Morphology of Methane Hydrate Formation in Porous Media - Energy

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Morphology of Methane Hydrate Formation in Porous Media Ponnivalavan Babu,† Daryl Yee,‡ Praveen Linga,*,† Andrew Palmer,§ Boo Cheong Khoo,∥ Thiam Soon Tan,§,⊥ and Pramoch Rangsunvigit# †

Department of Chemical and Biomolecular Engineering, §Department of Civil and Environmental Engineering, and ∥Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore ‡ Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, United Kingdom ⊥ Singapore Institute of Technology (SIT), Singapore 179104, Singapore # Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand S Supporting Information *

ABSTRACT: Experiments at 8.0 MPa and 277.15 K were carried out in different porous media, such as silica sand and activated carbon, to observe the formation and dissociation of methane hydrate in a specially designed crystallizer for mophology observation. In silica sand bed, we observed a clear hydrate front moving across the bed in the crystallizer at the experimental conditions with 50 and 100% water saturation. The hydrate crystals were observed to form in the interstitial pore space available between the silica sand particles. Whereas in activated carbon bed experiments, hydrates were observed to nucleate on the surface of the activated carbon grain and then dissociate within the stable hydrate formation region. For the first time, we were able to observe this behavior of transient hydrate crystal formation/dissociation in the stable hydrate region in porous media. We postulated that the particle size, pore space, and water saturation level may play a role in the above phenomenon. A clear hydrate front movement across the crystallizer and stable hydrate formation were observed when smaller sized activated carbon grains were used. In all of the experiments, the hydrate crystals were seen to form in the interstitial pore space between the porous media. Our results show that pore space and its interconnectivity play an important role in methane hydrate formation in porous media consisting of silica sand or activated carbon.

1. INTRODUCTION Natural gas hydrates are solid, crystalline, inclusion compounds composed of water, methane, and a small amount of other gases, with water molecules forming the cages via hydrogen bonds, trapping the gas molecules inside the cages at low temperatures and high pressures. Natural gas hydrates exist abundantly in nature and are located in marine sediments and permafrost regions all around the world. Each volume of hydrate can contain as much as 184 volumes of gas at standard temperature and pressure (STP), and hence, hydrates are considered a potential unconventional energy resource.1−3 They are estimated to contain more than half of world’s total organic carbon and twice as much as all other fossil fuels combined. The potential reserve of these hydrated gases are estimated to be over 1.5 × 1016 m3.4 It is estimated that the liberation and subsequent production of just 15% of the trapped gas in hydrates would provide the world with energy for 200 years at the current level of energy consumption. Several possible recovery methods investigated for recovery of natural gas from hydrates are depressurization, thermal stimulation, chemical inhibition, or a combination of these methods. Another potential method for recovering methane trapped in natural gas hydrates is via the exchange of methane in the stable hydrate deposits with carbon dioxide.5−9 If this approach can be successful, then there is a huge potential sink for storing carbon dioxide in hydrate deposits at the same time resulting in the production of methane from these hydrates. From a thermodynamic point of view, carbon dioxide hydrates are more stable than methane hydrates at a given temperature © XXXX American Chemical Society

and pressure. Reservoir-specific information along with laboratory data and models can be evaluated to assess the feasibility of production from natural reservoirs. The potential hazards of production are as follows: (1) Natural gas hydrates mainly consist of methane. Release of large quantities of methane into the atmosphere would substantially increase the greenhouse effect. (2) Decomposition of natural gas hydrates can place the sea floor at risk of damage or destruction and can generate underwater gas blowouts.10 Hence, to avoid the potential hazards during production, it is necessary to understand the mechanism of hydrate formation/dissociation in a porous media and also its stability. There are several laboratory studies in the literature pertaining to the three possible dissociation strategies to recover methane from naturally occurring hydrates.11−13 Kneafsey et al.14 studied the methane hydrate formation in the presence of silica sand by employing X-ray tomography and reported the spatial density changes during hydrate formation/ dissociation. Linga et al.15,16 and Haligva et al.17 studied the impact of sample size on the formation and dissociation of methane hydrates in porous media. Methane hydrate was found to dissociate in three stages for thermal stimulation and two stages for depressurization, and the initial dissociation rates were significantly impacted by the sample size.15,17 It is also noted that, in porous media, multiple nucleation events at Received: March 19, 2013 Revised: May 11, 2013

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dx.doi.org/10.1021/ef4004818 | Energy Fuels XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL SECTION

different times and different locations for methane hydrate formation were observed with the location of multiple thermocouples within the bed during hydrate formation experiments.16−18 This was later confirmed independently by Bagherzadeh et al.19 by employing magnetic resonance microimaging measurements. Falser et al.20 presented a new approach by employing line dissociation test via combining electrical heating and pressure reduction from a miniature and reported 3.8 times increased gas production compared to production by depressurization alone. Li et al.21 investigated the dissociation of methane hydrate in porous media in a threedimensional (3D) cubic hydrate simulator using the huff and puff method with different injection temperatures and injection times. They reported that higher energy efficiency can be obtained at the injection temperature of 130 °C and injection time of 5 min. Li et al.22−24 also investigated the gas production from methane hydrate in porous media by depressurization in a 3D cubic hydrate stimulator and pilot-scale hydrate stimulator. They reported three periods of gas production, namely, free gas, mixed gas, and gas production from hydrate dissociation in both the hydrate simulator.22,23 Understanding the methane hydrate process in porous media requires different measurement techniques, from which it is possible to obtain information from different perspectives.14,16,19 Understanding the fundamental behavior of hydrate formation and dissociation in porous media is an ongoing work. Bagherzadeh et al.19 employed a magnetic imaging resonance technique to visualize the formation of methane hydrates in an unconsolidated bed of silica sand particles and reported that hydrate formation was not uniform and that it was faster in a less water-saturated bed. They also reported that a smaller particle size of porous media lead to faster hydrate formation rates. Another approach to investigate the hydrate formation behavior at a microscopic scale is through morphological observations. Morphology is the study of the size and shape of hydrate crystals, whereby the length scales are larger than molecular and much smaller than macro-system dimensions.25−32 Recently, Jin et al.,33 on the basis of morphological observations, reported a two-step methane hydrate growth in sand bed that was not observed in bulk hydrate formation. The two-step methane hydrate growth behavior showed that the nature of hydrates in the pores changes with time during hydrate growth. The general characteristics of the gas uptake for hydrate formation reported by Jin et al.33 were similar to those reported in the literature.16,17 Jung and Santamarina34 employed optical, mechanical, and electrical measurements to monitor hydrate formation and growth in small pores in a capillary tube. They reported that faster growth rates cannot be possible only because of diffusive gas transport through the hydrate shell between the gas and liquid water and that water−hydrate volume expansion creates tensile discontinuities in the hydrate shell that facilitates transport of gas. There is limited information in the literature on the morphological behavior of methane hydrate formation in porous media. Understanding the mechanism of hydrate formation in porous media is also important for applications pertaining to carbon dioxide capture using the hydrate-based gas separation process by employing a fixed-bed approach with porous media or carbon dioxide sequestration as hydrates in porous media.5,7,35−37 The objective of this study is to understand the morphology of methane hydrate formation in porous media.

2.1. Materials. Silica sand supplied by Sigma-Aldrich was used. The sand used in this study was similar to the sand described elsewhere in the literature.16,35 Deionized and distilled water was used for the experiments. Activated carbon supplied by Carbokarn Co., Ltd., Thailand, was used. Table 1 summarizes the properties of the two porous media employed in this study.

Table 1. Properties of the Porous Media silica sand surface area (m2/g) total pore volume (cm3/g) average pore diameter (nm) activated carbon surface area (m2/g) total pore volume (cm3/g) average pore diameter (nm)

0.3499 0.1388 0.9 866.7 0.4757 2.19

2.2. Experimental Apparatus. The detailed description of the experimental setup is available in the literature.38,39 The schematic of the experimental setup is shown in Figure 1a. Briefly, the morphology apparatus consists of a crystallizer immersed in a temperaturecontrolled water bath coupled with an external refrigerator. The crystallizer was specially modified to investigate the morphology of hydrate formation in porous media. The crystallizer consists of a central and transparent cylindrical polyacrylic column and a pair of stainless-steel lids. The central column is a hollow cylinder with an inner diameter of 25 mm and a length of 75 mm. To improve the quality of the images and to view the interstitial spaces in porous media, we placed a cylindrical rod inside the hollow cylinder to enable a thin packing of porous media filled with water. Figure 1b shows the cross-sectional view of the crystallizer with the arrangement of the cylindrical rod and the thin packing of porous media. A thermocouple is inserted at the top of the crystallizer to monitor the temperature inside the crystallizer. A pressure transmitter and a pressure gauge are employed to measure the pressure inside the crystallizer. The data are recorded every 20 s by a data acquisition system connected to a personal computer for analysis. A microscope was used during the experiments to observe hydrate crystal morphology inside the crystallizer. 2.3. Experimental Procedure. The amounts of silica sand/ activated carbon placed in the crystallizer for each experiment and corresponding water saturation levels are given in Table 2. The volume of water required for 100% water saturation for silica sand and activated carbon is 0.217 and 0.48 cm3/g, respectively. On the basis of the total pore volume, required water for 50 and 100% water saturation were added to silica sand and activated carbon, respectively. All of the experiments were carried out at 8.0 MPa and 277.15 K. The uniform bed was set up in the crystallizer by tapping to eliminate any air pockets in the bed. Once the crystallizer bed was set up, the thermocouple was inserted and the crystallizer was closed. The crystallizer was placed inside the temperature-controlled water bath. To eliminate any air bubbles, the crystallizer was pressurized to 0.5 MPa and depressurized to atmospheric pressure thrice. Then, the crystallizer was pressurized to 8.0 MPa, and the temperature of the crystallizer was allowed to reach 277.15 K. Once the temperature and pressure reached the experimental conditions, this time was recorded as time zero for the formation experiment. Data were then recorded every 20 s. All hydrate formation experiments were carried out with a fixed amount of gas and water (batch manner). An external refrigerator maintained the temperature of the crystallizer to be constant. Meanwhile, a microscope coupled with a digital camera was used to observe the hydrate crystal growth in the porous media.

3. RESULTS AND DISCUSSION Experiments were carried out in two different porous media (silica sand and activated carbon) and also at different water B


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Figure 1. (a) Schematic of the experimental setup and (b) cross-section of the crystallizer, adapted/modified from the study by Lim et al.39

Although this set of images shows the hydrate growth front moving from the bottom of the sample upward, subsequent tests showed that this does not hold true all of the time, with the front being shown to be able to start from the left and move to the right and vice versa. Figure 3 shows the hydrate front movement from left side of the crystallizer to the right side. This growth phenomenon can be attributed to the randomness of nucleation itself. Bagherzadeh et al.19 reported that hydrate formation in the silica sand bed is quite inhomogeneous and that nucleation occurs at different positions inside the bed. Nucleation and, consequently, the first point of growth can occur anywhere throughout the sample depending upon the availability of the gas/water contact, and the growth front radiating outward will be dependent upon this initial first point or additional nucleation events occurring in the near vicinity. A video showing the hydrate formation for the images presented in Figure 3 is given in the Supporting Information for better visualization. For each experiment, hydrate front movement was observed in different directions. Similar results were observed for experiments at 100% water saturation in silica sand bed but are not presented here. From our study thus far on the hydrate formation in porous media with silica sand as a medium, it was difficult to observe the hydrate growth of water surrounding the silica sand particles at the particle level because of the lack of contrast between the sand particles and the water wetting the particles. To circumvent the color contrast problem encountered with the silica sand medium, activated carbon was used instead. 3.2. Activated Carbon Experiments. Similar to silica sand experiments, hydrate formation experiments were conducted at 50 and 100% water saturation levels, 8.0 MPa, and 277.15 K. With increased contrast between activated carbon and hydrate, hydrate growth can be clearly seen throughout the observed region. Figure 4 shows the sequential images of methane hydrate formation in a 50% water-saturated activated carbon bed at 8.0 MPa and 277.15 K. In Figure 4a, a yellow boxed region shows water droplets on the crystallizer wall. A close up of the boxed

Table 2. Type of Medium and Amount of Water Needed for the Different Experimentsa sample number 1 2 3 4 5 6

medium silica sand silica sand silica sand activated carbon activated carbon activated carbon

amount of medium (g)

amount of water (mL)

saturation (%)

solution state

4 4 4 2

0.44 0.44 0.87 0.50

50 50 100 50

fresh memory fresh fresh

2

1.00

100

fresh

3

0.70

100

fresh

a

All of the experiments were performed at 277.15 K with a starting pressure of 8.0 MPa.

saturation levels (50 and 100%). All formation experiments were carried out at 8.0 MPa and 277.15 K with fresh or memory water. Memory water refers to water that has experienced hydrate formation. The equilibrium pressure for methane hydrate formation at 277.15 K is 3.8 MPa.2,18 3.1. Silica Sand Experiments. Hydrate formation experiments were carried out at 8.0 MPa and 277.15 K at 50 and 100% water saturations in a silica sand bed. Figure 2 shows the sequential images of hydrate growth in sand at a 50% water saturation level. The images show the hydrate growth front moving from the bottom left all of the way to the top. It can be seen in the first frame (Figure 2a) the contrast between the grains saturated with water and the grains that are not. Panels b−d of Figure 2 show the hydrate front moving diagonally from the bottom of the crystallizer. As the front progresses, the water trapped between the grains is readily consumed, until no more water is left. When panels a and d of Figure 2 are compared, we could clearly observe that the water-saturated region in the bed in Figure 2a has translated to the hydrate region in Figure 2d. We have prepared a video to demonstrate the movement of the hydrate growth front, and it is available in the Supporting Information. C


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Figure 2. Sequential images of the formation of methane hydrate in sand with 50% H2O saturation at 277.15 K and 8.0 MPa. The time lapse after pressurization is shown below each image. (A video "ef4004818_si_001.avi" is provided as Supporting Information for better visualization).

Figure 3. Sequential images of the formation of methane hydrate in sand with 50% H2O saturation at 277.15 K and 8.0 MPa. The time lapse after pressurization is shown below each image. The hydrate front moves from the left side to the right side of the crystallizer. (A video "ef4004818_si_002.avi" is provided as Supporting Information for better visualization).

region in Figure 4a is shown in Figure 4b. After 250 s of experimental time, all of these droplets on the crystallizer wall became converted into hydrate crystals, as shown in Figure 4c. Similarly, when panels b and c of Figure 4 are compared, we can observe that several other droplets of water that were sticking on the crystallizer wall became converted to hydrate crystals. It is also noted that the hydrate crystals formed on the water droplets seen on the crystallizer wall were stable and did

not dissociate (as seen in the subsequent images presented after Figure 4b). Interesting hydrate formation behavior was also observed for the experiments conducted in the activated carbon bed. This is shown in Figure 5. Hydrate crystals were observed to form on the surface of the activated carbon but then subsequently dissociate after some period of time within the hydrate formation region (P and T in the hydrate stability region). D


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Figure 4. Sequential images of the formation of methane hydrate in activated carbon with 50% H2O saturation at 277.15 K and 8.0 MPa. The time lapse after pressurization is shown below each image.

Figure 5b depicts an expanded region marked in Figure 5a, showing some activated carbon grains that are wetted by water. At time t = 275 s (Figure 5d, shown in a rectangle box), we could observe hydrate formation on the surface of the activated carbon grain. Later in Figure 5e, we could see the hydrate crystal on the same grain partially dissociated at about 300 s. In Figure 5f, the hydrate crystal appears to have completely dissociated. We cannot see the hydrate layer, and it resembles the original image of the activated carbon grain in Figure 5b. In other words, we observed hydrate formation on the surface of the activated carbon grain (shown in the rectangle) and its subsequent dissociation on the surface within 150 s. Similarly in Figure 5f, hydrate crystals were observed on another activated carbon grain (shown in yellow ellipse), which subsequently dissociated at 800 s (Figure 5g). We observed this behavior happening at different locations at different times. A similar occurrence is shown in Figure 6, where it happens at a different location. Figure 6b is a close-up of another region. As seen in Figure 6d, at 275 s, we could observe hydrate formation on the surface of the grain, at 300 s (Figure 6e), the hydrate partially dissociates and, at 800 s (Figure 6g), almost all of the crystals appeared dissociated, except for a little portion that was stable and did not dissociate. This behavior was observed for all experiments conducted at a 50% water-saturated activated carbon bed at 8.0 MPa and 277.15 K. For the experiments conducted in this study, the crystallizer pressure did not drop below 7.0 MPa. The equilibrium hydrate formation pressure at the experimental temperature of 277.15 K is 3.8 MPa.2,18 This shows that the crystallizer pressure was in the stable hydrate region when this behavior was observed. To account for this behavior, we postulate three reasons: (1) The water wets the activated carbon particle on the surface, and the hydrate crystal formation on the surface of the activated carbon, although visible through a microscope, was not stable enough for continual hydrate growth to proceed, resulting in dissociation as the crystal becomes perturbed. On the other hand, the droplets on the crystallizer wall (shown in Figure 4) remained stable after nucleation throughout the experiments. (2) Smaller


E


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droplets that were present on the crystallizer wall became converted to hydrates and remained stable without being dissociated, suggesting that there was sufficient water/gas for the hydrate crystal nuclei to stabilize and completely engulf the water droplet there by being stable or not dissociating. Recently, Jung and Santamarina,34 on the basis of observation of methane hydrate formation in capillary pores, reported that the complex nucleation and growth processes can result in an unexpected phenomena, including transient hydrate formation/ dissociation. The occurrence of hydrate formation and dissociation processes within the hydrate stability region is defined as transient hydrate formation/dissociation within the hydrate stability region. Our study provides conclusive evidence of such a transient hydrate formation/dissociation happening on the surface of the activated carbon particles. Nucleation or induction point is a stochastic phenomenon, and it depends upon a lot of factors, such as driving force, gas/liquid contact, gas saturation in the liquid phase, etc. The droplets on the wall are able to sustain the nucleation point and continue to grow and remain stable, as seen in panels c and d of Figure 4, while the thin layer of water wetting the activated carbon particle was not able to sustain hydrate growth after nucleation, resulting in transient hydrate formation/dissociation. Specially designed nuclear magnetic resonance (NMR) micro-imaging experiments at the molecular level might show some further insights into this transient hydrate formation/dissociation behavior of methane hydrates in porous media. Even though this behavior of hydrate crystal formation/ dissociation was observed on the surface of the activated carbon grain, we did not observe a clear hydrate growth front, as was the case for the experiments conducted with silica sand. This may be due to the lack of interconnected pore spaces for hydrate formation and further hydrate growth in the activated carbon grains compared to that of the silica sand grains. It is also noted that activated carbon is porous in nature, whereas the silica sand16 used in this study is not porous. In addition, the silica sand particles were smaller in size than that of activated carbon. The scanning electron microscopy (SEM) images of sand and activated carbon are presented in Figure 8. As seen in the figure, the sand particles are smaller than the activated carbon particles on the same measurable scale. The schematic of interstitial pore space between larger irregularly shaped grains and smaller irregular shaped grains is shown in Figure 9. As seen from the schematic in Figure 9, unlike the larger irregularly shaped grains, the interstitial space for the smaller irregularly shaped grains is not uniform. This also means that the pore spaces between the large irregularly shaped grains might not be as interconnected as those between the smaller shaped grains. A smaller grain size would ideally lead to a more regular packing of the grains, resulting in more interconnected pore space and a larger surface area of contact. Hence, to improve the regularity of the pore spaces between the activated carbon grains, activated carbon was ground and crushed with a roller to obtain smaller grain sizes. After obtaining smaller and uniform activated carbon grains, hydrate formation experiments were conducted at 100% water saturation level, 8.0 MPa, and 277.15 K. Figure 10 shows the sequential images of hydrate formation in a 100% watersaturated crushed activated carbon with a more uniform grain size at 8.0 MPa and 277.15 K. A clear hydrate front growing can be observed starting from the bottom of the crystallizer and growing diagonally upward until it reaches the top of the bed. This hydrate front growth across the crystallizer was observed


pore space and less interconnectivity of pores were available for hydrate formation because of the large size of the activated carbon grains. (3) Partial saturation (50% water saturation) of the grains resulted in a lack of growth front after nucleation. Figure 7 shows the formation and dissociation of hydrate crystal on an activated carbon grain at 100% water saturation, 8.0 MPa, and 277.15 K. At time t = 650 s, no hydrate was observed in the circled region. Panels c and d of Figure 7 show that hydrate crystal formation appeared on the activated carbon grain at times t = 700 and 975 s, respectively, whereas at time t = 1175 s, the hydrate observed in the circled region had dissociated completely. For a better visualization of this behavior, we have prepared a morphology video, which is available as Supporting Information. This behavior of formation and dissociation of hydrate crystal on the activated carbon grain for both 50 and 100% water saturation confirms that water saturation has no role to play for this behavior. Results at different water saturation levels hint that the activated carbon grain becomes wetted by the water and the hydrate crystal formed on the surface of a grain is unable to sustain further growth because of the lack of interconnectivity of water/gas, which is needed for further hydrate growth, and hence, we believe the hydrate crystal dissociates. This view is further strengthened by the fact that, in Figure 4, the tiny water F


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Figure 7. Sequential images of the formation of methane hydrate in activated carbon with 100% H2O saturation at 277.15 K and 8.0 MPa. The time lapse after pressurization is shown below each image. The ringed grain highlights the formation and subsequent dissociation of the methane hydrate crystal. (A video "ef4004818_si_003.avi" is provided as Supporting Information for better visualization).

Figure 9. Schematic of how the interstitial space, highlighted in black, between (a) large grains compares to that of the space between (b) small grains. Note the irregular spacing as well as the sizing of the pore space between the irregularly shaped grains.

dissociation of the hydrate crystals around the activated carbon grains was not observed when the grains were crushed and reduced in size, suggesting that the interconnectivity of the pore space enabled the crystal nuclei to further grow and sustain hydrate growth thereafter (as seen by the hydrate growth front in Figure 10). The above results indicate that the lack of interconnected and sufficient-sized pore spaces in the larger irregularly shaped activated carbon grains has resulted in sporadic growth of hydrate crystals and the absence of a defined hydrate growth front. The transient hydrate formation/dissociation phenomenon that was observed at all experimental conditions in the larger irregular size activated carbon grains was not observed in

Figure 8. SEM images of silica sand and activated carbon.

for all experiments conducted with the reduced size activated carbon grains at 100% saturation. Similar to silica sand experiments, the nucleation of hydrate formation appeared at a random location in the bed. The transient formation/ G


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under the same conditions. We postulate that particle size, pore spaces, insufficient wetting of the grains may be the reason for this behavior in the activated carbon bed. When smaller size activated carbon grains were used, a clear hydrate growth front movement was observed similar to the silica sand bed. Our results also show that pore space plays an important role in hydrate formation and dissociation in porous media.

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ASSOCIATED CONTENT

S Supporting Information *

Morphology videos for the experiments presented in Figures 2, 3, and 7. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 65-6601-1487. Fax: 65-6779-1936. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Education’s AcRF Tier 1 (R279-000-386-112) and the Centre of Offshore Research and Engineering (CORE) in the National University of Singapore (R-302-501-008-112) for financial support.

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REFERENCES

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Figure 10. Sequential images of the formation of methane hydrate in crushed activated carbon with 100% water saturation at 277.15 K and 8.0 MPa. The time lapse after pressurization is shown below each image.

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4. CONCLUSION The morphology of methane hydrate formation in silica sand and activated carbon beds was observed at 8.0 MPa and 277.15 K for different water saturation levels in a specially designed morphology crystallizer. A clear hydrate front movement across the crystallizer was observed in the silica sand bed for both 50 and 100% water saturation levels. Interestingly, in activated carbon bed experiments at 8.0 MPa and 277.15K, we observed formation and dissociation of hydrate crystal on the surface of the grains in the stable hydrate formation region. The phenomenon was observed even for 100% water saturation H


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Morphology of Methane Hydrate Formation in Porous Media - Energy

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