Gas Hydrate Formation in a Variable Volume Bed of Silica Sand Particles

5496r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 5496–5507 : DOI:10.1021/ef900542mPublished on Web 09/28/2009

Gas Hydrate Formation in a Variable Volume Bed of Silica Sand Particles

Praveen Linga,† Cef Haligva,† Sung Chan Nam,‡ John A. Ripmeester,§ and Peter Englezos*,†

†Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3,Canada, ‡Energy Conversion Research Department, Korea Institute of Energy Research, Yuseong-gu, Daejeon 305-343, Korea,

and §Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada

Received May 27, 2009. Revised Manuscript Received September 8, 2009

Gas hydrate formation was studied in a new apparatus designed to accommodate three different sizevolume beds of silica sand particles. The sand particles have an average diameter equal to 329 μm. Thehydrate was formed in the water, which occupied the interstitial space of the water-saturated silica sandbed. A bulk gas phase was present above the bed (gas cap). Gas uptake measurements were carried outduring experiments at constant temperature. More than 74.0% of water conversion to hydrate wasachieved in all experiments conducted withmethane at 4.0 and 1.0 �C. An initial slow growth was followedby a rapid hydrate growth rate of equal magnitude for nearly all experiments until 43-53% of water wasconverted to hydrate. During the third and final growth stage, the final conversions were between 74 and98%and the conversion dynamics changed. Independent verification of hydrate formation in the sandwasachieved viaRaman spectroscopy andmorphology observations in experiments using the same sand/watersystem.

1. Introduction

Natural gas hydrates are non-stoichiometic, crystalline,inclusion compounds.1,2 They are known to occur naturallyin the earth.3 Potential reserves of natural gas hydrates areover 1.5� 1016m3 and are distributed all over the earth, in theArctic on land and offshore.4-8 A reliable assessment of thefeasibility of producing natural gas from the earth’s naturallyoccurring hydrates requires reservoir-specific information aswell as data, such as kinetics of hydrate formation anddissociation obtained in the laboratory.9 Only a few papersin the literature present laboratory data on the dynamics of

hydrate formation and decomposition (kinetics) investigatedin porous media. Spangenberg10 highlighted the urgent needto obtain laboratory data on hydrate formation in poroussediments. Melnikov and Nesterov11 studied the kinetics ofpropane hydrate formation in porousmedia and reported thatwater migration happens to the hydrate formation front.Chuvilin et al.12,13 reported that only a part of the ground-water present in the sediments transforms to hydrate and therest of the water freezes at subzero temperatures. Winterset al.14 concluded that methane hydrate cements unconsoli-dated Ottawa sand grains. A hydrate fraction of 0.35 isthought to be required for cementation.15 Kneafsey et al.9

studied hydrate formation in the methane-water system inthe presence of silica sand. They employed an X-ray-trans-parent pressure vessel, in which they monitored pressure,temperature, and the local density changes through X-ray-computed tomography. They observed that the rate of hy-drate formation is not alwaysproportional to the driving forcein the porous medium. They also concluded that there is aneed for multiple means of measurement, which is critical for

*To whom correspondence should be addressed. Telephone: 1-604-822-6184. Fax: 1-604-822-6003. E-mail: [email protected].(1) Davidson, D. W. Gas hydrates. In Water: A Comprehensive

Treatise; Plenum Press: New York, 1973; Vol. 2, pp 115-234.(2) Sloan, E. D., Jr. Clathrate Hydrates of Natural Gases, 2nd ed.;

Marcel Dekker: New York, 1998; p 754.(3) Makogon, Y. F.; Trebin, F. A.; Trofimuk, A. A.; Tsarev, V. P.;

Cherskii, N. Detection of a pool of natural gas in a solid (hydrated gas)state. Dokl. Akad. Nauk SSSR 1971, 196, 203–206.(4) Kvenvolden,K. Potential effects of gas hydrate on humanwelfare.

Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 3420.(5) Milkov, A. V.; Dickens, G. R.; Claypool, G. E.; Lee, Y. J.;

Borowski, W. S.; Torres, M. E.; Xu, W. Y.; Tomaru, H.; Trehu,A. M.; Schultheiss, P. Co-existence of gas hydrate, free gas, and brinewithin the regional gas hydrate stability zone at hydrate ridge (Oregonmargin): Evidence from prolonged degassing of a pressurized core.Earth Planet. Sci. Lett. 2004, 222 (3-4), 829–843.(6) Kvenvolden, K. A. Gas hydrates;Geological perspective and

global change. Rev. Geophys. 1993, 31 (2), 173–187.(7) Makogon, Y. F.; Holditch, S. A.; Makogon, T. Y. Natural gas

hydrates;A potential energy source for the 21st century. J. Pet. Sci.Eng. 2007, 56 (1-3), 14–31.(8) Klauda, J. B.; Sandler, S. I. Global distribution of methane

hydrate in ocean sediment. Energy Fuels 2005, 19 (2), 459–470.(9) Kneafsey, T. J.; Tomutsa, L.; Moridis, G. J.; Seol, Y.; Freifeld,

B. M.; Taylor, C. E.; Gupta, A. Methane hydrate formation anddissociation in a partially saturated core-scale sand sample. J. Pet. Sci.Eng. 2007, 56 (1-3), 108–126.(10) Spangenberg, E.Modelingof the influenceof gas hydrate content

on the electrical properties of porous sediments. J. Geophys. Res., [SolidEarth] 2001, 106 (B4), 6535–6548.

(11) Melnikov, V.; Nesterov, A.Modelling of gas hydrates formationin porous media. In the 2nd International Conference on Gas Hydrates,Toulouse, France, June 2-6, 1996; pp 541-548.

(12) Chuvilin, E. M.; Kozlova, E. V.; Makhonina, N. A.; Yakushev,V. S.; Dubinyak, D. V. Peculiarities of methane hydrate formation/dissociationP/T conditions in sediments of different composition. In the4th InternationalConference onGasHydrates,Yokohama, Japan,May19-23, 2002; pp 433-438.

(13) Chuvilin, E. M.; Makhonina, N. A.; Titenskaya, O. A.; Boldina,O. M. Petrophysical investigations of frozen sediments artificiallysaturated by hydrate. In the 4th International Conference on GasHydrates, Yokohama, Japan, May 19-23, 2002; pp 734-739.

(14) Winters, W. J.; Pecher, I. A.; Waite, W. F.; Mason, D. H.Physical properties and rock physics models of sediment containingnatural and laboratory-formed methane gas hydrate. Am. Mineral.2004, 89 (8-9), 1221–1227.

(15) Berge, L. I.; Jacobsen,K.A.; Solstad,A.Measured acousticwavevelocities of R11 (CCl3F) hydrate samples with and without sand as afunction of hydrate concentration. J. Geophys. Res., [Solid Earth] 1999,104 (B7), 15415–15424.

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Energy Fuels 2009, 23, 5496–5507 : DOI:10.1021/ef900542m Linga et al.

understanding hydrate behavior during hydrate formation/decomposition.Katsuki et al.16 reported that themorphologyof methane hydrate crystals formed in the porous mediumfilled with the methane-saturated liquid water depends uponthe magnitude of the mass transfer of the methane moleculesin the liquid water. Liu et al.17 reported that the methanehydrate formed in silica sand had similar characteristicsregarding cage occupancy and hydration number to bulkhydrate, indicating no influence of the particle size on hydratecomposition. The particle sizes of the sands studiedwere in therange of 53-75, 90-106, 106-150, and 150-180 μm.Recently, Zhou et al.18 presented hydrate formation data inthe water-saturated silica sand (particle size of the sand in therange of 100-500 μm) matrix conducted at 2.2 �C and 4137kPa in a 72L vessel. The authors reportedwater conversion tohydrate of 11% for the formation experiment.18

One of the aspects that was not examined in previouslaboratory studies with synthetic hydrate samples in porousmedia or samples recovered from the earth’s porous sedimentsis the impact of the sample size on the ratewithwhich the gas isconsumed during hydrate formation or released from thesample during thermal stimulation or depressurization. Itshould be noted, however, that Handa19 briefly discussedthe sample size of a naturally occurring hydrate in the contextof calorimetric measurements. Such information is useful ifthe collected data are to be used to test the validity ofnumerical simulation schemes for hydrate dissociation. Theobjective of this work is to study the kinetics of hydrateformation in a newapparatus, whichwas specifically designedto investigate the potential dependency of the results on thesize of the silica sand bed. Another unique feature of theapparatus is the placement of six thermocouples to monitorthe temperature in several locations in the bed.Understandinghydrate formation in sand sediments is important especially inview of the fact that drilling and seismic observations ofnaturally occurring hydrates point to preferred hydrateformation in sand-rich sediments.20

2. Experimental Section

UHP-grade methane (Praxair Technology, Inc.), silica sandwith an average diameter of 329 μm (diameter ranges from 150 to630 μm) supplied by Sigma-Aldrich (catalogue number 274739),and distilled and deionized water were used. Brunauer-Emmett-Teller (BET) surface area analysis of the sand showedthat the BET surface area of the sand used in this work is0.3499 cm2/g and that the sand has micropores with a specificpore volume of 0.000 152 cm3/g and an average pore diameter of0.90 nm. The pore volume and pore diameter were calculated

using the Horvath-Kawazoe method.21 Scanning electronmicroscopy (SEM) was performed using a Hitachi S-3000Nmicroscope. The contact angle of the sand was measured by firstgrinding the sand to fine powder and then pelletizing by applying∼100MPa pressure. The thickness of the pellet is approximately1 mm.

2.1. Apparatus. Figure 1 shows the schematic of the appara-tus. It consists of a crystallizer (CR), which is a cylindrical vessel(inner diameter, 10.16 cm; height, 15.24 cm) made of 316stainless steel. The thickness of the crystallizer wall is 3.81 cm.It has a volume of 1236 cm3. There is a reservoir (R) with avolume of 1000 cm3 that collects gas released during thedecomposition experiments. The crystallizer and reservoir areimmersed in a temperature-controlled water bath. The tempera-ture of the water bath is controlled by an external refrigerator(model 1187P, VWR Scientific). Two Rosemount smart pre-ssure transmitters (model 3051, Norpac controls, Vancouver,British Columbia, Canada) are employed for pressure measure-ment, with a maximum uncertainty of 0.075% of the span(0-15 000 kPa), i.e., 11 kPa. The temperatures of the hydrateand gas phases of the crystallizer were measured usingOmega (Omega Engineering, Stamford, CT) copper-constantanthermocouples (1/16th in diameter, stainless-steel shielded) withan uncertainty of 0.1 K. Seven thermocouples were located inthe crystallizer, with one in the gas phase and six in the silicasand bed. The data acquisition system (National Instruments) iscoupled with a computer to record the data as well as tocommunicate with the control valve during the experiment,and the software used for this purpose is LabView 8.0(National Instruments). The apparatus is also equipped with asafety pressure valve.

Figure 2a shows the location of the seven thermocouples thatwere placed in the crystallizer. To study the effect of the variablevolume of the silica sand bed on hydrate formation anddecomposition, two copper cylinders are placed inside thecrystallizer, as shown in Figure 2b. The reason that we chosecopper cylinders to place in the hydrate vessel and decrease thesample size is because heat-transfer resistance through copper(copper thermal conductivity = 400 W m-1 K-1 22) is mini-mized. Both copper cylinders fit precisely in the crystallizerwhen placed, as indicated in Figure 3. The first copper cylinder(CC1) has a diameter of 7.62 cm (3 in.), and the second one (CC2)has a diameter of 5.08 cm (2 in.). The wall thickness of bothcylinders is 1.27 cm (0.5 in.). It is noted that only four thermo-couples could be accommodated in the silica bed with both CC1

and CC2 present in the crystallizer (Figure 2b).2.2. Procedure. The amount of silica sand placed in the

crystallizer for each experiment was 914.1 g. The height of thesilica sand bed was 7.0 cm. The porosity of the sand bed wasdetermined to be 0.35. The volume of water required to fill thevoid space with water (100% water saturation) was found to be0.217 cm3/g, which is the interstitial or pore volume of the bed ofsand particles. Accordingly, 198.5 mL of water was added to thesand.

The procedure for the formation experiment was the samewhen one (CC1) or two copper cylinders (CC1 þ CC2) wereplaced in the crystallizer. The height of the silica sand bed wasfixed the same as 7.0 cm. Hence, the amount of the sand placedin the crystallizer was reduced to 513.7 and 228.5 g when one(CC1) or two (CC1 þ CC2) copper cylinders were present,respectively. The bedwas set up by splitting the required amountof sand and water into five equal parts and placing each in abatch order (sand þ water) to form a uniform bed and also toeliminate the presence of any pockets of air in the bed.

(16) Katsuki, D.; Ohmura, R.; Ebinuma, T.; Narita, H. Methanehydrate crystal growth in a porous medium filled with methane-satu-rated liquid water. Philos. Mag. 2007, 87 (7), 1057–1069.(17) Liu, C. L.; Lu, H. L.; Ye, Y. G.; Ripmeester, J. A.; Zhang, X. H.

Raman spectroscopic observations on the structural characteristics anddissociation behavior of methane hydrate synthesized in silica sandswith various sizes. Energy Fuels 2008, 22 (6), 3986–3988.(18) Zhou, Y.; Castaldi, M. J.; Yegulalp, T. M. Experimental inves-

tigation of methane gas production from methane hydrate. Ind. Eng.Chem. Res. 2009, 48 (6), 3142–3149.(19) Handa, Y. P. A Calorimetric study of naturally occurring gas

hydrates. Ind. Eng. Chem. Res. 1988, 27 (5), 872–874.(20) Riedel, M.; Collett, T.; Malone, M.; Akiba, F.; Blanc-Valleron,

M.; Ellis,M.; Guerin,G.; Hashimoto, Y.; Heuer, V.; Higasi, Y.; Holland,M.; Jackson, P.; Kaneko, M.; Kastner, M.; Kim, J.-H.; Kitajima, H.;Long, P.; Malinverno, A.; Myers, G.; Palekar, L.; Pohlman, J.;Schultheiss, P.; Teichert, B.; Torres, M.; Tr�ehu, A.; Wang, J.;Wortmann, U.; Yoshioka, H. Gas hydrate transect across northernCascadia margin. Eos, Trans. Am. Geophys. Union 2006, 87 (33), 325.

(21) Horvath, G.; Kawazoe, K. Method for the calculation of effec-tive pore-size distribution in molecular-sieve carbon. J. Chem. Eng. Jpn.1983, 16 (6), 470–475.

(22) Incropera, F. P.; DeWitt, D. P. Fundamentals of Heat and MassTransfer, 5th ed.; John Wiley and Sons, Inc.: New York, 2002.

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Once the crystallizer bed was set up, the thermocouples werepositioned and then the crystallizer was closed. To eliminate thepresence of any air bubbles in the system, the crystallizer waspressurized with methane to 2.5 MPa and depressurized toatmospheric pressure 3 times. The pressure in the crystallizerwas then set to the desired experimental pressure (8.0MPa), andthe temperature was allowed to reach the target temperature(approximately 5min). The pressure and temperature profiles ofthe crystallizer during this step for a typical formation experi-ment are shown in Figure 4. Once the pressure and temperatureof the system reached the experimental condition, this time wasrecorded as time zero for the formation experiment. Data wasthen logged in the computer every 20 s. All hydrate formationexperiments were carried out with a fixed amount of water andgas (closed system). The temperature in the crystallizer wasmaintained constant by an external refrigerator. When hydrateformation occurs, gas will be consumed and, hence, the pressurein the closed system drops. Typically, the experiment wasallowed to continue until there was no significant change inthe rate of the pressure drop in the crystallizer. The time that theexperiment finished is reported in Table 1. It is noted that eachformation experiment was conducted with fresh water and freshsand.

Calculation of the Amount of Gas Consumed during Hydrate

Formation. Pressure and temperature data were used to calcu-late the moles of methane consumed in the crystallizer (gasuptake). At any given time, the total number of moles of

methane in the system (CR and the connecting tubing) remainsconstant and equal to that at time zero. Thus, the number ofmoles of the gas that has been consumed for hydrate formationat time t = t is given by the equation

nH, t -nH,0 ¼ nG, 0 -nG, t ð1Þor

ΔnH, V ¼ nH, t -nH,0 ¼ PV

zRT

� �G,0

-PV

zRT

� �G, t

ð2Þ

where z is the compressibility factor calculated by Pitzer’scorrelation.23 On the basis of the number of moles of methaneconsumed (given by eq 2) and assuming a hydration numberequal to 6.1,24 the conversion ofwater to hydratewas calculated.

The rate of hydrate formation was calculated every 5 min byforward differentiation

dn

dt

� �t

¼ ntþΔt -nt

Δt, Δt ¼ 5 min ð3Þ

Subsequently, the average rate for every 30 min period wascalculated.

Figure 1. Apparatus.

(23) Smith, J. M.; Van Ness, H. C.; Abbott, M. M. Introduction toChemical Engineering Thermodynamics; McGraw-Hill, Inc.: New York,2001.

(24) Tulk, C. A.; Ripmeester, J. A.; Klug, D. D. The application ofRaman spectroscopy to the study of gas hydrates. Gas Hydrates 2000,912, 859–872.

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2.3. Procedure for Hydrate Characterization with Raman

Spectroscopy.Hydrate was synthesized in a glass vial kept insidea stainless-steel pressure vessel at 4.0 �C and 8.0MPa for 4 days.A straight glass vial having an outer diameter of 20 mm and awall thickness of 2 mm was filled with sand (20 g) and water(4.34 mL) and was placed along the vertical axes of an emptypressure vessel. The vessel was pressurizedwith themethane anddepressurized at a pressure below the equilibrium hydrateformation pressure 3 times to eliminate the presence of any airbubbles in the system.Hydrate was allowed to grow for 4 days ata desired temperature and pressure andwas recovered at a liquidnitrogen temperature for further analysis by Raman spectro-scopy. Three different portions (top, middle, and bottom sec-tions along the vertical axis) of the sand-hydrate matrix werechosen for Raman spectroscopic analysis. Each portion wascrushed with mortar and pestle in liquid nitrogen, and thehydrate was separated from silica sand for spectroscopic analy-sis. Hydrate was analyzed at atmospheric pressure and liquidnitrogen temperature with Raman spectroscopy. An ActonRaman spectrograph with fiber optics and equipped with a1200 grooves/mm grating and a charge-coupled device (CCD)detector was used in this study. An Ar-ion laser was used as theexcitation source, emitting at 514.53 nm. The laser was focusedon the sample by a 10�microscope objective. The spectrograph

was controlled with a computer, and the spectra were recordedwith a 1 s integration time over 50 scans.

2.4. Visualization of Hydrate Formation in the Sand þ Water

Matrix.A smaller scale morphology experiment was conductedto visualize hydrate formation within the sand/water matrix.This experiment was carried out in another apparatus, which isexplained in detail by Lee et al.25 26.0 g of silica sand was placedin a small glass vial. Accordingly, 5.65 mL of water was addedinto the sand in this case. Next, the vial was placed in thecrystallizer and pressurized with methane up to 8.0 MPa. Thetemperature of the system was kept constant at 4.0 �C. Thepressure of the system was continuously monitored. Gas con-sumed and water-hydrate conversion were calculated as in theother experiments. During this experiment, pictures were takenat several time intervals. ANikonD40 camera was connected toa Nikon (SMZ-2T, Japan) microscope for taking pictures.

3. Results and Discussion

Table 1 summarizes the hydrate formation experimentalconditions and results, indicating the induction time, amount

Figure 2.Cross-section of the crystallizer showing the (a) location of the thermocouples within the crystallizer without the copper cylinders and(b) arrangement of the copper cylinders (CC1 and CC2) in the crystallizer and the location of the thermocouples.

(25) Lee, J. D.; Susilo, R.; Englezos, P. Kinetics of structure H gashydrate. Energy Fuels 2005, 19 (3), 1008–1015.

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of methane consumed, and water conversion achieved. Asseen in Table 1, the water conversion ranged from 74.1 to82.4% for the experiments carried out at 1 and 4 �C in the full-size CR. However, the water-hydrate conversion at 7.0 �C is10.5 or 11%. This is likely due to the smaller driving force(deviation of pressure from equilibriumpressure). The drivingforce is 2.66, 4.15, and 5.09 MPa for the experiments at 7, 4,and 1 �C, respectively. On the basis of these observations, atemperature of 4.0 �C was selected as the experimentaltemperature for the experiments that were carried out withthe variable sand bed size (CC1 and CC1 þ CC2). This wasdecided to have a significant amount of hydrate in the cell forthe decomposition experiments that ensued.26 Finally, theuncertainties in the mole numbers (because of the uncertain-ties in the measurement of pressure and temperature) were

calculated and found to be less than 4.0% for all of theexperiments.

Gas Uptake and Temperature Profiles. Figure 5 shows thegas uptake measurement curve (moles of methane consumedper mole of water in the system) of a formation experimentcarried out at 7.0 �C along with the temperature profiles ofthe thermocouples located inside the bed. The data duringthe first 5 h are also shown in the expanded graph. Duringhydrate formation, heat is released, which can be seen in thefigure at 2.17 h (130 min). This time marks the nucleation orturbidity point. As seen in the expanded portion of the graph,hydrate nucleation occurs at approximately the same time(within 3.3 min) in the six thermocouple locations (moreprecisely, T5 and T6 at 130 min (2.17 h), T4 at 131.3 min(2.19 h), T3 at 132.0 min (2.20 h), and T1 and T2 at 133.3 min(2.22 h), respectively). The intensity of the observed peak canbe attributed to hydrate formation. In this case, all peaksappearwithin 3.3min and it is reasonable to assume that theyare primarily influenced by the heat released from hydrateformation. The CR is immersed in the constant temperature-controlled bath, and thus, the temperature of the system isgradually restored to its set point. Interestingly, there is no

Figure 4. Temperature and pressure profile of a typical experiment(experiment 11) before the start of the formation experiment. Stage1 represents the gas-filling stage, where by the gas is filled from the gascylinder to the crystallizer to reach Pexp (8.0 MPa). Stage 2 representsthe time required for the temperatures at different locations of the bedto reach the experimental temperature again (4.0 �C).

Table 1. Hydrate Formation Experimental Conditions along with Induction Times, Methane Consumed for Hydrate Formation, and Water

Conversion to Hydratea

end of the experiment

systemexperimentnumber

Texp

(�C)induction time

(first nucleation point) (min) time (h)CH4 consumed(mol/mol of H20)

water conversion tohydrate (mol %)

CH4/silica sand/water (CR) 1 7.0 130.0 92.5 0.0172 10.22 7.0 206.7 100.6 0.0180 11.03 4.0 18.3 71.6 0.1287 78.54 4.0 69.7 48.6 0.1261 77.05 4.0 61.3 29.0 0.1266 77.26 4.0 644.3 29.0 0.1215 74.17 1.0 123.7 65.3 0.1308 79.88 1.0 87.7 95.4 0.1350 82.4

CH4/silica sand/water (CR þ CC1) 9 4.0 546.3 61.2 0.1288 78.510 4.0 8.7 24.0 0.1244 75.911 4.0 40.3 24.0 0.1315 80.212 4.0 63.7 40.0 0.1381 84.3

CH4/silica sand/water (CRþ CC1 þ CC2) 13 4.0 11.0 46.9 0.1261 76.914 4.0 7.3 41.7 0.1410 85.915 4.0 8.0 46.7 0.1238 75.516 4.0 7.3 34.7 0.1603 97.8

aThe initial experimental pressure for all of the experiments was 8.0 MPa.

Figure 3. Arrangement of copper cylinders inside the crystallizer.

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substantial hydrate formation activity after the first 3 h,which is also manifested by the low (10.5%) water conver-sion to hydrate.

While all six thermocouples during the experiment at 7 �Cindicate a single nucleation time (at approximately 130min),the results at 4 �C from experiment 3 (Figure 6) show severalnucleation events occurring at different times in the experi-ment. The first temperature increase can be seen in section Ain the figure. In section B, the temperature increase is notlocalized and there is a larger consumption of gas for hydrateformation in that time period, while there are severaltemperature spikes in other time periods, which indicate that

nucleation sites are localized throughout the bed volume, ascan be seen in sections C, D, and F in the figure. Hence,hydrate formation happens at different locations at differenttimes and different rates (as seen in the insets A, B, C, D,and F in Figure 6). This multiplicity of hydrate nucleationsites appearing at different experimental times is perhaps thereason why a much higher percent of water converts tohydrate at 4.0 and 1.0 �C compared to the experimentscarried out at 7.0 �C. Kneafsey et al.9 formed methanehydrate at 6.2 MPa and 1.1 �C in the presence of silicasand with a particle size of 100-200 μm and reported aconversion of water to hydrate of at least 65% (using ahydration number of 5.75). They observed the temperaturein four locations inside the sand bed and reported a tempe-rature profile similar to that in Figure 6 but with someminorspikes afterward. They concluded that hydrate formationdoes not proceed at a constant rate as we also observed.

Experiments 11 and 13 were carried out at 4.0 �C with thepresence of one (CR þ CC1) and both copper cylinders (CRþ CC1 þ CC2), respectively. The uptake curves and thetemperature profiles are shown in Figure 7. For experiment11, one can see several temperature spikes during the hydrateformation experiment similar to the experiments conductedwithout copper cylinders at 4.0 �C. Interestingly, the secondhydrate formation event is much stronger (higher growthand higher temperature increase), similar to what was ob-served in Figure 6. On the other hand, a comparison ofFigures 6 and 7 also indicates that, as the bed size becomessmaller, the degree of temperature increase because of multi-ple nucleation and hydrate formation becomes smaller des-pite hydrate formation evident from the gas consumption.The conversion of water to hydrate for the three experimentspresented in Figures 6 and 7 are 78.5% (experiment 3),80.2% (experiment 11), and 76.9% (experiment 13). In ouropinion, as we decrease the silica sand bed with coppercylinders, heat is removed faster from the crystallizer to thewater bath and, as a result, the spikes become smaller. Thesame trend was observed for all of the other formationexperiments. As can be seen from the profiles of T3 and T4

Figure 6. Gas uptake measurement curve at 4.0 �C (experiment 3). The expanded graphs are for the regions A, B, C, D, and F, respectively,indicating hydrate formation happening at different locations at different times and different rates.

Figure 5. Gas uptake measurement curve at 7.0 �C (experiment 1).

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in Figure 7 (experiment 11), the activity of hydrate formationis different at these two regions in the crystallizer, althoughthey are at the samedistance from the cylindrical wall and thetop of the bed. The same observation applies to T1 and T2 aswell. Similar observations were made for all of the otherexperiments and suggest that there is spatial heterogeneity inhydrate formation. The observed temperature profiles at4 and 1 �C are then interpreted as instances of multiplenucleations, which do not appear in a symmetric fashion inthe bed. One has to note, however, that the response of aparticular thermocouple as a function of time could beattributed to nucleation or hydrate formation in its vicinityand also as a result of heat transfer from another location ofhydrate formation. To explore this further, the profiles fromspecific thermocouples are discussed.

Figure 8 shows the temperature profile for thermocouples5 and 6 from experiment 3. The thermocouple tips are locatedin the same plane (Figure 2a). Insets A and B show expandedregions of the temperature profiles. InA, there is a temperatureincrease for T5 and T6 at the same time, indicating that thisshould be due to hydrate formation at both locations. How-ever, the extent of increase is different. In B, there is a delay inthe increase of T6 as compared to T5, which could be possiblebecause of hydrate formation in the vicinity of T6 and the heattransfer from T5 to T6. However, the latter effect would besmall compared to hydrate formation. The temperature at T6

was calculated on the basis of heat conduction froma source atT5, and it was found that the maximum temperature increasewas 0.22 �C, which is less than that observed at T6 in Figure 8(inset B). The thermal conductivities of hydrate27 and water22

are similar 0.5 and 0.58Wm-1 K-1, respectively, whereas thatof quartz (sand) is an order of magnitude larger and that ofmethane is an order of magnitude smaller.9

Figure 9 shows the temperature profile for thermocouples5 and 6 during experiment 5. Insets A and B show expandedregions of the profiles. In A, there is a temperature increasefor T5 and T6 at the same time, indicating that this should bedue to hydrate formation at both locations. However, theextent of the increase is different, indicating differentamounts of hydrate formed. In B, one can see two spikes(“1” and “2”). The temperature increase at both thermo-couples in region 1 is simultaneous, indicating that hydrateformation at two different sites is occurring at the same time.In region 2, T5 increases first and T6 increases at a later time,but the extent of temperature increase inT6 is higher thanT5,which indicates that this is not due to heat transfer only.There is a larger effect as a result of hydrate formation in theregion at T6.

The occurrence of multiple nucleation sites can also beseen if one plots the rate of gas uptake as a function of time.Figure 10 shows the rate from experiment 3. Note that timezero in this plot corresponds to the first nucleation point(given in Table 1). As seen, the rate decreases after the firstinstance of hydrate formation (nucleation point) and is thehighest at the second instance of hydrate formation and thenthere are sporadic appearances of spikes in hydrate growth,which we interpret as new nucleation sites. The curves forthe other experiments at 4 and 1 �Calso show intense hydrategrowth followed by sporadic instances of increased hydrategrowth activity, leading to a final water conversion in thesame range. A similar observation was made for the otherexperiments as well as by Kneafsey et al.9

Figures 11, 12, and 13 show the rates of uptake for all ofthe experiments at 7, 4, and 1 �C, respectively. As seen,the two experiments at 7 �C show similar rates of uptake,which is not surprising from the fact that a single hydrateformation event was observed from the temperature and gasuptake profiles (Figure 5). However, in the other curves, therates cannot be superimposed. It would be difficult if notimpossible to model this behavior, owing to the randomcharacter of the appearance of the second and subsequentnucleation sites. This is in agreement with observationsthrough magnetic resonance imaging on water droplets.28

As pointed out by Moudrakovski et al.28 in quiescentsystems, the stochastic nature dominates. This is contraryto what happens in a well-stirred vessel, where because ofagitation the first nucleation acts as a hydrate seed generatorthat eventually causes nucleation throughout the bulk of thesystem. In such cases, where one has good control of thehydrodynamic conditions, the rate of hydrate growth can becorrelated.29-32

Figure 7.Gasuptakemeasurement curveswith thepresenceof one cop-per cylinder (experiment 11) and two copper cylinders (experiment 13).

(26) Linga, P.; Haligva, C.; Nam, S. C.; Ripmeester, J. A.; Englezos,P. Recovery of Methane from Hydrate Formed in a Variable VolumeBed of Silica Sand Particles. Energy Fuels 2009, DOI:10.1021/ef900543v.(27) Cook, J. G.; Leaist, D. G. An exploratory study of the thermal

conductivity of methane hydrate. Geophys. Res. Lett. 1983, 10 (5), 397–399.

(28) Moudrakovski, I. L.;McLaurin,G.E.;Ratcliffe, C. I.; Ripmeester,J. A. Methane and carbon dioxide hydrate formation in water droplets:Spatially resolved measurements frommagnetic resonance microimaging.J. Phys. Chem. B 2004, 108 (45), 17591–17595.

(29) Englezos, P.; Kalogerakis, N.; Dholabhai, P. D.; Bishnoi, P. R.Kinetics of formation of methane and ethane gas hydrates. Chem. Eng.Sci. 1987, 42 (11), 2647–2658.

(30) Englezos, P.; Kalogerakis, N.; Dholabhai, P. D.; Bishnoi, P. R.Kinetics of gas hydrate formation frommixtures ofmethane and ethane.Chem. Eng. Sci. 1987, 42 (11), 2659–2666.

(31) Clarke, M. A.; Bishnoi, P. R. Determination of the intrinsickinetics of CO2 gas hydrate formation using in situ particle size analysis.Chem. Eng. Sci. 2005, 60 (3), 695–709.

(32) Hashemi, S.; Macchi, A.; Servio, P. Gas hydrate growth model in asemibatch stirred tank reactor. Ind. Eng. Chem. Res. 2007, 46 (18), 5907–5912.

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An alternative but equivalent way of presenting the gasuptake is to plot the associated water conversion to hydrate.

Figure 14 shows the water conversion for all experimentswith the full-size bed. As seen at 1 and 4 �C, the water

Figure 9. Temperature (T5 and T6) data from experiment 5.

Figure 8. Temperature (T5 and T6) data from experiment 3.

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conversions are within the 74.1-82.4% range as opposed tothat at 7 �C, which is approximately 10.5%. Thus, it isnoteworthy that, despite the difference in the dynamics ofhydrate growth, the final water conversions at a particulartemperature are similar (Table 1).

Visualization and Characterization of Hydrate Formation

Inside the Bed. The temperature profiles provide indirectevidence for hydrate formation within the bed. Independentconfirmation for hydrate formation was obtained byRamanspectroscopy. It is noted that the hydrate samples were

prepared separately with the same sand and sand/water ratiofor the Raman spectroscopy analysis. Figure 15 showstypical Raman spectra of the C-H (2800-3000 cm-1) andO-H (3000-3400 cm-1) regions of the methane hydratesample. The peak doublet at approximately 2906 and 2917cm-1 indicates CH4 molecules encaged in large and smallcages, respectively. Two large, broad peaks at approximately3090 and 3220 cm-1 are due to the internal vibrationalmodesof water molecules making up the host lattice of the hydratestructure.24 Additionally, a minor band for methane waspresent at about 3050 cm-1, indicative of methane in thehydrate phase.33 Raman spectroscopy performed on differ-ent parts (or locations) of the sample confirmed the presenceof hydrate in the sand-hydrate matrix. Also, on the basis ofvisual observation of the hydrate sample when recovered forRaman analysis, the hydrate was found to exist everywherein the sand-hydrate matrix along the vertical axis.

The hydrate is expected to form in the interstitial space.However, this study did not make any attempt to delineatethe pore-scale distribution of the hydrate. The latter hasbeen discussed in the literature mainly through conceptual

Figure 10. Methane uptake curve and the average rate of methaneuptake (experiment 3).

Figure 11. Comparison of the rate of methane uptake per mole ofwater at 7.0 �C.



Figure 14.Water conversion to hydrate (%) for all of the formationexperiments conducted with the full-size bed.

(33) Kumar, R.; Linga, P.; Moudrakovski, I.; Ripmeester, J. A.;Englezos, P. Structure and kinetics of gas hydrates from methane/ethane/propane mixtures relevant to the design of natural gas hydratestorage and transport facilities. AIChE J. 2008, 54 (8), 2132–2144.

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models. Waite et al.34 reported that hydrates formed in thelab from unsaturated systems cement the sand grains, whileWinters et al.14 considered hydrates extracted from a naturalsite are part of a solid frame of the sediment. Clennell et al.35

discussed a conceptual model and concluded that the for-mation of interstitial hydrate occurs in coarse sedimentlayers and, with high gas flux, concentrated gas hydrate willform and water depletion may occur. Kleinberg et al.36

postulated four different growth habits of hydrates in porousmedia. Priest et al.37 interpreted the elastic wave attenuationmeasurements via a conceptual model of cemented graincontacts. It has been known for a long time that hydrateformation can consolidate a bed of sand particles.38

Kneafsey et al.9 postulated that capillary driven flow ofwater through a porous medium containing hydrate con-tributes to a moving hydrate formation front that woulddrag the water saturation front along. The existence of poresin the hydrate has been proposed.39-41 For example,

Staykova et al.41 observed sub-micrometer pores but didnot present any surface area data. On the basis of theporous hypothesis of the nature of hydrate, a capillary-driven water flow mechanism is also thought to operatewhen hydrates form in small water droplets exposed to agas atmosphere.25,42,43 The postulatedmechanism byKneaf-sey et al.9 probably takes place in our system too. The sand intheir experiment was partially water-saturated, whereas ourswas fully saturated. Both systems are water-wet. The contactangle of our sand was measured and found to be 36.5�.However, one concern we have with the capillarity-drivenmechanism is the potential to grow hydrate at the sand/bulkgas interface because of the abundance of gas there. Thisassumption was tested by carrying out a smaller scalemorphology experiment to visualize hydrate formationwith-in the sand/water matrix. During this experiment, pictures ofthe sand bed were taken at several time intervals and areshown in Figure 16. A dash mark is put on the vial as areference point to note any difference in the height of the bedmore easily. The gas phase, sand þ water, and gas/(sand þwater) interface are marked in the figure. The time elapseduntil the picture was taken and the water-hydrate conver-sion are reported along with the pictures. This figure showsthat hydrate formation occurs within the sediment becausethe gas/(sand þ water) interface remains constant duringhydrate formation. An increase in the bed size would meanhydrate formation on top of the silica sand bed.

SEM images revealed that the sand particles are notspherical. They exhibit various shapes, and their surface isrough. Figure 17 shows a collection of particles as well as arandomly chosen particle and progressive focus on its sur-face. The implication is that there is a distribution of volumesof interstitial spaces. Hydrate is believed to form preferen-tially in the largest spaces available.36 Because we havesaturated the sand with water, the methane gas availablefor hydrate formation is dissolved gas, except at the interfacebetween bulk gas (gas cap) and the sand/water matrix. Isdiffusion of methane through the liquid water enough toprovide the gas required to achieve the water conversionsreported in Table 1? Our experiments cannot directly answerthis question. Diffusion calculations on a water/methanequiescent system44 reveal that it is possible for methane todiffuse in water to the experimented depth (7 cm) and reachthe saturation levels required formethane hydrate formationfor the time our experiments were conducted. The calcula-tion procedure that Servio and Englezos42 used was em-ployed. Transport mechanisms other than diffusion throughthe liquid water might include transport through tortuouspaths formed by the porous hydrate in the porousmedium. Itis also conceivable that, because of the irregular shapes of thesand particles, water does not completely wet all availablesurfaces. Besides the chemical nature of the surface, thecontact angle also depends upon surface roughness.45

Roughness decreases the contact angle of hydrophilic sur-faces but increases that of hydrophobic ones. However, a

Figure 15. Typical Raman spectra formethane hydrate synthesized inthe silica sandmatrix showing the C-H stretch ofmethane encaged inhydrate cages as a doublet at 2906 and 2917 cm-1 (inset). Also note thebroad water band for O-H stretching between 3000 and 3500 cm-1.Hydratewas synthesized at 4 �Cand8MPa, and spectrawereobtainedat atmospheric pressure and liquid nitrogen temperature.

(34) Waite, W. F.; Winters, W. J.; Mason, D. H. Methane hydrateformation in partially water-saturated Ottawa sand.Am.Mineral. 2004,89 (8-9), 1202–1207.(35) Clennell, B. M.; Hovland, M.; Booth, J. S.; Henry, P.; Winters,

W. J. Formation of natural gas hydrates in marine sediments. 1.Conceptual model of gas hydrate growth conditioned by host sedimentproperties. J. Geophys. Res., [Solid Earth] 1999, 104 (B10), 22985–23003.(36) Kleinberg, R. L.; Flaum, C.; Straley, C.; Brewer, P. G.; Malby,

G. E.; Peltzer, E. T., III; Friederich, G.; Yesinowski, J. P. Seafloornuclear magnetic resonance assay of methane hydrate in sediment androck. J. Geophys. Res., [Solid Earth] 2003, 108 (B3), 2137.(37) Priest, J. A.; Best, A. I.; Clayton, C. R. I. Attenuation of seismic

waves in methane gas hydrate-bearing sand. Geophys. J. Int. 2006, 164(1), 149–159.(38) Cheng, W. K.; Pinder, K. L. Consolidation of sand formation

using Freon-11 gas hydrate. Can. J. Chem. Eng. 1976, 54 (5), 377–381.(39) Mori, Y. H.; Mochizuki, T. Mass transport across clathrate

hydrate films;A capillary permeation model. Chem. Eng. Sci. 1997,52 (20), 3613–3616.(40) Klapproth, A.; Techmer, K. S.; Klapp, S. A.; Murshed, M. M.;

Kuhs, W. F. Microstructure of gas hydrates in porous media. Phys.Chem. Ice 2007, 321–328.(41) Staykova, D. K.; Kuhs, W. F.; Salamatin, A. N.; Hansen, T.

Formation of porous gas hydrates from ice powders: Diffractionexperiments and multistage model. J. Phys. Chem. B 2003, 107 (37),10299–10311.

(42) Servio, P.; Englezos, P.Morphology study of structureHhydrateformation from water droplets. Cryst. Growth Des. 2003, 3 (1), 61–66.

(43) Servio, P.; Englezos, P. Morphology of methane and carbondioxide hydrates formed from water droplets. AIChE J. 2003, 49 (1),269–276.

(44) Haligva, C. Natural gas recovery from hydrates in a silica sandmatrix. University of British Columbia, Vancouver, British Columbia,Canada, 2008.

(45) Adamson, A.W.; Gast, A. P. Physical Chemistry of Surfaces, 6thed.; Wiley: New York, 1997.

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combination of surface roughness and surface chemistrymay lead to a transition from hydrophilicity to hydropho-bicity.46 Klapproth et al.40 examined hydrate formation inthree media (quartz, quartz þ kaolinite, and quartz þmontmorillonite) and reported a dependence of gas con-sumption on the porous media type.

Effect of the Bed Size on the Rate of Gas Uptake. McCal-lum et al.47 discussed the impact of the size of the hydrateformation vessel on induction time and hydrate formation/decomposition. Mesoscale experiments were conducted on a72 L seafloor process simulator (SPS) and were compared toa 450mL Parr vessel. They concluded that hydrates from theSPS consistently had less overpressures and shorter induc-tion times than the equivalent experiments in the Parr vessel.They reported that the large volume of the SPS was acontributing variable, which increased the likelihood of theinitial hydrate nucleation clusters to grow and accumulate.

However, they did not report any kinetic data (gas uptake or

hydrate formation rate) or the conversion of water to

hydrate. Thus, to the best of our knowledge, this work is

the only study that investigates the effect of vessel size in

laboratory hydrate formation in porous media.Figure 18 shows the percent conversion of water to

hydrate versus time for the experiments conducted at 4 �Cand the three bed sizes. Time zero represents the first

nucleation point. It is clearly evident from the figure that

the general shape of the gas uptake curves is similar for all

three bed sizes. An initial slow growth phase is followed by a

very rapid one, and then during the last stage, the rate of

water conversion progressively approaches its final value,

between 74 and 98%. The duration of the first stage is not

predictable, but in general, it lasted longer in the largest size

bed. Interestingly, the rate of water conversion is similar

during the rapid (second) stage for all experiments. During

the last stage, the rates begin deviating, probably as a

cumulative effect of the heterogeneous nature of the hydrate

nucleation combined hydrate growth with heat- and mass-

transfer effects.

Figure 16. Morphology experiment (8.0 MPa). Series of pictures showing the height of the bed at different times during hydrate formation.A red line is drawn in the image to indicate the gas/(sand þ water) interface.

Figure 17. SEM images of the silica sand.

(46) Kietzig, A. M.; Hatzikiriakos, S. G.; Englezos, P. Patternedsuperhydrophobic metallic surfaces. Langmuir 2009, 25 (8), 4821–4827.(47) McCallum, S. D.; Riestenberg, D. E.; Zatsepina, O. Y.; Phelps,

T. J. Effect of pressure vessel size on the formation of gas hydrates.J. Pet. Sci. Eng. 2007, 56 (1-3), 54–64.

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4. Conclusions

A new apparatus was set up to study methane hydrateformation and decomposition behavior in the interstitialspace of a bed of silica sand particles. The formation experi-ments were studied at three different temperatures of 7.0, 4.0,and 1.0 �C, respectively. Multiple nucleation points wereobserved for formation experiments carried out at 4.0 and1.0 �C. This resulted in achieving a high percent conversion ofwater to hydrates in the range of 74-78.5%. Hydrate forma-tion was found to appear in a heterogeneous manner

(spatially). Independent verification of hydrate formation inthe sand was achieved via Raman spectroscopy andmorphol-ogy observations in experiments using the same sand/watersystem. The effect of silica sand bed size on the hydrateformation was studied by installing one and two coppercylinders in the CR, thereby narrowing the size of the bed,with the height of bed kept constant at 7.0 cm. An initial slowgrowth was followed by a rapid hydrate growth rate of equalmagnitude for nearly all experiments until 43-53% of waterwas converted to hydrate. During the third and final growthstage the final conversions were between 74 and 98% and thegas uptake dynamics changed. It was concluded that, if onewishes tomodel the rate of hydrate formation in a porous bed,the dependency of the results on the size of the bed should betaken into account.

Acknowledgment. The financial support from the NaturalSciences and Engineering Research Council of Canada(NSERC) and Natural Resources Canada (NRCan) is greatlyappreciated. Dr. Nam would like to thank the Korean Govern-ment (MOEHRD) for the Korea Research Foundation Grant(KRF-2006-611-D00006).We also thankDr. RajnishKumar forhis help with the Raman spectroscopy experiments.

Nomenclature

nT = total number of molesnH =moles of the gas in the hydrateþ water phase of CRnG = moles of gas in the gas phase of CRnR = moles of gas in RVG = volume of the gas phase in CRVR = volume of Rz = compressibility factor(ΔnH,V)tend = number of moles consumed for hydrate for-

mation at the end of a formation experiment(dn/dt) = rate of hydrate formation

Figure 18. Percent conversion of water to hydrate for all of theformation experiments conducted at 4.0 �C and three different bedsizes. Time zero represents the nucleation point. Blue color linesrepresent the experiments for the largest bed size (CR). Black colorlines represent the experiments for themediumbed size (CRþCC1).Red color lines represent the experiments for the smallest bed size(CR þ CC1 þ CC2).

Gas Hydrate Formation in a Variable Volume Bed of Silica Sand Particles

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