Crystal Growth of Ionic Semiclathrate Hydrate Formed in CO2 Gas +

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Crystal Growth of Ionic Semiclathrate Hydrate Formed in CO2 Gas + Tetrabutylammonium Bromide Aqueous Solution System Shunsuke Koyanagi and Ryo Ohmura* Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan ABSTRACT: This article reports the visual observations of the formation and growth of ionic semiclathrate hydrate crystals in the system of aqueous solution of tetrabutylammnonium bromide (TBAB) and CO2 gas. The experimental thermodynamic conditions are (i) the temperature from 286 to 290 K for wTBAB = 0.40 and (ii) 282 to 288 K at wTBAB = 0.10, under the common pressure of 2.3 or 3.4 MPa, where wTBAB denotes the mass fraction of TBAB in the aqueous solution. The lower limit of the temperature was set to avoid the formation of simple TBAB hydrate containing no CO2. The hydrate crystals first formed in the bulk of the aqueous liquid as well as at the gas/liquid interface. The former crystals settled downward and the latter grew to form a hydrate film covering the interface. Both of the hydrate crystals then grew into the aqueous liquid. The morphology of hydrate crystals grown in the aqueous liquid varied distinctly depending on the system subcooling ΔTsub. ΔTsub ≡ Teq − Tex, where Teq is the equilibrium temperature and Tex is the system temperature. When ΔTsub is smaller than ∼2.0 K, hydrate crystals with wedge morphology were observed. At the range of ∼3.1 K > ΔTsub > ∼2.0 K, the wedge crystals were replaced by columnar shaped crystals. At ΔTsub > ∼3.5 K, the hydrate crystals with needle and sword-like morphology were observed. The size of the individual hydrate crystals decreased with increasing ΔTsub.

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INTRODUCTION Clathrate hydrates are crystalline solids consisting of host water molecules hydrogen-bonded to form cages that enclose different guest molecules within the cages and are usually stable at low temperature and high-pressure conditions. Clathrate hydrates may be positively utilized for various engineering practice because hydrates have large gas-storage capacity, large heat of formation/dissociation, and fractionation of the guest mixture. There are a number of studies on the positive use of these properties for the applications. The related technologies include storage of natural gas1 and hydrogen,2 ground/ocean sequestration of CO2,3−5 development of heat pump/refrigeration system utilizing the heat of hydrate formation/dissociation,6 and gas separation technology.7,8 One of the greatest obstacles for the development of the hydrate-based technologies is the high pressure required for the hydrate formation. The breakthrough on this problem may be the usage of semiclathrate hydrates formed with ionic guest substances such as tetrabutylammonium bromide (TBAB). These hydrates have the property of being formed around ambient temperature under atmospheric pressure. There are many phase-equilibrium studies, reported in the literature, on the remarkable reduction in the equilibrium pressure for hydrate formation by the use of the guests that form the ionic semiclathrate hydrates.9−13 It is important to understand the dynamic/kinetic characteristics of the hydrate crystal growth as well as the phase-equilibrium properties. The crystal morphology of hydrates is also one of the important factors. Thus, it is necessary to systematically figure out a dynamic behavior of the hydrate crystal growth and the variations in the hydrate crystal morphology. © 2013 American Chemical Society

An understanding on the crystal growth behavior of semiclathrate hydrates formed with ionic guest substances is still limited despite that some progress has been made for the past decade. The previous studies on the crystal growth of semiclathrate hydrates formed with ionic guest substances are reviewed below. Shimada et al.14,15 was the first to provide the detailed observation of the crystal growth of the simple ionic semiclathrate hydrate formed with TBAB. They observed and measured the growth rates of TBAB semiclathrate hydrate crystals growing from aqueous liquid under the atmospheric pressure. From their study, the crystal morphology varied distinctly depending on the system subcooling ΔTsub. Oyama et al.16 reported the phase-equilibrium diagram for TBAB hydrates and observation of TBAB hydrate crystals under atmospheric pressure. They observed two types of TBAB hydrate crystals at 282.5 K for wTBAB = 0.357. The two types of hydrate have different transmittance, refraction, and crystal morphology. One had columnar shape, and the other was an undefined form composed of thin crystals. Sakamoto et al.17 reported an experimental study on the formation of the two new semiclathrate hydrates with tetrabutylphosphonium chloride (TBPC) and tetrabutylammonium acrylate (TBAAc). From their study, the major morphology in both systems was a columnar shape, but hexagonal plate crystals were observed for wTBPC = 0.10 in the TBPC system. It was also confirmed that the Received: January 25, 2013 Revised: March 19, 2013 Published: March 19, 2013 2087

dx.doi.org/10.1021/cg4001472 | Cryst. Growth Des. 2013, 13, 2087−2093

Crystal Growth & Design

Article

Figure 1. Schematic diagram of the apparatus for hydrate crystal growth. The temperature inside test cell, T, was maintained within ±0.1 K at the prescribed level by keeping the test cell set at the ethylene glycol solution bulk. T was measured by a Pt-wire thermometer inserted through a port at the bottom of the test cell into the TBAB aqueous liquid phase. The pressure inside the test cell, P, was controlled by supplying the CO2 gas from a gas cylinder through a pressure-regulating valve. P was measured by a strain-gauge pressure transducer with the uncertainty of ±0.02 MPa. About 2.5 cubic centimeters of TBAB aqueous solution was poured in the lower half of the test cell to form a pool. The air in the test cell was replaced with a CO2 gas supplied from the high-pressure cylinder through the pressure-regulating valve, by repeating the pressurization of the cell with CO2 gas and evacuating it from the cell. After setting P at a prescribed level by placing the guest gas, T was decreased to form a hydrate (and possibly, simultaneously, ice) and then raised to a level higher by 1 K than Teq corresponding to the system pressure P, thereby dissociating the hydrate crystals. After visual confirmation of the dissociation of all the hydrate crystals, the test cell was manually oscillated to ensure the saturation of the aqueous liquid with CO2. Subsequently, T was set at a prescribed level Tex, which is lower than Teq in order to observe the formation and growth of the hydrate crystals in the test cell. The reason why we first form a hydrate is to shorten the induction time for hydrate nucleation by utilizing the memory effect21,22 of the prior hydrate formation. Because the effect of the memory on the crystal growth behavior was concerned, we performed some experiments without the memory effect and confirmed that crystal growth behavior and crystal morphology were identical with those of the memory effect. The experimental thermodynamic conditions are (i) the temperatures from 286 to 290 K for wTBAB = 0.40 and (ii) 282 to 288 K at wTBAB = 0.10, under the common pressure of 2.3 or 3.4 MPa, where wTBAB denotes the mass fraction of TBAB in the aqueous solution. The lower limit of the Tex was set at 286 K at wTBAB = 0.40 and 282 K at wTBAB = 0.10 to avoid the formation of simple TBAB hydrate containing no CO2 (Figure 2). We performed two or three experimental runs at some experimental conditions and confirmed the reproducibility of the crystal-growth behavior and crystal morphology.

hydrate crystals grown at greater ΔTsub are finer than those at smaller ΔTsub. Ye and Zhang9 reported the phase diagram of CO2 + TBAB double hydrate and visual observations of CO2 + TBAB double hydrate and simple TBAB hydrate. The morphology of CO2 + TBAB double hydrate crystals for w = 0.10 and 0.19 is similar but different from that of the simple TBAB hydrate. The size of the hydrate crystals depends on ΔTsub, but the morphology of that does not for w = 0.10 and 0.19. However, their observations of CO2 + TBAB double hydrate were exclusively conducted under the two w−T−P conditions ((i) w = 0.19, Tex = 283.2 K, and P = 2.525 MPa; (ii) w = 0.19, Tex = 283.6 K, and P = 3.793 MPa). They provided several snapshots of semiclathrate hydrate crystals grown in the aqueous liquid. The macroscopic behavior of crystal growth in the gas−liquid system was not investigated. Importantly, it is unknown in the study by Ye and Zhang whether the CO2 + TBAB semiclathrate hydrate crystals preferentially form at the gas/liquid interface as commonly observed in the ordinary clathrate hydrate forming systems.18,19 Thus, there is still a room for a further study on the dynamic behavior of the hydrate crystal growth and the variations in morphology of the CO2 + TBAB double hydrate crystals. The objective of this study is to observe the hydrate crystal-growth behavior and analyze the observations in terms of the driving force for the crystal growth.

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APPARATUS AND PROCEDURE

The liquid water used in the experiments was laboratory-made distilled liquid water. TBAB aqueous solution was prepared with a solid reagent of TBAB (99%, Aldrich Chemical Co.), and the gas was CO2 (99.995%, Taiyo Nippon Sanso Co.). TBAB is known to be a chemically stable compound around a room temperature under atmospheric pressure. The long-life stability of hydrate-forming TBAB aqueous solution was also confirmed in the course of the commercialization of the refrigeration system utilizing the TBAB hydrate as the cool energy storage medium.20 Figure 1 schematically illustrates the main portion of the experimental apparatus: a test cell made of a stainless-steel cylinder with a pair of flange-type glass windows of 20 mm in diameter, and a microscope-camera system. The inner space of the test cell to hold the test fluids and hydrate crystals was 60 mm in diameter and 2.0 mm in axial length.

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RESULTS AND DISCUSSION The experimental thermodynamic conditions are (i) the temperatures from 286 to 290 K for wTBAB = 0.40 and (ii) 282 to 288 K for wTBAB = 0.10, under the pressure of 2.3 or 3.4 MPa. 2088



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digital images hydrate crystal growth were captured every five minutes. The time for the completion of hydrate growth was determined by comparing the sequential images. The hydrate crystals first formed in the bulk of the aqueous liquid as well as at the gas/liquid interface (Figure 3a). Both of the hydrate crystals then grew into the aqueous liquid (Figure 3b−d). Subsequent nucleation was not observed in the bulk of the aqueous liquid, but the crystal growth continued (Figure 3e). The hydrate crystal growth finally stopped in 3.0 h (Figure 3f). Figure 4 shows sketches illustrating the crystal growth behavior. The hydrate crystals first formed in the bulk of the aqueous liquid as well as at the gas/liquid interface. Both of the hydrate crystals then grew into aqueous liquids. This process of hydrate formation and growth was commonly observed in all experimental systems. In terms of nucleation, it would be inferred that hydrate crystals nucleate on random point in the bulk of the aqueous liquid and at the gas/liquid interface. Figures 5 to 9 show the crystal growth under the various conditions. The difference in the experimental conditions between Figures 3 and 5 or Figures 6 and 7 is only the subcooling temperature ΔTsub. From the comparison of these figures, it was found that the crystal growth rate depends on ΔTsub and decreased with a decrease in ΔTsub. At wTBAB = 0.10, growth of the hydrate crystals continued for 2 h at ΔTsub = 6.6 K and 10 h at ΔTsub = 1.6 K. Similarly, at wTBAB = 0.40, the hydrate crystal growth completed in 4 h at ΔTsub = 3.9 K and 38 h at ΔTsub = 1.5 K. Figure 8 shows the sequential images at wTBAB = 0.10, Tex =282.9 K (4.6 K), P = 2.25 MPa. By comparing Figure 3 with Figure 8, we may know whether crystal growth and morphology depend on the experimental pressure or not. The experimental pressure of Figures 3 and 8 was approximately 2.3 and 3.4 MPa, respectively. The difference in general behavior and rate of TBAB + CO2 hydrate growth due to the difference in the pressure was not noticeable. The effect of the mass fraction difference may be known from the comparison of Figures 5 and 7. As comparing Figures 5 with 7, the difference in the process of crystal growth of TBAB + CO2 hydrate due to the mass fraction difference was not clearly observed. In the present study, no crystal growth was observed at ΔTsub < 1.0 K. Hydrate crystal growth at the interface of CO2 gas and TBAB aqueous solution was observed in the present study. The preferential formation of hydrate crystals at the guest fluid/liquid water interface is the well-known phenomenon commonly observed in the various hydrate-forming systems. There are a number of theoretical and experimental studies on the controlling mechanisms of the hydrate formation at guest/ water interfaces.23,24,33,34 However, there is no definite conclusion on the mechanism. Some of the previous studies indicate that the heat-transfer would be the rate-controlling process,25−28 while the significant effect of mass transfer of the guest compounds in the liquid was discussed in the other studies.29−31 The rate-controlling mechanism of semiclathrate hydrate formation at the interface of gaseous CO2/TBAB aqueous solution should be the future target of the similar detailed theoretical analysis. In the present study, hydrate crystal growth in the bulk of TBAB aqueous solution was also observed. Observation of similar hydrate crystal growth in the liquid water was also previously reported in the system with pure water and carbon dioxide, fluorocarbons, methane, or hydrocarbon mixtures. The crystal growth of TBAB + CO2 hydrate in the aqueous phase may be explained by considering mass transfer of dissolved CO2 to the hydrate-crystal surfaces in contact with aqueous liquid presaturated with CO2. The explanation is given below based

Figure 2. Experimental conditions and TBAB + CO2 hydrate phase diagram for the CO2 + TBAB + H2O system. The symbols denote the experimental data at different TBAB mass fractions: wTBAB = 0.400 (●), Deschamps et al.;10 wTBAB = 0.427 (⧫), wTBAB = 0.100 (○), Arjmandi et al.;11 wTBAB = 0.410 (■), wTBAB = 0.100 (□), Lee et al.;12 wTBAB = 0.100 (◊), Mohammadi et al.;13 wTBAB = 0.100 (Δ) Ye and Zhang;9 The dotted line or dashed−dotted line was the predicted line at wTBAB = 0.400 or wTBAB = 0100, respectively. (▲) and (▼) plots are the experimental conditions at wTBAB = 0.400. (+) and (×) plots are the experimental conditions at wTBAB = 0.100, respectively.

Observational experiments of crystal growth were performed at 21 temperature −pressure conditions as listed in Table 1. The representative observational results are shown here. Figure 3 shows the sequential images of TBAB + CO2 hydrate growing at wTBAB = 0.10, Tex = 284.3 K (4.6 K), and P = 3.40 MPa. The time was set as t = 0 when first hydrate crystal was observed. The Table 1. Experimental Conditions and ΔTsub exptl

wTBAB

P (MPa)

ΔTeq (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

0.10

2.3

4.6 3.6 2.6 1.6 0.6 6.6 5.6 4.6 3.6 2.6 1.6 0.6 3.0 2.1 1.7 0.9 3.9 3.2 2.6 1.5 0.9

3.4

0.40

2.3

3.4

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Figure 3. Sequential videographs of TBAB hydrate crystal growth at wTBAB = 0.10, P = 3.40 MPa, and Tex = 284.3 K (ΔTsub = 4.6 K). The time elapsed after the visual recognition of the first hydrate crystal in the experimental system is indicated below each videograph.

Figure 4. Schematic illustration of hydrate crystal growth behavior.

Figure 5. Sequential videographs of TBAB hydrate crystal growth at wTBAB = 0.10, P = 3.32 MPa, and Tex = 286.4 K (ΔTsub = 2.6 K).

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on the mass-transfer mechanism originally presented by Ohmura et al.32 The concentration of CO2 at the temperature before experiment, Tpri (a temperature higher by 1 K than Teq, the CO2−hydrate−aqueous liquid equilibrium temperature), is less than that at Teq. The concentration of CO2 at Tex (the experimental temperature at which the hydrate formation is observed) is less than that at Teq and Tpri. Consequently, the TBAB aqueous liquid phase is supersaturated with CO2 with respect to TBAB + CO2 hydrate formation when the experimental system is cooled to Tex. If a hydrate film formed at the gas/liquid interface, the CO2 concentration in TBAB aqueous solution adjacent to the hydrate film surface should have decreased, whereas the concentration in the bulk of the liquidwater phase hardly decreased. The difference in concentration in CO2 thus generated of the TBAB aqueous liquid phase must cause a mass transfer of CO2 from the bulk to the hydrate film and crystal surfaces, thereby resulting in the hydrate crystal growth in the aqueous liquid phase.

CRYSTAL MORPHOLOGY

The observed variations in the crystal morphology are arranged on the diagrams of P vs ΔTsub in Figure 10. Figure 10 summarizes the images observed at the time of the hydrate growth completion. On this figure, the images obtained at wTBAB = 0.10 may appear different from those at wTBAB = 0.40. It should be noted, however, that the apparent difference of the images is due to the difference in the refractive index of the aqueous solutions of different concentrations and is not substantive in terms of crystal morphology. The morphology of hydrate crystals grown in the aqueous liquid varied distinctly depending on the system subcooling ΔTsub. The three crystal morphologies listed below, wedge shape, polygonal columnar, and sword (needle)-shaped were observed. Wedge Shape. When ΔTsub is smaller than ∼2.0 K, hydrate crystals with wedge morphology were observed. As hydrate crystals grew, the edge of the hydrate crystals spread thinly in the across-the-width direction. The width and thickness of wedge2090



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Figure 7. Sequential videographs of TBAB hydrate crystal growth at wTBAB = 0.40, P = 3.37 MPa, and Tex = 288.1 K (ΔTsub = 2.6 K).



shaped hydrate crystals were 0.7−2.0 mm and less than 0.1 mm,

crystals were replaced by columnar-shaped crystals. The thickness of columnar-shaped hydrate crystals was 0.5−2.0 mm Sword (Needle)-Shaped. At ΔTsub > ∼3.5 K, hydrate crystals with needle and sword morphology were observed. Sword-shaped and needle-shaped crystals looked like pencils. As

respectively. Polygonal Columnar. At the range of ∼3.1 K > ΔTsub > ∼2.0 K, this crystal morphology was observed. The wedge 2091



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results obtained in the present study, as summarized in Figure 10, are consistent with the observations by Ye and Zhang.9 Figure 11 shows ΔTsub dependence of crystal size. The largest width of the observed hydrate crystals was used as the characteristic size to depict this figure. At wTBAB = 0.40, the size of the largest width of the hydrate crystals was ∼3.0 mm at ΔTsub ≈ 1.7 K and ∼1.0 mm at ΔTsub ≈ 3.5 K under the 3.4 and 2.3 MPa. Similarly, at wTBAB = 0.10, the size of the largest width of the hydrate crystals was ∼1.5 mm at ΔTsub = 2.6 K and ∼1.0 mm at ΔTsub = 4.6 K under the 3.4 and 2.3 MPa. The size of the individual hydrate crystals decreased with increasing ΔTsub. Besides, the pressure dependency of crystal morphology under 2.3 and 3.4 MPa was not observed.

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Figure 10. Summary of crystal growth observations.

AUTHOR INFORMATION

Corresponding Author

*(R.O.) Phone: +81-45-566-1813. E-mail: [email protected]. ac.jp. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Mori, Y. H. J. Chem. Ind. Eng. (China) 2003, 54 (Suppl.), 1−17. (2) Mao, W. L.; Mao, H. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 708− 710. (3) Brewer, P. G.; Friederich, G.; Peltzer, E. T.; Orr, F. M., Jr. Science 1999, 284, 943−945. (4) Ohmura, R.; Mori, Y. H. Environ. Sci. Technol. 1998, 32, 1120− 1127. (5) Tohidi, B.; Yang, J.; Salehabadi, M.; Anderson, R.; Chapoy, A. Environ. Sci. Technol. 2010, 44, 1509−1514. (6) Ogawa, T.; Ito, T.; Watanabe, K.; Tahara, K.; Hiraoka, R.; Ochiai, J.; Ohmura, R.; Mori, Y. H. Appl. Therm. Eng. 2006, 26, 2157−2167. (7) Seo, Y. T.; Moudravski, I. L.; Ripmeester, J. A.; Lee, J. W.; Lee, H. Environ. Sci. Technol. 2005, 39, 2315−2319. (8) Linga, P.; Adeyemo, A.; Englezos, P. Environ. Sci. Technol. 2007, 42, 315−320. (9) Ye, N.; Zhang., P. J. Chem. Eng. Data 2012, 57, 1557−1562. (10) Deschamps, J.; Dalmazzone, D. J. Therm. Anal. Calorim. 2009, 98, 113−115. (11) Arjmandi, M.; Chapoy, A.; Tohidi, B. J. Chem. Eng. Data 2007, 52, 2153−2158. (12) Lee, S.; Park, S.; Lee, Y.; Lee, J.; Lee, H.; Seo, Y. Langmuir 2011, 27, 10597−10603. (13) Mohammadi, A. H.; Afzal, W.; Richon, D. J. Chem. Eng. Data 2008, 53, 683−686. (14) Shimada, W.; Ebinuma, T.; Oyama, H.; Kamata, Y.; Takeya, S.; Uchida, T.; Nagao, J.; Narita, H. Jpn. J. Appl. Phys. 2003, 42, 129−131. (15) Shimada, W.; Shiro, M.; Kondo, H.; Takeya, S.; Oyama, H.; Ebinuma, T.; Narita, H. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 2005, 61, 65−66. (16) Oyama, H.; Shimada, W.; Ebinuma, T.; Kamata, Y.; Takeya, S.; Uchida, T.; Nagao, J.; Narita, H. Fluid Phase Equilib. 2005, 234, 131− 135. (17) Sakamoto, H.; Sato, K.; Shiraiwa, K.; Takeya, S.; Nakajima, M.; Ohmura, R. RSC Adv. 2011, 1, 315−322. (18) Tanaka, R.; Sakemoto, R.; Ohmura, R. Cryst. Growth Des. 2009, 9 (5), 2529−2536. (19) Servio, P.; Englezos, P. AIChE J. 2003, 49, 269−276. (20) Ogoshi, H.; Takao, S. JFE Techn. Rep. 2004, 3, 1−5. (21) Ohmura, R.; Ogawa, M.; Yasuoka, K.; Mori, Y. H. J. Phys. Chem. B. 2003, 107, 5289−5293. (22) Makogon, Y. F. Hydrates of Natural Gas; Penn Well Publishing: Tulsa, OK, 1981. (23) Mori, Y. H. Energy Convers. Manage 1998, 39, 1537−1557.

Figure 11. ΔTsub dependence of crystal size.

hydrate crystals grew, the hydrate crystals of polygonal columnar became sharper. The thickness of Sword-shaped hydrate crystals was 0.3−0.5 mm. The thickness of needle-shaped hydrate crystals was ∼0.1 mm. It was found from these results that the hydrate crystal morphology varied depending on the system subcooling temperature ΔTsub. The crystal shape changed from sword/ needle-like to polygonal columnar and then wedge with decreasing ΔTsub. In contrast with the simple TBAB hydrate,9,14−16 the difference in crystal morphology of TBAB + CO2 hydrate due to the difference in mass fraction was not observed. Besides, the pressure dependency of crystal morphology under the 2.3 and 3.4 MPa was not observed. It is likely that the hydrate crystals formed at the different pressure and mass fraction have different solid phase composition. However, it is difficult to discuss the effect of the difference in the hydrate phase composition on the crystal morphology as the hydrate phase compositions were not analyzed in the present study. Ye and Zhang9 observed the columnar crystals of TBAB + CO2 semiclathrate hydrate at ΔTsub = ∼6 K and wTBAB = 0.19. The 2092



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Crystal Growth of Ionic Semiclathrate Hydrate Formed in CO2 Gas +

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