Enhancement in Methane Storage Capacity in Gas Hydrates Formed

Article pubs.acs.org/JPCC

Enhancement in Methane Storage Capacity in Gas Hydrates Formed in Hollow Silica Pinnelli S. R. Prasad,* Yalavarthi Sowjanya, and Vangala Dhanunjana Chari Gas Hydrate Group, National Geophysical Research Institute, Council of Scientific & Industrial Research, Hyderabad-500 007, India S Supporting Information *

ABSTRACT: Gas hydrate based methodology plays a pivotal role in separation/storage. Some associated crucial issues are optimization of gas content and operational pressure and temperature and developing means of controlled gas liberation upon command and means of converting larger volumes of hydrates rapidly in a lighter (cheaper) medium. Accordingly, we carried out systematic studies, aiming to enhance the methane gas storage capacity in methane hydrates. We used hollow silica to improve the hydrate formation kinetics and efficiency. We observed over 90% hydrate conversion in a silica−water−methane system at moderately high pressure (5.0 MPa) and 278 K. Methane hydrate conversion in such a system is extremely fast, and this material is apt for multiple freezing−thawing cycles without noticeable reduction in the storage capacity. The volume storage capacity increased from 128 to 206 v (STP)/v by decreasing the combined mass of water and silica from 100 and 18 g in a fixed volume nonstirred reactor at pressures higher than 5.0 MPa.

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INTRODUCTION Gas hydrates, fiery ice, have the ability to store large volumes of naturally occurring fuel gas molecules, such as methane, as guests in its vacant water cages, and they exist in some moderately high pressure and low temperature zones across the permafrost and ocean bottom sedimentary strata.1 Earlier research on these materials was largely for academic curiosity; however, recent estimates reveal that the total energy contained in them will suffice ever-increasing requirements; of course some stringent challenges connected with formation kinetics and material properties have to be properly addressed.1−5 Gas hydrates (also known as clathrate hydrates) are nonstoichiometric inclusion compounds in which gaseous guest molecules are trapped in a host lattice, formed by water molecules in an icelike hydrogen-bonded framework, and they are an important source of natural gas.1−3 For example, methane hydrates represent a highly concentrated form of methane, with 1 m3 of idealized methane hydrate containing 0.8 m3 of water and more than 160 m3 of methane at standard temperature−pressure conditions.1 Methane gas storage/transportation in the form of synthetic gas hydrates is economically feasible.6,7 However, the main drawbacks are in the form of sluggish formation kinetics and lower conversion efficiency (hydrate yield). The common methods for increasing clathrate formation kinetics, for example, use of high pressures (driving force), vigorous mechanical mixing, surfactants, or micrometersized ground/sieved ice particles, can be adopted in the laboratory environment, but these may be less cost-effective and impractical in real gas storage applications. Therefore, for gas transportation conventional methods such as a pipeline © 2014 American Chemical Society

network and the compressed/liquefaction form are being used. However, for vehicular applications storage of fuel in compressed or liquefaction form is less preferred because of inherent handling difficulties involving high pressure or cryotemperature, and hence, the physisorptive methods are being investigated. The guiding factor for methane storage capacity is the U.S. Department of Energy (DOE) set target, which is 180 cm3 (STP) cm−3 at 298 K and 3.5 MPa [cm3 (STP) cm−3 = the standard temperature and pressure equivalent volume of methane per volume of adsorbent material].8 However, recently a new methane storage program was initiated with the following ambitious targets: 0.5 g of CH4/g of sorbent for gravimetric capacity and a new volumetric target equivalent to 263 cm3 (STP) cm−3, which is significantly higher than the previous target.9 It is not possible to reach such a target in pure gas hydrate systems even with 100% hydrate conversion leading to full cage occupancy.1,10,11 Moreover, the DOE-targeted pressure (3.5 MPa) and gas delivery temperature (298 K) are very much outside the methane hydrate stability zone.1,10,11 Storage of gaseous molecules such as hydrogen, methane, carbon dioxide, etc. in solids is yet another technologically important approach suitable for energy and environmental sector applications.4,8,12−14 Set targets pose some taut challenges in terms of suitable material synthesis and development. Several porous materials are being tested Received: December 3, 2013 Revised: March 24, 2014 Published: March 26, 2014 7759

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worldwide.4,14−18 Challenges are very prominent particularly for hydrogen storage applications because most of them have low operational temperatures (77 K) and lower storage (7 wt %) capacity.19 However, some porous materials have tested positively for methane storage applications at the laboratory scale.14−16 For example, zeolites have shown a storage capacity of 100−150 cm3 (STP) cm−3 13−15 at around 298 K and 3.5 MPa; however, this material has a lower surface area and highly hydrophilic nature; thereby moisture is preferentially adsorbed over a period of time. The carbon-based materials have also shown a capacity in the range of ∼50−160 cm3 (STP) cm−3.16,17 If both storage capacity and formation rate are taken into account, ∼278.0 K and ∼8.0 MPa are the most suitable conditions for methane to form hydrates in wet carbon.17 Metal−organic frameworks (MOFs) are another class of adsorbents, and some of them have shown storage capacity matching the DOE targets.18−20 MOFs could be synthesized with larger surface area and variable porosity.18−20 The methane gas uptake capacity of MOF (NU-111) was reported as 177 cm3 (STP) cm−3 at 298 K and 6.5 MPa. The same was increased to 239 cm3 (STP) cm−3 at 270 K and 6.5 MPa.21 However, the main concern, in their practical applications, is their sensitivity toward moisture, which exits ubiquitously, and some inherent structural defects.18−20 One subclass of MOFs, zeolitic imidazolate frameworks (ZIFs), also show greater storage capacity and stability in a water environment.22 The authors have shown that the methane storage capacity could be increased by about 56% in a wet ZIF-8 framework (with a water content of 35.13%) at 269.15 K and 2.85 MPa. The ideal volume storage capacity of the wet ZIF-8 framework was estimated to be more than 190 v/v at 3.0 MPa or so, which is 7% higher than the DOE target (180 v/v) for methane storage.22 Besides physisorption in porous materials, storage of gas in the form of hydrate is another promising approach. However, some of the associated critical challenges, such as insufficient water to hydrate conversions, lethargic formation kinetics, etc., need careful attention. Wang et al. found that water droplets with an average size of 20 μm, surrounded by hydrophobic fumed silica preventing immediate coalescence, can be used for hydrate formation.23 Such a system is popularly known as dry water. Normally the weight fractions of water and silica often used are 95% and 5%. In the past, solid or porous silica was also used to achieve rapid and efficient hydrate conversion in nonstirred reactor vessels, but the corresponding weight fractions were 18% and 82%.24,25 Therefore, porous or solid silica systems are less preferred because of the higher amount of silica and lower amount of water. Even though hydrate formation kinetics and total methane intake are attractive in dry water, there are some serious constraints, particularly on their preparative conditions; e.g., high-speed mixing is necessary for making free-flowing dry water, and there is also a significant reduction in methane intake during its reuse (inappropriate for recycling). The critical shortfalls with a dry water system, namely, stringent preparative conditions and reusability issues, can be avoided in a hollow silica system, and optimization of capillary water resulted in faster, better volumetric/gravimetric yields of methane gas hydrates (MGHs).26,27 The hollow structure of silica comprises an inner void surrounded by a thinner solid shell. Thus, hollow particles have superior properties as compared to solid ones, such as high porosity, low bulk density, high specific surface area, low thermal conductivity, capsulation capacity, etc.28 These materials have potential applications in controlled release

of drugs and biomolecules, photocatalysis, energy storage, sensing, etc.29,30 However, these materials have not been used for methane hydrate formation. In our previous work we demonstrated the use of this material in methane hydrates to achieve rapid and high conversion efficiency,26,27 yet the storage capacity is less than the DOE target and also the operational methane pressure is high. Therefore, the formation pressure suitable for a higher volumetric/gravimetric hydrate yield needs to be optimized. In this paper we report improved methane storage in the form of gas hydrates in a hollow silica system at moderately higher gas pressure. Another attractive feature of this system is its reusability with less stringent sample preparation.

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EXPERIMENTAL SECTION The silica powders with a mean diameter of 30−70 μm were obtained from Nanoshel (Intelligent Materials Pvt. Ltd.) (see Figure SI-2, Supporting Information). Deionized ultrapure water (Millipore, type 1) was used, and dissolved gases were removed by evacuation. The grain size and morphology of the silica were analyzed by field emission scanning electron microscopy (FESEM) (Carl Zeiss Ultra 55), whereas the grain size distribution was measured by the dynamic lightscattering (DLS) method in a water suspension using a particle size analyzer (Malvern Mastersizer 2000). The bulk density of the hollow silica was measured as 0.12 g cm−3, while the same for solid silica having similar particle size distributions (obtained from the same vendor) was 1.5 g cm−3 (see Figure SI-3, Supporting Information). The BET specific surface area of the hollow silica was 2.4 m2 g−1. High-resolution FESEM images also indicated that the voids in hollow silica have narrow pores (see the insets of Figure SI-2). Apparatus. Figure SI-4 (Supporting Information) shows a schematic of the experimental setup where the main part is an SS-316 cylindrical vessel, which can withstand gas pressures up to 20 MPa, and the volume of the vessel is 700 mL. Cold fluid (water + glycol mixture) was circulated around the vessel with the help of a circulator to bring the temperature inside the cell to a desired level and maintain it. A platinum resistance thermometer (Pt100) was inserted into the vessel to measure the temperature with an accuracy of ±0.2 K. The pressure in the vessel was measured with a pressure transducer (WIKA, type A-10 for a pressure range of 0−25 MPa with ±0.5% accuracy). Procedure. Fixed amounts of silica powder and water were mixed gently, and the resulting mixtures were pressurized to the desired levels (at 300 K) with methane gas (99.95% purity) in all the experiments to synthesize the hydrates. The atmospheric gases in the experimental cell were diluted by being purged with methane gas prior to the experiments. The methane gas was filled to a desired level using a Teledyne ISCO syringe pump to a pressure and temperature outside of the hydrate stability zone. Then the reactor was isolated from the ISCO pump/gas tank by closing the gas inlet valve. Subsequently, cold fluid from the chiller was circulated to bring the temperature of the reactor down, and hydrate formation was detected by a sharp pressure drop at a particular temperature. However, the system takes some time (approximately 30−60 min) to reach the gas hydrate stability zone for nucleation, and this depends on the initial driving force. The insignificant head-pressure drop in the reactor over a longer duration indicates saturation in hydrate conversion. The temperature and pressure were logged every 60 s of the time interval. Time zero in all our measurements 7760

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corresponds to a reactor temperature of 288 K. The molar concentration of methane gas (ΔnH,t) in the hydrate phase during the experiment at time t is defined by the following equation: ΔnH, t = ng,0 − ng, t = (P0V /Z0RT0) − (PV t / Zt RTt )

(1)

where Z is the compressibility factor calculated by the Peng− Robinson equation of state. The gas volume (V) was assumed as constant during the experiments; i.e., the volume changes due to phase transitions were neglected. ng,0 and ng,t represent the numbers of moles of feed (methane) gas at 288 K taken at zero time and in the gas phase at time t, respectively. The hydrate yield was computed from the observed methane gas consumption and the expected values with ideal stoichiometric compositions (8CH4·46H2O).26,27 A simpler formula involving the total methane gas consumed, hydration number (in the range of 5.85−6.1), and total water in the reactor is often used in the literature to compute the hydrate yield.22,24,25 We estimated the hydration number for methane hydrates in a hollow silica matrix as 5.94 using Raman spectroscopy.31 The hydrate yields estimated from both approaches had a maximum error of ±5.0%.

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RESULTS AND DISCUSSION Figure 1 shows the cooling/heating curves for the CH4−H2O− SiO2 system at different pressure conditions. We plotted the n(CH4)−T trajectory instead of the usual P−T curves, and n(CH4), the amount of methane in the vapor phase at any instant, was computed from recorded P−T logs and the real gas equation. We conducted experiments with three amounts of water + SiO2, and the volume of the reactor vessel was constant. As observed in our experiments, the amount of methane gas consumed during hydrate formation does not show a large variation in a certain pressure range, e.g., 5.0−9.4 MPa. However, rapid reduction in consumed methane is observed at lower pressures (∼2.4 MPa). We purposefully avoided experiments at still lower pressures as the hydrate formation temperature could cause the system to go into the ice phase. Our motivation is to study the hydrate yield and formation kinetics in the methane−water−silica system. It is generally agreed that pure MGH formation is sluggish, and often vigorous mechanical stirring is required to promote faster nucleation and growth.1 It is evident from Figure 1 that MGH formation in association with silica particles does not require such vigorous mechanical stirring and the MGH conversion is also significantly large (as seen from a huge pressure drop). Another concern is the reusability of the material, particularly silica. We did not change the water or silica for the experiments at different pressures, and the same material was used for several freezing−thawing cycles to indicate its reusability. At the end of each experiment the temperature was maintained above 310 K for 1−2 h to minimize “memory” effects.32,33 The amount of methane consumed in hydrates at different initial pressures, total MGH conversion, and observed volumetric and gravimetric storage capacity in several experiments are tabulated in Table SI-1 (Supporting Information). The hydrate yield was computed from the observed methane gas consumed in the experiments and the expected values with stiochiometric compositions (8CH4·46H2O).26,27 It is generally agreed that for higher hydrate conversions one needs to conduct the experiments at high pressures (driving

Figure 1. Trajectories showing variations in the methane gas content (nCH4) at different temperatures in methane−water−silica systems at three different pressures. The combined masses of silica and water in (a), (b), and (c) are 100, 36, and 18 g, respectively.

force) or cool the hydrate-forming system deeper (supercooling) into the hydrate-stable zone. Experimentally observed hydrate yields under different methane pressures are plotted in Figure 2. We also plotted hydrate yields in a pure methane hydrate system, i.e., without hollow silica, and these experiments were conducted in a stirred reactor to accelerate hydrate formation. However, the experiments with hollow silica were conducted in nonstirred conditions. The hydrate yield in the pure system shows a progressive increase with the driving force, 7761

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Figure 2. Observed changes in gas hydrate yields at different pressures in methane−water−silica systems. Filled circles represent the hydrate yield observed for pure methane hydrates, and the solid line is an empirical fit to these data.

and the behavior can be empirically fitted to a quadratic relation (shown as a continuous line) with a constraint of no hydrate formation without methane gas pressure. However, from the experimental data it is clearly seen that the increase in hydrate yield is initially rapid with pressure (≤4−5 MPa) but significantly slows at higher pressures. Another concern is the kinetics of hydrate formation. Figure 3 shows typical kinetic plots for methane intake (v/v) during cooling cycles in the H2O−SiO2 system over 1000 min measured at three different initial pressures. Details on the processes involved in the hydrate nucleation and growth mechanism are still not exactly known; 34,35 however, experimentally two main stages can be realized. They are initial nucleation and later on stable growth under favorable thermodynamic conditions. As can be seen from Figure 3, the methane intake is rather quick at high pressures, and the process attains 90% saturation in about 100−200 min depending on the amount of water + silica. However, at moderate pressure (4−5 MPa) conditions 90% saturation for methane intake occurred rather slowly. Furthermore, at still lower pressures (≤4.5 MPa), methane intake considerably slowed and also the total gas consumption rapidly decreased (see Table SI-1, Supporting Information). Both the volumetric and gravimetric yields also rapidly decreased. At still lower pressures the hydrate growth appeared to be initiated from the ice phase, and thus, we have not conducted experiments at such low pressures. It is well-known that the hydrate formation kinetics is much faster in the ice phase than in the water phase.36 As described in the Introduction, to make hydrate-based technology economical and industry friendly, a number of parameters need to be optimized. Generally, the hydrate formation process is sluggish, and higher hydrate conversion is possible at higher pressures. As shown in Figure 2, the hydrate yield is higher in the silica−water system even in the nonstirred conditions at moderately higher operational pressures. This could be because of the improved effective surface area of water and the gas molecules. It is also noticed that the hydrate conversion is not always proportional to the gas pressure. Furthermore, the formation kinetics is also relatively faster as shown in Figure 3. It is worth noting that the hydrate yield does not decrease significantly when the methane pressure is

Figure 3. Volumetric ratio of methane consumed with time for silica− water−methane systems. Other details are the same as those in Figure 1.

reduced to about 4−5 MPa, but in pure hydrates the drop is very significant. This could be because of channeling of water into the capillary spaces in porous silica and because after initial rapid growth the methane gas consumption in hydrates is faster. However, at moderate pressures (critical pressure) eventually the hydrate yield reaches the maximum value but on longer time scales. The total time for the process depends on the initial methane pressure. At still lower pressures the hydrate yields and kinetics are similar to those in a quiescent water system. In practice, one desires to obtain a larger amount of hydrate at lower pressures. As shown in Figure 3, it is possible to achieve larger conversion at moderate pressures. When the 7762

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ACKNOWLEDGMENTS We sincerely thank the Director of the National Geophysical Research Institute, Hyderabad, for his encouragement and permission to publish this paper. We acknowledge funding from the Department of Science & Technology (DST), Ministry of Earth Sciences (Gas Hydrates program), and Directorate of Hydrocarbons (NGHP). Financial support for Y.S. through a DST-WOS grant is thankfully acknowledged.

combined mass of SiO2 and water in a 700 mL reactor is 100 g, the maximum volumetric storage is about 128 v/v (at 8.84 MPa). This storage capacity is lower by about 29% than the DOE-targeted value. The desired DOE target for methane storage specifies the experimental pressure also to be lower (3.5 MPa). As shown in Figure 2, it is not possible to get higher hydrate conversion at lower pressures, but at moderately higher gas pressures it is possible to increase hydrate conversion. Also, by arbitrarily decreasing the total mass of water and matrix to 36 and 18 g, the storage capacity improved significantly. Hao et al.37 have shown that the load coefficient (volume of water/ volume of reactor) of a reactor vessel can also play a dominant role in hydrate formation. The maximum methane gas intake capacities in these systems, at pressures around 8.0 MPa, respectively are 151 and 206 v (STP)/v, and evidently the latter is ∼15% higher than the ideal storage capacity of MGH. Mu et al.22 have also reported higher methane storage (190 v/v at 3.0 MPa) in wet ZIF-8 porous materials, and the reason behind this could be the combination of physisorption and hydrate conversion. Furthermore, Liang et al.38 reported from the molecular dynamics simulation studies that the hydroxylated silica surfaces can serve as a source for attracting the methane molecules, which can help to promote hydrate growth. In particular, they concluded that the silanol groups at the surfaces have a lower polarity compared to water molecules; therefore, the silica surfaces provide a reasonable location for methane bubble formation. On the basis of our observations, we can postulate that a combination of physisorption on partly wet silica and hydrate conversion (both within the voids and at the outer periphery of hollow silica) could be a cause for higher methane capture.

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CONCLUSIONS In conclusion, our studies clearly demonstrate that rapid methane gas storage in the form of hydrates is possible at moderate pressures using a hollow silica matrix. Hollow silica has several advantages, such as no special sample preparation and reusablility for several hydrate formation−dissociation cycles. It is possible to achieve DOE set targets for methane storage in this relatively inexpensive material. Moreover, the material is insensitive to water, unlike MOFs, the main component of all hydrate formers. ASSOCIATED CONTENT

S Supporting Information *

Methane gas consumption, hydrate yield, and volumetric and gravimetric storage capacity measured in 90 experimental runs (Table SI-1), grain size distribution and FESEM images for hollow silica (Figure SI-2), photographs showing equal volumes of hollow and solid silica before and after addition of some amount of water (Figure SI-3), and schematic experimental setup (Figure SI-4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +914027012710. Fax: +914027171564. Notes

The authors declare no competing financial interest. 7763

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