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Natural gas storage and transportation within gas hydrate of smaller particle: Size dependence of self-preservation phenomenon of natural gas hydrate
Chemical Engineering Science 118 (2014) 208–213
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Natural gas storage and transportation within gas hydrate of smaller particle: Size dependence of self-preservation phenomenon of natural gas hydrate Hiroko Mimachi a,n, Satoshi Takeya b, Akio Yoneyama c, Kazuyuki Hyodo d, Tohoru Takeda e, Yoshito Gotoh b, Tetsuro Murayama a a
Mitsui Engineering and Shipbuilding Co., Ltd., 1 Yawatakaigandori, Ichihara, Chiba 290-8531, Japan National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Hitachi Ltd., Central Research Laboratory, 2520 Akanuma, Hatoyama, Saitama 350-0395, Japan d High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan e Allied Health Sciences, Kitasato University, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan b c
H I G H L I G H T S
Larger particles of NGH allowed a larger fraction of gas hydrate to be preserved. Dense NGHs over 0.50 mm could keep natural gases for 14 days at 253 K and 1 atm. Ice films on self-preserved NGHs over 1.0 mm had an equivalent thickness.
art ic l e i nf o
a b s t r a c t
Article history: Received 24 April 2014 Received in revised form 22 July 2014 Accepted 26 July 2014 Available online 1 August 2014
In this study, the preservation ability of natural gas hydrate (NGH) with different particle size from 0.50 mm to 30 mm was investigated. It is known that natural gas preservation does not occur when NGHs are in powder form and their size is under several tens of micrometers, while the dense bulk NGH was preserved after the initial rapid dissociation. It is important to investigate the required size of NGH particle for self-preservation to occur when NGH is used as a storage media of natural gases. Despite the difference in particle size, the experimental results herein revealed that the NGH sample was enveloped by an ice film caused by the initial rapid dissociation. The thickness of the ice film grown during the initial rapid dissociation was almost the same and did not depend on the particle size of NGH being over 1.0 mm and higher than that of methane hydrate. The thickness of the ice film necessary for the selfpreservation of NGH may be the reason why powder NGH does not show the self-preservation phenomenon. However, we revealed that NGH particles with diameters of more than 0.50 mm were preserved for two weeks. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Clathrate hydrate Natural gas hydrate Self-preservation Particle size Ice film
1. Introduction Due to the expansion of the worldwide demand for natural gas, production of natural gas from not only conventional gas fields but also unconventional gas fields, such as shale gas, is increasing. Therefore, ocean transportation of natural gas or liquefied natural gas (LNG) is the key for further development of the use of natural gas. Natural gas hydrate (NGH) is one of the clathrate compounds formed from water and natural gases, such as CH4, C2H6, C3H8, C4H10, etc., with natural compositions. NGH is a candidate for a
n
Corresponding author. Tel.: þ 81 436 41 1110; fax: þ 81 436 43 1785. E-mail address: [email protected] (H. Mimachi).
http://dx.doi.org/10.1016/j.ces.2014.07.050 0009-2509/& 2014 Elsevier Ltd. All rights reserved.
new media for natural gas storage and transportation because it can contain about 170 times as much natural gas in the form of gas hydrate (Gudmundsson and Borrehaug, 1996; Sloan, 2003). It is known that gas hydrates are usually stable under high pressures and at low temperatures. Some gas hydrates keep gases under atmospheric pressure and just below the melting point of ice though the surroundings are outside thermodynamically stable temperature and pressure conditions; this is called the selfpreservation phenomenon (Yakushev and Istomin, 1992). It has been reported that CH4 and CO2 hydrates show the preservation phenomenon (Stern et al., 2001; Kuhs et al., 2004; Shimada et al., 2005; Falenty and Kuhs, 2009). Additionally, the preservation phenomenon of these hydrates correlate to their size, that is, larger particles of CH4 or CO2 hydrates exhibit better stability
H. Mimachi et al. / Chemical Engineering Science 118 (2014) 208–213
(Takeya et al., 2005; Sun et al., 2011). Meanwhile, CH4 hydrate particles packed well with hydrate crystals showed better stability independently from particle sizes (Nakoryakov and Misyura, 2013), and it has been suggested that the thicknesses of ice films enveloping CH4 hydrate were independent from particle size when the particle size was at least several hundreds of micrometers (Stoporev et al., 2014). On the other hand, C2H6 and C3H8 hydrates do not show the preservation phenomenon, and a mixture of C2H6 or C3H8 with CH4 reduces the preservation ability of CH4 hydrate (Stern et al., 2003; Takeya and Ripmeester, 2008, 2010). When the CH4 þC2H6 þC3H8 hydrate system was formed from water with surfactant (Zhang and Rogers, 2008), the hydrate particles were compacted on the metal surfaces to minimize internal void spaces, and the consolidated hydrate mass exhibited high stability at atmospheric pressure and 268 K. Recently, we have found that mechanically packed dense NGH pellets without any additives can also be preserved for three weeks when stored at 253 K under atmospheric pressure, while powdered NGH did not show the preservation phenomenon (Takeya et al., 2012). These facts call into question the effect of particle size on gas hydrate stability and whether smaller NGH particles (less than several tens of mm in diameter) can be preserved in the same way as CH4 hydrates. Here, NGH particles from 0.50 mm to 30 mm stored at 253 K, which is outside their thermodynamically stable zone, were examined. The change of NGH particles depending on particle size under atmospheric air conditions was investigated over two weeks at 253 K. We report our findings on how the particle size of NGH affects its dissociation.
209
the θ/2θ step scan mode with a step width of 0.021 over a 2θ range of 6–601 using CuKα radiation and parallel beam optics (40 kV, 40 mA; Ultima III, Rigaku, Japan). Rietveld analysis using the RIETAN-FP program (Izumi and Momma, 2007) was conducted to analyze unit cell parameters of hydrate crystal, mass fraction of ice, and NGH using the same method as our earlier study (Takeya et al., 2012). Internal imaging of the NGH pellet sample was demonstrated by DEI to visualize the distribution of NGH coexisting with ice formed due to residual water or NGH dissociation
2. Experimental section 2.1. Sample preparation A cylindrical NGH pellet φ33 mm in diameter was formed by means of a semi-batch system with a reactor and one directional pelletizing machine. The system was pressurized to 4.6 MPa with methane gas, and then a gas mixture of simulated natural gas (89.8% CH4, 5.6% C2H6, 3.1% C3H8, 0.6% iso-C4H10, 0.8% n-C4H10, and under 0.1% iso-C5H12) was supplied and pressurized up to 5.5 MPa. The pressure was kept at 5.5 MPa during the formation process by supplying the gas mixture intermittently. NGH slurry was formed at 281 K, and then the slurry was dewatered and pelletized into a φ33 mm cylindrical mass. Then, the system was cooled to 253 K and depressurized to atmospheric pressure to obtain the NGH pellet. The NGH pellet was divided into two parts. The part for the measurements of initial NGH properties was kept under liquefied nitrogen immediately after the NGH pellet was obtained. The other part was kept at 253 K, and the storage test was started as soon as possible. 2.2. Measurements of initial NGH properties by GC, PXRD, and phase contrast X-ray imaging Part of the NGH pellet was arranged for the initial property measurements using gas chromatography (GC), powder X-ray diffraction (PXRD), and phase contrast X-ray CT by means of a diffraction enhanced imaging (DEI) technique. A portion of the NGH pellet was dissociated, and the gases from the NGH sample were collected after the initial rapid hydrate dissociation to obtain the correct gas composition in the sample by means of GC (Micro GC CP4900, Varian, Walnut Creek, CA). The other portion was ground into powder in an atmosphere of vapor from liquefied nitrogen at a temperature lower than 100 K to avoid NGH dissociation. PXRD measurement was performed at 123 K in
Fig. 1. Pellet and particle samples of NGH for storage test. (a) Side view of cylindrical NGH pellet that has just come out from the hydrate formation system. About quarter of left part of pellet was porous because this part was only cohered not pelletized. (b) Largest sample of φ33 30 mm2 for storage test. (c) Samples of NGH particles smaller than 20 mm for storage test. Diameters of particles were 10–20, 4.0–6.7, 1.0–4.0, and 0.50–1.0 mm from left to right.
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Fig. 2. Schematic diagram for storage test at 253 K. NGH samples were stored and weighed in same room at 253 K. Samples were stored in foamed polystyrene box to keep temperature constant.
Fig. 3. PXRD pattern of NGH obtained immediately after production. Plus signs ( þ ) denote observed intensities; solid line was calculated from best-fit model of Rietveld analysis. Bottom line shows deviation between observed and calculated intensities. Upper dashes represent calculated peak positions for structure II hydrate, and lower dashes represent those for hexagonal ice.
during the initial sample preparation process. DEI measurement with a spatial resolution of 0.040 mm was carried out using a 35 keV monochromatic synchrotron X-ray at a vertical wiggler beam line called BL-14C at the Photon Factory in Tsukuba, Japan. Phase map images consisted of 11 images obtained by scanning the analyzer crystal at each position. The scanning time for each position was one second, and the total measurement time was about 150 min. During each measurement, the NGH sample was immersed in methyl acetate (99.5%, Kishida Chemical Co., Japan) and kept at 1937 1 K to eliminate artifacts caused by the outer surface of the sample. More details on this can be found elsewhere (Takeya et al., 2011, 2012). 2.3. Storage test For the storage test, the NGH pellet was divided into five sizes. Fig. 1 shows the pictures of the NGH samples. The largest one was cut out from the cylindrical mass to a size of φ33 30 mm2, and the remainder of the pellet was cracked and sifted into 10–20, 4.0– 6.7, 1.0–4.0, and 0.50–1.0 mm diameters at 253 K. The largest sample was put into a 500 ml plastic case, and the other samples were put into 30 ml plastic cases under atmospheric air. The cases had a hole leading to the atmosphere to avoid increasing the internal pressure due to dissociated gases. When all samples were sifted and put into the cases, the storage test for two weeks at 253 K had begun. The weights of the NGH sample with container were measured by an electric scale placed in a 253 K room at the
beginning and end of the storage test (Fig. 2). Weighting accuracy was 70.010 g at 253 K. Then, the mass fraction of the NGH was estimated by the change in weight due to natural gas release from the dissociating NGH. At the end of the storage term, the samples were dissociated completely at room temperature to obtain the total gas weight and water weight of each sample.
3. Results and discussion PXRD measurement was performed to identify the crystal structure of the NGH used. Structure II with a lattice constant of 17.1160 (6) Å and Ice Ih were detected from the NGH sample at 123 K (Fig. 3). The gas composition of the sample obtained from gas chromatography was 84.5% CH4, 10.4% C2H6, 4.2% C3H8, 0.4% iso-C4H10, 0.4% n-C4H10, and under 0.1% iso-C5H12. Here, it was assumed that 90% of the small cages were occupied by only CH4, and the large cages were fully occupied by natural gases including CH4. Structure II hydrate is composed of 16 small cages and 8 large cages. Thus, the cage occupancies for the large cages were estimated to be 56.6% CH4, 29.1% C2H6, 11.8% C3H8, 1.1% isoC4H10, 1.2% n-C4H10, and residual iso-C5H12, and the small cages were occupied with 90% CH4. To simplify the analysis by the Rietveld method using PXRD data, gases in the large cages were assumed to be 56.6% CH4, 29.1% C2H6, and 14.3% C3H8. As a result, the mass fraction of the NGH sample stored in the vapor from liquefied nitrogen directly after production was estimated to be
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start point of the storage test, the mass fraction of NGH was about 75 wt%, and that of ice was about 25 wt% for the largest sample size of φ33 30 mm2. On the other hand, the mass fractions of NGH with smaller particles were as follows: 72 wt% for 10–20 mm, 59 wt% for 4.0–6.7 mm, 34 wt% for 1.0–4.0 mm, and 19 wt% for 0.50–1.0 mm. The smaller the particle was, the lower the initial mass fraction of NGH was. This is because the smaller NGH samples, which had larger specific surfaces, dissociated as soon as the NGH was shaped and exposed to atmospheric air at 253 K. The mass fraction of each sample after two weeks' storage was 74 wt%, 67 wt%, 57 wt%, 32 wt%, and 16 wt% in descending order. In other words, the losses of weight in the form of a ratio between the initial and final weight fraction of NGH in the storage test were 0.76 wt%, 6.9 wt%, 3.3 wt%, 6.7 wt%, and 17 wt%. It was notable that the largest particle dissociated with less than 1 wt% of NGH, and the smallest particle dissociated with 17 wt%, but more than 80% in gas volume of NGH was maintained after the two weeks' storage test within the smallest NGH particles. Vaporization of water did not be taken into account during storage and complete dissociation at the end of the storage term. As the amount of water vapor needed to saturate inside the plastic case at 253 K was little compared to the sample amount, the weight decrease caused by water vaporization was negligible during storage. However, water vaporization at room temperature might affect the total weight of water because the amount of water vapor for saturation increased at higher temperature. Water vaporization at room temperature might lead to a rise in the hydrate ratio at the beginning of the storage test, but weight changes during storage were compensated for in this experiment. These results reveal that the NGH within
Fig. 4. DEI images of NGH particle with ice film. Cross-section of axial plane (a) and 3D image (b) of NGH pellet were obtained from DEI. White regions in cross-section image (a) correspond to ice, which formed under atmospheric air at 253 K. Thickness of ice layer indicate with arrow was 0.30 mm. Ice, which is shown as light gray areas in (b), covered pellet surface.
81 wt% of NGH and 19 wt% of ice. Accordingly, the mean hydration number of the NGH sample used herein was analyzed to be 6.07. The hydration number analyzed is consistent with the one obtained in an earlier study. (Kumar et al., 2008). Fig. 4 shows non-destructive images measured by DEI system. The difference in density of the sample is reflected in the grayscale. The white region shows ice whose density was lower than the surroundings. Part of an original outer surface of the sample was shaved to remove the ice film under dry nitrogen vapor at a temperature below 150 K, which suppresses hydrate dissociation. Since the ice film with a thickness of about 0.30 mm was observed only at the original outer surface of the particle and then cut at 253 K, the inside of the sample was homogeneous without ice grains or pore spaces. By considering the spatial resolution of DEI, the pellet sample was produced with an internal homogeneity on a scale of more than 0.040 mm. Using the NGH particle samples divided from the cylindrical pellet at 253 K, the storage test was performed. Fig. 5 shows the change of mass fraction of NGH at 253 K over two weeks. At the
Fig. 5. Change of mass fraction of NGH for each diameter. Solid circles represent starting mass fraction of NGH cut or cracked from φ33 mm cylindrical pellet for storage test. Open squares represent mass fraction of NGH after two weeks' storage. Dotted line represents initial hydrate mass measured by PXRD.
Fig. 6. Calculated thickness of ice film on NGH particle for each diameter. Thickness of ice film ranged from about 0.25 to 0.40 mm for 2.5–30 mm diameters. In case of diameter of 0.75 mm, ice film was approximately half that of others.
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each particle with a different size was maintained during the two week storage test. In our previous study on the dissociation of NGH pellets (Takeya et al., 2012), it was shown that the outer surface was enveloped by an ice film initially, but the dissociation of NGH and growth of ice film or internal dissociation of the NGH pellet did not occur after the initial dissociation from the outer surface. Accordingly, as is noted above, the NGH within each particle with a different size was maintained after the initial rapid dissociation. Thus, even for small NGH particles, the ice film that envelops the NGH particle may prevent further NGH dissociation. Here, the thicknesses of the ice film on the NGH particle were estimated from the change of the mass fraction of NGH shown in Fig. 5 before and after size processing. For the estimation, the value of the mass fraction of NGH in the sample analyzed by PXRD measurement (81%) was adopted as the initial hydrate mass. We used averaged sample sizes with diameters of 30, 15, 5.4, 2.5, and 0.75 mm for
φ33 30, 10–20, 4.0–6.7, 1.0–4.0, and 0.50–1.0 mm samples, respectively. Then, the thicknesses of the ice film were estimated by spherical approximation to be about 0.25–0.40 mm except when the sample size was 0.75 mm (Fig. 6). Here, the fact that the similarity of the thickness of the ice film did not depend on particle size suggests that the hydrate was encased by ice, and this greatly hinders the dissociation process as has been reported so far. A calculated ice thickness of about 0.14 mm was obtained for the smallest NGH sample (0.75 mm in size). Since particles of smaller diameter led to a larger contact surface compared to particles of larger diameter, the actual surface area of this small sample may be smaller. Also, the ice film on the adjacent surface of NGH particles with smaller particle size could have a high sintering effect during the storage test (Blackford, 2007). Fig. 7 shows the schematic diagrams for forming the ice film on the NGH particles and sintering of ice films on the NGH particles with
NGH polycrystal
NGH
Sintering Ice film
Fig. 7. Schematic diagrams for forming ice film on NGH particles and sintering ice covering NGH particles. (a) NGH pellet that has just come out from semi-batch reactor system. (b) NGH particles just after being cracked and sifted. (c) Beginning of formation of ice films on new planes of NGH crystals. (d) NGH particles coated by ice films. (e) NGH particles larger than 1.0 mm are covered with ice films of about 0.25 mm or more. Clearance is indicated by arrows, and this represents thickness of ice layer on NGH. (f) Smaller NGH particles, such as those of 0.75 mm in diameter, have thinner ice compared to larger ones. But higher sintering effect of ice films enveloping smaller NGH particles can be expected. (g) NGH fine powder completely dissociated into ice.
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different diameters. Consequently, an ice film with a thickness of 0.14 mm could act like an ice film with a thickness of about 0.30 mm. In this study, dense NGH particles without internal pores could be stabilized over two weeks at 253 K irrespective of the size of the particle. The value of the ice thickness estimated from the mass fraction was in the same order of the thickness of the ice layer measured by the DEI method. It was also revealed that the thickness of the ice film required for stabilizing NGH was thicker than that for CH4 hydrate. The CH4 hydrate stored at 253 K and was tightly enveloped and stabilized by a layer of ice with an average thickness of 100 μm (Takeya et al., 2011). The thickness of the ice film necessary for self-preservation may be the reason why powder NGH does not show the self-preservation phenomenon even though its mechanism is remains to be explained. Also, understanding the sintering effect of NGH or the ice film enveloping the NGH particle will be necessary for hydrate-based natural gas storage.
4. Conclusion In this report, the preservation effects of NGH were investigated from the viewpoint of sample size for φ33 30, 10–20, 4.0–6.7, 1.0–4.0, and 0.50–1.0 mm at 253 K under atmospheric pressure. The NGH pellet used for the experiment was dense inside and there was an ice film about 0.30 mm, which was caused by the initial rapid dissociation. When the NGH particle diameters were more than 0.50 mm, gases were preserved for two weeks. It is required that the NGH sizes are more than 0.50 mm for NGH handling. The reason NGH requires a thicker ice film than that of CH4 hydrate to show self-preservation is still unclear. However, our results suggest a way to more effectively store natural gas in the form of NGH.
Acknowledgments Part of this study was carried out under Proposal nos. 2012I001 and 2013C307, which were approved by the High Energy Accelerator Research Organization.
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References Blackford, J.R., 2007. Sintering and microstructure of ice: a review. J. Phys. D: Appl. Phys. 40, R355. Falenty, A., Kuhs, W.F., 2009. “Self-preservation” of CO2 gas hydrates—surface microstructure and ice perfection. J. Phys. Chem. B 113, 15975. Gudmundsson, J.S., Borrehaug, A., 1996. Frozen hydrate for transport of natural gas. In: Proceedingsof the 2nd International Conference on Natural Gas Hydrates Toulouse, p. 415. Izumi, F., Momma, K., 2007. Three-dimensional visualization in powder diffraction. Solid State Phenom. 130, 15. Kuhs, W.F., Genov, G., Satykova, D.K., Hansen, T., 2004. Ice perfection and onset of anomalous preservation of gas hydrates. Phys. Chem. Chem. Phys. 6, 4917. Kumar, R., Linga, P., Moudrakovski, I., Ripmeester, J.A., Englezos, P., 2008. Structure and kinetics of gas hydrates from methane/ethane/propane mixtures relevant to the design of natural gas hydrate storage and transport facilities. AIChE J. 54, 2132. Nakoryakov, V.E., Misyura, S.Y., 2013. The features of self-preservation for hydrate systems with methane. Chem. Eng. Sci. 104, 1. Shimada, W., Takeya, S., Kamata, Y., Uchida, T., Nagao, J., Ebinuma, T., Narita, H., 2005. Texture change of ice on anomalously preserved methane clathrate hydrate. J. Phys. Chem. B 109, 5802. Sloan, E.D., 2003. Fundamental principles and applications of natural gas hydrates. Nature 426, 353. Stern, L.A., Circone, S., Kirby, S.H., Durham, W.B., 2001. Anomalous preservation of pure methane hydrate at 1 atm. J. Phys. Chem. B 105, 1756. Stern, L.A., Circone, S., Kirby, S.H., Durham, W.B., 2003. Temperature, pressure, and compositional effects on anomalous or “self” preservation of gas hydrates. Can. J. Phys. 81, 271. Stoporev, A.S., Manakov, A.Y., Altunina, L.K., Bogoslovsky, A.V., Strelets, L.A., Aladko, E.Y., 2014. Unusual self-preservation of methane hydrate in oil suspensions. Energy Fuels 28, 794. Sun, D., Shimono, Y., Takeya, S., Ohmura, R., 2011. Preservation of carbon dioxide clathrate hydrate at temperatures below the water freezing point under atmospheric pressure. Ind. Eng. Chem. Res. 50, 13854. Takeya, S., Uchida, T., Nagao, J., Ohmura, R., Shimada, W., Kamata, Y., Ebinuma, T., Narita, H., 2005. Particle size effect of CH4 hydrate for self-preservation. Chem. Eng. Sci. 60, 1383. Takeya, S., Ripmeester, J.A., 2008. Dissociation behavior of clathrate hydrates to ice and dependence on guest molecules. Angew. Chem. Int. Ed. 47, 1276. Takeya, S., Ripmeester, J.A., 2010. Anomalous preservation of CH4 hydrate and its dependence on the morphology of hexagonal ice. ChemPhysChem 11, 70. Takeya, S., Yoneyama, A., Ueda, K., Hyodo, K., Takeda, T., Mimachi, H., Takahashi, M., Iwasaki, T., Sano, K., Yamawaki, H., Gotoh, Y., 2011. Nondestructive imaging of anomalously preserved methane clathrate hydrate by phase contrast X-ray imaging. J. Phys. Chem. C 115, 16193. Takeya, S., Yoneyama, A., Ueda, K., Mimachi, H., Takahashi, M., Sano, K., Hyodo, K., Takeda, T., Gotoh, Y., 2012. Anomalously preserved clathrate hydrate of natural gas in pellet form at 253 K. J. Phys. Chem. C 116, 13842. Yakushev, V.S., Istomin, V.A., 1992. Gas-hydrates self-preservation effect, Physics and Chemistry of Ice. Hokkaido University Press, p. 136. Zhang, G., Rogers, R.E., 2008. Ultra-stability of gas hydrates at 1 atm and 268.2 K. Chem. Eng. Sci. 63, 2066.
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Natural gas storage and transportation within gas hydrate of smaller particle: Size dependence of self-preservation phenomenon of natural gas hydrate Hiroko Mimachi a,n, Satoshi Takeya b, Akio Yoneyama c, Kazuyuki Hyodo d, Tohoru Takeda e, Yoshito Gotoh b, Tetsuro Murayama a a
Mitsui Engineering and Shipbuilding Co., Ltd., 1 Yawatakaigandori, Ichihara, Chiba 290-8531, Japan National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan Hitachi Ltd., Central Research Laboratory, 2520 Akanuma, Hatoyama, Saitama 350-0395, Japan d High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan e Allied Health Sciences, Kitasato University, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan b c
H I G H L I G H T S
Larger particles of NGH allowed a larger fraction of gas hydrate to be preserved. Dense NGHs over 0.50 mm could keep natural gases for 14 days at 253 K and 1 atm. Ice films on self-preserved NGHs over 1.0 mm had an equivalent thickness.
art ic l e i nf o
a b s t r a c t
Article history: Received 24 April 2014 Received in revised form 22 July 2014 Accepted 26 July 2014 Available online 1 August 2014
In this study, the preservation ability of natural gas hydrate (NGH) with different particle size from 0.50 mm to 30 mm was investigated. It is known that natural gas preservation does not occur when NGHs are in powder form and their size is under several tens of micrometers, while the dense bulk NGH was preserved after the initial rapid dissociation. It is important to investigate the required size of NGH particle for self-preservation to occur when NGH is used as a storage media of natural gases. Despite the difference in particle size, the experimental results herein revealed that the NGH sample was enveloped by an ice film caused by the initial rapid dissociation. The thickness of the ice film grown during the initial rapid dissociation was almost the same and did not depend on the particle size of NGH being over 1.0 mm and higher than that of methane hydrate. The thickness of the ice film necessary for the selfpreservation of NGH may be the reason why powder NGH does not show the self-preservation phenomenon. However, we revealed that NGH particles with diameters of more than 0.50 mm were preserved for two weeks. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Clathrate hydrate Natural gas hydrate Self-preservation Particle size Ice film
1. Introduction Due to the expansion of the worldwide demand for natural gas, production of natural gas from not only conventional gas fields but also unconventional gas fields, such as shale gas, is increasing. Therefore, ocean transportation of natural gas or liquefied natural gas (LNG) is the key for further development of the use of natural gas. Natural gas hydrate (NGH) is one of the clathrate compounds formed from water and natural gases, such as CH4, C2H6, C3H8, C4H10, etc., with natural compositions. NGH is a candidate for a
n
Corresponding author. Tel.: þ 81 436 41 1110; fax: þ 81 436 43 1785. E-mail address: [email protected] (H. Mimachi).
http://dx.doi.org/10.1016/j.ces.2014.07.050 0009-2509/& 2014 Elsevier Ltd. All rights reserved.
new media for natural gas storage and transportation because it can contain about 170 times as much natural gas in the form of gas hydrate (Gudmundsson and Borrehaug, 1996; Sloan, 2003). It is known that gas hydrates are usually stable under high pressures and at low temperatures. Some gas hydrates keep gases under atmospheric pressure and just below the melting point of ice though the surroundings are outside thermodynamically stable temperature and pressure conditions; this is called the selfpreservation phenomenon (Yakushev and Istomin, 1992). It has been reported that CH4 and CO2 hydrates show the preservation phenomenon (Stern et al., 2001; Kuhs et al., 2004; Shimada et al., 2005; Falenty and Kuhs, 2009). Additionally, the preservation phenomenon of these hydrates correlate to their size, that is, larger particles of CH4 or CO2 hydrates exhibit better stability
H. Mimachi et al. / Chemical Engineering Science 118 (2014) 208–213
(Takeya et al., 2005; Sun et al., 2011). Meanwhile, CH4 hydrate particles packed well with hydrate crystals showed better stability independently from particle sizes (Nakoryakov and Misyura, 2013), and it has been suggested that the thicknesses of ice films enveloping CH4 hydrate were independent from particle size when the particle size was at least several hundreds of micrometers (Stoporev et al., 2014). On the other hand, C2H6 and C3H8 hydrates do not show the preservation phenomenon, and a mixture of C2H6 or C3H8 with CH4 reduces the preservation ability of CH4 hydrate (Stern et al., 2003; Takeya and Ripmeester, 2008, 2010). When the CH4 þC2H6 þC3H8 hydrate system was formed from water with surfactant (Zhang and Rogers, 2008), the hydrate particles were compacted on the metal surfaces to minimize internal void spaces, and the consolidated hydrate mass exhibited high stability at atmospheric pressure and 268 K. Recently, we have found that mechanically packed dense NGH pellets without any additives can also be preserved for three weeks when stored at 253 K under atmospheric pressure, while powdered NGH did not show the preservation phenomenon (Takeya et al., 2012). These facts call into question the effect of particle size on gas hydrate stability and whether smaller NGH particles (less than several tens of mm in diameter) can be preserved in the same way as CH4 hydrates. Here, NGH particles from 0.50 mm to 30 mm stored at 253 K, which is outside their thermodynamically stable zone, were examined. The change of NGH particles depending on particle size under atmospheric air conditions was investigated over two weeks at 253 K. We report our findings on how the particle size of NGH affects its dissociation.
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the θ/2θ step scan mode with a step width of 0.021 over a 2θ range of 6–601 using CuKα radiation and parallel beam optics (40 kV, 40 mA; Ultima III, Rigaku, Japan). Rietveld analysis using the RIETAN-FP program (Izumi and Momma, 2007) was conducted to analyze unit cell parameters of hydrate crystal, mass fraction of ice, and NGH using the same method as our earlier study (Takeya et al., 2012). Internal imaging of the NGH pellet sample was demonstrated by DEI to visualize the distribution of NGH coexisting with ice formed due to residual water or NGH dissociation
2. Experimental section 2.1. Sample preparation A cylindrical NGH pellet φ33 mm in diameter was formed by means of a semi-batch system with a reactor and one directional pelletizing machine. The system was pressurized to 4.6 MPa with methane gas, and then a gas mixture of simulated natural gas (89.8% CH4, 5.6% C2H6, 3.1% C3H8, 0.6% iso-C4H10, 0.8% n-C4H10, and under 0.1% iso-C5H12) was supplied and pressurized up to 5.5 MPa. The pressure was kept at 5.5 MPa during the formation process by supplying the gas mixture intermittently. NGH slurry was formed at 281 K, and then the slurry was dewatered and pelletized into a φ33 mm cylindrical mass. Then, the system was cooled to 253 K and depressurized to atmospheric pressure to obtain the NGH pellet. The NGH pellet was divided into two parts. The part for the measurements of initial NGH properties was kept under liquefied nitrogen immediately after the NGH pellet was obtained. The other part was kept at 253 K, and the storage test was started as soon as possible. 2.2. Measurements of initial NGH properties by GC, PXRD, and phase contrast X-ray imaging Part of the NGH pellet was arranged for the initial property measurements using gas chromatography (GC), powder X-ray diffraction (PXRD), and phase contrast X-ray CT by means of a diffraction enhanced imaging (DEI) technique. A portion of the NGH pellet was dissociated, and the gases from the NGH sample were collected after the initial rapid hydrate dissociation to obtain the correct gas composition in the sample by means of GC (Micro GC CP4900, Varian, Walnut Creek, CA). The other portion was ground into powder in an atmosphere of vapor from liquefied nitrogen at a temperature lower than 100 K to avoid NGH dissociation. PXRD measurement was performed at 123 K in
Fig. 1. Pellet and particle samples of NGH for storage test. (a) Side view of cylindrical NGH pellet that has just come out from the hydrate formation system. About quarter of left part of pellet was porous because this part was only cohered not pelletized. (b) Largest sample of φ33 30 mm2 for storage test. (c) Samples of NGH particles smaller than 20 mm for storage test. Diameters of particles were 10–20, 4.0–6.7, 1.0–4.0, and 0.50–1.0 mm from left to right.
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Fig. 2. Schematic diagram for storage test at 253 K. NGH samples were stored and weighed in same room at 253 K. Samples were stored in foamed polystyrene box to keep temperature constant.
Fig. 3. PXRD pattern of NGH obtained immediately after production. Plus signs ( þ ) denote observed intensities; solid line was calculated from best-fit model of Rietveld analysis. Bottom line shows deviation between observed and calculated intensities. Upper dashes represent calculated peak positions for structure II hydrate, and lower dashes represent those for hexagonal ice.
during the initial sample preparation process. DEI measurement with a spatial resolution of 0.040 mm was carried out using a 35 keV monochromatic synchrotron X-ray at a vertical wiggler beam line called BL-14C at the Photon Factory in Tsukuba, Japan. Phase map images consisted of 11 images obtained by scanning the analyzer crystal at each position. The scanning time for each position was one second, and the total measurement time was about 150 min. During each measurement, the NGH sample was immersed in methyl acetate (99.5%, Kishida Chemical Co., Japan) and kept at 1937 1 K to eliminate artifacts caused by the outer surface of the sample. More details on this can be found elsewhere (Takeya et al., 2011, 2012). 2.3. Storage test For the storage test, the NGH pellet was divided into five sizes. Fig. 1 shows the pictures of the NGH samples. The largest one was cut out from the cylindrical mass to a size of φ33 30 mm2, and the remainder of the pellet was cracked and sifted into 10–20, 4.0– 6.7, 1.0–4.0, and 0.50–1.0 mm diameters at 253 K. The largest sample was put into a 500 ml plastic case, and the other samples were put into 30 ml plastic cases under atmospheric air. The cases had a hole leading to the atmosphere to avoid increasing the internal pressure due to dissociated gases. When all samples were sifted and put into the cases, the storage test for two weeks at 253 K had begun. The weights of the NGH sample with container were measured by an electric scale placed in a 253 K room at the
beginning and end of the storage test (Fig. 2). Weighting accuracy was 70.010 g at 253 K. Then, the mass fraction of the NGH was estimated by the change in weight due to natural gas release from the dissociating NGH. At the end of the storage term, the samples were dissociated completely at room temperature to obtain the total gas weight and water weight of each sample.
3. Results and discussion PXRD measurement was performed to identify the crystal structure of the NGH used. Structure II with a lattice constant of 17.1160 (6) Å and Ice Ih were detected from the NGH sample at 123 K (Fig. 3). The gas composition of the sample obtained from gas chromatography was 84.5% CH4, 10.4% C2H6, 4.2% C3H8, 0.4% iso-C4H10, 0.4% n-C4H10, and under 0.1% iso-C5H12. Here, it was assumed that 90% of the small cages were occupied by only CH4, and the large cages were fully occupied by natural gases including CH4. Structure II hydrate is composed of 16 small cages and 8 large cages. Thus, the cage occupancies for the large cages were estimated to be 56.6% CH4, 29.1% C2H6, 11.8% C3H8, 1.1% isoC4H10, 1.2% n-C4H10, and residual iso-C5H12, and the small cages were occupied with 90% CH4. To simplify the analysis by the Rietveld method using PXRD data, gases in the large cages were assumed to be 56.6% CH4, 29.1% C2H6, and 14.3% C3H8. As a result, the mass fraction of the NGH sample stored in the vapor from liquefied nitrogen directly after production was estimated to be
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start point of the storage test, the mass fraction of NGH was about 75 wt%, and that of ice was about 25 wt% for the largest sample size of φ33 30 mm2. On the other hand, the mass fractions of NGH with smaller particles were as follows: 72 wt% for 10–20 mm, 59 wt% for 4.0–6.7 mm, 34 wt% for 1.0–4.0 mm, and 19 wt% for 0.50–1.0 mm. The smaller the particle was, the lower the initial mass fraction of NGH was. This is because the smaller NGH samples, which had larger specific surfaces, dissociated as soon as the NGH was shaped and exposed to atmospheric air at 253 K. The mass fraction of each sample after two weeks' storage was 74 wt%, 67 wt%, 57 wt%, 32 wt%, and 16 wt% in descending order. In other words, the losses of weight in the form of a ratio between the initial and final weight fraction of NGH in the storage test were 0.76 wt%, 6.9 wt%, 3.3 wt%, 6.7 wt%, and 17 wt%. It was notable that the largest particle dissociated with less than 1 wt% of NGH, and the smallest particle dissociated with 17 wt%, but more than 80% in gas volume of NGH was maintained after the two weeks' storage test within the smallest NGH particles. Vaporization of water did not be taken into account during storage and complete dissociation at the end of the storage term. As the amount of water vapor needed to saturate inside the plastic case at 253 K was little compared to the sample amount, the weight decrease caused by water vaporization was negligible during storage. However, water vaporization at room temperature might affect the total weight of water because the amount of water vapor for saturation increased at higher temperature. Water vaporization at room temperature might lead to a rise in the hydrate ratio at the beginning of the storage test, but weight changes during storage were compensated for in this experiment. These results reveal that the NGH within
Fig. 4. DEI images of NGH particle with ice film. Cross-section of axial plane (a) and 3D image (b) of NGH pellet were obtained from DEI. White regions in cross-section image (a) correspond to ice, which formed under atmospheric air at 253 K. Thickness of ice layer indicate with arrow was 0.30 mm. Ice, which is shown as light gray areas in (b), covered pellet surface.
81 wt% of NGH and 19 wt% of ice. Accordingly, the mean hydration number of the NGH sample used herein was analyzed to be 6.07. The hydration number analyzed is consistent with the one obtained in an earlier study. (Kumar et al., 2008). Fig. 4 shows non-destructive images measured by DEI system. The difference in density of the sample is reflected in the grayscale. The white region shows ice whose density was lower than the surroundings. Part of an original outer surface of the sample was shaved to remove the ice film under dry nitrogen vapor at a temperature below 150 K, which suppresses hydrate dissociation. Since the ice film with a thickness of about 0.30 mm was observed only at the original outer surface of the particle and then cut at 253 K, the inside of the sample was homogeneous without ice grains or pore spaces. By considering the spatial resolution of DEI, the pellet sample was produced with an internal homogeneity on a scale of more than 0.040 mm. Using the NGH particle samples divided from the cylindrical pellet at 253 K, the storage test was performed. Fig. 5 shows the change of mass fraction of NGH at 253 K over two weeks. At the
Fig. 5. Change of mass fraction of NGH for each diameter. Solid circles represent starting mass fraction of NGH cut or cracked from φ33 mm cylindrical pellet for storage test. Open squares represent mass fraction of NGH after two weeks' storage. Dotted line represents initial hydrate mass measured by PXRD.
Fig. 6. Calculated thickness of ice film on NGH particle for each diameter. Thickness of ice film ranged from about 0.25 to 0.40 mm for 2.5–30 mm diameters. In case of diameter of 0.75 mm, ice film was approximately half that of others.
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each particle with a different size was maintained during the two week storage test. In our previous study on the dissociation of NGH pellets (Takeya et al., 2012), it was shown that the outer surface was enveloped by an ice film initially, but the dissociation of NGH and growth of ice film or internal dissociation of the NGH pellet did not occur after the initial dissociation from the outer surface. Accordingly, as is noted above, the NGH within each particle with a different size was maintained after the initial rapid dissociation. Thus, even for small NGH particles, the ice film that envelops the NGH particle may prevent further NGH dissociation. Here, the thicknesses of the ice film on the NGH particle were estimated from the change of the mass fraction of NGH shown in Fig. 5 before and after size processing. For the estimation, the value of the mass fraction of NGH in the sample analyzed by PXRD measurement (81%) was adopted as the initial hydrate mass. We used averaged sample sizes with diameters of 30, 15, 5.4, 2.5, and 0.75 mm for
φ33 30, 10–20, 4.0–6.7, 1.0–4.0, and 0.50–1.0 mm samples, respectively. Then, the thicknesses of the ice film were estimated by spherical approximation to be about 0.25–0.40 mm except when the sample size was 0.75 mm (Fig. 6). Here, the fact that the similarity of the thickness of the ice film did not depend on particle size suggests that the hydrate was encased by ice, and this greatly hinders the dissociation process as has been reported so far. A calculated ice thickness of about 0.14 mm was obtained for the smallest NGH sample (0.75 mm in size). Since particles of smaller diameter led to a larger contact surface compared to particles of larger diameter, the actual surface area of this small sample may be smaller. Also, the ice film on the adjacent surface of NGH particles with smaller particle size could have a high sintering effect during the storage test (Blackford, 2007). Fig. 7 shows the schematic diagrams for forming the ice film on the NGH particles and sintering of ice films on the NGH particles with
NGH polycrystal
NGH
Sintering Ice film
Fig. 7. Schematic diagrams for forming ice film on NGH particles and sintering ice covering NGH particles. (a) NGH pellet that has just come out from semi-batch reactor system. (b) NGH particles just after being cracked and sifted. (c) Beginning of formation of ice films on new planes of NGH crystals. (d) NGH particles coated by ice films. (e) NGH particles larger than 1.0 mm are covered with ice films of about 0.25 mm or more. Clearance is indicated by arrows, and this represents thickness of ice layer on NGH. (f) Smaller NGH particles, such as those of 0.75 mm in diameter, have thinner ice compared to larger ones. But higher sintering effect of ice films enveloping smaller NGH particles can be expected. (g) NGH fine powder completely dissociated into ice.
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different diameters. Consequently, an ice film with a thickness of 0.14 mm could act like an ice film with a thickness of about 0.30 mm. In this study, dense NGH particles without internal pores could be stabilized over two weeks at 253 K irrespective of the size of the particle. The value of the ice thickness estimated from the mass fraction was in the same order of the thickness of the ice layer measured by the DEI method. It was also revealed that the thickness of the ice film required for stabilizing NGH was thicker than that for CH4 hydrate. The CH4 hydrate stored at 253 K and was tightly enveloped and stabilized by a layer of ice with an average thickness of 100 μm (Takeya et al., 2011). The thickness of the ice film necessary for self-preservation may be the reason why powder NGH does not show the self-preservation phenomenon even though its mechanism is remains to be explained. Also, understanding the sintering effect of NGH or the ice film enveloping the NGH particle will be necessary for hydrate-based natural gas storage.
4. Conclusion In this report, the preservation effects of NGH were investigated from the viewpoint of sample size for φ33 30, 10–20, 4.0–6.7, 1.0–4.0, and 0.50–1.0 mm at 253 K under atmospheric pressure. The NGH pellet used for the experiment was dense inside and there was an ice film about 0.30 mm, which was caused by the initial rapid dissociation. When the NGH particle diameters were more than 0.50 mm, gases were preserved for two weeks. It is required that the NGH sizes are more than 0.50 mm for NGH handling. The reason NGH requires a thicker ice film than that of CH4 hydrate to show self-preservation is still unclear. However, our results suggest a way to more effectively store natural gas in the form of NGH.
Acknowledgments Part of this study was carried out under Proposal nos. 2012I001 and 2013C307, which were approved by the High Energy Accelerator Research Organization.
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