Article pubs.acs.org/jced
Use of Hydrophobic Particles as Kinetic Promoters for Gas Hydrate Formation Jialin Wang, Ruijia Wang, and Roe-Hoan Yoon Center for Advanced Separation Technologies, Virginia Tech, Blacksburg, Virginia 24060, United States
Yongkoo Seol* National Energy Technology Laboratory, Morgantown, West Virginia 26507, United States ABSTRACT: Hydrophobic particles have been tested as kinetic promoters for gas hydrate formation. The experimental results obtained with stationary beds of sands show that methane (CH4) hydrate is more readily formed when the sand particles are hydrophobized by coating the surfaces with octadecyltrichlorosilane (OTS). The induction times decreased steadily with increasing water contact angles (θ) possibly due to the increased propensity of the water molecules in the vicinity of hydrophobic surfaces to form partial clathrates. The kinetics of CO2 hydrate formation has been studied using Teflon and hydrophobic silica particles to disperse either water in gas phase to obtain “dry water” or to disperse gas in water phase to obtain foam. The results show that hydrates are formed instantaneously due to the increased mass transfer rates at the gas/water interface and the formation of partial clathrates in the vicinity of hydrophobic surfaces.
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INTRODUCTION Various gases form hydrates in water under appropriate thermodynamic conditions. The best known of them is methane (CH4) hydrate. The amount of methane deposited in the continental margins and permafrost of the world is estimated to be 400 million trillion cubic feet (Tcf), which accounts for more than 50% of the total organic carbon on earth. Although recent estimates are much lower,1,2 the “best estimate” is considered to be 700 000 Tcf. A recent estimate based on a thermodynamic model is about 4.2 million Tcf (1.2 × 1017 m3) at STP.3 Compared to the enormous reserves of methane in the form of hydrate, the 2013 EIA estimate of the world’s wet natural gas reserve is only 2431 Tcf, including both shale and conventional gas.4 Thermodynamically, CO2 hydrate is more stable than CH4 hydrate, which suggests the possibility of extracting methane from a hydrate deposit by injecting CO2, a greenhouse gas.5,6 There are also possibilities of exploiting the new scientific knowledge of gas hydrate gained in recent years to develop efficient methods separating one-type of gas from another,7,8 transportation and storage of natural stranded gas and hydrogen,9−11 and desalination of seawater.12 It is well-known that the kinetics of gas hydrate formation is slow, which is a major technological barrier for using the science of gas hydrate for industrial applications. A fundamental reason for this problem lies in the facts that most of the gas molecules, particularly CH4, have low solubility in water and that their diffusion coefficients are small. At 273 K, its diffusion coefficient is ∼0.7·10−9 m2·s−1.13 Typically gas hydrates form at the gas/water interfaces where the gas concentration is higher © XXXX American Chemical Society
than in bulk water. Further, the gas hydrates forming at the interfaces become a physical barrier for further transfer of the gas molecules to aqueous phase. To overcome these problems, different approaches have been taken, including vigorous agitation, use of additives such as surfactants,14−17 hydrotropes,18 and semiclathrate former.7,19 Sodium dodecyl sulfate (SDS) is one of the most frequently used surfactants to accelerate the growth rate of gas hydrate, although the reason for improved kinetics is unclear.20,21 Other approaches to improving hydrate formation rate include increasing the gas/ water interfacial area by using water sprays,22 silica gel,23 and water-in-oil (or gas) emulsions.11,24 According to the kinetic model developed by Christiansen and Sloan,25 hydrate formation involves four different steps. In Step 1, water molecules form partial clathrates. As is wellknown, clathrates are formed in supercooled water. When gas molecules such as CH4 are introduced in Step 2, the partial clathrates become stronger due to guest−host interactions. For the case of CH4, the guest−host interaction is driven by the van der Waals force. In Step 3, the clathrates filled with guest molecules form agglomerates by sharing faces and edges of the clathrates to minimize the loss of entropy associated with forming H-bonded structures.26 In Step 4, the agglomerates Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: July 10, 2014 Accepted: September 23, 2014
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Kinetics Study of Gas Hydrate Formation. A multipressure vessel system was used to study the formation kinetics of CH4 hydrates in sand beds of different hydrophobicities. Five identical pressure vessels were connected to CH4 gas supply lines and a pressure accumulator. The vessels were placed in a water bath. To study the formation of CH4 and CO2 hydrates in the presence of dry water and foam stabilized by hydrophobic particles, a mini benchtop hydrate reactor, model 4560, Parr Instrument, was used. The 100 mL volume reactor was placed in a constant temperature bath, while the initial pressure was controlled by means of the pressure regulator of a gas cylinder. The pressure drop associated with hydrate formation was recorded as a function of time by means of a data acquisition system. Prior to each experiment, the hydrate reactor was purged with CH4 or CO2 for 15 min to flush out the air. The uncertainty of the pressure and temperature measurements were ± 0.2 MPa and ± 1.2 °C, respectively.
grow in size and become visible. The time it takes to go through the first three steps is referred to as induction time, which varies with the gas type, temperature, pressure, and agitation speeds employed for hydrate formation. In the present work, the possibility of using hydrophobic particles as kinetic promoters for CH4 and CO2 hydrates has been studied. The hydrophobic particles used in the present work included hydrophobic sands, micron-sized Teflon, and nanosized hydrophobic silica powder. The hydrophobicity of the sand particles were varied by controlling the octadecyltrichlorosilane (OTS) coating to study the effects of surface hydrophobicity on the kinetics of CH4 hydrate formation. The latter two hydrophobic particles were used effectively as solid surfactants that can disperse either water in gas phase or gas in water phase in a hydrate reactor. The results support the Christianson and Sloan’s model involving formation of partial clathrates.
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EXPERIMENTAL SECTION Materials. A Millipore Direct-Q3 (Millipore, MA) ultrapure water system was used to obtain pure water with a resistivity of 18.2 MΩ·cm−1 at 25 °C. CO2 (bone dry grade) and CH4 were purchased from Airgas. Silica sand with an average diameter of 200 μm was purchased from US Silica. Fine polytetrafluoroethylene (Teflon) powders of 1 and 12 μm diameters were obtained from Aldrich. Octadecyltrichlorosilane (OTS, 90+%) was purchased from Aldrich and used without further purification. Cyclohexane (HPLC grade) and chloroform (100%) were purchased from Fisher Scientific. Hydrophobic fumed silica powders of Aerosil R972 (16 nm) was supplied by Evonik Industries. The silica powders were hydrophobized by hexamethyl-disilazane27,28 and exhibited a water contact angle of 105°. Preparation of Hydrophobic Sand. The silica sand was hydrophobized by OTS adsorption. Samples of untreated hydrophilic sand were baked in an oven overnight at 100 °C to remove surface moisture. The dried samples were immersed in OTS-in-cyclohexane solutions. Control of surface hydrophobicity was achieved by varying the OTS concentration and immersion time. A hydrophobized sand was filtered and rinsed with chloroform, followed by annealing in oven at 120 °C for 24 h. The OTS-coated silica sands were pressed in a KBr die at 15 000 psi for 2 min without a binding agent to obtain flat shiny surfaces, which were then used for contact angle measurements. Droplets of water of 1−2 mm diameter were placed on a flat surface by means of a syringe. Equilibrium contact angles were measured using a goniometer (Rammé-Hart). Preparation of Dry Water and Foam. The Teflon and fumed silica particles were used to produce water-in-gas (dry water) and gas-in-water (foam) dispersions. Water-in-CO2 dispersions were prepared by mixing pure water and air in the presence of Teflon particles in a blender (Osterizer 10speed, 1.5 L) at 14 100 rpm for 180 s. The resulting dry water was placed in a hydrate reactor, and subsequently a stream of CO2 gas was injected into it to displace air with CO2. The amount of Teflon that was used to produce the CO2 dry water was 15% by weight. At this solid concentration, the dry water was most stable.29 The CO2-in-water dispersion (foam) was produced by agitating a mixture of 50 g of water, 0.1 g of Aerosil R972, and ∼50 mL of CO2 gas in a hydrate reactor with a turbine-type impeller at 685 rpm.
RESULTS AND DISCUSSION In the present work, a series of kinetics tests were conducted in pure water, in beds of OTS-coated silica sands of different hydrophobicity and in the presence of dry water and foam. For comparison, a kinetics test was also conducted in the presence of SDS, a well-known kinetics promoter. In each experiment, the changes in pressure were monitored as a function of time. Typically, the gas pressure inside the hydrate reactor remained constant before it dropped suddenly, signifying gas hydrate formation. The time it took before a significant pressure drop was taken as the induction time. Figure 1 shows the induction times for the formation of CH4 hydrate in the beds of silica sands of different contact angles. All
Figure 1. Effect of water contact angles of the silica sands on the induction times for the formation CH4 hydrate. The initial pressure was 80 MPa, and the temperature was 4 °C.
experiments were conducted in sand beds of 80% water saturation. With the untreated (hydrophilic) sand with a contact angle of 0°, the induction time was approximately 576 s. As the contact angle increased, the induction time decreased substantially as shown. With the OTS-coated sands with θ = 110°, the induction time was reduced substantially to 71 s. The induction time measurements became more reproducible with increasing contact angles. B
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The results presented in Figure 1 show that the kinetics of methane hydrate formation increases with increasing hydrophobicity of sand particles. A possible explanation may be that partial clathrates may be formed in the vicinity of hydrophobic surfaces, as has been suggested previously.30,31 According to the Christiansen and Sloan’s model,32 hydrate formation begins with the formation of partial clathrates, which is a slow process in bulk water. In the thin liquid films (TLFs) of water confined between hydrophobic particles, clathrate formation may be enhanced as will be discussed later in this section. Figure 2 shows the pressure versus time plots for the formation of CO2 hydrate in pure water and in dry water. The
Figure 3. Pressure vs time profiles for hydrate formation in dry water at 1.3 °C.
is negative for removal into the water phase. According to eq 1, smaller particles can be more readily detached under conditions of high-shear agitation and cannot form stable CO2-in-water emulsions. That the smaller particles were more efficient for the formation of CO2 hydrate may, therefore, be attributed to the high surface area of the hydrophobic particles, which in turn could more readily produce partial clathrates and hence increase the kinetics of hydrate formation. That the hydrate formation was more efficient at 15% than at 13% solids may also be ascribed to the larger surface area of the hydrophobic surfaces. Figures 4a and b show the effects of temperature and pressure, respectively, on CO2 hydrate formation using the water-in-CO2 dispersions (or dry water) stabilized by micronsized Teflon particles. As shown, the induction time decreased substantially as the temperature was reduced from 3.0 °C to 1.1 °C, which can be attributed to increased driving force.12 Figure 4b shows that the induction time observed at 3.14 MPa is substantially lower than at 2.64 MPa, which may be attributed to the increased driving force. Figure 5 shows the results obtained using a CO2-in-water dispersion (or foam) stabilized by nanosized hydrophobic silica (Aerosil R972). As shown, the pressure dropped immediately, indicating fast kinetics. In 60 min, the pressure dropped from 3 MPa to 1.25 MPa. The initial pressure drop observed with pure water was due to CO2 dissolution in water, and no hydrate was formed for a period of 16 h. It was found that the CO2 hydrate produced from foam exhibited porous structures. As CO2 gas dissipates into the surrounding water, empty cavities may be left behind, forming a porous structure. Figure 6 compares the methods of using dry water and foam for the formation CO2 hydrate. In the former, water is dispersed in CO2 gas phase, while in the latter CO2 gas is dispersed in water. Both of these approaches increased the kinetics of hydrate formation most probably due to the mass transfer rates of CO2 across the gas/liquid interface. It appears that the foam was more efficient than the dry water, which may be attributed to the curvature of interfaces involved. The pressure inside the CO2 bubble is higher than that of the surrounding liquid by 2γ/R, where γ is the interfacial tension and R is the bubble radius. The excess pressure due to the Laplace pressure should facilitate the mass transfer rate of CO2 across the gas/liquid interface as compared to the case of dry
Figure 2. Pressure vs time plots for CO2 hydrate formations in pure water and in Teflon-stabilized dry water at 1.1 °C. The amounts of water in these experiments were the same (30 g).
dry water was obtained using 1 μm Teflon particles as a stabilizer for the water-in-CO2 dispersion. The amount of the stabilizer was 15% by weight of water, which was found to be optimum. The reactor was cooled to 1.1 °C before pressurizing it to 3 MPa. No mechanical agitation was employed during the hydrate formation. When water was present as small droplets in the gas phase, CO2 hydrate began to form after 18 min, with the pressure dropping from 3 MPa to 1.2 MPa in 260 min. By contrast, no reaction occurred in pure water for 1080 min. The slight pressure drop observed during the first 100 min was due to the dissolution of CO2 in water. Figure 3 shows the pressure versus time curves for the formation of CO2 hydrate using 1 μm and 12 μm Teflon particles to produce water-in-CO2 dispersions for hydrate formation. As shown, the smaller particles were more efficient than the larger ones, which may suggest that the former produced smaller water droplets. It has been shown, however, that larger particles are more efficient for the formation of Pickering emulsions. The reason is simply because the larger particles are more difficult to be detached from the gas/water interfaces. Under conditions of high-shear agitation, hydrophobic particles can be detached from the CO2−water interface and, thereby, destabilize water-in-CO2 dispersions. The amount of energy (E) required to detach hydrophobic particles from a water-in-gas (or water-in-oil) emulsions can be obtained as follows,33 E = πr 2γg/w(1 ± cos θ )
(1)
where γg/w is the interfacial tension at the gas/water interface, r is the particle radius, and θ is the water contact angle. The sign inside the bracket is positive for removal into the gas phase and C
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Figure 4. (a) Temperature dependence of the pressure profiles for hydrate formation in dry water with 15% by weight Teflon (1 μm) and (b) pressure dependence of the profile for hydrate formation in dry water at 15% by weight Teflon (1 μm) at 1.3 °C.
Figure 7. Pressure−time profiles for hydrate formation in silica foam and water−SDS mixture at 1.5 °C.
Figure 5. Pressure vs time plots for CO2 hydrate formations in water and in the presence of foam stabilized by nanosize hydrophobic silica at 1.1 °C.
used at 0.1% by weight of SDS. The results obtained with the two different approaches are comparable. The particle-stabilized form may be advantageous over SDS in that hydrophobic silica are chemically inert and nontoxic and can be recycled after use. Further, no agitation is necessary once a foam is placed in a hydrate reactor. When a hydrophobe of less than ∼0.5 nm across, e.g., CH4, is placed in water, it occupies a small enough volume that can be easily accommodated without breaking H-bonds, while the water molecules forming a structure known as “iceberg” or clathrates around it with a significant entropy decrease.26,34 Thus, the hydrophobic hydration entails decreases in both enthalpy and entropy. In fact, dissolving small hydrocarbons such as ethane, propane, butane, and pentane in water is an exothermic process.35 However, the process is entropic, i.e., |TΔS| > |ΔH|, with ΔG > 0. To minimize the entropy loss, the clathrates filled with small guest molecules form “agglomerates” by sharing edges and faces.26 When a large hydrophobic particle is placed in water, the water molecules that are denied of forming H-bonds with the surface may form structures among themselves to minimize free energy. Evidence for this possibility has been shown in a series of thermodynamic studies of macroscopic hydrophobic interaction.30,36 The results showed that hydrophobic interaction entails decreases in both enthalpy and entropy, the entropy decrease representing the formation of partial clathrates or low density liquid.37−40 According to the
Figure 6. Comparison between dry water and foam.
water and accounts for the faster kinetics. Taking advantage of the Laplace pressure to improve the kinetics of hydrate formation should apply not only for CO2 hydrate but also for CH4 hydrate. Sodium dodecyl sulfate (SDS) is a well-known kinetics promoter. Figure 7 compares the performance of the SDS and nanosized hydrophobic silica as kinetics promoters for the formation of CO2 hydrate. The latter was formed in the presence of 1% by weight of Aerosil R972, while the former was D
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Christianson and Sloan’s kinetics model,25 the formation of partial clathrates is the first step in gas hydrate formation. If the partial clathrates are already present in the vicinity of hydrophobic surfaces, the first step may be shortened or even eliminated, resulting in a reduced induction time. As shown in Figure 1, the induction time decreased with increasing hydrophobicity, suggesting that partial clathrates are more readily formed in the TLFs confined between hydrophobic surfaces of higher contact angles. Recent X-ray diffraction41 and Monte Carlo simulation42 studies also showed that methane hydrate is more readily formed in the water confined between hydrophobic surfaces. The improved kinetics observed with the dry water and foam stabilized by hydrophobic particles may have dual benefits, i.e., increased mass transfer at the gas/water interface due to increased interfacial area,11 and the formation of partial clathrates in the vicinity of hydrophobic surfaces. That smaller particles are more efficient than the larger ones despite the higher probability of detachment supports the view that the partial clathrates may be formed in the vicinity of the hydrophobic particles and help reduce the induction time. For the case of using particle-stabilized foams, the Laplace pressure may also contribute to increased mass transfer rate.
(5) Lee, H.; Seo, Y.; Seo, Y.-T.; Moudrakovski, I. L.; Ripmeester, J. A. Recovering Methane from Solid Methane Hydrate with Carbon Dioxide. Angew. Chem., Int. Ed. 2003, 42, 5048−5051. (6) Ota, M.; Abe, Y.; Watanabe, M.; Smith, R. L., Jr.; Inomata, H. Methane recovery from methane hydrate using pressurized CO2. Fluid Phase Equilib. 2005, 228−229, 553−559. (7) Kang, S.-P.; Lee, H. Recovery of CO2 from Flue Gas Using Gas Hydrate: Thermodynamic Verification through Phase Equilibrium Measurements. Environ. Sci. Technol. 2000, 34, 4397−4400. (8) Klara, S. M.; Srivastava, R. D. U.S. DOE integrated collaborative technology development program for CO2 separation and capture. Environ. Progress 2002, 21, 247−253. (9) Kim, D.-Y.; Park, Y.; Lee, H. Tuning clathrate hydrates: Application to hydrogen storage. Catal. Today 2007, 120, 257−261. (10) Mao, W. L.; Mao, H.-K. Hydrogen storage in molecular compounds. Proc. Natl. Acad. Sci.. 2004, 101, 708−710. (11) Wang, W.; Bray, C. L.; Adams, D. J.; Cooper, A. I. Methane Storage in Dry Water Gas Hydrates. J. Am. Chem. Soc. 2008, 130, 11608−11609. (12) Kubota, H.; Shimizu, K.; Tanaka, Y.; Makita, T. Thermodynamic properties of R13 (CClF3), R23 (CHF3), R152a (C2H4F2), and propane hydrates for desalination of sea water. J. Chem. Eng. Jpn. 1984, 17, 423−429. (13) Guo, H.; Chen, Y.; Lu, W.; Li, L.; Wang, M. In situ Raman spectroscopic study of diffusion coefficients of methane in liquid water under high pressure and wide temperatures. Fluid Phase Equilib. 2013, 360, 274−278. (14) Karaaslan, U.; Parlaktuna, M. Surfactants as Hydrate Promoters? Energy Fuels 2000, 14, 1103−1107. (15) Mishal, Y.; Macchi, A.; Servio, P. In Effect of Gemini Surfactant on the Formation Kinetic Behavior of Methane Hydrate. Proceedings of 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, Canada, 2008. (16) Nguyen, H.; Phillips, J. B.; John, V. T. Clathrate hydrate formation in reversed micellar solutions. J. Phys. Chem. 1989, 93, 8123−8126. (17) Zhong, Y.; Rogers, R. E. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 2000, 55, 4175−4187. (18) Gnanendran, N.; Amin, R. The effect of hydrotropes on gas hydrate formation. J. Pet. Sci. Technol. 2003, 40, 37−46. (19) Li, S.; Fan, S.; Wang, J.; Lang, X.; Liang, D. CO2 capture from binary mixture via forming hydrate with the help of tetra-n-butyl ammonium bromide. J. Nat. Gas Chem. 2009, 18, 15−20. (20) Profio, P. D.; Arca, S.; Germani, R.; Savelli, G. Surfactant promoting effects on clathrate hydrate formation: Are micelles really involved? Chem. Eng. Sci. 2005, 60, 4141−4145. (21) Zhang, J. S.; Lee, S.; Lee, J. W. Does SDS micellize under methane hydrate-forming conditions below the normal Krafft point? J. Colloid Interface Sci. 2007, 315, 313−318. (22) Fukumoto, K.; Tobe, J.-i.; Ohmura, R.; Mori, Y. H. Hydrate formation using water spraying in a hydrophobic gas: A preliminary study. AIChE J. 2001, 47, 1899−1904. (23) Adeyemo, A.; Kumar, R.; Linga, P.; Ripmeester, J.; Englezos, P. Capture of carbon dioxide from flue or fuel gas mixtures by clathrate crystallization in a silica gel column. Int. J. Greenhouse Gas Control 2010, 4, 478−485. (24) Su, F.; Bray, C. L.; Tan, B.; Cooper, A. I. Rapid and Reversible Hydrogen Storage in Clathrate Hydrates Using Emulsion-Templated Polymers. Adv. Mater. 2008, 20, 2663−2666. (25) Christiansen, R. L.; Sloan, E. D. Mechanisms and Kinetics of Hydrate Formation. Ann. N.Y. Acad. Sci. 1994, 715, 283−305. (26) Stillinger, F. H. Water Revisited. Science 1980, 209 (4455), 451−457. (27) Forny, L.; Pezron, I.; Saleh, K.; Guigon, P.; Komunjer, L. Storing water in powder form by self-assembling hydrophobic silica nanoparticles. Powder Technol. 2007, 171, 15−24. (28) Forny, L.; Saleh, K.; Denoyel, R.; Pezron, I. Contact Angle Assessment of Hydrophobic Silica Nanoparticles Related to the
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SUMMARY AND CONCLUSIONS Three different types of hydrophobic particles have been tested as kinetics promoters for the formation of CH4 and CO2 hydrates. The tests conducted with stationary beds of hydrophobic sands showed that the induction times for the formation of CH4 hydrate decrease substantially with increasing hydrophobicity, which may be ascribed to the partial clathrates formed in the vicinity of hydrophobic surfaces. The results obtained using the water-in-gas and gas-in-water dispersions also showed substantial increases in induction times for the formation of CO2 hydrate. The improved kinetics observed with these dispersions may be due to the increased mass transfer across the gas/water interface as well as the formation of partial clathrates in the vicinity of hydrophobic surfaces.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 1-304-285-2029, e-mail:
[email protected]. Funding
The authors would like to acknowledge the financial support from the National Energy Technology Laboratory (NETL), U.S. Department of Energy (Project Number: RES1000024). Notes
The authors declare no competing financial interest.
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