Phase Equilibrium Data of Methane Hydrate in the Aqueous Solutions

Sep 27, 2016 - Key Laboratory of Natural Gas Hydrate,. ‡. Guangdong Key Laboratory of New and Renewable Energy Research and Development,,. §. Guang...
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Phase Equilibrium Data of Methane Hydrate in the Aqueous Solutions of Additive Mixtures (THF + TBAC) Wen-Zhi Wu,†,‡,§,∥ Jin-An Guan,†,‡,§ Xiao-Dong Shen,†,‡,§,∥ Ling-Li Shi,†,‡,§ Zhen Long,†,‡,§ Xue-Bing Zhou,†,‡,§ and De-Qing Liang*,†,‡,§ †

Key Laboratory of Natural Gas Hydrate, ‡Guangdong Key Laboratory of New and Renewable Energy Research and Development,, Guangzhou Institute of Energy Conversion,, Chinese Academy of Sciences, Guangzhou 510640, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China §

ABSTRACT: In this work, the phase equilibrium data of methane hydrate in tetrahydrofuran (THF) and tetra-n-butylammonium chloride (TBAC) mixed aqueous solutions were investigated in the temperature range of (279.6 to 301.5) K and pressure range of (3.74 to 11.62) MPa. The measurements were carried out at mass fraction of 0.019 THF with the mass fraction of 0.020, 0.050, 0.101, 0.201, and 0.340 TBAC, and 0.058 THF with 0.102, 0.202, and 0.340 TBAC. The phase equilibrium data were obtained by the isochoric pressure-search method. The results showed that the addition of THF + TBAC could remarkably increase the phase equilibrium temperatures compared to the equilibrium temperature of pure CH4 hydrate. However, the addition of THF + TBAC mixtures did not push the CH4 hydrates equilibrium temperature to a higher degree compared to the addition of single THF. In stabilizing hydrate crystals, the TBAC played a positive role when the concentration of THF was low, but a negative role when the concentration of THF became high. induction time, and slow generation rate.26−29 To reduce the difficulty in hydrate crystallization, researchers try to add hydrate promoters such as acetone, 1,4-dioxane, propylene oxide, and tetrahydrofuran (THF), which can enhance the hydrate forming temperature at a certain pressure.30−38 Seo et al.30 measured four cyclic ethers with a fixed concentration of 3 mol % relative to water. The stabilization effect on the formed hydrates was found to be in on the order of THF > propylene oxide >1,4-dioxane > acetone. THF molecules form sII hydrates with a stoichiometric ratio of THF·17 H2O alone and only occupy the large 51264 cages because of their large molecular size. 36,38 Lee et al.36 found that the THF concentration with a range of 3 to 6 mol % showed little significance to the hydrate dissociation boundaries, and the promotion effect was the most obvious at a concentration of 5.56 mol % THF. In 1940, Fowler et al.39 first discovered that adding quaternary ammonium salts (QAS) could remarkably reduce the formation pressure of gas hydrates, the QAS such as tetra-nbutyl ammonium bromide (TBAB), tetra-n-butyl ammonium chloride (TBAC), and tetra-n-butyl ammonium fluoride (TBAF) have gained great attention for potential use in natural gas storage and separation.40 In 1969, Jeffrey studied their structure by X-ray structural analysis and named them as semiclathrate hydrates (SCHs).41 As compared to the tradi-

1. INTRODUCTION Clathrate hydrates are nonstoichiometric solid compounds formed from water and guest molecules under suitable temperature and pressure conditions.1−4 The guest molecules include gases (such as methane, carbon dioxide, nitrogen, and hydrogen) and some volatile liquids (such as tetrahydrofuran, acetone, and cyclopentane).4 The host lattices formed from water molecules construct different-sized polyhedral cages through hydrogen bonds, while the guest molecules with suitable size can be trapped in the cavities.1−3 Three kinds of gas hydrates structures known as structure I (sI), structure II (sII), and structure H (sH) can naturally occur, which largely depend on the size and the properties of the guest molecules.3−6 With the development of science and technology, the gas hydrates are suggested to be used in many areas, such as gas separation,7,8 gas storage and transport,9−14 seawater desalination,15,16 carbon dioxide sequestration,17,18 cold energy storage,19−21 carbon dioxide replacement exploitation of natural gas hydrate technology,22,23 and so on. Natural gas hydrates (NGHs), primarily methane hydrates, have been widely recognized as a potential new energy resource.24 NGHs not only have high storage capacity, but are also highly safe. A 1 m3 volume of gas hydrates contains as much as 180 m3 of gas at standard temperature and pressure.25 In addition, methane produces less carbon dioxide per mole than any other fossil fuel resource and reduces the amount of anthropogenic emissions of carbon dioxide gas. But there are many inherent bottlenecks to commercialize the hydrate-based technology, such as high pressure, low temperature, long © XXXX American Chemical Society

Received: May 18, 2016 Accepted: September 16, 2016

A

DOI: 10.1021/acs.jced.6b00405 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 1. A high pressure reactor with an available volume of 152 mL was made of stainless steel, which could withstand high

tional gas hydrate structures in which the guest molecules are not physically bound to water lattices, the guest anions such as Br, Cl, and F join in building a polyhedral framework with the host water molecules by hydrogen bonding to encage the TBA+ cations in the SCH.42−46 For example, in the unit cell of TBAC SCH, composed of four pentakaidecahedral cavities (51263, P), 16 tetrakaidecahedral cavities (51262, T), 10 dodecahedral (512, D), the TBA+ cations are usually located in four-compartment cavities of 4T or 3T·P and the empty Dcages are used to encage suitable gas molecules. TBAC SCHs have higher equilibrium temperature than TBAB SCHs under atmospheric pressure,42 and the small cages in TBAC can store more gas molecules than those in TBAF SCH.46 Because the results of using single additives are limited, some researchers have investigated additive mixtures to achieve the maximum benefit. Although some researchers have measured the phase equilibrium conditions using dynamics additive mixtures sodium dodecyl sulfate (SDS) + dodecyl polysaccharide glycoside (DPG) + CH4 or dynamics and thermodynamic additive mixtures sodium dodecyl sulfate (SDS) + cyclopentane (CP) + CH4,10 the phase equilibrium data of thermodynamic additive mixtures on methane hydrate have not been investigated in detail, especially THF + TBAC + CH4 system. Therefore, the equilibrium conditions of the CH4 hydrate forming system containing THF + TBAC were measured using the isochoric pressure search method in the current work. The effect of THF and TBAC concentrations on equilibrium conditions was shown by a comparison to the equilibrium of pure CH4 hydrates. Also, the mechanism of these two additives in shifting equilibrium conditions of CH4 hydrate was preliminarily discussed.

Figure 1. Schematic diagram of experimental apparatus. PC, personal computer; DA, data acquisition; GC, gas cylinder; VP, vacuum pump; MS, magnetic stirrer; SS, stirring seed; TB, thermostat bath; R, reactor; TS, temperature sensor; PT, pressure transducer.

pressures up to 20 MPa and temperatures in the range of 233.2 to 383.2 K. The temperature inside the reactor was controlled by immersing the reactor into a thermostat bath. A platinum resistance thermometer (PT100) with an uncertainty of ±0.1 K and a pressure transducer (CYB-20S) with an uncertainty of ±0.02 MPa was used to measure the temperature and pressure in the reactor. The aqueous solutions inside the cell were driven by a magnetic driven stirrer throughout the experiments. The temperature and pressure inside the reactor during the experiments were recorded every 10 s by Agilent online data acquisition system. The phase equilibrium data of methane hydrate with different concentrations of THF and TBAC additive mixtures were measured by an isochoric pressure-search method.5,32,35,36,44 Prior to each test, the cell was rinsed by deionized water and the test solution in sequence. Then the cell was evacuated using a vacuum pump for 5 min, and approximately 40 mL of THF and TBAC aqueous solutions was loaded in the cell. Afterward, methane gas was introduced into the cell until a desired pressure was achieved. When the original temperature and pressure of the cell were stable, the stirrer was started and the temperature of the thermostatic bath was lowered to a supercooling value to form hydrate. The first appearance of the hydrate crystal in the liquid phase usually led to an abrupt increase in temperature since hydrate formation was an exothermic reaction, and the pressure dropped continuously for gas consumption. When the pressure became stable, hydrate formation was considered to be finished. Subsequently, the temperature was gradually increased at a step of 0.1 K and each step was kept constant for about 6 h to achieve a steady equilibrium state. For each round of experiment, both the temperature and pressure were recorded in the computer. Eventually, the phase diagram of P−T for hydrate formation and decomposition was obtained. In this way, the point in the phase diagram where the P−T curve slope changed sharply was considered to be the phase equilibrium point. At this point, all of the hydrate crystals have just completely dissociated. The next hydrate phase equilibrium point can be achieved by changing the original system pressure in the reactor and repeating the procedure above. The reliability and reproducibility of the data reported by this procedure were confirmed.

2. EXPERIMENTAL SECTION 2.1. Experimental Materials. The detailed information on the test materials used in this work was listed in Table 1. The Table 1. List of the Materials Used in this Experiment component

purity (w)a

supplier

tetra-n-butyl ammonium chloride tetrahydrofuran

98 wt %

Tokyo Chemical Industry

99.5 wt %

CH4

99.99 wt %

deionized water

resistivity of 18 MΩ

Tokyo Chemical Industry Co. Ltd. Guangzhou Yigas Gases Co. Ltd. made in the laboratory

a

w represents the mass fraction.

methane used in this work was supplied by Guangzhou Yigas Gases Co. Ltd., with a purity of 99.99 wt %. The TBAC and THF with a purity of 98 and 99.5 wt %, respectively, were purchased from Tokyo Chemical Industry Co. Ltd. These chemicals were used without any further purification. Deionized water with a resistivity of 18 MΩ was made in the laboratory to dilute THF and TBAC to the desired concentrations in the experiments. Appropriate quantities of THF, TBAC, and deionized water were weighed on an electronic analytical balance with an uncertainty of ±0.1 mg. Eight different aqueous solutions at the mass fraction of 0.019 THF with the mass fraction of 0.020, 0.050, 0.101, 0.201, 0.340 TBAC, and 0.058 THF with 0.102, 0.202, 0.340 TBAC were prepared. 2.2. Experimental Apparatus and Procedure. The schematic of the experimental apparatus was sketched in B

DOI: 10.1021/acs.jced.6b00405 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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3. RESULTS AND DISCUSSION 3.1. Phase Equilibrium Conditions of Methane Hydrate. In advance, the experimental phase equilibrium data of pure CH4 hydrate were measured and listed in Table 2 Table 2. Phase Equilibrium Data of Methane Hydrate in Pure Water

a

T/K

P/MPa

287.5 285.4 284.0 282.3 279.6

11.39 9.20 7.84 6.49 4.89

Uncertainties u are u(T) = ±0.1 K, u(p) = ±0.02 MPa. Figure 3. Phase equilibrium conditions for THF + TBAC + CH4 + H2O system and comparing with pure methane hydrate: ⬢, pure CH4 hydrate, this work; △, pure CH4 hydrate, ref 47; ▽, pure CH4 hydrate, ref 48; ◇, pure CH4 hydrate, ref 49; ◁, pure CH4 hydrate, ref 50; ▷, pure CH4 hydrate, ref 51; ■, 0.019 THF + 0.020 TBAC + CH4 + H2O, this work; ▲, 0.019 THF + 0.050 TBAC + CH4 + H2O, this work; ▼, 0.019 THF + 0.101 TBAC + CH4 + H2O, this work; ◆, 0.019 THF + 0.201 TBAC + CH4 + H2O, this work; ◀, 0.019 THF + 0.340 TBAC + CH4 + H2O, this work; ▶, 0.058 THF + 0.102 TBAC + CH4 + H2O, this work; ●, 0.058 THF + 0.202 TBAC + CH4 + H2O, this work; ★, 0.058 THF + 0.340 TBAC + CH4 + H2O, this work.

Table 3. Phase Equilibrium Data of CH4 Hydrate in Different THF + TBAC Concentrations Aqueous Solution wTHFb

wTBAC

T/K

P/MPa

wTHF

wTBAC

T/K

P/MPa

0.019

0.020

0.340

0.050

0.058

0.102

0.058

0.202

0.019

0.101

0.019

0.201

10.72 9.78 7.32 5.60 4.00 10.76 9.50 8.50 7.64 5.85 3.74 11.35 9.69 7.96 6.11 4.49 11.35 9.60 8.04 6.23 4.51

0.019

0.019

289.2 288.2 286.1 284.1 281.2 290.8 290.0 289.2 288.6 287.4 285.4 293.5 292.5 291.6 290.7 289.3 295.5 294.5 293.7 292.8 291.5

0.058

0.340

295.6 295.0 294.3 293.4 292.1 301.5 300.5 299.1 297.5 295.5 299.4 298.9 298.2 297.0 296.0 293.1 296.1 295.0 293.2 291.8 290.7

11.08 9.63 8.08 6.36 4.44 11.62 10.07 8.14 6.61 4.74 10.99 10.13 9.07 7.85 6.77 4.38 10.66 9.30 7.39 5.76 4.26

Figure 2. Phase equilibrium conditions for the CH4 + water system: ●, this work; △, ref 47; ▽, ref 48; ◁, ref 49; ◇, ref 50; □, ref 51.

and plotted in Figure 2 with the data reported in refs 47−51. It was observed that the data from the current study were consistent with the reference data. It showed that our experimental apparatus and approach were reliable. 3.2. Phase Equilibrium Conditions of Methane Hydrate in the Aqueous Solutions of Additive Mixtures (THF + TBAC). The addition of THF and TBAC mixtures with different concentrations led to the hydrate equilibrium temperature increasing from 2.5 to 14.5 K at a given pressure as compared to the CH4 + H2O system in Figure 3, suggesting that the formed hydrate crystal could remain stable at a higher temperature. For the solutions with a mass fraction of 0.019 THF, the equilibrium temperature gradually rose with the increasing salt concentration of TBAC from 0.020 to 0.340 (see Table 3). However, the relationship between the promotion effect and the concentration of TBAC did not seem to be linear. At the pressure of 7.3 MPa, as the salt concentration of TBAC sequentially increased from 0.020 to 0.201, the equilibrium temperature increased linearly up to 2.7 K; while the temperature only increased by 0.7 K when the salt concentration of TBAC was 0.340, suggesting that the hydrate thermodynamic stability was greatest at the TBAC concentration around 0.201. Although CH4 hydrate formed from TBAC solution was reported to contain 0.340 TBAC in the hydrate phase, the lowest equilibrium temperature did not appear at this concentration of TBAC. The stability of

a Uncertainties u are u(w) = ±0.001, u(T) = ±0.1 K, u(p) = ±0.02 MPa. bwTHF and wTBAC represent the mass fraction of tetrahydrofuran and tetra-n-butylammonium chloride, respectively.

semiclathrate hydrates was assumed to be weakened by excessive TBAC. When the mass fraction of THF was fixed at 0.058, TBAC showed a negative effect on stabilizing the hydrate structures. With the increasing concentration of TBAC from 0.102 to 0.340, the equilibrium temperature of the hydrate forming C

DOI: 10.1021/acs.jced.6b00405 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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system was found to decrease. The equilibrium temperature difference between the pure methane hydrate forming system and the system containing 0.102 TBAC was around 14.5 K. As the TBAC concentration increased to 0.202 and 0.304, the difference reduced down to around 12.5 and 9.4 K, respectively. TBAC + CH4 hydrates and THF + CH4 hydrates were assumed to form from THF + TBAC + CH4 + H2O system individually, which was similar to THF + TBAB + H2O system.19 It might be resulted from the different proportion of sII hydrates and semiclathrates with different concentrations. Another finding was that the phase equilibrium curve of the 0.058 THF + 0.340 TBAC + CH4 was intersected with the two curves of 0.019 THF + 0.201 TBAC + CH4 and 0.019 THF + 0.340 TBAC + CH4 at the pressure ranging from about 8 to 9 MPa and temperature ranging from 293.7 to 294.7 K. It indicated that the promotion effect of THF + TBAC mixtures on CH4 hydrate was not only affected by the concentration, but also greatly determined by both temperature and pressure. This fact might be related to the complex hydrates formation kinetics and mechanism in the THF + TBAC + CH4 + H2O system at different pressures and temperatures. In Figure 4 and Figure 5, the equilibrium data for different concentrations of THF + TBAC in this work were separately

Figure 5. Comparison of hydrate phase equilibrium for THF + TBAC + CH4 + H2O system and THF + CH4 + H2O: ■, 0.019 THF + 0.020 TBAC + CH4 + H2O, this work; ◆, 0.019 THF + 0.050 TBAC + CH4 + H2O, this work; ▲, 0.019 THF + 0.101 TBAC + CH4 + H2O, this work; ▼, 0.019 THF + 0.201 TBAC + CH4 + H2O, this work; ◀, 0.019 THF + 0.340 TBAC + CH4 + H2O, this work; ▶, 0.058 THF + 0.102 TBAC + CH4 + H2O, this work; ⬟, 0.058 THF + 0.202 TBAC + CH4 + H2O, this work; ★, 0.058 THF + 0.340 TBAC + CH4 + H2O, this work; ▽,0.019 THF + CH4 + H2O, ref 35; △, 0.041 THF + CH4 + H2O, ref 35; ◊, 0.042 THF + CH4 + H2O, ref 31.

were almost equal to the data of single additives of 0.3401 TBAC at low pressures.43 In the system containing 0.058 THF + 0.340 TBAC, the promotion effect was better than that of 0.019 THF + 0.340 TBAC and 0.4483 TBAC at high pressure; but was inferior to that of 0.019 THF + 0.201 TBAC and 0.2 TBAC under low pressure.44 If the concentration of TBAC was less than a stoichiometric proportion (mass fraction of 0.340), the phase equilibrium condition of THF + TBAC + CH4 was more thermally stable than that of single TBAC. This is due to the existence of free TBA+ and Cl− ions, which performed as inhibitors to disturb hydrogen bonding with water molecules.46 As seen in Figure 5, however, apart from the 0.058 THF + 0.102 TBAC and 0.019 THF + 0.340 TBAC, other concentrations showed inferior equilibrium conditions as compared with single added THF at the pressure range from 4 to 11 MPa. It could be inferred that there might be an optimum mixed concentration for THF and TBAC, which might be beneficial to the potential applications of gas hydrates such as gas separation and storage. It was confirmed that the stability of hydrate structures increased with the number of filled cavities and the number of hydrate cages occupied by the guest molecules.51 In view of pure THF and pure TBAC hydrates, they were reported to form sII and semiclathrate gas hydrates, respectively, and the phase equilibrium conditions largely depended on their concentrations.52−54 As shown in Figure 6, increasing the concentration would raise their equilibrium temperature. However, when the concentrations reached or surpassed the certain value, the equilibrium temperature would not grow. Such equilibrium curves could be named the equilibrium limits of THF hydrate and TBAC hydrates. In general, the temperature of the equilibrium limits of THF hydrate was higher than that of the TBAB hydrate as seen in Figures 4 and 5. It was interesting to see that the equilibrium temperature will be approaching the equilibrium limits of TBAC hydrate with the growing concentration of TBAC in the

Figure 4. Comparison of hydrate phase equilibrium for THF + TBAC + CH4 + H2O system and TBAC + CH4 + H2O: ■, 0.019 THF + 0.050 TBAC + CH4 + H2O, this work; ▲, 0.019 THF + 0.101 TBAC + CH4 + H2O, this work; ▼, 0.019 THF + 0.201 TBAC + CH4 + H2O, this work; ◆, 0.019 THF + 0.340 TBAC + CH4 + H2O, this work; ⬟, 0.058 THF + 0.102 TBAC + CH4 + H2O, this work; ▶, 0.058 THF + 0.202 TBAC + CH4 + H2O, this work; ★, 0.058 THF + 0.340 TBAC + CH4 + H2O, this work; ◇, 0.05 TBAC + CH4 + H2O, ref 26; ▽, 0.05 TBAC + CH4 + H2O, ref 44; ◁, 0.0997 TBAC + CH4 + H2O, ref 44; ○, 0.1007 TBAC + CH4 + H2O, ref 45; □, 0.2 TBAC + CH4 + H2O, ref 44; △, 0.3401 TBAC + CH4 + H2O, ref 43; ▷, 0.4483 TBAC + CH4 + H2O, ref 46.

compared with the published data of TBAC and THF to better analyze the synergistic effects. As shown in Figure 4, with the addition of THF, the phase equilibrium of 0.019 THF + 0.050 TBAC was more moderate than the single additives of 0.050 TBAC.26,44 Similar results were found in the systems containing 0.019 THF + 0.101 TBAC and 0.019 THF + 0.201 TBAC. Moreover, when adding 0.058 THF in the system containing 0.102 TBAC and 0.202 TBAC the equilibrium temperature increase was even more significant. However, it should be noted that the equilibrium conditions of 0.019 THF + 0.340 TBAC D

DOI: 10.1021/acs.jced.6b00405 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*Tel.: + 86 20 8705 7669. Fax: + 86 20 8705 7669. E-mail: [email protected]. Funding

This work was supported by the National Natural Science Foundation of China (41374149), Scientific cooperative project by CNPC and CAS(2015A-4813) Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3 rd ed.; CRC Press-Taylor & Francis: New York, 2008. (2) Sloan, E. D. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426, 353−359. (3) Koh, C. A. Towards a fundamental understanding of natural gas hydrates. Chem. Soc. Rev. 2002, 31, 157−167. (4) Englezos, P. Clathrate Hydrates. Ind. Eng. Chem. Res. 1993, 32, 1251−1274. (5) Liang, D. Q.; Du, J. W.; Li, D. L. Hydrate Equilibrium Data for Methane + tert-Butylamine + Water. J. Chem. Eng. Data 2010, 55, 1039−1041. (6) Hammerschmidt, E. G. Formation of gas hydrates in natural gas transmission lines. Ind. Eng. Chem. 1934, 26, 851−855. (7) Li, X. S.; Xu, C. G.; Chen, Z. Y.; Cai, J. Synergic effect of cyclopentane and tetra-n-butyl ammonium bromide on hydrate-based carbon dioxide separation from fuel gas mixture by measurements of gas uptake and X-ray diffraction patterns. Int. J. Hydrogen Energy 2012, 37, 720−727. (8) Eslamimanesh, A.; Mohammadi, A. H.; Richon, D.; Naidoo, P.; Ramjugernath, D. Application of gas hydrate formation in separation processes: A review of experimental studies. J. Chem. Thermodyn. 2012, 46, 62−71. (9) Martin, A.; Peters, C. J. Hydrogen Storage in sH Clathrate Hydrates: Thermodynamic Model. J. Phys. Chem. B 2009, 113, 7558− 7563. (10) Sun, Z. G.; Wang, R. Z.; Ma, R. S.; Guo, K. H.; Fan, S. S. Natural gas storage in hydrates with the presence of promoters. Energy Convers. Manage. 2003, 44, 2733−2742. (11) Kim, D. Y.; Park, Y. J.; Lee, H. Tuning clathrate hydrates: Application to hydrogen storage. Catal. Today 2007, 120, 257−261. (12) Khokhar, A. A.; Gudmundsson, J. S.; Sloan, E. D. Gas storage in structure H hydrates. Fluid Phase Equilib. 1998, 150−151, 383−392. (13) Chapoy, A.; Anderson, R.; Tohidi, B. Low-Pressure Molecular Hydrogen Storage in Semi-clathrate Hydrates of Quaternary Ammonium Compounds. J. Am. Chem. Soc. 2007, 129, 746−747. (14) Javanmardi, J.; Nasrifar, K.; Najibi, S. H.; Moshfeghian, M. Economic evaluation of natural gas hydrate as an alternative for natural gas transportation. Appl. Therm. Eng. 2005, 25, 1708−1723. (15) Javanmardi, J.; Moshfeghian, M. Energy consumption and economic evaluation of water desalination by hydrate phenomenon. Appl. Therm. Eng. 2003, 23, 845−857. (16) Park, K. N.; Hong, S. Y.; Lee, J. W.; Kang, K. C.; Lee, Y. C.; Ha, M. G.; Lee, J. D. A new apparatus for seawater desalination by gas hydrate process and removal characteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+). Desalination 2011, 274, 91−96. (17) Giavarini, C.; Maccioni, F.; Politi, M.; Santarelli, M. L. CO2 hydrate: Formation and dissociation compared to methane hydrate. Energy Fuels 2007, 21, 3284−3291. (18) House, K. Z.; Schrag, D. P.; Harvey, C. F.; Lackner, K. S. Permanent carbon dioxide storage in deep-sea sediments. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 12291−12295. (19) Li, G.; Liu, D. P.; Xie, Y. M. Study on thermal properties of TBAB−THF hydrate mixture for cold storage by DSC. J. Therm. Anal. Calorim. 2010, 102, 819−826.

Figure 6. Temperature phase boundaries, T, versus TBAC and THF mass fraction, w: □, pure TBAC + H2O at atmospheric pressure, ref 52; △, pure THF + H2O below the atmospheric pressure condition (not isobaric condition), ref 53; ∗, pure methane hydrate (P = 4.78 to 9.02 MPa), ref 50; ▲, 0.019THF + TBAC + CH4 (P = 4.64 MPa), this work; ▼,0.019 THF + TBAC + CH4 (P = 6.08 MPa), this work; ◀, 0.019 THF + TBAC + CH4 (P = 7.81 MPa), this work; ▶, 0.019 THF + TBAC + CH4 (P = 9.35 MPa), this work; ◆, 0.019 THF + TBAC + CH4 (P = 10.71 MPa), this work; ◇, 0.058 THF + TBAC + CH4(P = 4.64 MPa), this work; ○, 0.058 THF + TBAC + CH4 (P = 6.08 MPa), this work; ▷,0.058 THF + TBAC + CH4 (P = 7.81 MPa), this work; ◁, 0.058THF + TBAC + CH4 (P = 9.35 MPa), this work; ▽, 0.058 THF + TBAC + CH4 (P = 10.71 MPa), this work.

solution regardless of the THF concentrations. When THF concentration was low, the equilibrium temperature may be lower than the equilibrium limits of TBAC hydrate. Adding TBAC showed the promotion effect. However, when the concentration of THF concentration was high, the equilibrium temperature was higher than the equilibrium limits of TBAC hydrate. Since the structures of the formed hydrates were not determined, the interactions between THF and TBAC in hydrate phase were still unknown. More work in learning the structures of the THF + TBAC + CH4 hydrate is required.

4. CONCLUSIONS Three phase equilibrium conditions of CH4 hydrate forming systems containing THF + TBAC were measured with temperature ranging from 279.6 to 301.5 K. The significance of TBAC concentration on equilibrium conditions was discussed. The phase equilibrium data were obtained by an isochoric pressure-search method. The results showed that the addition of THF + TBAC mixtures caused the methane hydrate phase equilibrium boundaries to remarkably shift to higher temperatures and lower pressures. Moreover, the thermodynamic stability of THF + TBAC + CH4 mixed system was more stable than that of the TBAC + CH4 single additive system. The effect of additive mixtures on improving the hydrate’s equilibrium conditions was progressively strengthened with the increasing salt concentration from 0.021 TBAC to 0.340 TBAC in the solution mixed with 0.019 THF. However, such a trend was found to be opposite to that of the 0.019 THF + TBAC + CH4 system at 0.058 THF. In stabilizing hydrate crystals, the TBAC played a positive role when the concentration of THF was low, but a negative role when the concentration of THF was high. E

DOI: 10.1021/acs.jced.6b00405 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jced.6b00405 J. Chem. Eng. Data XXXX, XXX, XXX−XXX