10, 2012 1433

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Figure 1. Schematic of the continuous QCM system.

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up to 15 MPa. The pressure and temperature in the cell are measured by a pressure transmitter (Sensys/range: (0 to 7) MPa) and a k-type thermocouple probe (uncertainty = ± 0.1 K). We used a small, silver-coated quartz crystal (HC49/U, d > 1 cm) with a 5 MHz resonance frequency to detect phase transition conditions. For the nuclear magnetic resonance (NMR) analysis, a Varian 600 MHz solid-state NMR spectrometer (Unity Inova 600) was used in this study. Methods. After preparing the feed gas in the supply vessel, we first attempted to analyze the gas composition using gas chromatography. However, the broad and asymmetric SF6 gas17 peak hindered accurate analysis of the mole fractions in the SF6 and H2 mixture. For this reason, we only report the prepared feed gas composition in this study. With the “continuous” QCM method using the “memory effect”, we could measure the hydrate phase equilibrium even faster. Initially, the cell was cooled to 253.15 K to convert the water droplets into ice, and then the cell was pressurized to approximately (0.5 to 20) MPa with the prepared gas from the supply vessel. The temperature was raised to slightly above 273.15 K to ensure that all of the ice transformed into hydrate. Then, the cell was heated stepwise until the dissociation point was observed. After the first dissociation point, the temperature was reduced again by 3 K, and an additional pressure of 0.1 MPa was simultaneously introduced into the cell for the second phase equilibrium point so that the hydrate quickly reformed because of the memory effect. Then, the cell was again heated stepwise, and the second dissociation point was measured at another temperature and pressure. This step was repeated to allow the detection of several phase equilibrium points for the given composition of gases at a rapid rate of 1 h per point. For the NMR analysis, the powdered samples were placed in a 4 mm zirconia rotor loaded into an NMR variable temperature probe and the cross-polarization magic angle spinning technique (CPMAS) was used, with spinning speeds of (10 to 11) kHz. The temperature was maintained at 183.15 K.

RESULTS AND DISCUSSION

In this study, the gas phase composition in the cell is assumed to be consistent with that of the supply vessel because of the insoluble nature of H2 and SF6 gas and the small volume of droplets (2 μL). The three-phase equilibrium of the introduced gas mixture was measured using the QCM method for several different ratios of SF6 and H2, and the results are given in Table 1 and plotted in Figure 2. The experimental results obtained in this work were fitted by an Antoine-like equation, which is one of the most widely used for regression of the hydrate formation data, as shown in the following equation. ln p(MPa) = A +

B T (K)

(1)

The results indicate that the phase equilibrium data for pure SF6 hydrate showed good agreement with the reference data13 and that the hydrate formation pressures of the SF6 and H2 mixed gas are much lower than those of the THF and H2 mixtures.3 Because of the small volume of hydrate, it is not easy to analyze the composition of gases in the gas hydrate droplets using the conventional gas chromatography method. To ensure that hydrogen gas is present in the gas hydrates from the xH2 = 0.75 mixture gas, the NMR spectra of the hydrates from the SF6 + H2 mixed gas formed at high pressures of (4.5 and 5.5) MPa and at 279.65 K were measured (Figure 3), and the hydrogen resonance peak is obvious. To exclude any proton signals from water molecules, deuterium oxide (D2O) was used to synthesize the hydrate samples for the NMR experiments. The spectrum shows a peak at 6.4 ppm, which is caused by residual HDO protons that are included in the D2O as impurities. The resonance peak at 4.3 ppm that grows with increasing pressure represents hydrogen molecules that are stored in sII small cages.18,19 When THF is used as a promoting agent for hydrogen hydrate, a high pressure (above 6.0 MPa) is needed to form hydrate at a particular temperature,3 whereas SF6 can simultaneously form hydrate and store hydrogen even at a lower pressure. 1434

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Table 1. Phase Equilibrium Data for SF6 + H2 + Water Mixed System with Varying Mole Fractions xH2 of the Feed Gases mole fraction xH2

T/K

P/MPa

0.00

279.85 281.15 282.05 282.75 280.25 280.55 281.35 282.27 283.15 279.15 279.65 280.25 281.35 282.35 282.85 279.35 280.05 280.75 281.55 282.15 282.55 282.65 279.35 279.95 280.65 281.45 282.55

0.343 0.453 0.554 0.648 0.695 0.730 0.897 1.079 1.286 1.139 1.244 1.394 1.602 2.065 2.393 1.654 2.002 2.323 2.664 2.974 3.310 3.558 2.008 2.543 3.027 3.538 4.519

0.50

0.75

0.85

0.90

Figure 3. Solid-state 1H NMR spectra of xH2 = 0.75 mixed gas hydrate at 5.5 MPa (upper) and 4.5 MPa (lower) in the range of the chemical shift from (15 to −5) ppm.

moderate temperatures and pressures using solid-state NMR spectroscopy; when THF is used as a promoting additive, hydrate cannot form at such conditions.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-54-279-2271. Fax: +82-54-279-8298. E-mail: [email protected]. Funding

The authors thank the Ministry of Education, Science and Technology of Korea (MEST) for its financial support through the second phase BK21 program and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) through a grant provided by the Ministry of Knowledge Economy of Korea (MKE). This research was also supported by the Research Institute of Industrial Science & Technology (RIST) CO2 project. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 2. Phase equilibrium boundary of the SF6 + H2 mixed gas hydrate in this study; ▲, xH2 = 0.90; ■, xH2 = 0.85; □, xH2 = 0.75; ●, xH2 = 0.50; ○, pure SF6 hydrate; +, 0.056 mole fraction THF + H2 from Hashimoto et al.;3 ×, pure SF6 hydrate from Sugahara et al.13

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CONCLUSIONS In this study, the phase equilibrium for the SF6 + H2 + water mixed system with feed gases of various compositions were measured using the QCM method. The experimental data showed that the three-phase equilibrium conditions of the SF6 + H2 hydrate shift to a lower temperature and higher pressure as the SF6 content of the gas mixture increases. Additionally, we identified the presence of hydrogen in sII small cages even at 1435

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