Article pubs.acs.org/EF
Methane Hydrates in Spherical Silica Matrix: Optimization of Capillary Water V. Dhanunjana Chari,†,‡ B. Raju,†,§ P. S. R. Prasad,*,† and D. Narayana Rao§ †
Gas Hydrate Group, National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad 500 007, India ‡ Department of Physics, Osmania University, Hyderabad 500 007, India § School of Physics, University of Hyderabad, Hyderabad 500 046, India S Supporting Information *
ABSTRACT: Methane hydrate formation/dissociation behavior in the presence of a fixed amount (10 g) of silica grains with a mean diameter of 30−70 μm has been examined in a non-stirred reactor vessel. We systematically varied the amount of water from 10 to 200 g in steps and probed the methane hydrate formation kinetics and the overall hydrate conversion in a silica− water−methane system. Our results showed that the overall methane conversion monotonically increased from 6.14 to 67.82% by reducing the water content from 20 to 1 g/g of SiO2, while under similar experimental conditions, the hydrate conversion in a pure water system was 4.1%. Interestingly, the time taken for 90% of methane gas consumption in hydrate, in a silica−water− methane system, decreased from 450 to 100 min by reducing the total water content from 20 to 4 g/g of SiO2. Our results also indicated that the methane hydrates in the SiO2 matrix are useful for gas storage upon optimizing capillary water. Moreover, we observed that the water-optimized system shows faster formation kinetics and better hydrate conversion, which are critical for methane storage/transportation.
1. INTRODUCTION Gas hydrates (GHs) are the non-stoichiometric clathrate compounds consisting of water and lighter hydrocarbon molecules. These compounds are stable under moderately higher pressure and near ice-melting temperature. The resulting structures are ice-like crystalline in nature, where a group of water molecules form a cage by a hydrogen-bonding network, and the vacant cage space is occupied by suitable gas molecules.1,2 In nature, GHs are widely present in certain oceanic margins and permafrost regions.3−5 Approximately 160−180 times standard temperature and pressure (STP) equivalent gas can be contained in one unit of hydrate, and thus, GH-based technology is attractive for fuel gas storage and transportation. Presently, the natural gas is being transported by compressing it to higher pressures, but its operations are expensive and unsafe because of the inherent high-pressure condition.6 The other possible methods are adsorbed natural gas7 and GHs, which require lower operational pressures. Although adopting the GH-based technology is economically viable in storage/transportation of natural gas, this method is associated with severe drawbacks, such as sluggish formation and poor thermodynamic stability.8 The application of hydrate technology, especially for gas storage and transportation, has been critically challenged by a slow formation rate and low conversion efficiency of gas to solid hydrate, resulting in insufficient storage capacity. It has already been shown that hydrate formation in a pure water and natural gas system suffers from inadequate hydrate conversion and a slow rate of formation in storage options.1,2,8 However, the addition of some thermodynamic promoters9 and/or surfactants to the system provides plausible solution.10 The surfactants have a strong influence on the kinetics of gas dissolution in the © 2013 American Chemical Society
aqueous phase as well as increasing the overall rate of hydrate conversion.10−12 There are some methods other than the addition of chemicals that have been suggested previously to improve hydrate conversion, e.g., increasing the surface/volume ratio using a porous medium. Methane hydrate formation in certain porous materials is also attractive. Detailed studies on the methane hydrate formation mechanism in porous media are limited. Previous studies indicated that the addition of a porous medium, even in small quantities, can act as a kinetic promoter.13−18 Experimentation with different types of porous media, such as activated carbon,13−16 carbon nanotubes,17 silica gel,18 etc., revealed that the hydrate yield considerably depends upon specific surface area, pore volume, and size distribution of the particular porous medium. Experimental studies have been performed to investigate the effect of the particle size,19,20 pore size,21 sediment surface, and water saturation22−25 on methane hydrate formation in silica and natural sediments. The material suitable for fuel gas storage/transportation in the form of hydrates must have the following specifications in terms of the total weight of the material: the combined weight of the matrix and the hydrate should be low, and the overall amount of gas contained in the hydrates should be high. As already stated, water (host) and gas (guest) molecules are the essential components in GHs, and therefore, a sufficiently larger number of water molecules ensures the entrapment of a larger amount of guests in vacant cages under suitable thermodynamic conditions. Other concerns are with kinetics and thermodynamic stability, which require high pressure, low temperature, Received: March 6, 2013 Revised: June 26, 2013 Published: June 26, 2013 3679
dx.doi.org/10.1021/ef400397x | Energy Fuels 2013, 27, 3679−3684
Energy & Fuels
Article
Figure 1. Schematic diagram of the experimental setup.
silica−water−methane system. In addition, we have also focused our study in optimizing the amount of capillary water used for hydrate conversion.
etc. Partially water-saturated porous materials have been reported to show higher hydrate conversions, even though the absolute amount of gas stored was less because of the smaller amount of hydrate.14−16,26 On the other hand, ample methane gas could be adsorbed on the dry pores of nano or activated carbon matrix.13,14 Usually, adsorbed methane gas has less thermodynamic stability than the gas stored in hydrate form. An optimum amount of water was needed to achieve higher hydrate conversions in activated carbons; nevertheless, the maximum weight of water was always less than or equal to the weight of the matrix.27,28 Another concern was about the formation kinetics, which can extend over several days for hydrate formation, while the methane adsorption in porous silica medium is much faster.14 Similar problems can also prevail in a porous silica medium. A higher hydrate yield was reported with 58% or less water saturation (pore volume), and excess water results in slower hydrate formation.24 Furthermore, the hydrates formed within pores of less than 0.1 μm were found to be thermodynamically inhibited compared to the bulk hydrates.21 Usefulness of the dry water system for fuel gas storage has been recently demonstrated by Wang et al.29 Dry water is a free-flowing powder prepared by mixing water, hydrophobic silica particles, and air at high speed.30 Wang et al. found that the water droplets with an average size of 20 μm were surrounded by hydrophobic fumed silica, preventing immediate coalescence.29 Even though the methane hydrate formation kinetics and total methane intake are attractive for these systems, there are serious constraints, particularly in their preparative conditions; e.g., high speed mixing was necessary for making free-flowing dry water, and there is also significant reduction in methane intake during its reuse (inappropriate for recycling). The critical shortfalls, namely, stringent preparative conditions and reusability issues, were addressed in our previous paper.31 In the present study, we have used hollow spherical silica because of their low density, high pore volume, high surface permeability, and good thermal stability,32 which were found favorable for methane hydrate synthesis. The primary objective of the current investigation was to probe methane hydrate formation kinetics and hydrate yield in a
2. EXPERIMENTAL SECTION The silica powders with a mean diameter of 30−70 μm were obtained from Nanoshel (Intelligent Materials Pvt. Ltd.). Deionized ultrapure water (Millipore, type 1) was used, and dissolved gases were removed by evacuation. 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 light-scattering (DLS) method in water suspension using a particle size analyzer (Malvern Mastersizer 2000). 2.1. Apparatus. Figure 1 shows the schematic of the experimental setup, where the main part was a SS-316 cylindrical vessel, which can withstand gas pressures up to 20 MPa, and the volume of the vessel was 250 mL. Cold fluid (water + glycol mixture) was circulated around the vessel with the help of a circulator to bring and maintain the temperature inside the cell at a desired level. 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 a ±0.5% accuracy). 2.2. Procedure. A fixed amount of silica powder (10 g) was mixed with a variable amount of water from 10 to 200 g, and the resulting mixtures were pressurized to 9.35 MPa (at 300 K) with methane gas (99.95% purity) in all of the experiments to synthesize the hydrates. The silica and water were mixed thoroughly in the reactor with the help of a glass rod, and no rigorous blending was applied. The atmospheric gases in the experimental cell were diluted by purging with methane gas prior to the experiments. The methane gas was filled to a desired level using 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 down the temperature of the reactor, and hydrate formation was detected by a sharp pressure drop at a particular temperature. However, the system has taken some time (approximately 30−70 min) to reach into the gas hydrate stability zone for nucleation, and it depends upon the initial driving force and silica/water ratio. The insignificant head-pressure drop in the reactor over a longer duration indicates the saturation in hydrate conversion. The temperature and pressure were logged for every 60 s of the time interval. Time zero in 3680
dx.doi.org/10.1021/ef400397x | Energy Fuels 2013, 27, 3679−3684
Energy & Fuels
Article
all of our measurements 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 RT )
time (more than 48 h). However, the addition of a small amount of silica (10 g) to water (200 g) significantly improved the overall methane consumption, corroborating the earlier reports.20,31 Further stepwise reduction in the amount of water to 50 g (Table 1 and Figure 3) showed a gradual increase in the methane uptake, and also the time taken for the 90% hydrate saturation was reduced at least by a factor of 7. Although the hydrate formation process is stochastic in nature, typically, the gas consumption during hydrate phase evolution can broadly be divided into different regions, viz., nucleation, growth around the seed nuclei of critical dimension, and subsequent growth by diffusion.1,2 As shown in Figure 3c, the gas consumption was slower and occurred after 40 min in a pure or water-rich system. However, the methane consumption was rather rapid and faster in systems with sufficient water for saturating/partly saturating the SiO2 pores. Considerable reduction in free headspace available for methane molecules could also contribute to the lesser methane gas intake, even though an identical initial gas pressure at 288 K was maintained in all of the experiments. During investigation, we observed that the driving force was not always proportional to the overall hydrate yield, particularly with a porous medium. More experiments under isobaric conditions would be helpful for further understanding. However, as shown in Table 1, both the volumetric and gravimetric yields are at a maximum around 4− 6 g of H2O/g of SiO2. The amount of water needed for saturating the capillary spaces in a fixed amount of silica was measured by a simple experiment, and the details are described in the Figure SI.1 of the Supporting Information. The capillary space in 10 g of silica matrix was nearly saturated by 4−6 g of H2O/g of SiO2, and the excess water formed a layer at the bottom of the cell. Nearly uniform average methane gas consumption (0.09 g of CH4/g of H2O) indicated that the water filled in the capillary spaces predominantly converted to hydrates and the excess water had no role in the hydrate conversion. Methane gas can also be adsorbed on the pores in dry/ partially dry silica, and this could be much more prominent in water-depleted systems. Typical methane adsorption isotherms at 275 K for pressures up to 11 MPa were classified as type I.33 We measured the methane adsorption in dry silica (employed in our study) to estimate the adsorptivity relevant to our p−T conditions. The p−T trajectory for methane gas alone and that with dry silica was shown in Figure SI.2 of the Supporting Information. An identical p−T path during cooling and warming cycles clearly indicates the absence of hydrate formation. The reduction in methane gas pressure is primarily due to gas compression/adsorption. From our data, it can be seen that ∼3.2 mmol/g of SiO2 is adsorbed in dry silica, and this is broadly in agreement with the earlier report.34 From the Figure 3c, it is clear that the methane gas consumption is much faster when the sufficient water is present to fill all of the capillary spaces. As already described, the induction time for hydrate seed nucleation and subsequent growth is extremely faster in the water-optimized system. Furthermore, n−T (amount of methane gas in vapor phase− temperature) trajectories shown in Figure 3 (insets a and b) indicate that the methane consumption in the water-saturated (1:4) system is primarily due to the hydrate conversion, whereas the consumption in the water-depleted system (1:2) can be due to the combination of gas adsorption in dry pores and hydrate conversion. The expected variations in the gasphase pressure as a result of hydrate formation are rather
(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 because of phase transitions were neglected. ng,0 and ng,t represent the number 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 to the expected values with ideal stoichiometric compositions (8CH4·46H2O).20 In the literature, different authors have used different hydration numbers in the range of 5.85−6.1, and this variation was mainly attributed to the cage occupancy factors.1,2 Throughout the study, we have used the ideal hydration number assuming full occupancy of cages. While calculating the volumetric/gravimetric yields, we considered the total volume/ mass of water and SiO2 used in the experiments.
3. RESULTS AND DISCUSSION Figure 2 shows the FESEM images and grain size distribution of the hollow silica (before and after pressurizing with methane
Figure 2. FESEM images and grain size distribution of silica samples used for methane hydrate synthesis. Insets a, b, and e are the virgin samples under different magnifications, while insets c and d are the images of samples taken after several freezing and thawing cycles of methane hydrate synthesis.
gas). Before pressurizing with methane gas, the silica particles were spherical, with a mean diameter of 30−70 μm, and appeared to be hollow with a brittle shell and narrow pores (inset of Figure 1a). Upon pressurizing the silica−water system with methane gas, redistribution occurred in the silica particle sizes and the larger silica grains (with higher modal abundance) were broken into smaller fragments. However, such breakages for smaller silica grains were insignificant (insets c and d of Figure 2). In Figure 3, we have plotted the variation in methane gas consumption per volume of water per volume of sediment (SiO2). As expected, methane gas conversion to methane hydrate in a pure water system was very low. We observed smaller variation in the head pressure even after keeping the system within the hydrate stability zone for a sufficiently longer 3681
dx.doi.org/10.1021/ef400397x | Energy Fuels 2013, 27, 3679−3684
Energy & Fuels
Article
Figure 3. Volumetric ratio of methane consumed with time for different silica/water ratios. Insets a and b show the graph between the measured methane contents in the vapor phase (mmol/cm3) with the temperature. Inset c shows the enlarged version of the shaded portion of the main figure.
Table 1. Observed Relation among the Hydrate, Volumetric, and Gravimetric Yields for the Hydrates Synthesized with Different Silica/Water Ratiosa methane hydrate yield silica/water ratio 1:0 1:1 1:2 1:3 1:4 1:5 1:6 1:8 1:10 1:15 1:20 pure MH a
consumed CH4 (g) 0.52 1.00 1.84 2.72 3.63 4.46 4.97 6.25 6.30 4.65 1.76 0.55
(±0.02) (±0.04) (±0.08) (±0.08) (±0.12) (±0.12) (±0.13) (±0.12) (±0.13) (±0.12) (±0.16) (±0.02)
hydrate yield (%) 67.82 63.00 62.19 62.12 61.16 58.00 52.82 42.16 22.44 6.14 4.10
(±2.48) (±2.50) (±1.67) (±1.91) (±1.55) (±1.75) (±1.28) (±1.23) (±0.75) (±1.42) (±0.25)
volumetric yield (v/v) 96.16 (±3.8) 105.0 (±4.6) 110.0 (±3.2) 114.8 (±3.9) 114.0 (±3.1) 107.0 (±3.5) 103.0 (±2.6) 84.0 (±2.5) 41.8 (±1.4) 12.3 (±1.5) 7.20 (±1.0)
gravimetric yield (%) 4.76 5.77 6.37 6.77 7.07 6.73 6.29 5.19 2.82 0.83 0.54
(±0.06) (±0.20) (±0.20) (±0.10) (±0.08) (±0.12) (±0.10) (±0.13) (±0.12) (±0.10) (±0.05)
Possible errors associated with our measurements are shown in parentheses.
depleted system (see insets a and b of Figure 3). These experiments indicated that the reduction in methane gas pressure is primarily due to hydrate conversion for systems with a water content greater than 4 g/g of SiO2, while the gas adsorption also contributes to the total pressure drop in the water-depleted system. Our experiments show that the formation kinetics is faster for the water-saturated systems. Also, there is no notable change in the formation kinetics for water-depleted systems. The absolute amount of methane gas storage is remarkably low in water-depleted systems because the porous structure of the matrix is partly saturated with the water molecules. However, in a water-rich system, the hydrate conversion factor decreases and the kinetics are comparatively slower. It is well-known that the formation of the hydrate mostly occurs at the water and gas interface, and in the case of water in sediments, the hydrate formation was kinetically promoted because of an increase in
sudden because of the phase change (see inset a of Figure 3), while it is rather gradual in the gas adsorption process (see inset b of Figure 3). Observed slope variations for n−T trajectories outside the hydrate stability zone are tabulated in Table SI.1 of the Supporting Information. A higher slope could indicate the methane adsorption in the water-depleted system. Therefore, the pressure drop in the hydrate stable zone can be due to adsorption as well as hydrate formation in the water-depleted system, while the change in the slope became increasingly smaller in water-saturated and water-rich systems. Moreover, the phase change of methane gas and liquid water into the solid gas hydrate is typically a first-order transition and, thus, occurs with a finite hysteresis loop width. Typically, this was correlated with the volume fraction of the hydrates,35 even though the hysteresis loop width depends upon other factors, such as the rate of temperature variations. However, the loop width in the water-saturated system is comparatively larger than the water3682
dx.doi.org/10.1021/ef400397x | Energy Fuels 2013, 27, 3679−3684
Energy & Fuels
Article
the effective surface area.20 Once a certain amount of water converts to hydrates, it forms a sufficiently thick layer over the water phase and, hence, acts as a significant mass-transfer barrier for further hydrate growth; the subsequent conversion is diffusion-controlled.36,37 However, controversy exists whether it is the transport of the guest or the host (water) molecule through the crystal film that controls further hydrate growth. Recent high-resolution confocal Raman spectroscopy38 has provided direct evidence for the mobility of H2O molecules through the hydrate film, and thus, the hydrate growth is controlled by the diffusion of water molecules. To use the hydrates for methane gas storage and transportation applications, it is important that the absolute gas amount trapped in hydrates must be high and, at the same time, the total weight (combined weight of hydrate and matrix material) of the system should be low; i.e., the hydrate carrying porous medium must be saturated/supersaturated with water, so that most of the water is converted into hydrates. Earlier reports showed that the hydrate yield is high only when the pores are partially saturated with water.14 Therefore, the absolute amount of gas stored in such a system is less because of the higher matrix/water ratio. Recently, it was shown that use of dry water29 or activated carbons13−16 could be advantageous for improving the water/matrix ratio and, thereby, the hydrate yields (absolute methane carrying capacity). However, certain operational difficulties, as described in the Introduction, need some careful attention. We attempted a systematic study to determine the gravimetric and volumetric yields with a fixed amount of silica and a variable amount of water at some fixed gas pressures. Figure 4 shows the
hinders the mobility of free water (beneath the hydrate layer), and further growth is slow.36,37 In the case of the water-rich system, both the volumetric and gravimetric yields are low because the amount of water converted into hydrate is less. Whereas for the water-depleted system, the kinetics is fast, methane consumption can be due to gas adsorption and hydrate formation. In these systems, the hydrate yield is very high but the amount of methane gas stored is relatively less (see Table 1). Therefore, the volumetric and gravimetric yields for water-depleted systems are lower than 1:4 to 1:6 silica/water ratios. However, when the silica/water ratio was 1:4 (just watersaturated), the formation kinetics was similar to that of the water-depleted systems and also consumption of the methane gas resulted in higher hydrate yields (Figure 4 and Table 1). Furthermore, it would be difficult to identify the time boundary for hydrate nucleation and growth stage, particularly when the water in the reactor was 4−6 g/g of SiO2 because there was a marginal variation in the hydrate yield factor for these ratios. Our visual observations (see the Supporting Information) showed that 4 g of H2O/g of SiO2 was just sufficient for filling the capillary space and there was no excess water layer for 5−6 g of H2O/g of SiO2. The hydrate formation process could possibly be explained by the approach reported by Jin et al.39 According to this approach, the hydrate conversion in the system could occur in two stages: first, all of the water within the capillary will be converted into hydrates, and then, some microcracks would have developed around the thin hydrate film because of volume change. A small amount of excess water will fill the cracks and also induce some partial melting of the hydrate layer. Furthermore, its conversion into hydrates will be slow because it is diffusion-controlled. Indeed, we observed 90% saturation in methane gas consumption for the 6 g of H2O/g of SiO2 system in 200 min. The same for the watersaturated (4 g of H2O/g of SiO2) system was relatively faster. With a further increase in the water content to 1:15 and 1:20, the system resembled silica particles suspended in water. The formation kinetics and hydrate yield in excess water systems closely resembled that of the quiescent water system. Similar results were reported in earlier studies using activated carbons as matrix.14 Therefore, it is evident from Figure 4 that, as the water content in the system decreases, the volumetric and gravimetric yields increase and appear to be saturated from 1:4 to 1:6. However, at lower water ratios, even though the hydrate yield is high, the gravimetric yield is decreased by 25% when compared to 1:4.
4. CONCLUSION Storing methane gas in the form of hydrate on silica hollow spheres with a variable amount of water was investigated. To improve the volumetric and gravimetric hydrate yields, it is very important to optimize the water content. Although the kinetics was fast and the hydrate yield was high in the water-depleted system (
