Effect of Hydrate Shell Formation on the Stability of Dry Water - The

Jan 5, 2015 - Graduate School of EEWS and Department of Chemical and Biomolecular Engineering (BK 21+ program), Korea Advanced Institute of Science an...
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Effect of Hydrate Shell Formation on the Stability of Dry Water Juwoon Park,‡ Kyuchul Shin,§ Jakyung Kim,† Huen Lee,*,‡ and Yutaek Seo*,† †

Ocean Systems Engineering Division, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Graduate School of EEWS and Department of Chemical and Biomolecular Engineering (BK 21+ program), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea § Department of Applied Chemistry, Kyungpook National University, Daegu 702-701, Republic of Korea

Nobuo Maeda,∥ Wendy Tian,∥ and Colin D. Wood*,∥ ∥

CSIRO Manufacturing Flagship, Clayton, VIC 3168, Australia S Supporting Information *

ABSTRACT: This study investigates the effect of gas hydrate formation on the stability of dry water (DW) particles when they are exposed to high pressure methane at low temperatures. The DW particles are prepared by mixing water with hydrophobic silica nanoparticles at high speed to form a water-in-air inverse foam. A high pressure autoclave was used to determine the hydrate equilibrium conditions and formation characteristics including hydrate onset time, subcooling temperature, and initial growth rate. In comparison to bulk water, the equilibrium conditions for methane hydrate are shifted to higher temperatures and low pressures, suggesting that the silica nanoparticles promote the hydrate equilibrium conditions. The surface-to-volume ratio between the gas and the water encapsulated by the silica nanoparticles is increased in comparison to bulk water which enhances the kinetics of methane hydrate formation without the need for vigorous mixing. However, after multiple cycles of hydrate formation and dissociation, the hydrate fraction decreases exponentially and approaches 0.22, which is approximately 20% of the hydrate fraction formed during the first cycle. From the data presented, it was concluded that the hydrates form a shell on the DW particles. Dissociation of this hydrate-shell generates a free water phase that cannot be reabsorbed into the DW particles which causes the exponential reduction in the hydrate fraction. PXRD confirms that structure I methane hydrate is formed with a lattice parameter of 1.1827(1) nm. Raman spectroscopy confirms that the hydrate-shell covers the DW particles as evidenced by the presence of two peaks for methane at 2901 and 2913 cm−1, which indicates that the methane exists in large and small cages, respectively. These results suggest that the particles are covered with a hydrate-shell when methane hydrates are formed. Therefore, the hydrophobic silica is rearranging during hydrate formation, and after dissociation of the hydrate, free water is expelled. This free water cannot absorb back into the particles due to the hydrophobic surface.

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INTRODUCTION Dry water (DW) is a water-in-air inverse foam produced by mixing water with hydrophobic silica nanoparticles.1,2 The DW particles are a free-flowing powder where coalescence of the water droplets is prevented by the hydrophobically modified silica coating at the water−air interface. As such, they have a higher surface-to-volume ratio than bulk water. Therefore, the finely dispersed water droplets lead to greatly enhanced kinetics of clathration in a gaseous system, resulting in increased rate of methane hydrate formation in comparison to bulk water.2−5 This has potential as a material for gas storage in a static unmixed system. Another application for DW is to carry out heterogeneous gas−liquid reactions without the need to stir or agitate the reaction,6 thus providing a significant energy saving for reactions. Alkaline solutions can also be encapsulated in the form of a dry base which was recently shown to rapidly capture carbon dioxide without any mixing.7 Therefore, the key feature © 2015 American Chemical Society

for a DW system is the greater surface area available for gas− liquid contact, even in the absence of mixing. Gas hydrates, or clathrate hydrates, are nonstoichiometric crystalline compounds that form when gas molecules are incorporated into crystal structures formed by water molecules through hydrogen bonding.8 Light hydrocarbons such as methane and ethane are well-known hydrate-forming gas molecules at low temperatures and high pressures. There has been considerable interest in developing methods for efficient gas storage based on the fact that each volume of hydrate can contain as much as 170 volumes of gas under standard conditions.8,9 Gas separation based on hydrate formation has also been studied due to the selective enclathration of gas Received: October 21, 2014 Revised: December 11, 2014 Published: January 5, 2015 1690

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The Journal of Physical Chemistry C molecules under specific conditions.10−14 For the energy industry their formation in offshore flowlines transporting hydrocarbons is a serious concern;8,9,15,16 thus, there have been extensive studies to prevent the formation of hydrates by injecting glycol solutions17,18 or alternative materials.19−22 There are several issues related to studying hydrates, but formation kinetics is central to understanding the mechanism of gas enclathration in a host framework formed by water molecules.23−25 As the formation of hydrates involves transport of gas molecules from the gas phase to the aqueous phase, the formation rate in bulk water is slow as observed in experiments using a high pressure autoclave, especially when the system is not stirred. There have been many methods employed to increase the formation rate including the use of vigorous mixing devices, the addition of a promoting material such as tetrahydrofuran (THF) or sodium dodecyl sulfate (SDS), and increasing the gas−water contact area by using finely ground ice particles.26,27 However, all of these methods have disadvantages such as energy-intensive operation, reduced gas storage capacity due to incorporation of a promoter in the hydrate cages, and the ice particles are only effective in the first cycle, requiring additional steps to re-form the particles. We have previously shown10,11 that pure water adsorbed in the pores of silica gel could be used to form hydrates. In bulk water the hydrate formation was limited by the gas−water contact at the interface; however, gas molecules could easily penetrate into a bed of silica gel particles saturated with water. The dispersed water in the pores of the silica gel enhanced the contact surface area between the gas molecules to form gas hydrates within the pores. 1H NMR microimaging showed that the hydrate yield was over 85% after 1 h when a steady state was reached. Approximately 90% of this yield was achieved after ∼20 min of reaction time.10 Increasing the gas−water contact was a promising method to enhance the formation rate. However, the volume penalty was high for silica gel particles because only half of the volume of the silica gel can be occupied by the water. Conversely the DW particle itself is composed of 95 wt % water and 5 wt % silica nanoparticles; thus, the volume penalty can be minimized. In the case of porous materials, Linga et al. investigated a silica sand and polyurethane (PU) foam in a fixed bed system to reveal how the porous materials affect hydrate kinetics and gas uptake. The PU form system and the silica sand system showed a fast hydrate growth rate and improved hydrate formation with good reproducibility. They suggested that the enhanced kinetics results from highly connected interstitial pores.28−30 Dispersed water particles in the form of DW can be used as an alternative strategy. The average size distribution for these particles is approximately 50 μm, which increases the diffusion of gas into the aqueous phase compared to bulk water or ice.6 Although the formation kinetics is greatly increased in DW, the stability during hydrate formation is not adequate and needs to be resolved for implementation as a gas storage solution.2,3,5 The DW can be reused after several cycles of hydrate formation and dissociation; however, the storage capacity degrades significantly. For example, the reduction in pressure due to hydrate formation decreased from ∼5.5 MPa to ∼3.2 MPa when the system went through three cooling/heating cycles with methane.3 It was postulated that partial agglomeration of the water droplets would occur during the freezing and warming process. Aggregation of dry solution powders was also observed in the presence of a surfactant, and the powders did not recover their original properties.5 The incorporation of a

hydrocolloid gelling agent was reported to increase the recyclability of the DW system.3 However, the stability of the DW upon hydrate dissociation still remains challenging. Despite the advantages of DW, only a few studies have been carried out concerned with the stability of these systems. In this work, a series of experiments have been carried out to investigate the effect of hydrate formation on the stability of DW. We performed several cycles of hydrate formation and dissociation in order to investigate the decrease in the formation rate and the amount of gas that was consumed, which was estimated from the reduction in pressure upon the formation of hydrates. In addition, Raman spectroscopy and powder X-ray diffraction (PXRD) were used to identify the structural characteristics of the gas hydrates formed on the surface of the DW. Thermodynamic equilibrium conditions were also measured to study the dissociation temperature and pressure of the hydrates formed on the surface of the DW particles. These studies provide a better understanding of the hydrate formation characteristics in water droplets surrounded by hydrophobic silica nanoparticles, i.e., dry water.

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EXPERIMENTAL SECTION Materials. Pure methane gas (UHP grade, 99.999%) was supplied by Special Gas (Korea). The hydrophobic silica nanoparticles (HDK18) were kindly supplied by Wacker Chemie (Australia). Synthesis of Dry Water (DW). The free-flowing DW powder was prepared by mixing 10 g of hydrophobic silica nanoparticles (HDK18) with 190 g of water at 19 000 rpm for 60 s in a domestic blender. Formation Kinetics and Equilibrium Measurements. The experimental apparatus is designed to measure changes in pressure and temperature during hydrate formation. The DW (100 g) was loaded into a cell with an internal volume of 460 mL. A circulator bath was connected to the cell to provide the cooling fluid and the temperature of the fluid was controlled by a preprogrammed bath. A platinum resistance thermocouple was used to monitor the temperature of the gas and the DW phase inside the autoclave with an accuracy of 0.15 °C. The pressure was measured by a pressure transducer with an accuracy of 0.1 bar, and the range was 0−200 bar. A two-blade impeller attached to a solid shaft which was magnetically coupled to a motor (BLDC 90) was used to vigorously mix the contents of the reactor. The impeller was not used for all experiments as the objective of using the DW is to form gas hydrates without vigorous mixing. The impeller was used for confirming the formation of the hydrates from the free water phase which was expelled from the DW after several cycles of hydrate formation and dissociation. When the impeller was used, a torque sensor (TRD-10KC) with a platinum-coated connector measured the torque on the stirrer with an accuracy of 0.3%. Temperature, pressure, and torque data were recorded using a data acquisition system. A detailed description of the experimental apparatus was outlined in our previous work.18,21 The DW was added to the autoclave, and then the top housing was mounted on the autoclave body. The temperature of the bath was maintained at 24 °C, as no hydrate formation occurs at this temperature under the pressure conditions being studied. After being purged three times with pure methane, the autoclave was pressurized to the desired pressure, 115 bar in this work. The cell was subsequently cooled to 5 °C over 60 min and kept at this temperature for 10 h. During the whole experiment, the pressure and temperature were continuously 1691

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volume of gas phase, and z is the compressibility factor calculated using Peng−Robinson equation of state. The hydrate fraction, Φhyd in the liquid phase at the end of each cycle is calculated using the equation:

monitored. Figure 1a,b shows the temperature and pressure changes that were measured during the experiment when

Φ hyd =

Vhyd + (Vw − Vw,conv)

where Vw is the volume of water, Vw,conv is the volume of the water converted to hydrate, and Vhyd is the volume of hydrate calculated from the molecular weight and density of hydrates calculated at a given time. This study was performed over a constant duration in each cycle to investigate how hydrate formation affects the stability of the DW. Although hydrate formation was not completed in Figure 1, the hydrate fraction changed slightly at the end of each cycle due to reduced growth rate. To the best of our knowledge, there have been limited studies concerned with the effect of DW on the equilibrium conditions of methane hydrates.3 To experimentally investigate these parameters in DW, pressure and temperature profiles were generated during continuous cooling and heating cycles while changing the initial pressure from 120 to 60 bar. Hydrate formation and dissociation were confirmed by a reduction in the pressure upon hydrate formation and when the cooling curve returned due to hydrate dissociation. The temperature and pressure conditions where the heating curve meets the cooling curve identified the hydrate equilibrium condition for methane hydrate formed on the surface of the DW. PXRD and Raman Spectroscopy. The PXRD pattern was recorded on a Rigaku D/Max-RB diffractometer using graphitemonochromatized Cu Kα radiation (λ = 0.15406 nm) in a θ/2θ scan mode. The sample was transferred to a holder in liquid nitrogen, and the holder was then quickly loaded onto the Xray stage which was cooled to 93 K using a nitrogen stream. The experiment was carried out in a step mode fashion with a fixed time of 3 s and a step size of 0.02° for 2θ = 5° to 55°. The obtained pattern was refined by a nonstructural Le Bail fitting method using a profile matching method within FULLPROF.31 The Raman spectra were recorded using a Horiba Jobin-Yvon ARAMIS high resolution dispersive Raman microscope equipped with an electrically cooled (203 K) CCD detector. The excitation source was an Ar-ion laser emitting a 514.53 nm line, and the laser intensity was typically 30 mW. The experimental temperature was kept at 93 K by a Linkam TMS 94 temperature controller during the measurements.

Figure 1. Pressure and temperature changes with time during the hydrate formation and dissociation for four cycles.

ramping the temperature between 24 and 5 °C for four cycles. Time zero indicates the start of the cooling process. Hydrate formation was accompanied by an increase in temperature and a decrease in pressure due to the exothermic formation of gas hydrates consuming the gas. The hydrate onset time was identified by a sudden decrease in pressure. The initial growth rate of the hydrate was calculated during the linear gas consumption stage. The hydrate fraction in the liquid phase was estimated from the amount of consumed gas during the hydrate formation. Twelve cycles of forming and dissociating the hydrate were carried out to examine the degradation of the hydrate formation kinetics. The growth rate and hydrate fraction at the final stage were compared for each cycle. To dissociate the hydrate between cycles, the temperature was increased to 24 °C for 3 h. The mole of gas consumed during the formation of hydrate was calculated from the pressure difference between the pressure at the measurement moment and the calculated pressure with the assumption that no hydrate was formed as follows: ΔnH, t =

Vhyd

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RESULTS AND DISCUSSION Dry water (DW) is free-flowing powder prepared by rapid mixing of hydrophobic silica with water in a conventional domestic blender. Previous studies on the formation of hydrates in DW focused on the enhanced growth rate and the storage capacity for methane.2,3 The capacity for methane storage was found to be affected by the temperature of hydrate formation, and the highest capacity obtained was between 273 and 277 K. In addition, varying the ratio of silica to water used to prepare the DW had no significant effect on the formation rate of hydrate, as the particle size was mostly affected by the mixing speed rather than the silica content. Interestingly, Carter et al. reported the deviation of the hydrate dissociation temperature in DW from the equilibrium curve for methane hydrate formed in bulk water.3 Hydrate equilibrium conditions were shifted to a higher temperature and lower pressure region,

⎛ PexpVcell ⎞ ⎛ PcalVcell ⎞ ⎜ ⎟ − ⎜ ⎟ ⎝ zRT ⎠t ⎝ zRT ⎠ t

where, ΔnH,t is the moles of gas consumed for hydrate formation at a given time, and Pcal is the calculated pressure assuming no hydrate. Pexp is the measured pressure, Vcell is the 1692

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particles. There was no self-preservation effect as witnessed by Hu et al.4 because there was a continuous evolution of methane from the hydrate during the dissociation process. Therefore, we believe that the hydrate equilibrium temperature was promoted by the DW particles. Interestingly, for methane hydrate, a promotion of the equilibrium conditions can be achieved by adding promoters such as THF and 1,4-dioxane that form the structure II hydrate together with methane.39 Other gaseous molecules such as ethane and carbon dioxide may also induce a promotion effect by forming a mixed hydrate with methane while maintaining structure I hydrate.40 However, as described above, no additional hydrate guests were used in this work. In an attempt to increase the gas−liquid contact when silica gel was used (not DW), a depression in equilibrium temperature was observed, as more driving force was required to form hydrates inside the nanosize pores of the silica gel.10,41,42 This mechanism is different from that of DW because hydrates would form on the surface of the particles, resulting in hydrate shell-covered particles. Therefore, in order to gain a better understanding of the hydrate formation characteristics in DW, additional investigations are required that are related to the interaction between hydrophobic silica and the encapsulated water phase with a hydrate shell. The hydrate formation process was studied during 12 cycles of cooling and heating, and the obtained hydrate onset time, subcooling temperature, and initial growth rate in each cycle are presented in Table 1. Figure 3 shows hydrate fraction profiles

indicating the promotion effect of DW. The deviation was explained in terms of the high heating rate, 2.0 K/h, and the self-preservation effect of methane hydrate during the dissociation process. However, the hydrate equilibrium curves need to be confirmed before investigating the formation kinetics of hydrates in DW. Therefore, our initial experiments were carried out with DW having 5 wt % silica and 95 wt % water to determine the hydrate equilibrium temperature and pressure. This is typically achieved using a continuous cooling and heating rate of 0.1 K/h, so in this study that was employed to measure the hydrate equilibrium conditions. Figure 2 shows the measured hydrate equilibrium conditions of methane hydrate in DW in addition to the predicted

Table 1. Hydrate Onset Time, Subcooling Temperature, and Growth Rate Constant for Methane Hydrates Formed in Dry Water (DW) (5 wt % silica:95 wt % water)

Figure 2. Hydrate equilibrium conditions of methane hydrate: (), predicted methane hydrate equilibrium curve; (▽), methane hydrate equilibrium point with bulk water (McLeod et al., ref 38); (●), methane hydrate equilibrium point with dry water (DW).

cycle 1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th 12th average std deviation

equilibrium conditions of methane hydrate in bulk water using MultiFlash software (version 4.4).32 The Cubic Plus Association (CPA) equation of state is used along with the transport property models:33 Lorenz−Bray−Clark (LBC) model for viscosity,34 Chung−Lee−Starling model for thermal conductivity,35 and McLeod−Sugden model36,37 for interfacial tension. The methane hydrate equilibrium conditions in bulk water was obtained in the literature.38 The thermodynamically stable region for hydrates is on the left side of the equilibrium conditions (hydrate stability region); however, no hydrate can exist on the right side of the conditions (hydrate free zone). The calculated equilibrium temperature for methane hydrate in bulk water was 13.9 °C at 111.45 bar; however for DW, the equilibrium temperature was shifted to the right to 14.9 °C. The resulting increase of equilibrium temperature from the original value, ΔTeq, was 1.0 °C, indicating a promotion effect. The hydrate equilibrium measurement was also carried out with a fresh DW sample at a lower pressure, and the measured equilibrium temperature at 87.5 bar was 13.1 °C, resulting in a ΔTeq of 1.4 °C. The additional equilibrium measurement determined an equilibrium temperature of 10.6 °C at 59.7 bar, resulting in a ΔTeq of 2.4 °C. Therefore, the measured equilibrium conditions show a promotion of 1.0−2.4 °C from the measured equilibrium temperature in the studied pressure range. The promotion of the equilibrium temperature, ΔTeq, in this work is lower than in the literature data.2−4 The heating rate during the measurements was maintained at 0.1 K/h in order to induce slow dissociation of hydrate on the surface of the DW

Tonset (°C)

ΔTsub (°C)

tonset (min)

kgrowth (min−1)

4.8 8.6 5.4 5.6 5.9 5.9 5.8 8.8 5.3 7.0 5.5 5.4

9.98 6.1 9.3 9.1 8.8 8.8 8.9 5.9 9.4 7.7 9.2 9.3 8.5 1.3

66.8 35.3 42.0 39.6 37.6 37.3 37.5 37.3 41.6 32.1 42.0 42.6 41.0 8.7

0.0760 0.0382 0.0723 0.0715 0.0572 0.0594 0.0576 0.0524 0.0568 0.0543 0.0603 0.0591

Φhyd,final 0.77 0.49 0.48 0.43 0.38 0.33 0.31 0.27 0.26 0.26 0.26 0.22 -

with time when hydrates form in DW. The formation process is similar to that for bulk water which is characterized by three distinct stages of hydrate formation, (i) hydrate nuclei formation, (ii) growth of hydrate particles, (iii) achieving steady-state conditions due to consumption of hydrate-forming materials.43 The formation of hydrate nuclei is typically detected using spectroscopic methods, and no decreases in pressure or changes in temperature occur as small amounts of gas are consumed. However, it is believed that the growth of hydrate particles occurs shortly after the nucleation stage, which results in a large decrease in pressure and an increase in temperature. In this study, pressure decreases and rapid increases in temperature were observed during the cooling of the fluids, which we consider the hydrate onset (Figure 1). The 1693

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0.08 except for the the second cycle, but the fraction was significantly reduced from 0.77 in the first cycle to 0.22 in the 12th. The limiting step for hydrate growth would be the transportation of water or gas molecules through the hydrate shell. In the case of hydrate formation in bulk water, mass transfer limitations dictate the growth rate and the amount of hydrate formed, because the solubility of gases in water is very low and consequently gas hydrate only forms at the surface where the gas concentration is the highest. This formation of a gas hydrate shell at the surface then prevents mass transfer of gas because the permeability of gas through a solid like gas hydrate is very low. For DW in a gas phase, the high surface area between the gas and water would be maintained; therefore, a fast growth rate was observed for 12 cycles. However, the formation of hydrate destabilizes the DW, causing a separate free water phase to form. After 12 cycles of hydrate formation and dissociation, a separate free water phase can be observed inside the autoclave which has “drained” from the dry water particles (see Figure S1, Supporting Information). These studies indicate that a hydrate shell is formed on the surface of the particles, which affects the interaction between the silica and the water phase. The formation of a hydrate-shell on the dry water particles lowers the chemical potential of the water molecules inside the hydrate cages which consequently increases the chemical potential differential between them and the free water molecules remaining in the dry water particle, which is causing water to be drained out of the dry water particles. Due to the hydrate shell, the dry water maintains its shape; however, during the dissociation of the shell, some amount of water from the hydrates is converted to the free water phase (outside the particles) while the other water rearranges with the silica nanoparticles. Overall, this process results in transport of some of the water from the dry water particle to the exterior, forming the free water phase that was observed. Destabilization of the DW upon formation and dissociation of hydrates was already reported in the literature. Wang et al.2 presented the reduction of methane storage capacity due to hydrate formation, and Carter et al.3 also observed the partial agglomeration of water due to free water that was produced. One hypothesis to describe the destabilization process in DW5 suggested that the DW particles partially aggregated to form bigger DW particles during the dissociation of the hydrate layer. However, this hypothesis does not explain how a free water phase was evolved by partial agglomeration of the dry water particles during the freezing and warming process. In a recent study Farhang et al.44 suggest that high pressure and stirring caused coalescence of larger dry water particles whereas the smaller dry water particles remained unchanged in the system under study. As discussed above and reported in the literature,3 destabilization of the DW accompanies the evolution of free water and silica with agglomeration of the DW droplets. We believe that the nature of the surface of the silica nanoparticles is changed during the formation and dissociation of hydrate; therefore, more studies are required to elucidate the effect of hydrate layer on the interaction between water and hydrophobic silica particles in order to increase the stability of the DW. Figure 4 shows the decrease in hydrate fraction with the number of cycles. As discussed previously, the hydrate fraction in the 1st cycle was 80%; however, it drops to 50% in the second cycle. If all of the water in the hydrate shell on the

Figure 3. (Top) Hydrate fraction profiles for each cycle during hydrate formation in dry water (DW) with time, tonset (t = 0). (Bottom) Enlarged region for rapid growth of hydrate.

time for hydrate onset was defined as the difference between the hydrate onset time and the time when the temperature becomes lower than the hydrate equilibrium temperature, which is indicative of the delay in hydrate onset. The hydrate onset temperature Tonset was typically measured below the hydrate equilibrium temperature, and the subcooling temperature is defined by the difference between Teq and Tonset to indicate the subcooled temperature before the hydrate formation occurred in the water phase. Figure 3 shows that the observed gas consumption was rapid during the growth of the hydrate particles. Considering the gas−water interface in DW, hydrate formation will occur and proceed on the surface of the particles while consuming water and gas. When fitting the experimental results with an exponential function, y = y0 + a (1 − exp(−kgrowthx)), the growth rate constant, kgrowth in the first cycle was 0.0760 min−1. An inflection point occurred at 136 min, then the gas consumption curve showed slow growth. The resulting hydrate fraction from the amount of consumed gas was 0.77. In the second cycle, the growth rate constant was 0.0382 min−1, and the hydrate fraction was reduced to 0.49. It is noted that the initial growth rate of the second cycle is similar to that of first cycle; however, a subsequent inflection point appeared early with a slow increase of the hydrate fraction as shown in the first cycle. The resulting hydrate fraction was 0.49 and similar to those from the third and fourth cycles. We presumed that the evolved water from the first cycle may not be separated from the DW phase completely, and induce the mass transfer limitation. While cycling the formation and dissociation of hydrates, the growth rate constant was in the range of 0.05− 1694

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provides a destabilizing rate constant of k1 = 0.36. Previous work forming DW particles containing maleic acid argued that the solution remained stable at relatively high concentrations for at least one month. However, once the DW went through the hydrogenation reaction, the resulting samples were found to be aggregated and less flowable. Using a dry base for the adsorption of CO2 also showed a loss of DEA from the dry base during the regeneration cycles.7 To increase the stability of DW, the silica content can be increased (although this was not reported to assist in all cases3) or by adding a solution of a hydrocolloid gelling agent.3 However, there have been scarce studies to analyze the destabilization rate quantitatively. Here we carried out 12 cycles of hydrate formation and dissociation cycles to investigate the destabilization rate during the cycles. The obtained results suggest that the DW particles lose their stability with an exponential function, and the final hydrate fraction approaches 20%. The obtained destabilization rate constant would be useful to quantify the stability of the DW and could be compared with the rate constant obtained from modified DW systems obtained by adding a gelling agent or a polymer. The evolved free water from the DW particles remained separated from the DW particles due to the hydrophobic silica nanoparticles. The autoclave we employed is equipped with an overhead stirrer; thus, free water and DW could be stirred to try to regenerate the DW. Figure 5 shows the pressure, temperature, and torque changes during the operation of the

Figure 4. Hydrate fraction changes while repeating the cycles of hydrate formation and dissociation.

surface of the DW was expelled resulting in free water upon dissociation, the hydrate fraction in the second cycle would be less than 20%, as it is likely that a negligible amount of hydrate would be formed from the free water without mixing. This strongly suggests that a rearrangement of the water and silica nanoparticles is occurring during the formation and dissociation of hydrate in the DW. In the additional cycles, the hydrate fraction decreases with an exponential decay function, y = y0 + a exp(−k1 x). Fitting the hydrate fraction data in Figure 4

Figure 5. Pressure, temperature, hydrate fraction, and torque change with 600 rpm stirring for 2 h. After mixing for 2 h, pressure and temperature are further monitored until hydrate formation is completed. 1695

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The Journal of Physical Chemistry C impeller at 600 rpm for 2 h. It is worth noting that hydrate formation and dissociation cycles were already repeated 12 times. Upon commencing the stirring at 600 rpm, the temperature rapidly increased to 9 °C accompanied by a sudden decrease in pressure as seen in Figure 5, indicative of hydrate formation due to mixing of the DW and the free water phase. The torque increased rapidly and maintained a high value during the mixing, characteristic of hydrate formation in bulk water. As such, at that stage it was unclear if the free water had been reabsorbed back into the DW. The resulting hydrate fraction after mixing was 65%. The temperature was then increased to dissociate the hydrate, and then the hydrate formation experiment was carried out again without mixing. The obtained hydrate fraction in the next cycle was 21% which is close to the last hydrate cycle (before commencing stirring), indicating that the hydrate was formed only in the free bulk water due to the mixing. Therefore, the DW cannot be regenerated by mixing the free water and hydrophobic silica at 600 rpm. This result indicates that the regeneration of DW requires higher mixing, at least 19 000 rpm as tested in this work. The DW prior to hydrate formation exists as fine particles with an average droplet size of 13 μm as shown in Figure 6a.

Figure 7. Powder X-ray diffraction pattern of methane hydrate with DW. Green ticks show the reflections of CH4 hydrate (structure I hydrate, upper row), hexagonal ice (lower row).

structure (lattice parameter a = 1.1827(1) nm) with some unreacted hexagonal ice phase (lower row ticks in Figure 7). Broad signals from silica particles were not observed in this measurement, indicating that a hydrate shell covered the DW particles. Raman spectroscopic observation also supports that the hydrate shell covers the surface of DW. Figure 8a shows two Raman spectra: one obtained from the DW sample collected after the hydrate formation (blue line) and the other obtained from hydrate-free DW for comparison (red line). The blue line shows two characteristic peaks of structure I methane hydrate at 2901 and 2913 cm−1 (Figure 8b for enlarged spectrum), and some additional peaks originated from O−H stretching of water molecules in the region 2960−3500 cm−1. The intensity ratio of I2901 to I2913 is ∼3.1, which is close to the stoichiometric value of cage number in structure I hydrate (2 small and 6 large cages per 46 waters). On the other hand, the characteristic peaks from silica at 493 and 703 cm−1 (red line in Figure 8a) are not observed in the DW with hydrate (Figure 8c for enlarged spectra). These Raman spectroscopic results confirm again the formation of a hydrate shell on the surface of DW. Although there may be a chance for the water molecules to be rearranged with the hydrophobic silica particles during the dissociation of hydrate shell, the amount of water encapsulated in DW again would be limited, as high speed mixing is required to regenerate the whole water phase. In order to increase the stability of DW during the hydrate formation and dissociation cycles, the nature of the surface of DW must be improved to overcome the mechanism of free water evolution described above.

Figure 6. Images of DW particles before hydrate formation (a), after hydrate dissociation (b).

This average droplet size is only an estimate due to the irregular shape of the particles. The image in Figure 6b was taken after the hydrate formation and dissociation, showing that the fine DW particles existed together with larger aggregated particles. This image was taken after 12 cycles of hydrate formation and dissociation; therefore, the free water phase was already separated from the hydrophobic DW phase. The bigger particles are likely to result from the aggregation of the free silica particles generated by the release of the water as in the case with fumed silica. The optical images of the DW particles indicate that the shape of the particles may be retained even after the numerous cycles of hydrate formation and dissociation. There would be an aggregation of particles through silica rearrangement around the water droplets dissociated from the hydrate phase. It seems that part of the evolved free water phase is separated from the DW particles due to the hydrophobic characteristics of DW (Figure S1); however, small amounts of free water still remain between the DW particles. More quantitative analysis will be carried out to find out how much free water would be expelled during a single cycle of hydrate formation and dissociation. PXRD and Raman spectroscopy were carried out to analyze the characteristics of methane hydrate formed on the surface of DW. Figure 7 represents the PXRD pattern and its Le Bail fitting of the sample collected after the hydrate formation of fresh DW with methane. The pattern clearly shows cubic Pm-3n

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CONCLUSIONS Dry water (DW) particles were prepared by mixing water with hydrophobic silica nanoparticles. Due to the high surface-tovolume ratio of the DW particles, the hydrate formation kinetics were enhanced when the particles were exposed to methane at low temperatures and high pressures. A high pressure autoclave was used to determine the hydrate equilibrium conditions, hydrate onset time, subcooling temperature, growth rate constant, and hydrate fraction. Interestingly, the hydrate equilibrium conditions were measured at 1.0−2.4 °C higher than in bulk water, indicating the promotion effect on the equilibrium conditions by encapsulating water with hydrophobic silica nanoparticles. More studies will be carried out to investigate the interaction between the water and silica 1696

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Figure 8. (a) Raman spectra of methane hydrate in DW and DW (inset is the Raman focusing area image.). (b) Enlarged Raman spectrum in 2880− 2930 cm−1 to indicate the methane in the hydrate large (51262) and small (512) cages. (c) Enlarged green circle region of part a to reveal Si−O−Si stretching band at 493 and 703 cm−1.

to understand the hydrate promotion. A hydrate shell was formed on the surface of DW particles and results in destabilization of the adsorbed layer of silica nanoparticles during its dissociation. The hydrate fraction obtained in the first cycle was 0.77; however, it decreased to 0.49 in the second cycle, which is about 36% reduction in a single cycle. While cycling the formation and dissociation 12 times, the hydrate fraction continued to decrease and approached 0.22 following an exponential function with the destabilization rate constant k1 = 0.36. A free water phase was formed, resulting in a separate phase below the DW particles. This clearly indicates the destabilization of the DW due to the hydrate formation and dissociation. The data indicate that the silica nanoparticles were excluded from the hydrate shell during the hydrate formation and are rearranged with water during the hydrate dissociation. It is worth noting that not all water evolves from the hydrate shell as free water because the hydrate fraction was 0.49 in the second cycle while it was 0.77 in the first cycle. An additional experiment with vigorous mixing at 600 rpm showed that additional hydrate could be formed; however, this was accompanied by an increase in torque during the mixing. Therefore, the hydrate formation was occurring in the free water phase rather than in DW particles. The PXRD pattern for DW samples after the formation of hydrates indicate that the cubic structure I was formed with a lattice parameter of

1.1827(1) nm. The Raman spectra obtained on the surface of DW samples suggest that the particle was covered with a hydrate shell, where two peaks for methane appear at 2901 and 2913 cm−1, indicating methane in large and small cages, respectively. The O−H stretching in the region between 3000 and 3500 cm−1 represents the water molecules structuring the hydrate cages. These results suggest that hydrate-shell formation and dissociation would induce the destabilization of the layer of hydrophobic silica nanoparticles surrounding the water phase, which induces the free water to drain out of the DW as a separate phase.

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ASSOCIATED CONTENT

S Supporting Information *

A recovered dry water sample after 12 cycles of hydrate formation and dissociation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: +82-42-350-1521. E-mail: [email protected]. *Telephone: +82-42-350-3917. E-mail: [email protected]. *Telephone: +613-9545-8160. E-mail: [email protected]. 1697

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Technology Innovation Program (10045068) funded by the Ministry of Trade industry & Energy (MI, Korea). This work was partially supported by the Global Leading Technology Program of the Office of Strategic R&D Planning (OSP) funded by the Ministry of Trade industry & Energy (10042424).

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