Phase Equilibrium for Ionic Semiclathrate Hydrates Formed in the

Phase Equilibrium for Ionic Semiclathrate Hydrates Formed in the System of Water ... This article is part of the In Honor of E. Dendy Sloan on the Occ...
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Phase Equilibrium for Ionic Semiclathrate Hydrates Formed in the System of Water + Tetra‑n‑butylammonium Bromide Pressurized with Carbon Dioxide Takayuki Kobori,† Sanehiro Muromachi,‡ and Ryo Ohmura*,† †

Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan Methane Hydrate Research Centre, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, 305-8569, Japan

‡

ABSTRACT: This paper reports the vapor−liquid−hydrate three phase equilibrium conditions of ionic semiclathrate hydrates formed in the system of water + tetra-nbutylammonium bromide pressurized with carbon dioxide. The three phase equilibrium temperatures were measured in the ranges from 278.8 to 287.4 K of temperature and from 0.05 to 0.45 of mass fraction of tetra-n-butylammonium bromide (wTBAB). Measurements were performed at the system pressures of 0.3 MPa, 0.6 MPa, and 1 MPa. For the three system pressures, the equilibrium temperatures increased with the increase in wTBAB for wTBAB < 0.35 and decreased for wTBAB > 0.40. The highest equilibrium temperatures were obtained at wTBAB = 0.35 and 0.40. The equilibrium temperatures of the tetra-n-butylammonium bromide + carbon dioxide hydrates were higher than those of the simple tetra-nbutylammonium bromide hydrates formed under atmospheric pressure. The improvement of the thermodynamic stability of the tetra-n-butylammonium bromide + carbon dioxide hydrates is ascribed to the incorporation of carbon dioxide molecules into the hydrate cages. The data obtained in the present study may be utilized to specify the thermodynamic conditions to form the tetran-butylammonium bromide + carbon dioxide hydrates without forming the simple tetra-n-butylammonium hydrates.

1. INTRODUCTION Clathrate hydrates (hydrates) are crystalline compounds consisting of hydrogen-bonded (host) water molecules, which form cage-like structures, and (guest) other molecules, for example, noble gases and hydrocarbons encapsulated in the cages. Hydrates form mostly at lower temperature under high pressure. The formation conditions of hydrates differ depending on guest substances. For example, at approximately 3 MPa of a system pressure, carbon dioxide hydrates and methane hydrates form at approximately 280 K and 274 K of a system temperature, respectively.1 Carbon dioxide capture utilizing hydrates based on the different formation conditions were investigated.2−4 Hydrates have also a property of storing gases such as natural gases,5 hydrogen,6 and ozone.7,8 Semiclathrate hydrates formed with ionic guest substances, for example, quaternary-ammonium and -phosphonium salts, crystallize at higher temperatures under lower pressures compared to the canonical clathrate hydrates. Anions of the ionic guest substances are also parts of the hydrate cages. There are two types of the ionic semiclathrate hydrate cages: large supercages such as tetrakaidecahedron + pentakaidecahedron face-shared cages and small dodecahedral cages.9−11 In these small cages, a guest gas molecule such as, methane, carbon dioxide, hydrogen, or nitrogen can be incorporated.13,15 Previous studies have reported the feasibility of carbon dioxide capture technology in industries,2−4 for example, the postcombustion carbon capture in power plants and steel making plants, the precombustion in the integrated coal gasification combined cycle power plant and natural gas fields. © XXXX American Chemical Society

In particular, the semiclathrate hydrates formed with tetra-nbutylammonium bromide have been well investigated as media to separate carbon dioxide because these hydrates crystallize at ambient temperature (273 K to 285 K) under atmospheric pressure. Dyadin and Udachin12 first reported the phase equilibrium conditions of the simple tetra-n-butylammonium bromide hydrates formed under atmospheric pressure. This study summarized the phase equilibrium conditions in the form of a temperature (T)−composition diagram and revealed that the equilibrium temperatures of the tetra-n-butylammonium bromide hydrates varied with the mass fraction of tetra-nbutylammonium bromide (wTBAB). Sato et al.18 reported a T−w diagram with the explicit measurement uncertainties and compared T−w data of the tetra-n-butylammonium bromide hydrates measured in the several previous studies. The pressure−temperature diagram of the tetra-n-butylammonium bromide + carbon dioxide hydrates have been reported in the previous studies.4,14−17 We processed these literature data and summarized in the form of a T−w diagram shown in Figure 1. Mohammadi et al.15 and Ye and Zhang14 reported the equilibrium temperatures for wTBAB from 0.05 to Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: June 26, 2014 Accepted: October 9, 2014

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2.2. Apparatus. The experimental apparatus is schematically shown in Figure 2. A 0.75 cm3 aliquot of the tetra-nbutylammonium bromide aqueous liquids with wTBAB from 0.05 to 0.45 was each injected in a test tube. The test tubes were set in a stainless steel high pressure cell equipped with 12 × 8 cm2 glass windows (1). The inner volume of the cell is approximately 1500 cm3. The cell was immersed in a water bath (6). The water temperature was maintained by an immersion cooler and a PID-controlled heater. Two platinum resistance thermometers (Pt100) (3) were inserted into the cell. One was set at the center of the cell and the other was set near a wall of the cell to confirm whether the gradient of temperature distribution inside exists. For measurements of the phase equilibrium temperatures of the system, the temperature of the water bath was measured by the other platinum resistance thermometer after making sure that the temperature inside the cell was equilibrated within ± 0.1 K. The pressure in the cell was measured by a strain-gauge pressure transducer, PHB-A-2MP (2). The carbon dioxide pressure in the cell was maintained at (0.3, 0.6, and 1) MPa through the experiments. The measurement uncertainties with 95 % coverage for the pressure, temperature, and mass fraction were ± 3.2 kPa, ± 0.1 K and ± 8.0·10−4, respectively. The dissociation processes of the tetra-n-butylammonium bromide + carbon dioxide hydrates were observed with a digital camera (Canon, EF180 mm F3.5L Macro USM). 2.3. Experimental Method. The phase equilibrium temperatures were measured for wTBAB from 0.05 to 0.45 at 0.3 MPa, 0.6 MPa and 1 MPa of carbon dioxide pressures. The test tubes were set inside the cell, and then the cell was closed. The cell was flushed with pure carbon dioxide and evacuated by a vacuum pump for five times to decrease the partial pressure of the residual air in the cell until less than 1.0·10−3 Pa. In each test tube, a nickel coated neodymium magnet was put to stir the tetra-n-butylammonium bromide solutions by using another magnet from outside the cell. The temperature of the water bath was first set above the equilibrium temperature of the tetra-n-butylammonium bromide hydrates. To crystallize the tetra-n-butylammonium bromide + carbon dioxide hydrate crystals, liquid nitrogen flowed into a copper tube which was in contact with the bottom of the test tubes to cool the tetra-nbutylammonium bromide aqueous liquids. After the crystallization, the system temperature was incrementally increased with a step of 0.1 K. At each step, the system temperature was maintained for at least 6 h, and images of crystals were obtained by the digital camera to observe the dissociation processes. If hydrate crystals started to dissociate within 6 h, the system temperature was maintained for another 6 h. If no noticeable dissociation was visually observed within 6 h, the system temperature was increased. The dissociation was continued from 6 h to 72 h at each step. As the system temperature increase repeated, the tetra-n-butylammonium bromide + carbon dioxide hydrate was completely dissociated. The system temperature just before the temperature at which all the crystals dissociated was determined as the vapor−liquid−hydrate three phase equilibrium temperature.

Figure 1. Temperature−composition diagram estimated by the interpolation of previous studies4,14−17 in the system of tetra-nbutylammonium bromide + water pressurized with carbon dioxide: gray markers, p = 0.3 MPa; white markers, p = 0.6 MPa; black markers, p = 1 MPa; circles, Duc et al.;4 squares, Ye and Zhang;14 rhombus, Mohammadi et al.;15 triangles, Lin et al.;16 inversed triangles, Li et al.17 p: pressure of carbon dioxide in the system.

0.55. There is a significant scatter and no consistency between the two data sets. At wTBAB = 0.05, the difference in the equilibrium temperatures reported in the previous studies4,14,15,17 reaches approximately 1.2 K. Further measurements to renew the T−w diagram are needed. We performed accurate measurements of the T−w data in this system. Present equilibrium data were compared with those of the simple tetra-n-butylammonium bromide hydrates and the effect of carbon dioxide pressures on the thermodynamic stability of tetra-n-butylammonium bromide hydrates was clarified. The measured data were compared to the literature data, and the reliability of this study was discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. A tetra-n-butylammonium bromide reagent with a ≥ 99 % purity supplied by Sigma-Aldrich and deionized and distilled water were used. The tetra-n-butylammonium bromide aqueous liquids were prepared with the mass fraction of tetra-n-butylammonium bromide (wTBAB) in the range from 0.05 to 0.45. In Table 1, wTBAB is converted to the mole fraction of tetra-n-butylammonium bromide (xTBAB). Carbon dioxide supplied by Japan Fine Products Corp. with the purity of ≥ 99.995 % was used. Table 1. Mass Fraction of Tetra-n-butylammonium Bromide (wTBAB) and the Mole Fraction of Tetra-n-butylammonium Bromide (xTBAB) in the Aqueous Liquids wTBAB

xTBAB

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

2.9·10−3 6.2·10−3 9.8·10−3 0.014 0.018 0.023 0.029 0.036 0.044

3. RESULTS AND DISCUSSIONS The vapor−liquid−hydrate three phase equilibrium conditions in the system of water + tetra-n-butylammonium bromide pressurized with carbon dioxide to 0.3 MPa, 0.6 MPa and 1 MPa were measured in this study. The mass fraction of tetra-nbutylammonium bromide (wTBAB) in the aqueous liquids was

Expanded uncertainties U with 95% coverage are U(wTBAB) = ± 8.0· 10−4; U(xTBAB) = ± 4.4·10−5. B

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Figure 2. Experimental apparatus: (1) high pressure cell; (2) strain-gauge pressure transducer; (3) platinum resistance thermometer; (4) copper tube; (5) camera (Canon, EF 180 mm F3.5L Macro USM); (6) temperature controlled water bath.

from 0.05 to 0.45. The phase equilibrium temperatures (Teq) are summarized in Figure 3 and Table 2. Figure 3 shows the

Table 2. Phase Equilibrium Temperatures of Semiclathrate Hydrates Formed in the System of Water + Tetra-nbutylammonium Bromide Pressurized with Carbon Dioxide for the wTBAB from 0.05 to 0.45a Teq/K wTBAB

atmospheric pressure18

p = 0.3 MPab

p = 0.6 MPab

p = 1 MPab

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

281.0 282.4 283.2 284.6 285.3 285.9 285.8 285.8

278.8 281.6 283.1 283.9 284.6 285.9 286.5 286.5 286.4

280.7 283.2 284.6 285.3 285.9 285.9 286.9 286.9 286.2

282.3 285.0 286.1 286.9 287.1 287.3 287.3 287.4 287.2

Expanded uncertainties U with 95 % coverage are U(wTBAB) = ± 8.0· 10−4; U(Teq) = ± 0.1 K; U(p) = ± 3.2 kPa. bNominal value: 0.3 MPa, 0.299 MPa to 0.301 MPa; 0.6 MPa, 0.600 MPa to 0.604 MPa; 1 MPa, 0.999 MPa to 1.003 MPa.

a

Figure 3. T−w diagram of the semiclathrate hydrates formed in the system of tetra-n-butylammonium bromide + water pressurized with carbon dioxide: □, p = 0.3 MPa (this work); ◊, p = 0.6 MPa (this work); △, p = 1.0 MPa (this work); ●, The simple tetra-nbutylammonium bromide semiclathrate hydrates formed under atmospheric pressure.18 p: system pressure.

for p = 1 MPa. These two equilibrium data are consistent with the first measured data within ± 0.1 K that corresponds to the actual measurement uncertainty of temperatures. From these results, the reliability of the present measurements was supported. For the system pressure (p) = 0.3 MPa, 0.6 MPa, and 1 MPa at wTBAB = 0.05, the lowest equilibrium temperature was 278.8 K, 280.7 K, and 282.3 K, respectively. For p = 0.3 MPa and 0.6 MPa at wTBAB = 0.35 and 0.40, the highest equilibrium temperature was 286.5 K and 286.9 K, respectively. For p = 1 MPa, the highest equilibrium temperature was 287.4 K. In Table 3, for p = 0.3 MPa, the ΔTeq was approximately 0.6 K for wTBAB from 0.05 to 0.45 except at wTBAB = 0.25. For p = 0.6 MPa, ΔTeq varied with wTBAB. At wTBAB = 0.10, ΔTeq was 2.1 K. For wTBAB from 0.20 to 0.30, ΔTeq decreased with the increase in wTBAB. ΔTeq was approximately 1.0 K for wTBAB from 0.35 to 0.40. At wTBAB = 0.45, ΔTeq decreased to 0.4 K. For p =

temperature−composition diagram in this system. The equilibrium temperatures were measured from 278.8 K to 287.4 K at the three system pressures. At wTBAB = 0.05, the equilibrium temperature was the lowest and increased with the increase in wTBAB. For wTBAB > 0.40, the equilibrium temperature decreased with the increase in wTBAB. Equilibrium temperatures of the tetra-n-butylammonium bromide hydrates formed under atmospheric pressure18 were together shown in Figure 3. For 0.05 < wTBAB < 0.45, all of the equilibrium temperature data were higher than those of the simple tetra-nbutylammonium bromide hydrates. The increase in the equilibrium temperatures from the simple tetra-n-butylammonium bromide hydrates was defined as ΔTeq and was summarized in Table 3. To ensure the measurement reliability, we twice performed the measurements at wTBAB = 0.05 and 0.40 C

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Table 3. Increase (ΔTeq) in the Phase Equilibrium Temperatures of Tetra-n-butylammonium Bromide + Carbon Dioxide Hydrates from Those of the Simple Tetra-nbutylammonium Bromide Hydrates Formed under Atmospheric Pressure for wTBAB from 0.10 to 0.45a ΔTeq/K wTBAB

p = 0.3 MPab

p = 0.6 MPab

p = 1 MPab

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

0.5 0.7 0.7 0.0 0.6 0.6 0.7 0.6

2.1 2.2 2.1 1.3 0.6 1.0 1.1 0.4

3.9 3.7 3.7 2.5 2.0 1.4 1.6 1.4

Expanded uncertainties U with 95 % coverage are U(wTBAB) = ± 8.0· 10−4; U(Teq) = ± 0.1 K; U(p) = ± 3.2 kPa. bNominal value: 0.3 MPa, 0.299 MPa to 0.301 MPa; 0.6 MPa, 0.600 MPa to 0.604 MPa; 1 MPa, 0.999 MPa to 1.003 MPa.

a

1 MPa, ΔTeq at wTBAB = 0.10 was 3.9 K. ΔTeq decreased with the increase in wTBAB. ΔTeq was approximately 1.4 K for wTBAB from 0.35 to 0.45. For any of the system pressures, ΔTeq has a positive value. This temperature increase may be ascribed to the encapsulation of carbon dioxide molecules into cages of the tetra-nbutylammonium bromide semiclathrate hydrates. Very recently, the hydrate phase composition of the tetra-n-butylammonium bromide + carbon dioxide hydrates was determined to be tetran-butylammonium bromide·38H2O·1.85CO2 at p = 1.08 MPa and at wTBAB = 0.10 by single crystal X-ray diffraction measurements.21 In Figure 3, the deflection of the T−w curve was observed for p = 0.3 MPa and p = 0.6 MPa. At p = 0.3 MPa, for wTBAB from 0.05 to 0.25, the T−w equilibrium data exhibit a continuous curve. For wTBAB from 0.25 to 0.30, the equilibrium temperature increased sharply. For wTBAB from 0.25 to 0.45, T−w equilibrium data exhibit another continuous curve. At p = 0.6 MPa, for wTBAB from 0.05 to 0.30, T−w equilibrium data exhibit a continuous curve. For wTBAB from 0.30 to 0.35, the equilibrium temperature increased sharply. For wTBAB from 0.30 to 0.45, T−w equilibrium data exhibit another continuous curve. Sato et al.17 reported the deflection of the T−w curve of the tetra-n-butylammonium bromide hydrates at wTBAB = 0.20. Gaponenko et al.19 and Shimada et al.20 performed the single crystal structure analysis by X-ray diffraction measurements and reported the polymorphism of the tetra-n-butylammonium bromide hydrates formed under atmospheric pressure. The deflection of the T−w curve is related to the different thermodynamic stabilities of the hydrates having different hydration numbers. The deflection of the T−w curve at 0.3 MPa and 0.6 MPa in this study suggest the polymorphism of the tetra-n-butylammonium bromide + carbon dioxide as is the case without carbon dioxide. In Figure 4, the equilibrium temperatures measured in this study were compared with the literature data4,14−17 for each system pressure. The equilibrium temperatures of the previous studies were interpolated with quadratic function and were illustrated in the form of the T−w diagram. Figure 4 panels a, b, and c show each comparison of the equilibrium temperatures when the system pressure was elevated to 0.3 MPa, 0.6 MPa, and 1 MPa, respectively. In Figure 4a, at wTBAB = 0.05 the

Figure 4. Comparison of the phase equilibrium temperatures of tetran-butylammonium bromide + carbon dioxide hydrates between this study and interpolated literature data: gray ○, Duc et al.;4 gray □, Ye and Zhang;14 gray △, Mohammadi et al.;15 gray ◊, Lin et al.16; gray ▽, Li et al.;17 black □, ◊, △ (this work): (a) p = 0.3 MPa; (b) p = 0.6 MPa; (c) p = 1.0 MPa. p: system pressure.

present data were approximately 0.7 K lower than the data reported by Duc et al.4 At wTBAB = 0.25, the present data was 0.2 K higher than the data reported by Mohammadi et al.15 In Figure 4b, at wTBAB = 0.05, the data of this study were approximately 1.3 K lower than the data reported by Li et al.17 and approximately 1 K lower than the data measured by Duc et al.4 At wTBAB = 0.05, 0.25 and 0.30, the current data were approximately 0.1 K to 0.8 K lower than the data reported by Mohammadi et al.15 At wTBAB = 0.05 and 0.10, the data of this study was approximately 0.1 K to 0.2 K lower than the data by Ye and Zhang.14 In Figure 4c, at wTBAB = 0.05, the data measured in this study were consistent with the data of Ye and Zhang14 and approximately 1.0 K lower than the data reported D

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by Li et al.17 For wTBAB = 0.05, 0.10, 0.15, 0.25, and 0.35, the data measured in this study were approximately 0.3 K to 0.9 K lower than the data reported by Mohammadi et al.15 At wTBAB = 0.10, the present data were approximately 0.2 K higher than data reported by Ye and Zhang.13 Uncertainties of temperature in the previous studies14,15,17 were ± 0.1 K and the mutual uncertainty with this study was ± 0.2 K. The difference between the equilibrium temperatures of this study and those of the previous studies mostly exceeded the mutual uncertainty. These temperature−composition diagrams may be efficiently utilized for the application of the tetra-n-butylammonium bromide + carbon dioxide hydrates to the carbon dioxide separation. In the processes of carbon dioxide capture using hydrates, only the tetra-n-butylammonium bromide + carbon dioxide hydrates should be formed because the simple tetra-nbutylammonium bromide hydrates can form without encapsulating carbon dioxide molecules. The measured T−w data show the conditions to form the tetra-n-butylammonium bromide + carbon dioxide hydrates without forming the tetra-n-butylammonium bromide hydrates for the system pressure up to 1 MPa. The temperature, pressure, and wTBAB for the efficient operation of the carbon dioxide separation can be determined. In addition, for wTBAB from 0.35 to 0.40, the operation temperature can be set at the nearest value to ambient temperature.

REFERENCES

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4. CONCLUSIONS The temperature−composition phase equilibrium diagrams of the ionic semiclathrate hydrates formed in the system of tetran-butylammonium bromide + water pressurized with carbon dioxide to 0.3 MPa, 0.6 MPa, and 1 MPa were measured in this study. For wTBAB from 0.05 to 0.40, the phase equilibrium temperatures increased as wTBAB increased. The equilibrium temperatures were the highest at wTBAB = 0.35 and 0.40 and decreased with the increase in wTBAB from wTBAB = 0.40. The deflection of the T−w curve was observed for p = 0.3 and 0.6 MPa though it disappeared at 1 MPa. This fact suggested that the polymorphism of the tetra-n-butylammonium bromide + carbon dioxide hydrates existed under 1 MPa of a carbon dioxide pressure. The equilibrium temperatures of the tetra-nbutylammonium bromide + carbon dioxide hydrates for p = 0.3 MPa, 0.6 MPa, and 1 MPa were higher than those of the simple tetra-n-butylammonium bromide hydrates formed under atmospheric pressure. The formation conditions of the tetran-butylammonium bromide + carbon dioxide hydrates without the simple tetra-n-butylammonium bromide hydrate formation were measured. In these conditions the carbon dioxide capture would be operated because the formation of the simple tetra-nbutylammonium bromide hydrate formation without carbon dioxide encapsulation is avoided.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +81 45 566 1495. E-mail: [email protected]. Funding

This study was supported by a Keirin-racing-based researchpromotion fund from the JKA Foundation and by JSPS KAKENHI Grant No. 25289045. Notes

The authors declare no competing financial interest. E

dx.doi.org/10.1021/je500589z | J. Chem. Eng. Data XXXX, XXX, XXX−XXX