Energy Fuels 2010, 24, 2390–2403 Published on Web 03/22/2010
: DOI:10.1021/ef901403r
Methane Hydrate Synthesis from Ice: Influence of Pressurization and Ethanol on Optimizing Formation Rates and Hydrate Yield Po-Chun Chen,*,†,‡ Wuu-Liang Huang,‡ and Laura A. Stern§ †
Central Geological Survey, Post Office Box 968, Number 2, Lane 109, Hua-Hsin Street, Chung-Ho, Taipei, Taiwan 235, Republic of China, ‡Institute of Geosciences, National Taiwan University, Post Office Box 13-318, Number 1, Section 4, Roosevelt Road, Taipei, 106 Taiwan, Republic of China, and §United States Geological Survey (USGS), 345 Middlefield Road, Menlo Park, California 94025 Received November 19, 2009. Revised Manuscript Received February 25, 2010
Polycrystalline methane gas hydrate (MGH) was synthesized using an ice-seeding method to investigate the influence of pressurization and ethanol on the hydrate formation rate and gas yield of the resulting samples. When the reactor is pressurized with CH4 gas without external heating, methane hydrate can be formed from ice grains with yields up to 25% under otherwise static conditions. The rapid temperature rise caused by pressurization partially melts the granular ice, which reacts with methane to form hydrate rinds around the ice grains. The heat generated by the exothermic reaction of methane hydrate formation buffers the sample temperature near the melting point of ice for enough time to allow for continuous hydrate growth at high rates. Surprisingly, faster rates and higher yields of methane hydrate were found in runs with lower initial temperatures, slower rates of pressurization, higher porosity of the granular ice samples, or mixtures with sediments. The addition of ethanol also dramatically enhanced the formation of polycrystalline MGH. This study demonstrates that polycrystalline MGH with varied physical properties suitable for different laboratory tests can be manufactured by controlling synthesis procedures or parameters. Subsequent dissociation experiments using a gas collection apparatus and flowmeter confirmed high methane saturation (CH4 3 nH2O, with n = 5.82 ( 0.03) in the MGH. Dissociation rates of the various samples synthesized at diverse conditions may be fitted to different rate laws, including zero and first order.
exploration activities,7-10 laboratory synthesis studies have been conducted to better understand the thermodynamics and kinetics of MGH formation and dissociation,11-13 and to simulate its natural occurrence.14 Characterization of the formation and dissociation of the hydrate from MGH-bearing sediment samples with diverse characteristics may also have bearing on assessments of hydrate occurrences.15-18 Consequently, a variety of laboratory tests on synthetic MGH-bearing specimens have
1. Introduction Methane gas hydrate (MGH), also called methane clathrate hydrate, is a non-stoichiometric compound with a structure consisting of a network of H2O molecules hydrogen-bonded together in an ice-like manner within which methane molecules are encaged. Methane hydrate has received considerable interest for its potential applications not only as a future source of natural gas energy1-3 but also for the storage and transportation of conventional natural gas.4-6 In addition to intensive field
(10) Bourry, C.; Chazallon, B.; Charlou, J. L. Free gas and gas hydrates from the Sea of Marmara, Turkey: Chemical and structural characterization. Chem. Geol. 2009, 264, 197–206. (11) Circone, S.; Kirby, S.; Stern, L. A. Thermal regulation of methane hydrate dissociation: Implications for gas production models. Energy Fuels 2005, 19, 2357–2363. (12) Circone, S.; Kirby, S. H.; Stern, L. A. Thermodynamic calculations in the system CH4-H2O and methane hydrate phase equilibria. J. Phys. Chem. B 2006, 110, 8232–8239. (13) Pang, W. X.; Xu, W. Y.; Sun, C. Y.; Zhang, C. L.; Chen, G. J. Methane hydrate dissociation experiment in a middle-sized quiescent reactor using thermal method. Fuel 2009, 88, 497–503. (14) Eaton, M.; Mahajan, D.; Flood, R. A novel high-pressure apparatus to study hydrate-sediment interactions. J. Pet. Sci. Eng. 2007, 56, 101–107. (15) Anderson, R.; Llamedo, M.; Tohidi, B.; Burgass, R. W. Experimental measurement of methane and carbon dioxide clathrate equilibria in mesoporous silica. J. Phys. Chem. B 2003, 107, 3507–3514. (16) Winters, W. J.; Pecher, I. A.; Waite, W. F.; Mason, D. H. Physical properties and rock physics models of sediment containing natural and laboratory-formed methane gas hydrate. J. Am. Mineral. 2004, 89, 2121–1227. (17) Priest, J. A.; Best, A. I.; Clayton, C. R. I. A laboratory investigation into the seismic velocities of methane gas hydrate-bearing sand. J. Geophys. Res. 2005, 110, B4. (18) Waite, W. F.; Kneafsey, T. J.; Winters, W. J.; Mason, D. H. Physical property changes in hydrate-bearing sediment due to depressurization and subsequent repressurization. J. Geophys. Res 2008, 113, B7.
*To whom correspondence should be addressed: Central Geological Survey, Post Office Box 968, Number 2, Lane 109, Hua-Hsin Street, Chung-Ho, Taipei, Taiwan 235, Republic of China. E-mail: burt@ moeacgs.gov.tw. (1) Sloan, E. D. Clathrate Hydrates of Natural Gases, 2nd ed.; Marcel Dekker, Inc.: New York, 1998; p 705. (2) Collet, T. S. Gas hydrates as a future energy resource. Geotimes 2004, 49 (11), 24–27. (3) Boswell, R. Resource potential of methane hydrate coming into focus. J. Pet. Sci. Eng. 2007, 56, 9–13. (4) Gudmundsson, J. S. Gas storage and transport using hydrates. Offshore Mediterranean Conference, Ravenna, Italy, 1997. (5) Sloan, E. D. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426, 353–359. (6) Javanmardi, J.; Nasrifar, Kh.; Najibi, S. H.; Moshfeghian, M. Economic evaluation of natural gas hydrate as an alternative for natural gas transportation. Appl. Therm. Eng. 2005, 25, 1708–1723. (7) Chen, D. F.; Cathles, L. M., III.; Roberts, H. H. The geochemical signatures of variable gas venting at gas hydrate sites. Mar. Pet. Geol. 2004, 21, 317–326. (8) Chatti, I.; Delahaye, A.; Fournaison, L.; Petitet, J. P. Benefits and drawbacks of clathrate hydrates: A review of their areas of interest. Energy Convers. Manage. 2005, 46, 1333–1343. (9) Hester, K. C.; Dunk, R. M.; White, S. N.; Brewer, P. G.; Peltzer, E. T.; Sloan, E. D. Gas hydrate measurements at Hydrate Ridge using Raman spectroscopy. Geochim. Cosmochim. Acta 2007, 71, 2947–2959. r 2010 American Chemical Society
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been performed to measure their physical properties, including strength characteristics and rheology, acoustics, thermal and electric conductivity, petrophysics, etc.19-23 Such laboratory information aids exploration, exploitation, and development of gas hydrate as an alternative energy source.24 To gain further insight into clathrates formed under varying conditions, well-characterized, synthetic MGH specimens with diverse grain characteristics, phase distributions, concentrations, and sediment contents are required. Using ice grains as seeds for the growth of hydrate crystals, Stern et al.25-27 synthesized large volumes of polycrystalline gas hydrate aggregates of uniform grain size and random crystallographic orientation, with a final water/guest molar ratio close to 5.89. This method promotes virtually full reaction of all ice (>98.5 vol %) in the sample chamber to gas hydrate28 and has been successfully used to test specimens for a variety of purposes.19,23,29-32 The present study aims not only to apply this previous work to search for the optimal growth parameters for efficient synthesis of methane hydrate and its underlying reaction mechanism but also to place constraints on the conditions required for manufacturing specimens with a variety of specific characteristics. In conventional ice-seeding synthesis, granular ice, with or without the addition of sand grains or sediments, is pressurized by methane gas followed by warming the reactants above the ice melting point to conditions near the methane hydrate stability boundary. Melting of the ice grains and concurrent formation of methane hydrate effectively result in conversion of ice to hydrate while maintaining a uniform polycrystalline sample texture.
Mixed or layered sediments within the ice pack also maintain their original positioning in the final sample. Previous work showed that efficient synthesis can be promoted by starting with relatively fine-grained ice ( 270 K. (b) MGH yield measured by the flowmeter for five repeated runs (R1-R5 in Table 2) confirmed that the Tinitial significantly affects the MGH yields. Figure 10. Comparison of the MGH yields (yield IA in Table 3) during pressurization with the same Tinitial at 258.3 K for samples at various conditions. HD, highly compacted; FG1 and FG2, ice grain size e 180 μm; R2, normal grain size (180-250 μm) and rapid pressurization; R1, duplicated experiment of R2; MS, mixture of ice grains and sands; S2, slow pressurization rate; S1, duplicated experiment of S2; E4, with ethanol. The detailed run conditions were listed in Table 3.
same trend was displayed by the second group of samples (Table 2). This suggests that, by properly selecting the initial system temperature, a significant savings of experimental time and energy can be achieved in MGH synthesis using the ice-seeding method. However, our results also showed that the more MGH formed during pressurization, the less consolidated the final synthetic MGH aggregates. The granular aggregates of the synthetic MGH sample formed by pressurization only showed a lack of MGH cementing between grains. The amount of cementing increased when synthesis was completed over repeated heating/cooling cycles. This provides a guideline for synthesizing different granular MGH aggregates with a variety of structural characteristic properties for laboratory testing. While most of the experiments were conducted by rapidly applying pressure into the sample reactor, two similar experiments (runs S1 and S2) were carried out by slow pressurization up to around 17 MPa over 1440 min. The MGH yield synthesized over a total run time of 1440 min before the end of the pressurization was nearly double that of the rapid pressurization over 960 min. These duplicated runs showed very similar MGH yields, 25.7 and 23.1% for S1 and S2, respectively (Table 3 and Figure 10). The sample
temperature, which in this case did not deviate much from the gas temperature during the pressurization, rose only slightly to 260.7 K from the initial temperature of 258 K and then slowly decreased (Figure 11a). Note that the temperatures of the sample never exceeded the ice melting temperature in the slow pressurization stage, suggesting that the MGH formation proceeded mostly by reacting gaseous methane with solid ice.42 The high MGH yield, while unexpected from the solid ice-gas reaction, may be mainly attributed to its Tinitial (260.7 K) and partly to the continuing pressurization, which aids the diffusion of methane through the MGH rind to react with the ice inside. (42) Staykova, D. K.; Kuhs, W. R.; Salamatin, A. N.; Hansen, T. Formation of porous gas hydrates from ice powder: Diffraction experiments and multistage model. J. Phys. Chem. B 2003, 107, 10299–10311.
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Figure 11. Effect of the pressurization rate on the temperature-pressure history of MGH formation and dissociation: (a) slow pressurization (run S1) and (b) rapid pressurization (run R2). The MGH yields (yield IA) of the synthetic samples before dissociation are 25.67 and 11.81% for a and b, respectively. Note that the sample temperature never exceeded the ice melting point for the slow pressurization, although the yield is higher than that for the rapid pressurization.
3.4. Composition of the Synthetic MGH. Gas clathrate hydrates, being non-stoichiometric materials, contain cage vacancies that can in some cases lead to measurably higher molar water contents relative to theoretical formulas, e.g., CH4 3 5.75H2O for structure I methane hydrate.4,43 This study confirmed and quantified the composition of synthetic MGH by direct measurement of the methane released during dissociation, which was performed using seven individual samples under similar experimental conditions but varying the mass of the starting ice (Table 2). The ice samples were first completely converted into MGH, which was verified by the absence of a supercooled freezing signal as the P-T of the
samples passed across the H2O solid-liquid boundary. Compositions of samples were then calculated from the cumulative methane released from MGH during dissociation relative to the mass of the starting ice seeds. The stoichiometric number (n) ranges from 5.81 to 5.82, with an average of 5.82, which indicates nearly perfect saturation of methane in the hydrate, consistent with previous determination using similar methods and run conditions.39,43 3.5. Effects of Ice Grain Size, Degree of Compaction, and Mixing with Sand and Ethanol Additive. The experiments using grain sizes smaller than that used for most experiments (small than 180 μm, labeled runs FG1 and FG2) had considerably lower MGH yield during pressurization. This seems to be in conflict with the conventional thought that a reactant with finer grains, thus possessing a larger surface/volume ratio,
(43) Circone, S.; Kirby, S. H.; Stern, L. A. Direct measurement of methane hydrate composition along the hydrate equilibrium boundary. J. Phys. Chem. B 2005, 109, 9468–9475.
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Figure 12. Effect of the grain size and ethanol additive on the temperature-pressure history of MGH formation and dissociation: (a) smaller ice grains during rapid pressurization (run FG1) and (b) standard ice grains with ethanol additive during rapid pressurization (run E4). The MGH yields (yield IA) of the synthetic samples before dissociation are 2.97 and 90.95% for a and b, respectively. Note that the sample temperature was close to but never exceeded the ice melting point for the ethanol additive run (E4), although the yield was very high.
should react at a faster rate. One possible factor that may retard the rate is a lower permeability of the fine-grained aggregate, which limits the rate of methane penetration into the interior of the aggregate. In comparison to standard runs, it was noticeably harder to load the same mass (54.8 g) of finer ice grains into the holder. The additional force required to pack the ice grains may effectively reduce permeability by either fracturing some of the grains, such that smaller fragments occupy junctions between larger grains, or locally increasing the adhesion of ice grains and contact area at the grain-grain contact surface. Similarly, the standard sample designed with higher compaction (i.e., higher packing density and lower pore volume) also resulted in lower MGH yield (run HD), likely because of low permeability as well. In contrast, ice plus sand aggregates gave higher MGH yields, probably because of higher permeability as a result of less adhesion of the ice grain. The sand/ice grain contacts may also serve as nucleation sites for MGH formation.
In one experiment, ethanol accidentally contaminated the sample by leaking into the reactor from the bath and was found to dramatically enhance the MGH yield after pressurization (stage I). Five experiments (runs E1-E5 in Table 3) with 2 mL of ethanol added to the bottom of the reactor were performed to verify this finding (the presence of residual ethanol was confirmed by opening the reactor immediately after the evacuation in blank experiments). One experiment (run E4), with an initial temperature at 270.2 K, showed that about 90% of the starting ice can be converted into MGH within 600 min (Figure 12b), confirming that trace amounts of ethanol can dramatically enhance MGH formation rates in ice-seeding experiments. The pressure drop of the gas in the reactor during MGH formation (run E4 in Figure 12b) shows that most of the MGH formed within the first 300 min. Except for run E5, the higher the initial temperature used, the more MGH yield, contrary to the trend observed in the absence of ethanol (see section 3.3). The MGH aggregate synthesized in the presence of ethanol 2399
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Figure 13. Dissociation rate of some representative synthetic samples with 100% MGH. The methane accumulation from MGH dissociation for samples (a) F3 and (b) F5 were fitted to the zero-order rate law (light lines for data and dark line for modeling). Samples (c) E4 and (d) S1 were fitted to the first-order rate law.
Figure 14. SEM image of the synthetic MGH aggregate, showing that the morphology of newly formed (not stored in liquid nitrogen for a long time) MGH inherits the granular mosaic texture of the original ice granular aggregate. The hollow of each grain indicates the incomplete replacement of ice grains by MGH. Some pores between original ice gains are filled with MGH, suggesting that some water, in addition to forming the MGH rinds along the original ice grain surfaces, migrated from grain interiors to nearby pore spaces, where it formed MGH “cement”.
was also found to be highly cemented (Figure 5). Our experiments thus demonstrate that a polycrystalline cemented sample at a yield of more than 90% MGH can be manu-
factured in the presence of a small amount of ethanol within a few hours solely by pressurization and without externally heating the ice above its melting point. 2400
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Figure 15. Schematic diagrams showing the formation process of MGH from ice seeds in stage I before heating (see Figure 4): (a) initial ice seed grain, (b) thin rind of MGH formed around the ice grain by reacting methane with partially melting ice as the temperature increases during rapid pressurization, (c) methane continued to react with water to thicken the MGH rind, while the temperature buffered at the ice point (period A), probably in gas-water contact or through a permeable or microfractured MGH rind, and (d) MGH rind continues to form at a much slower rate probably through diffusion after the temperature dropped back to Tinitial (period B).47
temperature, as a result of the endothermic MGH dissociation, was distinct. The time period of the observed temperature deviation corresponds nicely with that of the duration of MGH dissociation as measured by the flowmeter (Figures 11 and 12), further confirming the effect of heat transfer because of the endothermic reaction.
MGH Dissociation Kinetics and Temperature-Time History. Flowmeter measurements show that samples formed by different formation methods often exhibit significantly different rates of dissociation. Our preliminary kinetic modeling revealed that the dissociation rates for those runs (F1-F7) using heating cycles to achieve 100% MGH yields were fitted approximately to a zero-order reaction. The representative runs (F3 and F5) show the rate constants of 1.793 10-5 and 2.595 10-5 s-1, respectively (panels a and b of Figure 13). In contrast, MGH samples synthesized in the presence of ethanol (run E4 in Figure 13c) or with slow pressurization (run S1 in Figure 13d) exhibit a first-order rate law with rate constants of ∼0.7 10-3 and ∼0.4 10-3 s-1, respectively. The zero- and first-order rate laws with similar rate constants were previously reported for the dissociation of sediment-bearing MGH.44 However, other samples with low MGH yield showed significantly different rate laws. The effects on the dissociation rates of the variation of yields, distribution, and cemented of polycrystalline MGH in the samples are currently under investigation. The T-t history during the dissociation of MGH was also monitored. It has been shown that the temperature of the gas in the reactor rapidly drops from its initial temperature to about 190-250 K, depending upon the runs, as soon as the residual methane pressure was vented to 0.1 MPa. The temperature then rose as soon as MGH dissociation began, first very rapidly and then gradually back to its initial temperature over a period of about 100 min. The temperature of the sample only dropped to 240 K before rising again. A significant deviation in the sample temperature toward the lower side relative to the reactor
4. Discussion The results presented above show that, by pressurizing the reaction vessel with methane gas alone, a measurable amount of methane hydrate can be formed even before externally raising the system temperature above the melting point of ice by the heating plate (Figure 10). In the absence of external heating, the rapid temperature rise by pressurization triggered the ice melting and the concurrent MGH formation, followed by continuous warming by self-generated heat from the exothermic MGH formation. The heat released from reacting 1 mol of water to form MGH is 9.07 kJ,32 which is higher than the uptake of heat from melting 1 mol of ice (6 kJ). The positive net heat prevented the sample from dropping back to the system temperature and, instead, maintained it slightly above the ice-freezing temperature for a period of time for further MGH formation. This phenomenon has not been previously recognized. Other factors that may assist the reaction at these conditions include possible microfracturing of the hydrate rind because of volumetric changes associated with the ice-hydrate reaction, as well as the temperature being just within the known “premelting” zone of ice, where H2O molecules on the outer layer of ice grains may form a quasi-liquid layer.45
(44) Kono, H. O.; Narasimhan, S.; Song, F.; Smith, D. H. Synthesis of methane gas hydrate in porous sediments and its dissociation by depressurizing. Powder Technol. 2002, 122, 239–246.
(45) Dash, J. G.; Fu, H.; Wettlauger, J. S. The premelting of ice and its environmental consequences. Rep. Prog. Phys. 1995, 58, 115–167.
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been proposed previously by Henning et al. to describe the growth of CO2 hydrate from deuterated ice at isothermal conditions below the ice point. The formation of MGH proceeds at a faster rate when the methane gas directly contacts and reacts with melting ice. A further methane-water reaction may proceed via the permeable MGH rind (Figure 15) or an incompletely covered grain surface (period A). After liquid water froze as the system temperature decreased below the freezing point, the overall reduction in the formation rate during period B may be attributed to the lack of liquid water or an ice/liquid interface. The methane-ice reaction may take place via the reaction between methane and water vapor sublimated from inner ice cores.47 Therefore, the overall MGH formation rate in stage I is mainly attributed to the reaction of methane with melting ice. Several factors will influence the yield of MGH during stage I, including Tinitial, ice grain size, degree of compaction, and mixing with sand and/or ethanol additive. The more MGH formed during this stage may have resulted in thicker MGH rinds with less residual ice; thus, fewer cycles were needed to achieve complete MGH formation in the subsequent external heating step (stage II). Thicker rinds may also help contain melt within a core from migrating to adjacent pore spaces to form MGH cement. We speculate that a lower residual ice amount may also lower the volumetric increase associated with H2O to hydrate conversion or the volumetric decrease during the melting of ice to water. In turn, the less volumetric change may reduce the chance of hydrate rind fracturing during heating. In contrast, a thinner rind, as forms at higher Tinitial, for example, may be more vulnerable to fracture during subsequent heating, thus allowing more liquid water to migrate to nearby pore spaces and leaving hollow MGH encasements (Figure 16). The description above may provide an explanation to the finding that the hollow shells were typically found in densely packed samples or in dense regions of samples, such as at their base,28 because lower MGH yields were found in the high compaction and high Tinitial runs in this work. It is an interesting and maybe a valuable discovery that trace amounts of ethanol vapor could dramatically enhance MGH formation rates and amount in ice-seeding experiments during stage I. Different from runs without ethanol, the higher Tinitial used, in general, resulted in an increased MGH yield (E1-E5 in Table 3). We also confirmed that those few residual ice cores of higher MGH yield samples could convert almost completely to MGH more easily during stage II with a shorter heating period and lower temperature above the ice point, thus offering potential for more efficient and cost-effective manufacturing of methane hydrate for storage and transport of natural gas. Although we have not yet determined the final distribution of the ethanol in the system following the reaction, we observed no obvious shift of the three-phase equilibrium of MGH formed with or without ethanol. The rate enhancement by ethanol vapor in our experiments is interesting when compared to previous studies that used ethanol as an inhibitor in liquid þ gas þ ethanol conditions by shifting the MGH equilibrium boundaries to higher pressures and lower temperatures.48 On the other hand,
Figure 16. Proposed models showing schematically the formation processes of two MGH aggregates with significantly different cementations. (a) Less cemented product: a thick MGH rind was formed by pressurization under lower initial temperature (see Figure 4) during stage I (before external heating). In this case, during stage II, less water from within the grain migrated to a nearby pore space to react with methane; thus, less MGH or ice cements formed. (b) More cemented product: a thin MGH rind was formed by pressurization under higher Tinitial during stage I. The melt may be liberated to the pore space by breaking the thin MGH rinds during heating, thus forming more MGH or ice cements.
We infer that the rapid and simultaneous rise in pressure and temperature facilitates the formation of the MGH rind around ice grains (Figures 14-16), which prevents liquid water from amassing and flowing to the base of the sample and, thus, keeps the grains in their intact structure (a small amount of water has been suggested previously by Stern et al.28 to migrate or be expelled from the original ice grain cores, but it typically forms hydrate in adjacent or nearby grain junctions). The two distinctly observed formation rates, first fast (period A in Figures 4 and 9) and then very slow (period B), may indicate different formation mechanisms. A two-stage formation model has also
(47) Melnikov, V. P.; Nesterov, A. N.; Reshetnikov, A. M.; Zavodovsky, A. G. Evidence of liquid water formation during methane hydrates dissociation below the ice point. Chem. Eng. Sci. 2009, 64, 1160–1166. (48) Mohammadi, A. H.; Richon, D. Experimental gas hydrate dissociation data for methane, ethane, and propane þ 2-propanol aqueous solutions and methane þ 1-popanol aqueous solution systems. J. Chem. Eng. Data 2007, 52, 2509–2510.
(46) Henning, R. W.; Schultz, A. J.; Thieu, V.; Halpern, Y. Neutron diffraction studies of CO2 clathrate hydrate: Formation from deuterated ice. J. Phys. Chem. A 2000, 104 (21), 5066–5071.
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Yousif et al. pointed out that, although methanol and ethylene glycol are known to suppress hydrates when added in adequate amounts to water, they tend to enhance the rate and amount of hydrate formation when present in small concentrations (ethylene glycol with a concentration of as low as 2 wt % shows a dramatic increase in the gas consumption). The hypothesis proposed is that the presence of alcohol will both intensify the hydrogen bonding with and among the water molecules and encourage hydrate structure production. Although their formation processes are different from ours, this hypothesis may also have relevance to the system with hydrate formation from melting ice grains and in the presence of ethanol vapor. Additionally, in the case of hydrate formation from ice, the possible depression of the ice point may in fact be part of the reason for the enhanced formation rate. A catalytic effect of ethanol on methane hydrate formation was also recently reported by Hao et al.,50 but in that study, MGH was formed from methane þ liquid water in a spraying reactor and not from granular ice under static conditions. A further study is currently ongoing to investigate the catalytic mechanism of ethanol and the physical properties of the resulting MGH.
follows: (1) In the ice-seeding method, nearly 13% of ice can be converted to methane hydrate during period A by pressurization of methane gas alone. (2) The MGH yields during pressurization varied with the initial temperatures (Tinitial) as well as grain size and porosity of the sample. The addition of trace amounts of ethanol dramatically enhanced the formation of the polycrystalline MGH aggregate. (3) The more MGH formed during stage I, the faster the residual ice or water converted into MGH in the subsequent heating, thus requiring a lower heating temperature, shorter time, and less repeated cycles to complete the MGH formation. (4) The more MGH formed in stage I, the thicker the MGH shell and the less the hydrate grains cemented/annealed together during subsequent heating, leading to less cementation of the synthetic aggregates. The extent of the cementation increased if synthesis was completed in repeated cycles of heating. (5) A variety of MGH dissociation rates for the synthetic polycrystalline MGH, which were fit to kinetic models, including zero and first order, were found for MGH synthesized under diverse conditions at yields of 100% or less. (6) This study has demonstrated that aggregate samples with different structural characteristics can be manufactured by properly controlling the synthetic procedures. Our results provide a few optimal growth parameters for synthesizing polycrystalline MGH aggregate samples, making a variety of samples with different modes of hydrate distribution suitable for various laboratory tests.
5. Conclusions The present study has unveiled some important observations during pressurization of methane gas that have improved our understanding of MGH formation processes and provided a guidance to more efficiently synthesize the granular MGH aggregate for a variety of purposes. The major results derived from the experiments can be summarized as
Acknowledgment. The experiments were carried out at the Gas Hydrate Laboratory at the Central Geological Survey, Taiwan. We thank Dr. Stephen Kirby (USGS) for consultation on experiments, John Pinkston (USGS) for technical assistance, and Dr. I-Ming Chou (USGS) for reviewing the manuscript. The research was mainly supported by the gas hydrate research fund of the Central Geological Survey (to Po-Chun Chen) and partially by the Earth Sciences Section, National Sciences Council (NSC) of the Republic of China (NSC Grant 95-2116-M-002010-MY3 to W.-L. Huang).
(49) Yousif, M. H. Effect of underinhibition with methanol and ethylene glycol on hydrate-control process. SPE Prod. Facil. 1998, 13, 184–189. (50) Hao, W. F.; Sheng, W.; Fan, S. S.; Wang, J. Q. Experimental investigation of methane hydrate formation in a spraying reactor. J. Wuhan Univ. Technol., Mater. Sci. Ed. 2007, 29, 39–43.
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