Phase Equilibria and the Crystallographic Properties of TBAB–CO2

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Phase Equilibria and the Crystallographic Properties of TBAB−CO2 Semiclathrate Hydrates Xuebing Zhou,†,‡,§ Zhen Long,† Yong He,† Xiaodong Shen,† and Deqing Liang*,† †

Key Laboratory of Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China ‡ Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China § Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, PR China ABSTRACT: Understanding the crystal properties of semiclathrate hydrates is important for their potential application in gas storage and separation. In this work, the phase equilibria of TBAB−CO2 hydrates were measured and hydrates formed from the solutions where TBAB concentrations ranged from 0.01 to 0.32 mass fraction were analyzed using Raman and powder X-ray diffraction. Results showed that the equilibrium pressure of TBAB−CO2 hydrate was more sensitive to temperature than that of simple CO2 hydrate, and the equilibrium temperature also got increased with a rise in TBAB concentration. By measuring the hydrate samples formed at 274 K and 3 MPa, the simple CO2 hydrates could be found distributed randomly and discontinuously in the samples, while the TBAB−CO2 hydrate crystals were relatively large. The simple CO2 hydrate was found to coexist with TBAB− CO2 hydrate, and the TBAB·38H2O was the only semiclathrate hydrate structure regardless of the initial concentrations of TBAB. When the TBAB concentration was high, the simple CO2 hydrate was assumed to grow competitively with TBAB−CO2 hydrate.

1. INTRODUCTION It is now widely accepted that the rising levels of carbon dioxide (CO2) in the Earth’s atmosphere as a consequence of burning fossil fuels is responsible for global warming. Carbon capture and sequestration (CCS) is considered to be one of the principal means to reduce the excessive emission of CO2 to the atmosphere. Semiclathrate hydrates are renowned to have great potential for CCS.1−3 These solid compounds mostly consist of ionic salts, gas and water molecules, and their forming or melting process can be operated at room temperature and a moderate pressure ranging from 273 to 300 K.4−6 Therefore, they are suggested to be more reusable and energy saving than traditional CCS materials such as lime or limestone slurry.7,8 Taking advantage of their large capacity of latent heat, semiclathrate hydrates are also proposed to be applied in air conditioning of living environments.9−12 Canonical clathrate hydrates consist of hydrogen-bonded host water cages and encage guest molecules inside via van der Waals interactions. Each guest molecule is encaged into different types of cages ordered in three-dimensional structures according to the size of the guest molecules. For example, methane is preferentially encaged in the pentagonal 512 dodecahedral (D) cage, while CO2 that has relatively large size prefers to occupy the 51262 tetrakidecahedral (T) cage. Different from the canonical gas hydrates where the host lattice is wholly water molecules, the semiclathrate hydrates allow some guest molecules to form a part of the host lattice.13,14 The tetra-n-butylammonium (TBA) and phosphonium (TBP) salts © XXXX American Chemical Society

are well-known as semiclathrate hydrate formers where the quaternary ammonium cations can fit into hydrate cages and anions are included in the hydrate lattice by replacing one or two water molecules.2,6,15 This type of hydrate structure significantly lowers the equilibrium pressure when adding extra gas molecules into the hydrates. Meanwhile, the D cage is reported to be only empty cages formed in semiclathrate hydrates so that gas molecules with different size and shape tend to have a significant difference in cage occupancy.16−20 Among the semiclathrate hydrates being investigated, tetra-nbutylammonium bromide (TBAB) is one typical hydrate former due to its broad availability and low toxicity.21−23 TBAB hydrates can be stably preserved at atmospheric pressure and room temperature. However, the phase equilibrium and the crystal structure are greatly influenced by the TBAB concentrations in the aqueous phase. Now, at least four types of TBAB hydrate structures have been found which are TBAB· 38H2O, TBAB·32H2O, and TBAB·24H2O.24 TBAB·24H2O, TBAB·26H2O, and TBAB·32H2O are tetragonal structures that consist of 4 51263 (P) cages, 16 51262 (T) cages, and 10 512 (D) cages per ideal unit cell, whereas TBAB·38H2O belongs to an orthorhombic structure which contains 4 P, 4 T, and 6 D cages per ideal unit cell.25 These types of semiclathrate hydrates are reported to have different enthalpies of fusion and gas storage Received: October 9, 2017 Accepted: February 23, 2018

A

DOI: 10.1021/acs.jced.7b00884 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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capacities.6,26 Rodionova et al.24 found that the dissociaition enthalpies of TBAB hydrates increased with a rise in hydration number. Fukumoto et al.27 concluded that the hydration number was inclined to increase when the TBAB concentration in the aqueous phase decreased. However, different types of crystals are found to not crystallize alone; a clear view of one semiclathrate hydrate is hard to achieve. One cannot expect only one type of hydrate is obtained during a simple crystallization, especially when hydrate-based technology is applied in industrial field.23,28 Therefore, it is necessary to understand the properties of the mixed clathrate hydrates. Considering the importance of TBAB concentrations on the crystal structures of the formed gas hydrates, three phase equilibria of the TBAB−CO2 hydrates were measured in this work. The dependence of the crystallographic properties of TBAB−CO2 semiclathrate hydrate on TBAB concentration was studied using Raman spectroscopy and powder X-ray diffraction (PXRD). The hydrate samples were prepared at 274 K, 3 MPa where both TBAB−CO2 semiclathrate hydrate and simple CO2 hydrate were available for crystallization so that the relationship between the TBAB−CO2 semiclathrate hydrate and simple CO2 hydrate during crystallization could be investigated.

an internal volume of 1.0 L was connected to the reactor which was used to precool the gas and provide a certain amount of gas to the reactor. The reactor and the gas reservoir were both immersed in a thermostatic bath. A Setra smart pressure transducer (model SS2, Boxborough, MA) and a platinum resistance thermometer (PT 100) were inserted in the reactor and the gas reservoir, respectively. The uncertainties of the pressure transducers and the platinum resistance thermometers were ±0.01 MPa and ±0.1 K, respectively. This hydrate forming system was not only available to equilibrium measurements but was also used to prepare the hydrate samples used for microscopic measurements. The crystallographic properties of the hydrate samples in this work were charactorized by Raman spectroscopy and a powder X-ray diffractometer, respectively. The Raman spectrometer (Horiba, LabRam HR) was used to measure the gas distributions in the hydrate phase. The spectrometer was equipped with a single monochromator of 600 grooves/mm grating and a multichannel air-cooled charge-couple device (CCD) detector. An Ar+ laser operating at 532 nm with a maximum power of 50 mW was used. The silicon crystal standard of 520.7 cm−1 was employed to calibrate the subtractive spectrograph. A cooling stage (Linkam BCS) was used to prevent samples from melting during measurements. A PXRD (X’Pert Pro MPD) using Cu Kα radiation (40 kV, 40 mA) was employed to measure the structures of hydrate samples. The samples were scanned at a scanning rate of 2°/ min from 2θ = 5 to 80° with a step of 0.017°. The samples were paved on a stage chilled by liquid nitrogen so that the hydrate samles were not assumed to melt significantly during scanning. 2.3. Procedure. In this work, hydrate equilibrium conditions for CO2, TBAB and water system was measured by the isochoric pressure search method, which was commonly used in blind reactors.30−33 In the first step, about 40 mL of aqueous solution was loaded in the reactor. The reactor was then sealed and immersed in the thermostatic bath together with the gas reservoir. To eliminate air, the hydrate forming system was first evacuated by a vacuum pump for 10 min. In the second step, the gas reservoir was loaded by CO2 to the desired value and the data collector was started to record the pressure and temperature of the reactor and the gas reservoir. When the pressure became stable, a certain amount of CO2 was injected from the gas reservoir, while the stirrer in the reactor was started at a speed of 300 rpm to agitate the aqueous phase. In the third step, the temperature of the system was decreased to form hydrates. Usually, the hydrate formation in a constant volume reactor was indicated by a rapid and continuous pressure drop. As the pressure became stable, the temperature was allowed to increase slowly and the formed hydrate started to dissociate. In the vicinity of the dissociation point, the temperature was set to increase 0.1 K/h to avoid the occurrence of metastable hydrate crystals. The point where the slope of pressure−temperature plot data changed sharply was taken to be the final hydrate dissociation point. Figure 2 depicted a typical isochoric curve for measuring the dissociation point of TBAB−CO2 semiclathrate hydrate. Measurement uncertainties of this system were ±0.01 MPa and ±0.1 K for pressure and temperature, respectively. The hydrate samples used for Raman and PXRD measurements were prepared by the same hydrate forming system as used in hydrate equilibrium measurements. The loading process of TBAB solution and gaseous CO2 is the same as that noted above. The initial hydrate forming conditions were set at 274 K

2. EXPERIMENTAL METHODS 2.1. Materials. The reagents of research-grade TBAB (0.98 mass fraction purity, Tokyo Chemical Industry Co., Ltd.) without further purification and the deionized water made in the laboratory (18.0 MΩ·cm at about 298 K) were used. Six kinds of aquesous solutions with the TBAB concentrations ranging from 0.01 to 0.32 mass fraction were prepared. The uncertainties of the TBAB concentrations were controlled within ±5 × 10−4. The CO2 (0.999 mole fraction purity) was purchased from the Puyuan Gas company (Guangzhou, China). The major impurity in CO2 was N2 which is not easily captured by semiclathrate hydrates compared with CO2.29 Therefore, the results would not be affected by the gas impurities. 2.2. Apparatus. The schematic diagram of the experimental apparatus was illustrated in Figure 1. A custom-made high pressure reactor with an internal volume of 100 mL was used. The reactor was made of stainless steel and can withhold 10 MPa. A magnetic driven stirrer was placed in the reactor to agitate the solution at a speed of 300 rpm. A gas reservoir with

Figure 1. Schematic diagram of the hydrate forming system: (1) reactor, (2) gas reservoir, (3) thermostatic bath, (4) thermometer, (5) pressure transducer, (6) data acquisition, (7) magnetic stirrer, (8) vacuum pump, (9) gas cylinder, (10) pressure relief valve, (11) safety valve, and (12−15) needle valves. B

DOI: 10.1021/acs.jced.7b00884 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 2. Schematic diagram of the isochoric pressure search method used in the hydrate equilibrium measurements.

Figure 3. Three-phase equilibrium data for TBAB + CO2 hydrates. Symbols show aqueous concentrations: ●, pure water; ■, 0.01 TBAB; red ▲, 0.05 TBAB; blue ▼, 0.10 TBAB; green ◆, 0.15 TBAB; brown ◀, 0.20 TBAB; magenta ▶, 0.32 TBAB; ○, pure water;36 red +, 0.05 TBAB;37 red ×, 0.05 TBAB;38 red ∗, 0.05 TBAB;30 blue +, 0.10 TBAB;37 blue ×, 0.10 TBAB;38 blue ∗, 0.10 TBAB;30 green ∗, 0.16 TBAB;30 brown +, 0.19 TBAB;37 magenta +, 0.32 TBAB;37 magenta ∗, 0.32 TBAB.30

and 3 MPa which were high enough to form a simple CO2 hydrate. Although TBAB−CO2 hydrates were reported to dramaticaly increase the equilibrium temperature, subcoolings required for hydrate nucleation could not be ignored, which could significantly prolong the crystallization.34 A high initial forming pressure helped to shorten the crystal nucleation, which was an essential step in commercial applications. To attain the equilibrium state, the samples were kept in the reactor at least 5 days. In the first 2 days, a fast conversion from liquid to solid hydrate phase took place acompanied with a noticeable pressure decrease. During the remaining 3 days, the pressure became stable and water and gas migration were allowed to repair the structural defects in the hydrate phase so that the formed hydrates could gain more stable structures and more proper gas distributions. After that, the reactor was first moved from a thermostatic bath to a cryogenic glovebox chilled by liquid nitrogen and quickly depressurized. Then, samples were shifted out from the reactor and finely grinded in liquid nitrogen. At last, the hydrate samples were preserved in liquid nitrogen. During the Raman and PXRD measurement, hydrate samples were shifted to the sample stages precooled at about 230 K where the hydrate dissociation was negligible.35

Clausius−Clapeyron equation could be used to predict the potential structural transition which was written as39 d ln P /d(1/T ) = −ΔHdiss/z R

(1)

where P and T refer to the absolute pressures and temperatures of hydrate equilibrium, z refers to the compressibility factor, and R is the universal gas constant. The ln P−1/T logarithmic planes for each TBAB concentration were shown in Figure 4. The slope of the ln P−1/T did not vary significantly in a pressure range from 1 to 6 MPa as the TBAB concentration ranged from 0.01 to 0.32 mass fraction. On the basis of the Clausius−Clapyron equation, a change of slope in the ln P−1/ T line suggested a change in the dissociation enthalpy of the solid associated with a structural change.39 However, no inflection point was found in Figure 4. The structural transition was thus not indicated to appear regardless of the changes in the TBAB concentrations which was different from the TBAB− N2 hydrates.39 According to the literature, the basic structure of the formed semiclathrate hydrate structure was also assumed to be orthorhombic.35 3.2. Microscopic Measurements. Raman spectra obtained in the hydrate samples formed from different TBAB solutions were shown in Figure 5. On each sample surafce, 8− 12 measuring spots were chosen randomly. As a result, the crystal structure was not found to be uniform. About two types of spectra had been discovered when the TBAB concentration was below 0.20 mass fraction. The TBAB−CO2 hydrates were characterized by the C−H stretching modes of the butyl groups in the spectral range from 2700 to 3050 cm−1, while the spectra having no peaks at around 2900 cm−1 belonged to simple CO2 hydrate. Therefore, TBAB−CO2 hydrates could coexist with simple CO2 hydrates in dilute TBAB solutions. However, when TBAB concentrations came up to 0.20 and 0.32 mass fraction, the simple CO2 hydrates were not found and the intensities of the C−H stretching modes of the butyl groups were greatly enhanced. The TBAB−CO2 hydrates were suggested to consititute the majority of the solid phase.

3. RESULTS AND DISCUSSION 3.1. Equilibrium Measurements. The equilibrium data were summerized in Figure 3 and Table 1. As seen, the equilibrium pressure and temperature range were controlled within 280−300 K and 1−6 MPa, respectively. The measured equilibrium points were generally in accord with literature values.30,36−38 As the concentration of TBAB increased from 0.01 to 0.32 mass fraction, the equilibrium temperature was found to increase evidently at a certain pressure. The slopes of the equilibrium curves of the TBAB−CO2 hydrates were also higher than that of pure CO2 hydrate. In this case, the equilibrium pressure was found to be more sensitive to temperature. If applied in gas storage, the TBAB−CO2 hydrates would dissociate easily which was induced by a minor fluctuation in temperature. From another aspect, the TBAB− CO2 hydrate dissociation could be precisely controlled by the CO2 vapor pressure which was suggested to be more suitable for air conditioning. Because of the close correlation between the hydrate crystal structure and the hydrate dissociation enthalpies (ΔHdiss), the C

DOI: 10.1021/acs.jced.7b00884 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Affected by the Fermi resonance effect, the CO2 molecules trapped in the structure I hydrate could be recognized by two intense peaks at 1381.3 and 1277.5 cm−1, which were generally in accord with the literature.40−42 Although CO2 molecules were proved to be able to occupy both the 512 and 51262 cages in structure I, it was still difficult to distinguish the split of CO2 peaks only from the Raman spectra.43,44 In the spectra of TBAB−CO2 hydrates, the CO2 peaks were located at 1381.2 and 1374.6 cm−1. It was reported that the CO2 molecules could only be encaged in the 512 cages in the semiclathrate hydrates so that the CO2 peaks would not deviate significantly from those in a simple CO2 hydrate. Judging from the split of the Raman peaks at 1319.2 and 1454.7 cm−1, the structure of the formed semiclathrate hydrates would be orthorhombic.17,35 TBAB·26H2O was also reported to encage the CO2 molecules, but no typical spectrum of TBAB·26H2O was found during the measurements, suggesting that TBAB·38H2O performed better than TBAB·26H2O in absorbing CO2 molecules.17 Another phenomenon found during the measurements was the distribution forms of the different types of hydrate crystals. The simple CO2 hydrates, as shown in Figure 7a, usually distributed randomly and discontinuously in the samples formed from dilute TBAB solutions. It should be ascribed to two aspects. First, the CO2 hydrates cannot grow into large crystals due to their high sensitivity to the ambient environment.45 Combined with agitation, CO2 hydrate crystals can only be randomly distributed. Second, the amount of CO2 hydrate cannot be very large if the TBAB concentration is high. Once formed, the simple CO2 hydrates could be easily mixed with other types of crystals. To the contary, the crystals of TBAB−CO2 hydrates were relatively large and had a flat cross section which could be easily distinguished from the simple CO2 hydrates, as seen in Figure 7b. Considering that the semiclathrate hydrates could remain stable without the participation of gas molecules and the stoichimetric ratio between water and TBAB molecules was also relatively constant, the formed crystals could grow more large and less porous comparing to simple CO2 hydrates. It should be noted that whether the CO2 molecules would take effect on the growth of TBAB−CO2 hydrates, especially when TBAB hydrates could not grow alone, was still unkown. With a large and suitable size, TBAB performed better than CO2 in stabilizing the crystal structures, but the water clusers such as 512 and 51262 cages used for hydrate nucleation were assumed to be produced around CO2 molecules more easier.46−48 PXRD patterns for the hydrate samples formed from different TBAB solutions were shown in Figure 8. The pattern of a simple CO2 hydrate formed from pure water was also obtained for comparison. According to the literature, the diffraction peaks at 2θ = 27.0 and 28.1° were assigned to the Miller index (hkl) of (320) and (321) of structure I, while the diffraction peaks at 2θ = 8.4, 10.9, and 26.4° were assigned to the structure of TBAB·38H2O.35,49 It was clear to see that the structure of TBAB·38H2O gradually dominated the hydrate phase with an increase in TBAB concentration which agreed well with the results obtained from Raman measurements. However, the diffraction peaks of structure I hydrate could still be observed in all of the hydrate samples. It should be noted that the TBAB concentration in TBAB·38H2O crystals was 0.32 mass fraction. Therefore, if some water molecules were consumed for the crystallization of structure I hydrate, the crystallization of TBAB·38H2O would face a water shortage in 0.32 TBAB solution. Unfortunately, the amount of structure I

Table 1. Equilibrium Data of TBAB−CO2 Hydrates mass fraction of TBAB

temperature (K)

pressure (MPa)

0.01

279.1 280.1 280.7 281.3 282.0 282.5 283.6 284.2 284.8 285.3 286.1 286.7 286.9 285.4 286.5 287.3 288.0 288.6 289.2 289.6 286.3 287.2 288.6 289.3 290.0 290.4 286.7 287.8 288.8 289.5 290.2 290.6 287.1 288.3 289.1 289.9 290.6 291.0 291.4

1.48 1.86 2.15 2.50 2.98 3.36 1.51 1.73 2.07 2.42 3.15 3.79 4.27 1.32 1.71 2.20 2.70 3.31 4.02 4.58 1.21 1.63 2.52 3.20 3.98 4.51 1.08 1.45 1.99 2.51 3.17 4.02 0.96 1.31 1.76 2.25 2.81 3.34 3.92

0.05

0.10

0.15

0.20

0.32

Figure 4. Natural logarithms of pressure (ln P) versus reciprocal of temperature (1/T) for TBAB−CO2 hydrates formed from different TBAB solutions. Dashed lines represent linear correlation.

Figure 6 showed the Raman spectra of typical simple CO2 hydrates and TBAB−CO2 hydrates obtained from the samples. D

DOI: 10.1021/acs.jced.7b00884 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 5. Raman spectra of the hydrate samples formed from different TBAB solutions: (a) 0.01 TBAB, (b) 0.05 TBAB, (c) 0.10 TBAB, (d) 0.15 TBAB, (e) 0.20 TBAB, (f) 0.32 TBAB. The spectra of simple CO2 hydrates were marked by red.

Figure 6. Raman spectra of the typical simple CO2 hydrates and TBAB−CO2 hydrates.

Results from the microscopic measurements showed that simple CO2 hydrates could coexist with TBAB−CO2 hydrates, while the three-phase equilibrium of the TBAB solutions was found to be determined by the TBAB−CO2 hydrates according to the equilibrium measurements. On the other hand, in the hydrate based gas separation process, the hydrate forming pressure should be strictly controlled to avoid the formation of simple CO2 hydrates which were relatively weak in gas separations.

hydrates was not significant, and the diffraction pattern obtained from 0.32 TBAB solution did not show new peaks compared to the other diffraction patterns formed from TBAB solution. In this case, it was still difficult to determine whether the residual TBAB transformed into TBAB crystals or some other types of TBAB hydrates. In terms of the kinetics of crystallization, the presence of simple CO2 hydrates that formed from 0.32 TBAB solution suggested that simple CO2 hydrates could grow competitively with TBAB−CO2 hydrates and coexist under high subcooling conditions. E

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no structural transition was assumed to appear. By measuring the hydrate sample formed at 274 K and 3 MPa, a simple CO2 hydrate and TBAB−CO2 hydrate were found to coexist by Raman spectra and powder X-ray diffraction patterns and TBAB·38H2O was the only semiclathrate hydrate structure. In Raman measurements, simple CO2 hydrates were found in the samples formed from the solutions where TBAB concentrations were below 0.20 mass fraction, and they distributed randomly and discontinuously. However, the crystals of TBAB−CO2 hydrates appeared to be relatively large and had a flat cross section. Powder X-ray diffraction patterns revealed that simple CO2 hydrates could even exist in the samples formed from 0.32 TBAB solution. Simple CO2 hydrates were suggested to grow simultaneously with TBAB−CO2 hydrates during crystallization.

Figure 7. Photograph of the simple CO2 hydrate crystals and the TBAB−CO2 hydrate crystals in the samples: (a) CO2 hydrate crystal, (b) TBAB−CO2 hydrate.

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AUTHOR INFORMATION

Corresponding Author

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

Deqing Liang: 0000-0001-7534-4578 Funding

This work was supported by the National Natural Science Foundation of China (51706230, 41473063), CAS Program (KGZD-EW-301). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Kim, S.; Seo, Y. Semiclathrate-based CO2 capture from flue gas mixtures: An experimental approach with thermodynamic and Raman spectroscopic analyses. Appl. Energy 2015, 154, 987−994. (2) Xu, C.-G.; Zhang, S.-H.; Cai, J.; Chen, Z.-Y.; Li, X.-S. CO2 (carbon dioxide) separation from CO2−H2 (hydrogen) gas mixtures by gas hydrates in TBAB (tetra-n-butyl ammonium bromide) solution and Raman spectroscopic analysis. Energy 2013, 59, 719−725. (3) Park, S.; Lee, S.; Lee, Y.; Seo, Y. CO2 capture from simulated fuel gas mixtures using semiclathrate hydrates formed by quaternary ammonium salts. Environ. Sci. Technol. 2013, 47, 7571−7577. (4) Shi, L.-l.; Liang, D.-q. Thermodynamic model of phase equilibria of tetrabutyl ammonium halide (fluoride, chloride, or bromide) plus methane or carbon dioxide semiclathrate hydrates. Fluid Phase Equilib. 2015, 386, 149−154. (5) Shi, L. L.; Yi, L. Z.; Shen, X. D.; Wu, W. Z.; Liang, D. Q. Dissociation Temperatures of Mixed Semiclathrate Hydrates Formed with Tetrabutylammonium Bromide Plus Tetrabutylammonium Chloride. J. Chem. Eng. Data 2016, 61, 2155−2159. (6) Deschamps, J.; Dalmazzone, D. Dissociation enthalpies and phase equilibrium for TBAB semi-clathrate hydrates of N2, CO2, N2 + CO2 and CH4 + CO2. J. Therm. Anal. Calorim. 2009, 98, 113−118. (7) Rochelle, G. T. Amine Scrubbing for CO2 Capture. Science 2009, 325 (5948), 1652−1654. (8) Tohidi, B.; Yang, J. H.; Salehabadi, M.; Anderson, R.; Chapoy, A. CO2 Hydrates Could Provide Secondary Safety Factor in Subsurface Sequestration of CO2. Environ. Sci. Technol. 2010, 44, 1509−1514. (9) Zhou, H.; Vasilescu, C.; Infante Ferreira, C. Heat transfer and flow characteristics during the formation of TBAB hydrate slurry in a coil heat exchanger. Int. J. Refrig. 2016, 64, 130−142. (10) Oshima, M.; Kida, M.; Jin, Y.; Nagao, J. Dissociation behaviour of (tetra-n-butylammonium bromide+tetra-n-butylammonium chloride) mixed semiclathrate hydrate systems. J. Chem. Thermodyn. 2015, 90, 277−281. (11) Oshima, M.; Kida, M.; Nagao, J. Thermal and Crystallographic Properties of Tetra-n-butylammonium Bromide + Tetra-n-butylam-

Figure 8. Powder X-ray diffraction patterns of hydrate samples formed from different TBAB solutions. Asterisks and arrows represent the peaks corresponding to the hexagonal ice and the TBAB·38H2O hydrate with CO2, respectively.

4. CONCLUSIONS The phase equilibrium conditions of TBAB−CO2 hydrate were measured at the TBAB concentration ranging from 0.01 to 0.32 mass fraction. The crystal properties of the hydrates formed at 274 K and 3 MPa were also measured by PXRD and Raman spectroscopy. Results showed that the slope of the phase equilibrium curves was found to be changed compared with the simple CO2 hydrates due to the existence of TBAB and the equilibrium temperature also increased with a rise in TBAB concentration. The slope of the ln P−1/T did not vary significantly in a pressure range from 1 to 6 MPa as the TBAB concentration increased from 0.01 to 0.32 mass fraction; F

DOI: 10.1021/acs.jced.7b00884 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jced.7b00884 J. Chem. Eng. Data XXXX, XXX, XXX−XXX