Methane Hydrate Formation and Dissociation in the Presence of

Article pubs.acs.org/jced

Methane Hydrate Formation and Dissociation in the Presence of Hollow Silica Pinnelli S. R. Prasad* Gas Hydrate Group, National Geophysical Research Institute (Council of Scientific & Industrial Research), Hyderabad 500 007, India ABSTRACT: A large number of natural gas hydrate deposits have been discovered worldwide so far, and the paramount interest is to evaluate them as an alternate source of energy. Further, the architecture of clathrate compounds finds many applications in gas separation, storage, and transportation processes. We examined methane hydrate (MH) formation/ dissociation behavior in the presence of hollow silica grains in stirred and nonstirred reactor vessels and have observed an identical value for hydrate conversion. Thus, the stirring is avoidable in the process. Further, the results also show that the overall methane conversion in SiO2−H2O−CH4 has steadily increased, and the mole fraction of methane varies from 0.010 to 0.092 by varying the mole fraction of methane in the vapor phase in the range 0.017 to 0.283. Thereafter, the hydrate conversion is much slower even when the mole fraction in the vapor phase reached 0.647. The gas intake capacity in MH has not improved significantly at higher pressures (≥4 MPa to 5 MPa) in the water-saturated silica system. The effect of the load factor (varying CH4 vapor phase mole fraction with the addition of CH4 gas at a fixed amount of water) is significant on MH, and the CH4 mole fraction in hydrates was 0.142.

1. INTRODUCTION Recent reports on the availability of huge amounts of methane gas in the form of natural gas hydrate deposits in favorable geological locations around the globe, both under the permafrost and the oceanic sediments, have triggered interest among several governments, resulting in enhanced research activities connected with gas hydrates (GH).1−4 Approximately 160−180 times of STP equivalent gas can be contained in one unit of hydrate,5 and thus, GH based technology finds a place in several industrial applications such as fuel gas storage and transportation,6 separation of the gas molecules from mixtures,7,8 methane gas extraction by molecular substitution,9 sequestration of greenhouse gas,10,11 etc. In particular, natural gas is being transported presently in a compressed form, though this method is expensive and unsafe due to the inherent highpressure condition.12 The other possible methods are adsorbed natural gas13 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.5 The application of hydrate technology, especially for gas storage and transportation, has been critically challenged by the slow rate of formation coupled with the low conversion efficiency of the gas to a solid hydrate, resulting in insufficient storage capacity. Addition of some thermodynamic promoters 14 and/or surfactants to the hydrate system appears to provide a plausible solution.15 The surfactants exert a strong influence on the kinetics of gas dissolution in the aqueous phase as well as in increasing the overall rate of hydrate conversion. Increasing the effective surface/volume of gas and water using a porous © XXXX American Chemical Society

medium also improves hydrate conversion. Methane hydrate formation in certain porous materials is also attractive, though detailed studies on methane hydrate formation mechanism in a porous media are limited. Previous studies have indicated that the porous material added even in small quantities can act as a kinetic promoter.16,17 Experimentation with different types of porous media such as activated carbon, carbon nanotubes, and silica gel, etc., reveal that the hydrate yield depends considerably on specific surface area, pore volume, and the size distribution of the particular porous medium.18−20 In an industrial application, based on gas hydrate technology, several steps are involved such as hydrate formation, hydrate separation, hydrate pelletizing, transportation, storage, and regasification. Optimization of the cost and time at each step is essential for achieving the set goals, for example it is certainly advantageous if hydrate formation occurs in a rapid and efficient manner. The formation kinetics could certainly be improved significantly by suitably modifying the hydrate crystallization chambers.21 Similarly, efficient hydrate conversion was also achieved in reactors with a fixed silica bed.22 The material suitable for fuel gas storage/transportation in the form of hydrates must have the following specifications in terms of total weight of the material, for instance the combined weight of the matrix and the hydrate should be low, and at the Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: June 28, 2014 Accepted: September 16, 2014

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dx.doi.org/10.1021/je500597r | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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MPa, and volume of the vessels were 100 mL (stirred) and (250, 400, and 700) mL (nonstirred). A cold fluid (water + glycol mixture) was circulated around the vessel with the help of a circulator to bring and maintain temperature inside the cell at a desired level. A platinum resistance thermometer (Pt100) was inserted into the vessel to measure temperature with an accuracy of ± 0.5 K, while pressure in the vessel was measured with a pressure transducer (WIKA, type A-10 for pressure range 0 to 25 MPa with ± 0.5 % accuracy). 2.2. Procedure. A fixed amount of silica powder was mixed with variable amounts of water, and the resulting mixtures were pressurized (@300 K) with methane gas (99.95 % purity) in all of the experiments to synthesize hydrates. 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, and methane gas was filled to the desired level using the Teledyne ISCO syringe pump at a 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, a 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 min to 70 min) to reach into the gas hydrate stability zone for nucleation, and it appears that is dependent on the initial driving force and the silica-to-water ratio. The insignificant head-pressure drop in the reactor observed over a longer duration indicates saturation in hydrate conversion. The temperature and pressure were logged for every 60 s of the time interval. The molar concentration of methane gas (ΔnH,t) in the hydrate phase during the experiment at time t is defined by the following equation:

same time 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 ensure appreciable entrapment of larger amount of guests in the vacant cages under suitable thermodynamic conditions. As already stated, the other concerns are with kinetics and the thermodynamic stability, which require high pressure and low temperature conditions. Partially water saturated porous materials have been reported to show higher hydrate conversion, although the absolute amount of gas stored was low because of the smaller amount of hydrate22,23 and an excess water content in such systems significantly retard hydrate formation.23 Wang et al. have found that water droplets with an average size of 20 μm, surrounded by hydrophobic fumed silica for preventing immediate coalescence, can be used for hydrate formation,24 and such a system is popularly known as the dry water. Normally the weight fractions of water and silica which are often used are 95 % and 5 %, respectively. Even though hydrate formation kinetics and the total methane intake are attractive in dry waters, there are serious constraints, particularly in terms of their preparative conditions; e.g., high speed mixing is essential for making free-flowing dry water, and there is also significant reduction in methane intake during its reuse (thus inappropriate for recycling). The critical shortfalls with a dry water system, namely, the stringent preparative conditions and reusability issues, can be avoided in a hollow silica system. The hollow structure of silica comprises of an inner void surrounded by a thinner solid outer 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, and capsulation capacity, etc.25 Optimization of capillary water and operating pressure has been reported earlier resulting in faster, better volumetric/ gravimetric yields of methane gas hydrates.26−28 In this paper we review certain advantages/disadvantages connected with the usage of hollow silica in methane hydrate formation.

ΔnH, t = ng,0 − ng, t = (P0V /Z0RT0) − (PV t / Zt RTt )

where Z is the compressibility factor calculated by the Peng− Robinson equation of state. The gas volume (V) was assumed as being constant during the experiments; i.e., volume changes due to 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).26 In literature, different authors have used different hydration numbers in the range of 5.75 to 6.1, and this variation has been mainly attributed due to the cage occupancy factors.5 Throughout this study, we have used the ideal hydration number (5.75) assuming full occupancy of the cages. 2.3. Pore Water. In order to obtain the appropriate ratio of water for saturating the pore spaces in SiO2 grains, we have conducted a simple experiment. A fixed amount of silica (10 g) was measured into a glass beaker, and the required amount water was added slowly. We have observed that, when the ratio of silica to water was between 1:1 to 1:3, almost the total content of water was trapped within the pores of silica. The free-flowing nature of silica was reduced gradually, and it turned into a thick paste as more and more water was added to the system. Upon increasing the silica to water ratio from 1:4 to 1:6, the system appeared to be saturated with water.27 In particular, for 1:4 we could not observe excess water layer at the bottom of the glass beaker, and therefore we assume that for 1:4 ratio the system could be just saturated, whereas in case of

2. EXPERIMENTAL SECTION The source and purity of the materials used in the present study are summarized in Table 1. These materials were used without Table 1. Information on the Chemicals Used in the Present Study chemical name

source

purity (mole fraction)

methane water silica

Linde India Ltd. Millipore Nanoshel

≥ 0.9995 ≥ 0.9999 ≥ 0.999

(1)

further purification. The silica powders were obtained from Nanoshel (Intelligent Materials Pvt. Ltd.). Deionized ultrapure water (Millipore, type 1) was used, and the dissolved gases were removed by evacuation. All grain size and morphology of silica were analyzed by the 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 set up where the main part was a SS-316 cylindrical vessel, which can withstand gas pressures up to 15 B



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Figure 1. Schematic diagram of the experimental setup. 1. CH4 Gas cylinder, 2. ISCO Pump, 3. Inlet port for Gas, 4. Outlet/Vacuum port, 5. Magnetic stirrer assembly, 6. Temperature sensor, 7. Pressure gauge and transducer, 8. Data acquisition and control, 9. Inlet for cold fluid, 10. Outlet for cold fluid, 11. Closed cycle refrigerant fluid circulator (LAB CAMPANION) and 12. Computer.

1:6 a negligible amount of water was observed at the bottom of the glass beaker. Further, a higher amount of water, e.g., 1:8 or 1:10, it appeared that a part of the water got dispersed into the interstitial spaces of the grains, while the excess amount of water slowly settled at the bottom of the beaker as shown in Figure 2.

Figure 2. Photographs showing the pore water saturation in hollow silica.

3. RESULTS AND DISCUSSION The silica particles which were spherical with a mean diameter of (30 to 100) μm, appeared to be hollow with a brittle shell and narrow pores (see insets of Figure 3). Upon pressurizing with CH4 gas, the larger silica grains (having a higher modal abundance) in the SiO2−H2O system were broken into smaller fragments. Meanwhile, such breakages of the smaller silica grains were insignificant.27 The bulk density of hollow silica was measured as 0.10 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. The BET specific surface area of the hollow silica was 2.4 m2·g−1. Figure 4 shows experimental results in stirred and nonstirred reactors in the H2O−CH4 system in the presence of hollow silica. Experiments were conducted in slurry (with 0.10 weight fraction of SiO2) to have the effect of stirring. A significant drop in the head-pressure in a narrow temperature zone indicates the formation of MHs in both the reactors.26−28 The n(CH4)− temperature trajectory has been plotted instead of usual PTtrajectory for easy interpretation of data using the procedure described earlier.26−28 It is clearly observed that the mole fraction of methane consumed in hydrates is 0.07, indicating a hydrate conversion efficiency of ∼40 %. Thus, our experiments

Figure 3. FESEM images (insets) and grain size distribution of silica samples used for methane hydrate synthesis.

demonstrate that the process of stirring could be eliminated in the slurry. The observed hydrate dissociation pattern (Figure 4) within the stirred reactor shows a closer match to the predicted behavior for the CH4−H2O system with CSMGem,5 while the same in the nonstirred reactor shows significant variations, which we believe are due to inherent inefficiency of the heattransfer process.29 The next concern is with the kinetics of the hydrate formation in this system. Figure 5 depicts the observed kinetics of hydrate formation in the stirred and the nonstirred reactors for in SiO2 slurry. For comparison we have also conducted experiments in the H2O−CH4 system. As expected, hydrate conversion is extremely slow and inefficient in the nonstirred reactor in comparison to that in the stirred reactor. However, there is a significant improvement in the formation kinetics in the SiO2 slurry system. Overall the consumption of methane in hydrates is the same (Xhyd = 0.07) in both the reactors and the kinetics is also faster. It takes about ∼140 and ∼300 min, respectively, in stirred and nonstirred vessels to attain 90% of the saturation value. Further, the kinetics of hydrate formation in the absence of SiO2 is also very slow, and C



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Figure 4. Experimental n(CH4)−temperature trajectories in the SiO2− H2O−CH4 system in the nonstirred and the stirred reactor vessels. n(CH4) was computed from pressure (vapor phase) and temperature logs. Red and blue points represent the data during cooling and warming cycles. The black line shows the phase boundary curve generated using CSMGem software.

Figure 5. Kinetics of methane hydrates formation in the SiO2−H2O− CH4 system (red points) in a nonstirred and a stirred reactor vessel. For comparison the same in H2O−CH4 system is also shown in black.

marginally changed upon decreasing the content of water to 10 g. A considerable reduction in free headspace available for the methane molecules could also contribute to the lesser intake of methane gas, although identical initial gas pressures were maintained in all of the experiments. Another reason for lower hydrate conversion could be the impermeable nature of the hydrate layer.30−32 In order to gain more insight into the process of hydrate conversion in the SiO2−CH4−H2O system, we have conducted another set of experiments by varying the CH4 pressure and fixing the amount of SiO2 + H2O in the reactor. The second set of experiments were conducted by fixing the mass of SiO2 (20 g) and H2O (80 g) and varied the methane gas pressure in step in the range of (2.3 to 8.5) MPa. We have observed that the overall gas consumption at low pressures is Xhyd = 0.004 (hydrate conversion 2.7 %) and the same is increased to 0.094 (hydrate conversion 64 %) at higher gas pressures. It is generally agreed that hydrate conversion is higher at a higher driving force.5,33 Parameters like subcooling (difference between equilibrium temperature and experimental temperature at a given pressure) or the difference in gas pressure (or fugacity) with respect to the hydrate equilibrium pressure (or fugacity) at a given temperature are used as the driving parameters, analogues to solubility or crystallization process. However, it was suggested recently that the formation rate is controlled by gas−liquid mass transfer rather than crystal growth.34,35 Recently it has been shown that gas consumption

it takes ∼400 min in a stirred reactor. Conversion of water to hydrate is much less in the nonstirred reactor. The next immediate concern would be to investigate the possibility of improving the hydrate yield in the SiO2−CH4− H2O system. Understandably we have used an excess amount of water, compared to pore saturation, while preparing the slurry. In earlier literature the utility of water saturated silica gels or fixed bed reactors are well-known for rapid (few hours) and efficient hydrate conversion. However, these are not suitable for methane gas storage/transportation applications, because of higher SiO2/H2O (weight) ratios.22 The typical SiO2/H2O ratio in such s system was 4.59, and therefore they exhibit poorer gravimetric yields.22 On the other hand, the hollow silica has good thermal and mechanical stability, and low density and thermal conductivity demonstrated superior adsorption capacity for water and thereby could help improving both volumetric and gravimetric hydrate yields.25 As expected, in the nonstirred reactor, conversion of methane gas to methane hydrate in pure water system is very low (see Figure 5). However, as shown in Figure 6 and Table 2, the addition of a small amount of silica (10 g) to water (200 g) notably improved the methane consumption (e.g., Xhyd = 0.010 or hydrate yield of 6.0%), corroborating earlier reports.16,17 Further stepwise reduction in the amount of water to 40 g showed a gradual increase in the methane uptake (e.g., 0.092 or hydrate yield of 62 %). Thereafter, the uptake of gas is D



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different values of the driving force, and an exponential relationship similar to that of pure water can be fitted to the observed behavior. However, there is an apparent saturation in gas intake in the presence of SiO2, possibly because of the following reason. It has been stated recently that the surface area reaction is not only dependent on the volume fraction of gas phase but also on the mass content of the aqueous phase, as water saturation also exerts an influence on it. Further, the effective occupancy of the pore volume in terms of the occupancy by the mobile phases (i.e., water and gas) gradually declined when the hydrate particles are deposited in the voids of the porous media as a solid phase.34 In other words, the solid hydrate particles seals the pores and forms an impermeable layer preventing further saturation of water and gas phases, thus enabling them to get converted into hydrates. Therefore, the overall consumption of methane gas in a porous medium can attain an apparent saturation, as seen from curves 1 and 2, at given gas and water fractions. The overall consumption of methane gas in both these experiments is far below than the expected gas intake for 100 % conversion. Since the amounts of gas, water, and silica play a critical role in influencing the hydrate conversion efficiency, it is desirable to optimize them in a given reactor. Hao et al.36 have shown that the load coefficient (volume of water/volume of reactor) of a reactor vessel can also play a dominant role in methane hydrate formation using additives such as SDS. Therefore, we have carried out a few more experiments, varying the combined mass of silica and water (with fixed ratio of 1:4 or 5). When the combined weight of SiO2 and water in a 700 mL reactor is 100 g, the maximum gas intake was Xhyd = 0.094 (@ 8.84 MPa). As noted, the gas intake was lower at 2.77 MPa (Xhyd = 0.004), and it varied marginally (0.090 to 0.094) when the mole fraction of gas is increased from 0.241 to 0.382 ((4.63 to 8.84) MPa). We arbitrarily decreased the total weight of water and silica to (36 and 15) g, and this will reduce the total amount of reaction mixture and thereby the thickness (height of capillary) of porous material in the reactor. The gas intake also varied similarly with driving force (curves 3 and 4 in Figure 6), and the maximum gas intake reached to Xhyd = 0.107 and 0.142, respectively, at a gas pressure of 8.3 MPa. Moreover, the process of hydrate conversion under such conditions is also extremely fast, and it takes typically about (1.5 to 2.0) h for 95 % conversion. Thus, these studies demonstrate the possibility

Figure 6. A plot shows the mole fraction of methane gas in hydrates as a function of driving force (mole fraction of methane in vapor phase) in the SiO2−H2O−CH4 system. The inset shows a similar plot for the H2O−CH4 system. Curve 1 (green) in the main plot shows the observed variations with different amounts of water, (10 to 200) g, and 10 g of SiO2 in the reactor. While the curve 2 (blue) depicts, the variations in the gas intake were observed with 20 g of SiO2 and 80 g of H2O. The methane gas pressure was varied in the range of (2.3 to 8.5) MPa. The experimental conditions for curve 3 (red) and 4 (black) were similar to curve 2, with reduced amounts of SiO2 and H2O. They are (6 g, SiO2; 30 g, H2O) and (3 g, SiO2; 15 g, H2O), respectively, for curve 3 and curve 4.

during the process of hydrate conversion is exponentially proportional to the driving force.35 We define the molar fraction of methane (in vapor phase) gas in the CH4−H2O system as the driving parameter; the mole fraction of gas consumed during hydrate conversion in a stirred reactor is shown in the inset of Figure 6 as a baseline, and it matches well with an exponential relationship. In Figure 6, we have plotted the mole fraction of gas consumed in the SiO2−H2O−CH4 system at various driving parameter values. As already explained curves 1 and 2 are the results obtained in our two sets of experiments, wherein in the first we varied the amount of water by keeping the SiO2 and CH4 pressure constant. It is worth noticing that the overall consumption of methane gas showed similar variations at

Table 2. Observed Mole Fraction of Methane Gas at Different Driving Force Values in the SiO2−H2O−CH4 System SiO2 (10 g) (curve 1)

SiO2 (20 g) (curve 2)



X (CH4)vapa

X (CH4)hyda

X (CH4)vapa

X (CH4)hyda

X (CH4)vapa

X (CH4)hyda

X (CH4)vapa

X (CH4)hyda

0.647 0.462 0.361 0.283 0.232 0.191 0.139 0.100 0.047 0.017

0.101 0.094 0.092 0.092 0.091 0.085 0.080 0.066 0.034 0.010

0.382 0.352 0.338 0.319 0.271 0.241 0.212 0.193 0.166 0.147 0.124

0.094 0.087 0.089 0.086 0.089 0.090 0.078 0.074 0.046 0.020 0.004

0.619 0.590 0.540 0.499 0.468 0.436 0.391 0.358 0.317 0.286

0.107 0.104 0.105 0.104 0.097 0.092 0.086 0.088 0.059 0.011

0.777 0.745 0.726 0.692 0.651 0.617 0.601 0.571 0.551 0.531 0.506 0.470

0.142 0.13 0.128 0.118 0.119 0.114 0.087 0.105 0.091 0.068 0.074 0.018

a Estimated uncertainties for the mole fraction of methane in vapour phase (X (CH4)vap) and in hydrate phase (X (CH4)hyd) respectively are 0.035 and 0.015. Tabulated values are the average of at least three repetitive measurements.

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of higher hydrate conversion with smaller amounts of silica and water in a reactor vessel at moderately higher methane pressures.

4. CONCLUSIONS In summary, this paper describes several advantages associated with the use of hollow silica in methane hydrate synthesis. The hydrate conversion efficiencies are nearly identical (Xhyd = 0.07) in both stirred and nonstirred reactors in slurries with hollow silica. Thus, an important step, namely, stirring, could be avoided during the hydrate formation. However, the formation kinetics in silica slurry under the nonstirred condition is slower. By using an optimum amount of water, i.e., (4 to 5) g H2O/g SiO2, it is possible to increase the hydrate yield and formation kinetics as well. It is also possible to achieve higher and rapid hydrate conversion in a water optimized hollow silica system by providing enough empty spaces for methane gas.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-40-2701 2710; fax: +91-40-2343 4651; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author sincerely thanks the Director of the National Geophysical Research Institute, Hyderabad, for his encouragement and permission to publish this paper. The author appreciates the help of Mr. V. D. Chari and Mrs. Y. Sowjanya in conducting experiments. This is a contribution to GEOSCAPE Project of NGRI under the 12th Five Year Scientific Program of CSIR.

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Methane Hydrate Formation and Dissociation in the Presence of

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