pubs.acs.org/Langmuir © 2009 American Chemical Society
Gas Storage in “Dry Water” and “Dry Gel” Clathrates Benjamin O. Carter,† Weixing Wang,†,‡ Dave J. Adams,† and Andrew I. Cooper*,† †
Department of Chemistry and Centre for Materials Discovery, University of Liverpool, Crown Street, Liverpool, L69 7ZD, U.K. and ‡School of Chemical and Energy Engineering, South China University of Technology, Guangzhou, 510640, China Received August 21, 2009. Revised Manuscript Received October 20, 2009 “Dry water” (DW) is a free-flowing powder prepared by mixing water, hydrophobic silica particles, and air at high speeds. We demonstrated recently that DW can be used to dramatically enhance methane uptake rates in methane gas hydrate (MGH). Here, we expand on our initial work, demonstrating that DW can be used to increase the kinetics of formation of gas clathrates for gases other than methane, such as CO2 and Kr. We also show that the stability of the system toward coalescence can be increased via the inclusion of a gelling agent to form a “dry gel”, thus dramatically improving the recyclability of the material. For example, the addition of gellan gum allows effective reuse over at least eight clathration cycles without the need for reblending. DW and its “dry gel” modification may represent a potential platform for recyclable gas storage or gas separation on a practicable time scale in a static, unmixed system.
Introduction Gas hydrates, or gas clathrates, are nonstoichiometric, crystalline inclusion compounds composed of a hydrogen-bonded water lattice which traps small molecules within polyhedral cavities. There is currently significant interest in the use of gas hydrates for the storage, transport, separation, and capture of a range of gases.1-11 For example, one volume of methane gas hydrate (MGH) can yield approximately 180 v/v STP methane,2 leading to the suggestion that it may be economically feasible to transport natural gas in a hydrated form. However, there are a number of key issues that need to be overcome, including trapped, unreacted interstitial water in the hydrate mass, thermal stability of the clathrate,12 and the slow rates of formation for many clathrates.9,13,14 The slow formation rates arise from the fact that the reaction is a gas-solid or gas-liquid interfacial phenomenon.15-18 Hence, formation *Corresponding author. E-mail:
[email protected]. (1) Struzhkin, V. V.; Militzer, B.; Mao, W. L.; Mao, H. K.; Hemley, R. J. Chem. Rev. 2007, 107, 4133. (2) Sloan, E. D. Nature 2003, 426, 353. (3) Aaron, D.; Tsouris, C. In 13th Symposium on Separation Science and Technology for Energy Applications; Marcel Dekker, Inc., New York, 2003; p 321. (4) Kang, S. P.; Lee, H. Environ. Sci. Technol. 2000, 34, 4397. (5) Duc, N. H.; Chauvy, F.; Herri, J. M. Energy Convers. Manage. 2007, 48, 1313. (6) Lee, S.; Liang, L. Y.; Riestenberg, D.; West, O. R.; Tsouris, C.; Adams, E. Environ. Sci. Technol. 2003, 37, 3701. (7) Seo, Y. T.; Moudrakovski, I. L.; Ripmeester, J. A.; Lee, J. W.; Lee, H. Environ. Sci. Technol. 2005, 39, 2315. (8) Linga, P.; Adeyemo, A.; Englezos, P. Environ. Sci. Technol. 2008, 42, 315. (9) Zhang, J.; Lee, J. W. Ind. Eng. Chem. Res. 2009, 48, 5934. (10) Chatti, I.; Delahaye, A.; Fournaison, L.; Petitet, J. P. Energy Convers. Manage. 2005, 46, 1333. (11) Di Profio, P.; Arca, S.; Germani, R.; Savelli, G. In 1st Conference on European Fuel Cell Technology and Applications (EFC2005); American Society of Mechanical Engineering: New York, 2005; p 49. (12) Wang, W. X.; Carter, B. O.; Bray, C. L.; Steiner, A.; Bacsa, J.; Jones, J. T. A.; Cropper, C.; Khimyak, Y. Z.; Adams, D. J.; Cooper, A. I. Chem. Mater. 2009, 21, 3810. (13) Stern, L. A.; Kirby, S. H.; Durham, W. B. Science 1996, 273, 1843. (14) Ribeiro, C. P.; Lage, P. L. C. Chem. Eng. Sci. 2008, 63, 2007. (15) Englezos, P.; Kalogerakis, N.; Dholabhai, P. D.; Bishnoi, P. R. Chem. Eng. Sci. 1987, 42, 2647. (16) Englezos, P.; Kalogerakis, N.; Dholabhai, P. D.; Bishnoi, P. R. Chem. Eng. Sci. 1987, 42, 2659. (17) Hawtin, R. W.; Quigley, D.; Rodger, P. M. Phys. Chem. Chem. Phys. 2008, 10, 4853. (18) Servio, P.; Englezos, P. AIChE J. 2003, 49, 269.
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rates are inversely proportional to the thickness of the hydrate zone. The rate of nucleation and growth can be increased by operating at either higher pressure or lower temperatures, but such conditions may be undesirable for many applications. Alternatively, a promoter such as tetrahydrofuran,4,8,19 cyclopentane,9 or an alkyl ammonium salt20,21 can be added. However, an obvious disadvantage of this approach is that the promoter occupies some of the cavities in the clathrate structure and hence the volume available for the trapping of gas is necessarily reduced. An alternative strategy to increase the kinetics of clathrate formation is to increase the interfacial contact between liquid water and the gas. A number of methods have been used to increase this contact, including the use of high pressures, vigorous mixing, grinding of ice particles,22,23 surfactants,24-27 and supports such as silica28,29 or a high surface area emulsion-templated polymer30,31 to generate a thin, supported water layer in contact with the gas. We recently demonstrated that “dry water” (DW),32 a freeflowing powder prepared by mixing water, hydrophobic silica particles, and air at high speeds, can be used to dramatically enhance methane uptake rates in MGH.33 Dry water is effectively a water-in-air emulsion (more correctly an inverse foam) consisting of water droplets surrounded by a network of hydrophobic (19) Delahaye, A.; Fournaison, L.; Marinhas, S.; Chatti, I.; Petitet, J. P.; Dalmazzone, D.; Furst, W. Ind. Eng. Chem. Res. 2006, 45, 391. (20) Arjmandi, M.; Chapoy, A.; Tohidi, B. J. Chem. Eng. Data 2007, 52, 2153. (21) Chapoy, A.; Anderson, R.; Tohidi, B. J. Am. Chem. Soc. 2007, 129, 746. (22) Takeya, S.; Shimada, W.; Kamata, Y.; Ebinuma, T.; Uchida, T.; Nagao, J.; Narita, H. J. Phys. Chem. A 2001, 105, 9756. (23) Kuhs, W. F.; Klapproth, A.; Gotthardt, F.; Techmer, K.; Heinrichs, T. Geophys. Res. Lett. 2000, 27, 2929. (24) Zhong, Y.; Rogers, R. E. Chem. Eng. Sci. 2000, 55, 4175. (25) Okutani, K.; Kuwabara, Y.; Mori, Y. H. Chem. Eng. Sci. 2008, 63, 183. (26) Karaaslan, U.; Uluneye, E.; Parlaktuna, M. J. Pet. Sci. Eng. 2002, 35, 49. (27) Lin, W.; Chen, G. J.; Sun, C. Y.; Guo, X. Q.; Wu, Z. K.; Liang, M. Y.; Chen, L. T.; Yang, L. Y. Chem. Eng. Sci. 2004, 59, 4449. (28) Anderson, R.; Chapoy, A.; Tohidi, B. Langmuir 2007, 23, 3440. (29) Anderson, R.; Llamedo, M.; Tohidi, B.; Burgass, R. W. J. Phys. Chem. B 2003, 107, 3507. (30) Su, F.; Bray, C. L.; Tan, B.; Cooper, A. I. Adv. Mater. 2008, 20, 2663. (31) Su, F.; Bray, C. L.; Carter, B. O.; Overend, G.; Cropper, C.; Iggo, J. A.; Khimyak, Y. Z.; Fogg, A. M.; Cooper, A. I. Adv. Mater. 2009, 21, 2382. (32) Binks, B. P.; Murakami, R. Nat. Mater. 2006, 5, 865. (33) Wang, W. X.; Bray, C. L.; Adams, D. J.; Cooper, A. I. J. Am. Chem. Soc. 2008, 130, 11608.
Published on Web 11/25/2009
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fumed silica nanoparticles which prevent droplet coalescence.32 After preparation, dry water is a stable free-flowing powder which can be readily handled under ambient conditions;for example, by pouring from the blending apparatus into a suitable reaction vessel. The water domains are of the order of 50 μm, meaning that diffusion of gas into the clathrate structure is significantly enhanced compared to bulk water or ice. The preparation method is also much more facile and scalable than, for example, grinding and size-selective sieving of ice particles.34 Here, we expand on our original work33 and demonstrate that DW can be used to increase the kinetics of formation for gas clathrates of gases other than methane. We also show that the stability of the DW powder and its recyclability can be increased significantly via the inclusion of a gelling agent.
Experimental Section Materials and Methods. The hydrophobic silica nanoparticles (5-30 nm, aggregates 100-1000 nm) were kindly supplied by Wacker Chemie (HDK grade H18, prepared by treatment of hydrophilic silica with dichlorodimethylsilane, replacing surface Si-OH with Si-O-Si(CH3)2. Residual SiOH content relative to hydrophilic silica: 25%). Glass beads (diameter: 3 mm) were purchased from Sigma-Aldrich. Sodium dodecyl sulfate (SDS, certified purity >98%) was purchased from Sigma-Aldrich. Gellan gum powder (average molecular weight 500 000 Da, purity 100.00%, product code BIPG434) was purchased from Apollo Scientific. The morphologies of the various dry water samples were observed using an Olympus CX41RF Microscope, fitted with a Linkam FDCS 196 variable temperature stage. Photographs were taken with an Olympus C-5060 digital camera. Synthesis of Dry Water. Deionized water (95 mL) was poured into a blender (Breville, glass jug blender, BL18, 1.5 L, total air volume with lid fitted estimated to be 1.7 L) and H18 (5 g) was added to the water. Mixing was carried out at the highest speed setting (average speed of 19,000 rpm) for 90 s, in three 30 s bursts to minimize droplet dissociation due to heat generated while mixing. The volume ratio of air to water/silica was the same in each DW preparation. The material was produced as a freeflowing “dry” white powder which could easily be poured from one vessel into another without any residues. Dry water containing gellan gum was prepared in the same fashion, using solutions at the appropriate concentrations at blending temperatures in the range 333-343 K to ensure complete dissolution of the gelling agent. The blender was prewarmed with hot water before addition of the gellan gum solutions. To prepare the higher concentration “dry gels”, solutions of gellan gum in water at the specified concentration were prepared as described above. After cooling, the gels were blended with silica at room temperature for 3 min at 19 000 rpm (10 wt %) or, for the 20 wt % gels, at 19 000 rpm for 90 s, and then at 37 000 rpm for 30 s (37 000 rpm blending carried out in a Vitamix 2-Speed blender, 1.4 L). The 10 and 20 wt % figures are in relation to the total system mass. Clathrate Hydrate Formation. To carry out the gas uptake kinetic experiments, 20.0 g of dry water (or control samples consisting of either glass beads (19.5 cm3) or unmixed water and silica (19 cm3 þ 0.5 cm3)) was loaded into a 68.0 cm3 high pressure stainless steel cell (New Ways of Analytics, L€ orrach, Germany). SDS solutions were prepared at room temperature in deionized water. The steel cell was washed five times with deionized water between SDS experiments. The temperature of the coolant in the circulator bath was controlled by a programmable thermal circulator (HAAKE Phoenix II P2, Thermo Electron Corporation). The temperature of the compositions in the high pressure cell was measured using a Type K Thermocouple (Cole-Parmer, -250-400 °C). (34) Strobel, T. A.; Taylor, C. J.; Hester, K. C.; Dec, S. F.; Koh, C. A.; Miller, K. T.; Sloan, E. D. J. Phys. Chem. B 2006, 110, 17121.
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Figure 1. Schematic diagram of experimental setup. The gas pressure was monitored using a High-Accuracy Gauge Pressure Transmitter (Cole-Parmer, 0-20.7 MPa). Both thermocouple and transmitter were connected to a Digital Universal Input Panel Meter (Cole-Parmer), which communicates with a computer. Prior to experiments, the cell was slowly purged with gas (Methane (UHP 99.999%); CO2 (liquid withdrawal): BOC Gases, Manchester, UK; Krypton (compressed): Air Liquide, Birmingham, U.K.) three times at atmospheric pressure to remove any air, and then pressurized to the desired pressure at the designated temperature. The temperature (T, K), pressure (P, MPa) and time (t, min) were automatically interval-logged using MeterView 3.0 software (Cole-Parmer). Using this setup it was possible to obtain high resolution data (for example, 2 s between individual [T, P, t] points, 120 000 data points in a 2000 min experiment). The apparatus is shown schematically in Figure 1.
Pressure-Temperature (P-T) Dependence Measurements. Studies of the onset and extent of clathration were typically carried out by pressurizing a 20 g sample as required for the experimental system, before ramping slowly (2 K/h) from 293 to 273 K and back, logging temperature and pressure at 10 min intervals. At the onset of clathration, an exotherm was observed due to the heat of crystallization, accompanied by a drop in system pressure (ΔP) as the gaseous guest was incorporated into the solid clathrate hydrate. Conversely, clathrate dissociation could be observed as a sudden rise in pressure upon release of enclathrated guest, the P-T curve departing from an ideal gas-type relationship. Recyclability of samples was examined through back-to-back repeats of the ramping cycle: a drop in system capacity would be evidenced by a reduction in ΔP as less guest was incorporated in the clathrate. Capacity was measured directly by conducting only the cooling ramp of the cycle, before quickly venting the vessel of nonenclathrated gas and resealing. After warming back to 293 K, the volume of gas released from the sample was measured via volumetric displacement of water. Kinetic Measurements. The rate of guest uptake at a constant temperature was studied by observing gas pressure change as a function of time. Typically, a 20 g sample was cooled to and held at 273 K. The vessel was then pressurized with gaseous guest as required and sealed, before logging temperature and pressure at 10 min intervals over a period of hours or days as appropriate. This method does not allow for quantitative analysis of the reaction rate, given the subsequent variation in driving force DOI: 10.1021/la903120p
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Figure 2. Free-flowing DW powder prepared by aerating 5 g of hydrophobic silica nanoparticles H18 and 95 g of water at 19 000 rpm for 90 s. The powder is photographed flowing through a funnel. (i.e., pressure) as the reaction proceeds within a closed system. Instead, in order to provide a qualitative assessment of the kinetic enhancement of clathration, the capacity of the system was measured as described below based on the time required to reach 90% of equilibrium capacity. Calculation of Capacity. In the context of this work, capacity is defined volumetrically, in terms of the number of volumes of guest gas released per unit volume of hydrate material (v/v). As well as direct measurement of gas volumes released, capacity was also calculated relative to the pressure change within the reaction vessel. The free space volume of the vessel was obtained by subtracting the sum volume of methane clathrate hydrate, unreacted water and H18. Taking into account nonideality factors, GASPAK v3.41 software (Horizon Technologies, USA) was employed to calculate the methane enclathration capacity, according to the pressure and the temperature. We assume that the liquid and gas phases inside the vessel are exclusively formed from the water and the guest gas, respectively, neglecting any dissolution of the guest gas into the liquid phase and any mixing of the water vapor in the gas phase. This assumption is based upon the low vapor pressure of water, and the limited solubilities of methane and krypton in water at clathrate formation temperatures (0.000046 and 0.000089 mol fraction in water, respectively, at 273 K).35,36 The higher solubility of CO2 (0.001462 mol fraction in water, 273 K)37 is reflected in the increased uptake observed for the CO2 hydrate bulk system (below). The temperature inside the vessel is assumed to be uniform throughout the operation.
Results and Discussion As described previously,33 DW was prepared by rapid mixing of hydrophobic silica (H18), water, and air in a conventional blender. The droplet size was altered by varying the speed at which the mixing was carried out.33 DW is formed as a freeflowing powder (Figure 2), which is stable in a sealed polypropylene bottle under ambient conditions for 1 month. Following our previous report on the formation of MGH in dry water, the formation of a wider range of gas clathrates was investigated using dry water. We also investigated in more detail the formation of MGH in dry water, focusing in particular on the effects of temperature and the initial formation pressure. Effect of Temperature on the Kinetics of MGH Formation. A key advantage of the DW system is the increased rate of formation of the MGH relative to bulk systems in the absence of any mechanical mixing or agitation. The rate of formation was found to be fast over a range of formation temperatures (Figure 3), with the time required to reach 90% of total methane uptake (t90) measured at 160 min at 273 K. The final capacity of the system was strongly affected by the temperature of MGH formation, with the highest capacity being obtained between 273 and 277 K. (35) Yamamoto, S.; Alcauskas, J. B.; Crozier, T. E. J. Chem. Eng. Data 1976, 21, 78. (36) Weiss, R. F.; Kyser, T. K. J. Chem. Eng. Data 1978, 23, 69. (37) Carroll, J. J.; Slupsky, J. D.; Mather, A. E. J. Phys. Chem. Ref. Data 1991, 20, 1201.
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Figure 3. Methane uptake kinetics in DW-MGH at different formation temperatures (initial pressure: 10.0 MPa). A formation temperature of 273 K was found to be optimal under these conditions.
A formation temperature of 273 K was found to be optimal with the gas uptake capacity and rate falling off both above and below this temperature. Given that the dry water sample is cooled prior to pressurizing with methane, gas uptake occurs across a gas/solid rather than a gas/liquid interface at temperatures below 273 K. DW droplets survive freezing, although they are somewhat unstable over the whole freeze-thaw cycle (see Figure S1, Supporting Information). It has been shown elsewhere that at a fixed methane pressure, the interfacial rate constant for MGH formation in powdered ice is significantly lower at 263 K as compared to 270 K, although the permeation coefficient is identical at these temperatures.38 It was also demonstrated recently that different crystal morphologies (sword-like, triangular, or large polygons) are formed above 273 K, depending on the clathration temperature.39 Different crystal morphologies may result in different barriers to permeation, and hence differences in diffusion rates. Ratio of Silica to Water. Figure 4 shows the cooling/heating curves for CH4-DW systems using DW prepared at two different ratios of water to silica (see also Figure S2, Supporting Information). The droplet size in particle-stabilized systems is known to be controlled by the process of limited coalescence. Assuming the solid silica particles are totally and irreversibly adsorbed at the interface (there are no free silica particles or aggregated silica particles observed in the continuous phase by optical microscopy at 100 magnification), the inverse average droplet radius varies linearly with the amount of particles.40,41 We therefore expect that the droplet size in the DW systems will be reduced at lower ratios of water to silica, although this is difficult to measure directly and quantitatively without perturbing the system. At higher particle concentrations, the density of dry water is known to be lower.32 It should be noted that irreversible adsorption of silica is dependent on such factors as particle size and wettability, as well as the surface tension of the liquid. Irreversible adsorption will occur only if the adsorption energy is greater than the thermal (38) Kuhs, W. F.; Staykova, D. K.; Salamatin, A. N. J. Phys. Chem. B 2006, 110, 13283. (39) Tanaka, R.; Sakemoto, R.; Ohmura, R. Cryst. Growth Des. 2009, 9, 2529. (40) Frith, W. J.; Pichot, R.; Kirkland, M.; Wolf, B. In 1st International Conference on Multiscale Structures and Dynamics of Complex Systems; American Chemical Society: Washington, DC, 2007; p 6434. (41) Leal-Calderon, F.; Schmitt, V. Curr. Opin. Colloid Interface Sci. 2008, 13, 217.
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Figure 4. P-T plots for cooling and heating under CH4 pressure (temperature ramp: 2.0 K/h) for dry water prepared using different ratios of hydrophobic silica to water. For each ratio, three different initial pressures are shown. The sI MGH phase boundary curve (blue curve) was generated using CSMHyd software.44
energy of the system, through a combination of these factors. Binks and Murakami suggest that this is the case for a dry water system;32 it is unknown what effect gas clathration has on the adsorption of the silica particles. Imaging the DW droplets by optical microscopy was carried out for samples produced at various blending speeds,33 but water tends to be lost during imaging and the droplet size is in any case rather too small for size quantification via this method. Likewise, scanning electron microscopy (SEM) in vacuo was not possible for these systems. High-speed freeze-drying also yielded no useful results since the silica shell (which is not covalently bonded) does not maintain its integrity in the absence of water. Estimates of DW droplet size were therefore made using a temperatureregulated optical microscope stage, set to ∼278 K, although even here it proved difficult to image sufficient droplets to provide a reliable quantitative size distribution before significant water loss occurred by evaporation. In an unmixed bulk control system, the P-T relationship for CH4 and water approximated to the ideal gas law during a continuous cooling/heating cycle. There was no evidence for substantial MGH formation or dissociation under these conditions (data not shown). In contrast, MGH formation and subsequent dissociation occurred in all cases when particulate DW was employed as shown by the dramatic pressure drop upon cooling and the rapid pressure rise upon heating, respectively (Figure 4). In all cases, the CH4 uptake was high (approximately 175 v/v). MGH formation occurred when cooling to 270.0 K at a rate of 2.0 K/h with an associated exotherm at approximately 279 K which results from the heat of crystallization for the clathrate. The formation of DW-MGH (as compared to the unmixed system) can be attributed to the highly dispersed water phase which has a large surface area/volume ratio compared to the bulk water case. MGH dissociation occurred on warming. The dissociation temperature was strongly dependent on the pressure of the system, with a higher pressure resulting in a higher dissociation temperature. This dissociation temperature closely follows the phase boundary curve for structure-type I (sI) MGH.42 We also see however a deviation from the phase boundary curve as warming of the sample continues, suggesting that DW-MGH is metastable beyond the accepted pressure-temperature range. (42) Sloan, E. D.; Koh, C. A. In Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press: Boca Raton, FL, 2008.
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There are two potential explanations for this: the first is that the 2.0 K/h heating rate is proceeding too quickly, given the induction or lag time between heat applied to the system and subsequent detection of clathrate dissociation through rising pressure. This would suggest that the detection of dissociation cannot keep up with the heating rate. This explanation is however undermined by the fact that the onset of dissociation itself falls accurately and reproducibly along the MGH phase boundary curve. The second potential explanation is the widely documented but little-understood hydrate “anomalous self-preservation” effect,42,43 which can cause both methane and CO2 hydrates to remain stable for extended periods outside their pressuretemperature stability range. As such, this phenomenon is by no means limited to DW systems, although directly comparable data showing this in a P-T plot format (Figure 4) were not found in the literature. As can be seen in Figure 4 (and Figure S2), varying the amount of silica used to prepare the dry water prior to exposure to CH4 had no discernible effect on the formation or dissociation temperatures of the MGH at different pressures. The limited coalescence effect on droplet size described above will only be maintained if the silica adsorbed per unit liquid area remains constant. While we cannot verify this given the irregular shapes of the silica particles, the data from Figure 4 suggest that by 10 wt % silica loading, we have passed beyond the threshold of the limited coalescence effect. As such, there is no advantage in this respect in increasing the silica loading above 10 wt % and indeed this introduces an additional gravimetric penalty. The kinetics of methane uptake were enhanced significantly using DW as compared to bulk water (Figure 5). At 273 K, a very low uptake was observed for bulk water over 1000 min. With DW, a rapid uptake of methane was observed. No significant kinetic differences were observed between systems in which DW was prepared using different ratios of silica to water, in keeping with the data presented in Figure 4. We previously reported that the kinetics of gas uptake were different when different blending speeds were used to prepare dry water at a fixed ratio of silica to water.33 We attributed this to the droplet sizes being reduced as (43) Stern, L. A.; Circone, S.; Kirby, S. H.; Durham, W. B. J. Phys. Chem. B 2001, 105, 1756. (44) Free software download from: http://hydrates.mines.edu/CHR/Software. html.
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Figure 5. Methane uptake kinetics in DW-MGH with different ratios of silica to water. In all cases, temperature: 273.2 K. Initial pressure: 8.5 MPa.
the mixing speed was increased. Here, the lack of variation in uptake rate between samples prepared at different ratios of silica to water implies that droplet size does not vary significantly as a function of silica content during mixing. It therefore appears that blending speed has a greater impact on droplet size compared to the ratio of silica to water. While DW dramatically promotes the rate at which MGH is formed, a key practical question is the reuse and recyclability of the system. It would be advantageous, for example, to form a powder by blending which could be used for multiple gas storage and discharge cycles without remixing. We previously reported that the DW system can be reused after MGH dissociation, but that the storage capacity and kinetics degrade significantly after a few cycles.33 This stems from partial agglomeration of the water droplets which are destabilized by the freezing and warming process (Figure S1). Reblending the DW results in regeneration of the original enclathration kinetics. In principle, increased silica content might give rise to greater stability and recyclability but this was not found to be the case. In all cases, partial agglomeration of water droplets occurred upon warming. The volume of separated bulk water thus evolved was 1.8 ( 0.3 mL for all ratios of silica to water. Direct recycling of the system (without reblending) led to some drop-off in gas uptake capacity, as reported previously.33 Gellan Gum: “Dry Gel” Formation. In order to address the issue of stability for reuse, we prepared DW using a solution of hydrocolloid gelling agent in place of water. Such hydrocolloids are known to form gels at relatively low concentrations.45 We examined gellan gum, a high molecular weight polysaccharide gum which has been shown to have relatively little effect on the surface tension at the water-air interface,46 as a means of increasing the stability of DW. A hot 1.0 wt % gellan gum solution could be blended as before to yield droplets which were imaged by optical microscopy (Figure 6b, c.f., DW in Figure 6a). Optical measurements taken from a cold microscope stage (278 K) indicated an average droplet size of 79 ( 23 μm for 1.0 wt % gellan gum blended at 19 000 rpm, as compared to 52 ( 14 μm for DW prepared without gellan gum. In both cases, the size distribution was calculated from at least 130 droplets. P-T experiments with methane revealed that this “dry gel” did not lead (45) Piculell, L. Curr Opin. Colloid Interface Sci. 1998, 3, 643. (46) Paunov, V. N. Langmuir 2003, 19, 7970.
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Figure 6. Optical microscope images for (a) dry water, (b) DW with 1 wt % gellan, and (c) DW with 3 wt % gellan. All samples contained 5 wt % silica and were photographed at 100 magnification at 278 K.
Figure 7. (a) Free-flowing powder prepared from blending gellan gum gel (10 wt %) with silica (5 wt %); (b) free-flowing powder prepared from blending gellan gum gel (20 wt %) with silica (10 wt %); (c) optical microscope image of powder shown in part a; (d) optical microscope image of powder shown in part b, both at 100 magnification.
to significant improvement for the recyclability of the material nor for the gas uptake capacity with respect to pure DW methane clathrate (Figure S3). As for DW,33 a gradual loss of capacity was observed over repeat cycles. Increasing the concentration of gellan to 3 wt % resulted in the formation of DW with an average droplet size of 163 ( 46 μm (Figure 6c, calculated from 130 droplets). The more viscous gel solution also yielded some larger “dry gel” droplets (of the order of millimeters) as a secondary species. As stated above, droplet size determination of DW and dry gel presents a particular challenge. The polydisperse, nonspherical nature of DW droplets is exacerbated in dry gels, as can be seen in Figure 6. As such, any estimate of sample surface area based on droplet measurements would be imprecise. The estimates provided for droplet size are based on optical measurements of the longest dimension of each droplet and sampling of 130 droplet images still represents only a very small proportion of the DW or dry gel sample. These factors combined would suggest that the size estimate in each case is likely Langmuir 2010, 26(5), 3186–3193
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Figure 8. (a) Methane uptake kinetics in water, DW-MGH and dry gels prepared at 10 and 20 wt % gellan gum, at 273.2 K with different ratios of silica to water (initial pressure: 8.5 MPa). (b) Recyclability of dry gel for methane storage (10 g dry gel prepared from 10 wt % gellan gum with 5 wt % silica). (c) Recyclability of dry gel for methane storage (10 g dry gel prepared from 20 wt % gellan gum with 10 wt % silica).
to be weighted toward a higher average figure. This may go some way toward explaining why, despite larger measured av. droplet sizes, dry gel samples still display kinetic clathration enhancement approaching the order of pure DW (below). The 3 wt % gellan gum DW composite performed better in terms of methane capacity and cyclability than those prepared at 1 wt % (Figure S4). Over five successive cycles, only a small reduction in ΔP was observed (1.23 MPa), suggesting that the dry gel maintains its integrity despite multiple cycles of freezing and thawing. In addition, the 3 wt % dry gel droplets remain discrete following clathration and release a volume of methane equivalent to a capacity of 144 v/v. These findings show that gelling agents can impart a level of cyclability previously unseen in clathrate hydrate systems, without the need for repeated stirring or a porous support. In a variation on these experiments, a gellan gum gel was preprepared and then blended at room temperature in the presence of silica, rather than using a hot solution of gellan gum. The preprepared gel blended in this way generated a freeflowing powder with a visual appearance and flow properties which were similar to DW (Figure 7a,b). Two systems were prepared: one at a concentration of 10 wt % gellan gum with 5 wt % silica and one at 20 wt % gellan gum with 10 wt % silica. It should be noted that attempts to prepare dry water equivalents with similar concentrations of gellan gum in hot solution failed due to the high viscosity of these systems. In addition, as blending was carried out, cooling was unavoidable due to the entrainment of air into the solution, leading to an inhomogeneous system. As such, a high quality dry gel not prepared using this technique. Langmuir 2010, 26(5), 3186–3193
By comparison, at room temperature the blender blades could cut the “solid” preprepared gels with little mechanical resistance, apparently retaining sufficient mobility to form DW-like droplets. The 10 wt % gellan gum system appeared qualitatively similar to the original DW system as imaged by microscopy and consisted of well-defined droplets with an average diameter of 64 ( 15 μm (as from 151 droplets; Figure 7c). The 20 wt % gellan gum system had a similar morphology, but also contained elongated, stringlike structures (Figure 7d). As such, the average droplet size was not calculated. The methane clathration properties of both systems were examined and gas uptake kinetics for both systems were found to be similar to the DW system (Figure 8a). The final gas capacities were however found to be lower than observed for the neat DW system (156 v/v for 10 wt % gellan gum; 130 v/v for 20 wt % gellan gum). This decrease in storage capacity was offset by a remarkable improvement in recyclability over many cycles (Figure 8b,c) without the need for any reblending between runs. Indeed, the 20 wt % gum system in particular was found to be perfectly recyclable over eight heating/cooling cycles, a major improvement over the original DW system. This “dry gel” method therefore results in a system which combines the benefits of fast kinetics, easy handling, good gas storage capacity, and excellent recyclability. Clathrate Formation in Dry Water with CO2 and Kr. The formation of gas clathrates using dry water is not restricted to MGH. We have also extended this study to the formation of gas clathrates of CO2 and krypton. It has been proposed that it may be possible to recover ocean floor natural gas hydrates in situ by displacing the trapped methane with CO2, with the additional DOI: 10.1021/la903120p
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Figure 9. P-T dependence for H2O-CO2 system during cooling and heating (temperature ramp: 2.5 K/h).
potential benefit of sequestering CO2.47,48 Another potential application is the sequestering of CO2 from flue gases,4,7,8 although it is likely that energy costs associated with compression will prove prohibitive. As for MGH, a key issue is the kinetics of clathrate formation.7,9 Again, the P-T plot for CO2 in an unmixed system containing water and hydrophobic silica approximated to the ideal gas law during a continuous cooling/ heating cycle;that is, very little CO2 was observed. In the case of DW, a rapid pressure drop was observed on cooling with an associated exotherm which can be ascribed to CO2 clathration (Figure S5). As for the MGH, the formation and dissociation temperatures are related to the pressure of the system (Figure 9). There is a maximum pressure under which the gas clathrate can be formed, which is defined by the gas-liquid phase boundary for CO2 as shown in Figure 9. The CO2 gas hydrate formation rate was significantly enhanced in the dry water system in comparison to unmixed systems (Figure 10). The clathration rate for the unmixed system was however higher than that observed the MGH system, probably because of the greater solubility of CO2 in water compared to methane.49,50 The maximum CO2 capacity for the system was found to be ∼150 v/v. On the basis of structural data for sI CO2 hydrate,51 the expected maximum should be comparable with that of sI MGH (180 v/v). This reduced figure for CO2 hydrate capacity might be a result of incomplete occupancy of the sI CO2 hydrate cages51 or the cocrystallization of structure-type II (sII) CO2 hydrate alongside the expected sI phase.9,52,53 We also examined the formation of krypton gas hydrates using DW. Unlike methane and carbon dioxide where sI clathrate structures are formed, krypton clathrates are known to form sII structures.54,55 Cooling DW under a pressure of krypton resulted in the characteristic exotherm associated with clathration (Figure S6). (47) Clarke, M. A.; Bishnoi, P. R. Chem. Eng. Sci. 2005, 60, 695. (48) Shindo, Y.; Lund, P. C.; Fujioka, Y.; Komiyama, H. Int. J. Chem. Kinet. 1993, 25, 777. (49) Lekvam, K.; Bishnoi, P. R. Fluid Phase Equilib. 1997, 131, 297. (50) Valtz, A.; Chapoy, A.; Coquelet, C.; Paricaud, P.; Richon, D. Fluid Phase Equilib. 2004, 226, 333. (51) Udachin, K. A.; Ratcliffe, C. I.; Ripmeester, J. A. J. Phys. Chem. B 2001, 105, 4200. (52) Staykova, D. K.; Kuhs, W. F.; Salamatin, A. N.; Hansen, T. J. Phys. Chem. B 2003, 107, 10299. (53) Fleyfel, F.; Devlin, J. P. J. Phys. Chem. 1991, 95, 3811. (54) Miyoshi, T.; Imai, M.; Ohmura, R.; Yasuoka, K. J. Chem. Phys. 2007, 126, 234506. (55) Montano, P. A.; Linton, J.; Thieu, V.; Halpern, Y. J. Synchrotron Rad. 2001, 8, 972.
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Figure 10. CO2 uptake kinetics in DW at 273.2 K (prepared at 19 000 rpm for 90 s; initial pressure, 3.3 MPa).
Figure 11. Krypton uptake kinetics in DW at 273.2 K (prepared at 19 000 rpm for 90 s; initial pressure, 4.5 MPa).
On rewarming, dissociation of the krypton clathrate was observed. An increase in the rate of clathration was also observed as compared to the unmixed control system (Figure 11). It is therefore clear that DW can be used to increase the kinetics of formation of both sI and sII clathrates. Comparison of Dry Water with Surfactant Promoters. As noted above, a key advantage of dry water is the increased gas uptake kinetics that can be achieved resulting from the increased gas-liquid surface area. As discussed above, other methods can increase gas clathration kinetics. Vigorous mixing increases gas-liquid contact but requires substantial and sustained energy input. Promoters such as THF4,8,19 or cyclopentane9 can be added. This is effective but both THF and cyclopentane are volatile and highly flammable. THF is also toxic. Grinding and sieving of ice to form high surface areas22,23 produces small crystals but this is somewhat laborious. The particles also have a tendency to sinter and, once melted, all advantages of the particulate nature are lost. An alternative method of enhancing mass transport is to use a high surface area support, such as a polyHIPE.28-31 Alternatively, the use of surfactants such as SDS to promote clathrate formation is widely documented.24,25,56 SDS (56) Zhang, J. S.; Lee, S.; Lee, J. W. Ind. Eng. Chem. Res. 2007, 46, 6353.
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hydrate cages are formed,27 culminating in a situation where the hydrate formation pressure in remaining liquid water is elevated beyond the level of the experiment by the concentration of SDS. Perhaps more seriously, even at the low concentrations used here, SDS solution hydrates are subject to copious and persistent foaming upon degassing. This could have practical implications regarding the use of SDS as a promoter on larger scales. Interestingly, in our hands, SDS could also be used to promote the formation of krypton clathrates, but not CO2 clathrates. There are limited examples of the formation of CO2 hydrates in the presence of surfactants.57 Indeed, recent work suggests that SDS has no effect on the mass transfer rate of CO2 across the gas-liquid interface and that this surfactant in fact inhibits the enclathration of CO2.58 By contrast, dry water can be used effectively to promote the rate of enclathration with CO2.
Conclusions Figure 12. Kinetic plot of initial methane uptake in dry water (5 wt % H18) and in SDS solutions.
solutions with concentrations as low as 242 ppm (0.02 wt %, 0.0007 M) have been shown to enhance the rate of clathrate formation up to 700-fold.24 Indeed, we find that SDS enhances the kinetics of clathrate formation to a greater extent than dry water, as shown in Figure 12. The t90 value for 0.1 wt % (0.0035 M) SDS solution was 18 min, as compared with 160 min for DW under similar conditions. In practical terms, however, the use of SDS presents some additional challenges. We found that the maximum methane capacity of the hydrate system was slightly retarded by the use of the surfactant (162.7 v/v). Previous studies have also reported less than complete methane uptakes (160 v/v) for hydrates based on SDS solutions.25 This may be attributed to the effective linear increase in SDS solution concentration as solid (57) Hunter, S. E.; Li, L. X.; Dierdorf, D.; Armendinger, T. Ind. Eng. Chem. Res. 2006, 45, 7275. (58) Zhang, J.; Lee, J. W. Ind. Eng. Chem. Res. 2009, 48, 4703.
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In conclusion, we have demonstrated that dry water can be used to increase the kinetics of formation of a range of gas hydrates including CH4, CO2, and Kr. We have also shown that the incorporation of a hydrocolloid gelling agent dramatically increases the recyclability of the system, and this “dry gel” approach may have applications in other areas which require efficient contacting of aqueous solutions and gases;for example, in gas separations or in the chemical conversion of CO2 by catalytic processes. Acknowledgment. We thank EPSRC (EP/C511794/1; EP/ F06229X/1; EP/G006091/1) for funding. We also acknowledge Wacker Chemie for providing hydrophobic silica. AIC is a Royal Society Wolfson Research Merit Award holder. Supporting Information Available: Figures showing photographs of samples and P-T plots. This material is available free of charge via the Internet at http://pubs.acs.org.
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