ARTICLE

Carbon dioxide-induced liberation of methane fromlaboratory-formed methane hydratesKristine Horvat and Devinder Mahajan

Abstract: This paper reports a laboratory mimic study that focused on the extraction of methane (CH4) from hydrates coupledwith sequestration of carbon dioxide (CO2) as hydrates, by taking advantage of preferential thermodynamic stability of hydratesof CO2 over CH4. Five hydrate formation-decomposition runs focused on CH4CO2 exchange, two baselines and three with hostsediments, were performed in a 200 mL high-pressure Jerguson cell tted with two glass windows that allowed visualization ofthe time-resolved hydrate phenomenon. The baseline pure hydrates formed from articial seawater (75 mL) under 64006600 kPa CH4 or 28003200 kPa CO2 (hydrate forming regime), when the bath temperature was maintained within 46 C andthe gas/liquid volumetric ratio was 1.7:1 in the water-excess systems. The data show that the induction time for hydrateappearance was largest at 96 h with CH4, while with CO2 the time shortened by a factor of four. However, when the secondarygas (CO2 or CH4) was injected into the system containing preformed hydrates, the entering gas formed the hydrate phaseinstantly (within minutes) and no lag was observed. In a system containing host Ottawa sand (104 g) and articial seawater(38 mL), the induction period reduced to 24 h. In runs withmultiple charges, the extent of hydrate formation reached 44% of thetheoretical value in thewater-excess system,whereas the valuemaximizedat 23% in thegas-excess system.TheCO2hydrate formationin a system that already contained CH4 hydrates was facile and they remained stable, whereas CH4 hydrate formation in a systemconsisting of CO2 hydrates as hosts were initially stable, but CH4 gas in hydrates quickly exchanged with free CO2 gas to formmorestable CO2 hydrates. In all ve runs, even though the systemwas depressurized, left for over aweek at room temperature, andushedwith nitrogen gas in between runs, hydrates exhibited the memory effect, irrespective of the gas used, a result in contradictionwiththat reported previously in the literature. The facile CH4CO2 exchange observed under temperature and pressure conditions thatmimic naturally occurring CH4 hydrates show promise to develop a commercial carbon sequestration system.

Key words: sediment hosted hydrates, gas exchange in hydrates, methane hydrate, carbon dioxide hydrate, carbon sequestration.

Rsum : Cet article prsente les rsultats dune tude visant a` reproduire en laboratoire les conditions dextraction dumthane(CH4) a` partir dhydrates et, simultanment, la squestration du dioxyde de carbone (CO2) sous forme dhydrates, en tirant partide la plus grande stabilit thermodynamique des hydrates de CO2 par rapport aux hydrates de CH4. Nous avonsmen cinq essaisde formation-dcomposition dhydrates centrs sur lchange entre le CH4 et le CO2, dont deux essais de rfrence et trois,dans des sdiments contenant des hydrates. Les essais ont t raliss dans une chambre Jerguson a` haute pression de 200 mlmunie de deux fentres de verre permettant lobservation du phnomne de transformation des hydrates en fonction du temps.Les hydrates purs de rfrence se sont forms a` partir de leau de mer articielle (75 ml) sous une pression de CH4 de 6400 a`6600 kPa ou de CO2 de 2800 a` 3200 kPa (conditions de formation dhydrates) lorsque la temprature du bain tait maintenueentre 4 C et 6 C et que le rapport volumtrique gaz/liquide tait denviron 1,7 : 1 dans les systmes o leau tait en excs. Lesdonnes rvlent que la priode dinduction requise pour lapparition de lhydrate tait la plus longue, soit 96 h, dans le cas duCH4, tandis quelle tait quatre fois plus courte dans le cas du CO2. Toutefois, lorsque le gaz secondaire (le CO2 ou le CH4) taitinject dans le systme contenant des hydrates dja` forms, le gaz entrant sest tout de suite transform en hydrate (en quelquesminutes); aucun dlai na t observ. Dans un systme contenant le substrat dans du sable dOttawa (104 g) et de leau de merarticielle (38 ml), la priode dinduction tait rduite a` 24 h. Dans les essais a` charges multiples, lhydrate sest form dans uneproportion atteignant 44 % de la valeur thorique en systme o leau tait en excs, tandis que la proportion a atteint une valeurmaximale de 23 % en systme o le gaz tait en excs. La formation dhydrates de CO2 dans un systme qui contient dja` deshydrates de CH4 tait aise, et les hydrates forms sont demeurs stables. linverse, les hydrates de CH4 forms dans le systmecompos pralablement dhydrates de CO2 taient initialement stables, mais ont rapidement vu leur CH4 gazeux schangercontre le CO2 gazeux libre pour former des hydrates de CO2 plus stables. Dans les cinq essais, mme dans le cas dun systme taitdpressuris, laiss a` temprature ambiante pendant plus dune semaine et purg avec de lazote gazeux entre les essais, leshydrates ont manifest l' effet mmoire , quel que soit le gaz employ, un rsultat qui est en contradiction avec les rsultatspublis auparavant. Lchange ais entre le CH4 et le CO2 observ en conditions de pression et de temprature imitant lesconditions naturelles de formation des hydrates de CH4 semble un moyen prometteur de raliser un systme commercial desquestration du carbone. [Traduit par la Rdaction]

Mots-cls : hydrates dorigine sdimentaire, changes gazeux dans les hydrates, hydrate de mthane, hydrate de dioxyde decarbone, squestration du carbone.

Received 7 December 2014. Accepted 15 March 2015.

K. Horvat. Materials Science and Engineering, Stony Brook University, Stony Brook, NY 11794, USA.D. Mahajan. Sustainable Energy Technologies Department, Brookhaven National Laboratory, Upton, NY 11973, USA.Corresponding author: Devinder Mahajan (e-mail: dmahajan@bnl.gov).This article is part of a Special Issue in honour of Dr. John Ripmeester and his outstanding contributions to science.

Pagination not final (cite DOI) / Pagination provisoire (citer le DOI)1

Can. J. Chem. 93: 19 (2015) dx.doi.org/10.1139/cjc-2014-0562 Published at www.nrcresearchpress.com/cjc on 15 April 2015.

Can.

J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.c

om b

y SU

NY

AT

STO

NY

BRO

OK

on

06/1

4/15

For p

erso

nal u

se o

nly.

IntroductionWith the world population expected to reach over 10 billion by

2050,1 the world is facing an ever increasing need for new energysources. Increasing greenhouse gas levels in the atmosphere ne-cessitate the use of carbon neutral energy sources. Current esti-mates show that over 30 trillionm3 ofmethane (CH4) is trapped ashydrates in marine and permafrost environments.2 The presenceof vast CH4 hydrate reserves have been known for decades, buttheir extraction, especially from CH4-rich but dispersed marinehydrates, remains a challenge. Gas production by external stimu-lation using techniques such as steam injection, gas depressuriza-tion, or hydrate-inhibitor injection are now well-established.3 Inmarine systems, CH4 harvesting from hydrate may result inseaoor instability.45 At temperatures below 10 C, carbon diox-ide (CO2) hydrates are more thermodynamically stable at lowerpressures than CH4 hydrates,6 and some work has been reportedon the CO2CH4 exchange. Both CH4 and CO2 gases form structuresI hydrates,7 and the exchange is a fascinating approach by itself:CH4 is liberated while CO2 is sequestered. However, large-scaleapplication of the exchange concept requires a thorough under-standing of immediate surroundings (sediments) of natural hy-drate and other environmental concerns.3

Laboratory studies to test the feasibility of CH4 release fromhydrates by CO2 gas injection are mostly limited to analyticalinvestigation tools, such as NMR, Raman, and MRI, to provide abasic understanding of the exchange process.8 Raman spectros-copy established that upon liquid CO2 injection into preformedCH4 hydrates, there was essentially mol to mol correlation be-tween liberated CH4 and CO2 loss due to CO2 hydrate formation.In one experiment, 70 mmol CH4 was recovered from hydratedecomposition, while 71 mmol CO2 was consumed to form hy-drates, indicating that CH4 guest molecules were replaced by CO2in the hydrate structure during the gas exchange.6 Another study9

measured CH4 recovery to be 8.3 mol% in 206 h using gas chroma-tography (GC). An in situ magnetic resonance imaging (MRI) mea-surement of a Bentheim sandstone hydrate core sample yielded50%85% CH4 gas recovery after ushing three times with CO2using GC.10 Another MRI study of brine solutions in Bentheimsandstone showed that CH4 hydrate spontaneously converted toCO2 hydrate when exposed to liquid CO2, and the hydrates werenot found to dissociate to liquid water during the exchange.11

Since CH4 hydrates located in permafrost regions are easier toaccess than those in the marine environment, the technology forcarbon sequestration has reached larger-scale eld testing in per-mafrost regions. Studies have shown that there are an estimated2.4 trillion m3 of recoverable gas from accessible hydrate accu-mulations in the North Slope of Alaska alone.12 In 2012, aConocoPhilips-led test showed success when a 23 mol% CO2 inN2 mixture was injected to release over 24 210 m3 of CH4 from ahydrate reservoir in Alaska. CO2 was preferred over N2 in thehydrate phase, as about 70% of the N2 gas injected was recovered,while only 40% of the 1376 m3 of CO2 injected was retrieved.13

Recently, a marine eld test was performed off the coast of Cali-fornia to exchange CO2 in CH4 hydrate. In the experiment, pureCH4 hydrate was brought to the seaoor from a depth of 690 mand then enclosed in a cylinder lled with a 25% CO2 75% N2 gasmixture. The CH4 hydrate was found to dissociate to form amixedgas phase, though no CO2 hydrate was found to form under thesesimulated conditions.14 Thus, the CH4CO2 exchange reactionmerits further research to develop a feasible method to mine CH4from natural hydrates. In this paper, we describe several laboratory-scale experiments to understand the CH4CO2 hydrate exchangekinetics.

Materials and methods

GasesPressurized cylinders of CH4 and CO2 gases were ordered from

Scott Specialty Gases and Praxair, respectively.

Articial seawaterArticial seawater used to simulate natural medium was pre-

pared according to Kester et al.15

Hydrate former unit descriptionThese experiments were performed in a 200 mL Jerguson cell

tted with two 12 inch long borosilicate windows in the FlexibleIntegrated Study of Hydrates (FISH) unit at Brookhaven NationalLaboratory (BNL) (Fig. 1) and described in detail elsewhere.16 Thepresence of a pressure transducer at the top of the cell and twothermocouples, located in the gas and liquid phases inside thecell, allowed for accurate pressuretemperature monitoring dur-ing runs. The cell capabilities allowed us to: (i) measure the per-centage of hydrates formed in the system, (ii) visualize hydrateformation and observe their morphology, and (iii) measure com-position of free gas above the liquid phase to quantify the mixedCO2CH4 system. We completed ve runs, as noted in Table 1, inthe Jerguson cell without stirring the cell. The rst two experi-ments used only 75 mL of articial seawater while the last threeused 38 mL articial seawater and 104 g of 110 m Ottawa sand,17

which was 99.0%99.9% silicon dioxide.18 The cell was pressurizedwith gas owing through the bottom such that that the enteringgas bubbled through the articial seawater. The cell was initiallycharged with either CO2 or CH4 gas, and after the hydrates startedto form, the second gas was injected into the cell. For example, byinitially forming CH4 hydrates, and then injecting CO2 gas, theCH4CO2 exchange in hydrates could be monitored. In experi-ments with preformed CH4 hydrate, the pressure in the cell wasreduced by depressurizing the cell below 4000 kPa prior to CO2injection to insure that CO2 would be injected as a gas rather thana liquid. The phase diagram for CO2 and CH4 hydrates in articialseawater (Fig. 2) served as a guide to follow theoreticalmonitoringof the experimental system. We also conducted two runs thatinvolved the formation of CO2 hydrate followed by a charge withCH4 gas to allow monitoring the stability of CO2 hydrates if theywere to come into contact with CH4 gas, as could potentially occurin a free CH4 gas zone or if CO2 hydrates formed near a CH4 plume.During the depressurization cycle, the system pressure was low-ered in multisteps, where in each step the pressure was reducedseveral hundred kPa and the system was then allowed to stabilizefor several minutes to an hour prior to subsequent pressure re-duction. This process was repeated until the system was totallydepressurized.

Modes of unit operationThe knownmole stoichiometry of water/gas in CH4 hydrate and

CO2 hydrate are 5.75:1 and 6:1 respectively. The unit was operatedin either water-excess or gas-excess mode, depending on themoleratio of water/gas added to the system. These data were used tocalculate theoretical maximum hydrate from which the extent ofhydrate saturation was calculated by the gas consumed during agiven run.

Gas analysisGC samples were taken periodically during runs and analyzed

for CH4 and CO2 on a Gow Mac series 580 gas chromatographtted with a Supelco Carbonex 1000 45/60 (1.5 m 0.32 mm)packed column using helium as carrier gas.

Pagination not final (cite DOI) / Pagination provisoire (citer le DOI)2 Can. J. Chem. Vol. 93, 2015

Published by NRC Research Press

Can.

J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.c

om b

y SU

NY

AT

STO

NY

BRO

OK

on

06/1

4/15

For p

erso

nal u

se o

nly.

Results and discussion

Run 1: Baseline run with no host, CH4 followed byCO2 chargeFreshly prepared articial seawater (75 mL) was syringed into

the Jerguson cell, and the cell was pressurized to 7065 kPa withCH4 gas at room temperature and then cooled till the bath tem-perature was about 4 C. After several days, an observed pressuredrop of over 2000 kPa was attributed solely to hydrate formation.

After repressurization to 6114 kPa with CH4, the pressure droppedto 5252 kPa. Together, total gas consumed corresponded to 44% ofthe theoretical hydrate value in this excess-water system after twogas charges. At this point, the cell was switched to the CH4CO2exchange mode by depressurization to 2866 kPa CH4 and thenpressurized with CO2 gas three times until the cell pressure in-creased to 4652 kPa, resulting in instant gas hydrate formation.This was necessitated by the fact that at partial pressures above4000 kPa, CO2 is in liquid phase. GC samples taken over 16 hshowed >99% CH4 (

were seen in the cell, and the cell was charged with additionalCO2 gas. Upon repressurization to 3997 kPa at 5.2 C, liquid CO2wasobserved above the aqueous layer in the cell. After two hours,transparent needle-like CO2 hydrateswere observed at the bottomof the cell, and after an additional hour CO2 hydrates were seen atthe aqueousliquid CO2 meniscus. These hydrates appeared morelike solid ice, unlike the needle-like hydrates still at the bottom ofthe reactor. One hour later, the gaswas discharged from the cell toreduce cell pressure to a point where liquid CO2 was no longerstable, and many of the hydrates present dissociated. The cell wasagain repressurized to 3273 kPa with CO2 gas that resulted ininstant hydrate formation during charging. No further pressuredrop was noted after the cell was sealed. Shortly thereafter, thecell was operated in the CO2CH4 exchange mode by rst depres-surization to 1301 kPa and repressurized to 6265 kPa with CH4,wherein instantaneous CH4 hydrate formation was observed dur-ing CH4 charging. Figure 4 shows recorded images of mixed CO2CH4 hydrates, and the hydrates that formed instantly after theCH4 injection are clearly seen as small spheres beneath the gasliquid interface. Though the physical appearance of hydrates ofCH4 and CO2 is indistinguishable, the time resolved analysis offree gas above the aqueous phase by GC was used to quantifyhydrates in themixed gas system. Gas samples taken from the cellone hour after the CH4 charge established the CH4/CO2 ratio to be35%:65% that changed to 50%:50% over the next 26 h. At this point,a stepwise depressurization of the system was initiated. The GCanalysis indicated that initially CO2 dominated the gas phase, butas the cell slowly depressurized, the amount of CH4 in the gasphase increased. The stability of CH4 hydrates was noted, evenwhen the partial pressure of CH4 was below the hydrate equilib-rium curve (Fig. 2), indicating that complete CH4 hydrate decom-position may be a slow phenomenon.

Run 3: CH4 followed by CO2 charge hosted in Ottawa sandRun 3 was conducted with Ottawa sand (104 g) as the host that

was fully saturatedwith articial seawater (38mL) and to ll spaceabove the sand pack. After cooling to 4 C, the cell was pressurizedwith 6617 kPa CH4 gas, when hydrates were observed at the gasliquid interface in about 24 h. Over time, the hydrate grew down-wards into the solution until they reached the top of the sand pack(Fig. 5). The pressure drop corresponded to 11% conversion in 48 hin this excess-gas system. At this point, the cell was depressurizedbelow the hydrate equilibrium conditions to allow complete hy-drate dissociation. The cell was repressurized to 6596 kPa andwithin 10min hydrates were observed as a thin lm on part of theglass window and at the gasliquid interface. The pressure de-creased continuously as hydrates grew from the gasliquid inter-face downwards into the solution, until the pressure stabilized at5810 kPa, which corresponded to 12% gas conversion into hydratesover 2 days. The observed relatively fast hydrate formation is at-tributed to the memory effect.21

Similar phenomenon of instantaneous hydrate formation wasobserved during subsequent CO2 charges in the partial depressur-ization/repressurization cycle. It is notable that in these runs, thehydrate formed above the sand pack; few if any, hydrates formedin the sand pack most likely due to capillary inhibition.22 GCanalysis indicated pure CH4 for several hours but 3 days after theCO2 injection, the gas phase CH4/CO2 ratio was 86.5%:13.5%. Sub-sequently, hydrate dissociation was induced by depressurization.A nal GC samplewas taken at 1287 kPa pressure and 3.7 C (belowthe hydrate stability zone) that established the gasphase CH4/CO2ratio of 15%:85%, suggesting that the cell was mostly composed ofCO2 hydrates prior to dissociation.

Fig. 3. Image of mixed CH4CO2 hydrates captured during Run 1.The images were taken at 312 h when the cell pressure was 4135 kPaat 3.2 C. The corresponding gas phase composition in the cell was99% CH4:1% CO2. Note that that mixed gas hydrates have lled theentire cell viewing region.

Fig. 4. Image taken at 52.8 h, at cell pressure of 6038 kPa at 4 C.The corresponding gas phase composition was 41% CH4:59% CO2. Gashydrates can be clearly seen. The spherical hydrates in the lowerportion of the image formed instantly when CH4 was added to thecell.

Pagination not final (cite DOI) / Pagination provisoire (citer le DOI)4 Can. J. Chem. Vol. 93, 2015

Published by NRC Research Press

Can.

J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.c

om b

y SU

NY

AT

STO

NY

BRO

OK

on

06/1

4/15

For p

erso

nal u

se o

nly.

Run 4: CO2 followed by CH4 charge hosted in Ottawa sandIn this run, the Ottawa sand/articial seawater from previous

run were reused after ushing with N2 gas, except that the addi-tion of the two gases was reversed to establish the effect of addedCH4 gas to preformedCO2 hydrates. After pre-cooling to 6 C, the cellwas pressurized to 3211 kPawith CO2, when transparent, needle-likeCO2 hydrates were observed on top of the sand pack in less than 7 h.After 24h, thehydrates had spread to the gasliquid interface, and inthe next 24 h hydrates were visible within the host sand. The cellpressure stabilized and corresponded to 11% conversion of CO2 intohydrates in this excess-water system. Another CO2 charge resulted ininstantaneous CO2 hydrate formation. Shortly thereafter, CH4 wasinjected to increase the cell pressure to 5314 kPa at 3.1 C to observethe CO2CH4 exchange. The GC analysis established the CH4/CO2ratio as follows: 6.4%:93.6% (5min); 30.4%:69.6% (36min). These data

are consistent with the liberation of CH4 from the hydrate phaseshortly after CO2 was introduced in to the system and displaced CH4to form CO2 hydrates. A second charge of CH4 increased the cellpressure to 6134 kPa and the measured CH4/CO2 ratio was 34.9%:65.1% (5min) that increased to 47.9%:52.1% over 3 h. It is known that,initially, CH4 molecules ll both the small and large hydrate cages,but as time elapses themajority of large hydrate cages are lledwithCO2 that results in increased CH4 concentration in the gas phase.23 Astep-wise depressurization of the cell to bring about hydrate dissoci-ation initially resulted in an increase of CH4 in the gas phase, but asthe pressure decreased the gas phase became richer in CO2. Whenthepressurewas reduced to2859kPa, thegasphase compositionwas53.7% CH4:46.3% CO2. However, as the pressure further decreased to812 kPa, the gas phase became richer in CO2 (65.7%), as the CO2hydrates began to dissociate. Figure 6 is an image of mixed gas hy-drates seen above and in the sand pack.

Fig. 5. Time-resolved images of hydrate formation during Run 3. (1) Taken 20.1 h after the pre-cooled cell was pressurized with 6382 kPa CH4at 3.5 C. Prior to (1), the cell was charged with 6617 kPa with CH4 gas. (2) At 194.8 h under 2728 kPa at 3.6 C. Prior to (2), after CO2 hydratesformed for several days, the cell was partially depressurized, left cooling for 3 days, and then partially depressurized and repressurized severaltimes. (3) At 236 h under 2542 kPa at 3.6 C. Prior to (3), the CO2 gas was charged twice. Shortly after (3), the cell was depressurized in steps tobring about complete dissociation of hydrates.

Pagination not final (cite DOI) / Pagination provisoire (citer le DOI)Horvat and Mahajan 5

Published by NRC Research Press

Can.

J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.c

om b

y SU

NY

AT

STO

NY

BRO

OK

on

06/1

4/15

For p

erso

nal u

se o

nly.

Run 5: CH4 followed by CO2 charge hosted in Ottawa sandIn Run 5, the Ottawa sand and articial seawater from Run 4

were re-used to establish the exchange of CO2 gas with preformedhydrates of CH4. The cell was ushed with N2 gas, pre-cooled to3 C, and then pressurized to 6651 kPa with CH4 gas. Within min-utes, a thin coating of ice-like hydrates was seen on the cell glasswindows above the gasliquid interface that spread to the gasliquid interface after 1 h and then moved downwards into thesolution. The noted pressure drop corresponded to 6% hydratesaturation in this excess-gas system. After 26 h, the second chargewith CH4 corresponded to an additional 2% gas conversion of CH4into hydrates. The cell was partially depressurized, then repres-surized with CO2 up to 4149 kPa, and then 1 h later, the cell wasagain partially depressurized and charged with CO2. Multiple GCsamples over 3 h established the presence of a pure CH4 gas phase.Upon quick depressurization to 1791 kPa (below the hydrate sta-bility zone), GC analysis established the gas phase rich in CO2(69.1%), indicating the presence of CO2 trapped as hydrate becameunstable during depressurization. Further depressurization to405 kPa increased CO2 (81.8%) and decreased CH4 (18.2%) in the gasphase. Figure 7 shows images of the hydrate growth throughoutrun 5.

Comparison of data from runsThis study consisted of 5 experimental runs to understand a

CH4CO2 system in which gas phase CO2 is pumped into a natural

CH4 hydrate reservoir that results in sequestered CO2 as hydratewith concomitant liberation of CH4. Specically, the completed5 runs focused on understanding the extent and rates of the CH4CO2 exchange phenomenon. The exact conditions of all experi-ments are listed in Tables 1 and 2. Runs 1 and 2 were baseline(without host sediments), while runs 35 included Ottawa sand asa host. In runs 1, 3, and 5, the cell was initially charged with CH4,and once CH4 hydrate formed, CO2 was added to the cell. Theopposite sequence was repeated in runs 2 and 4, wherein CO2 gaswas initially added to the cell, and once CO2 hydrates were ob-served, CH4 was added to the system.We extracted induction time data for hydrate formation (the

time when these could be seen by the naked eye) from completedruns to better understand its dependence on the nature of hydrateforming gas (CH4 and CO2), host sediment, and memory effect.The data in Table 2 show that the induction time for CH4 hydrateappearance was the largest at 96 h (Run 1), while with CO2 thetime shortened to 24 h (Run 2). Runs 1, 3, and 5 followed the samesequence; initially, CH4 gas was added to the system, and oncehydrates were visible in the reactor, CO2 gas was added. CH4 hy-drates formedmuchmore quickly (24 h) in the presence of Ottawasand (Run 3) than without it (4 days for Run 1). Similarly, Run 4, inwhich CO2 hydrates were formed and then CH4 gas was injectedwithin the host sand, had a shorter hydrate induction time (7 h)than Run 2 (24 h), in which no porous media was present. The

Fig. 6. Images of hydrate formation taken at 30.4 h under 6010 kPa at 3.1 C. The corresponding gas phase composition was 48% CH4:52% CO2.Mixed hydrates lled much of the reactor above the same pack though few hydrates were seen in the sand pack.

Pagination not final (cite DOI) / Pagination provisoire (citer le DOI)6 Can. J. Chem. Vol. 93, 2015

Published by NRC Research Press

Can.

J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.c

om b

y SU

NY

AT

STO

NY

BRO

OK

on

06/1

4/15

For p

erso

nal u

se o

nly.

presence of porous media is known to affect the stability of gashydrates. Higher pressures and (or) lower temperatures areneeded to form hydrates in sand systems.22 In one instance,Handa and Stupin24 found that systems containing CH4 or pro-

pane hydrates in silica gel required equilibrium pressures 20%100% higher than systems without sediments to form hydrates.The lack of hydrate formationwithin theOttawa sand pack hereinis in agreement with these studies. The shortened induction time

Fig. 7. Time-resolved images of hydrate formation during Run 5. (1) Taken 25 h after the pre-cooled cell was initially pressurized with6382 kPa CH4 at 3.5 C. Prior to (1), the reactor was charged with 6651 kPa with CH4 gas. After CH4 hydrates formed in 26 h, the cell wasrepressurized with CH4 gas to 6686 kPa. (2) At 46.7 h under 6548 kPa of CH4 at 2.9 C. After this image was taken, the cell was partiallydepressurized and repressurized with CO2 twice. (3). Taken at 52.2 h under 3797 kPa at 3.0 C. Shortly after (3), the cell was depressurizedquickly to induce hydrate dissociation.

Pagination not final (cite DOI) / Pagination provisoire (citer le DOI)Horvat and Mahajan 7

Published by NRC Research Press

Can.

J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.c

om b

y SU

NY

AT

STO

NY

BRO

OK

on

06/1

4/15

For p

erso

nal u

se o

nly.

observed in runs with Ottawa sand are likely due to sand particlesacting as the nucleation sites for hydrate formation. It is likelythat sand particles became stuck to the walls near the gasliquidinterface in the cell during gas bubbling through the sand packduring charges, and these impurities affected hydrate nucleationand induction times.25

A comparison of pure hydrates of CH4 and CO2 in studies foundthat CO2 hydrates had shorter induction times than CH4 hy-drates.2627 The gas hydrate phase diagram (Fig. 1) shows that inarticial seawater, CO2 hydrates are stable at higher temperaturesand lower pressures than CH4 hydrates. In agreement with previ-ous studies2627 and thermodynamics, the induction times inRuns 2 and 4, wherein CO2 hydrates were initially formed, wereshorter than the induction times for CH4 hydrates (Runs 1 and 3).Interestingly, the shortest induction time was noted in Run 5,

wherein CH4 hydrates formed nearly instantly after the cell waspressurized. This speedy formation is likely due to the hydratememory effect. Studies21,28 have shown that hydrate-forming so-lutions canmaintain a memory of their hydrate structure whenwarmed up slightly above the hydrate stability region.29 There aretwo theorieswhy this occurs: (i) a hydrate frame assembly remainsintact in the solution, either as a partial or ordered congurationonce hydrates had formed and (ii) some gas is left dissolved in thesolution after dissociation.29 For Runs 35 in which the sameOttawa sand and articial seawater were utilized for hydrate for-mation, the induction timewas signicantly reduced even thoughdifferent gases (CH4 or CO2) were initially used to form hydrates.In Run 3, CH4 hydrates formed after about 24 h, while in Run 4,CO2 hydrates formed in 7 h, and in Run 5, CH4 hydrates formedwithin minutes. In between each of these runs, the cell was com-pletely depressurized and ushed with nitrogen gas. Mazloumet al.30 found that the depressurization of a system of natural gashydrates, followed by repressurization with fresh natural gas, didnot result in the hydratememory effect for gas hydrate formation.It was found that the hydrate memory effect was destroyed bydepressurization to atmospheric pressure, but reducing the pres-sure of the system even slightly would still result in the hydratememory effect.30 In addition, if the solution is warmed above 2521

or 28C29 or for too long a time period (several hours29), thisretention effect will not occur. In between each of these experi-ments the cell was left uncooled for extended periods of timeranging from 7 days to 2 months. All of these factors are contraryto the decreased hydrate formation time observed hereinwith thesystems composed of CH4 and CO2.The sequence of gas addition to the cell affected hydrate stabil-

ity. In all experiments, once gas hydrate had formed, any addi-tional gas charge, independent of the gas (CO2 or CH4), resulted innear instant hydrate formation. This fast formation indicates thatif CO2 is to be sequestered in a CH4 hydrate reservoir, it is likelythat CO2 hydrates will form instantly. In addition, whether CO2 orCH4 was the initial gas for hydrate formation, when a second gaswas charged into the reactor, initially the gas phase of the cell wasmostly, if not entirely, composed of the initial system gas, whilethe newly injected gas entered the hydrate phase. For systems

where the cell was initially lled with CH4 and then charged withCO2 gas, the gas phase of the cell composed of pure CH4 for severalhours after the CO2 injection.Whereas for systems originally pres-surized with CO2 and then injected with CH4, the gas phase wasinitially mostly composed of CO2, but over a much shorter timeperiod, the percentage of CH4 in the gas phase increased, indicat-ing that CO2 gas exchanged with CH4 gas in the hydrate structure.Uchida et al.23 reported similar results with the following expla-nation: during initial stages of the hydrate formation, CH4 mole-cules are able to occupy both small and large hydrate cages, butover time the larger cages mostly trap CO2 gas, resulting in anincreased CH4 in the gas phase.23 The phase eld simulationmod-els have yielded similar results.31 Overall, whether CH4 or CO2 gasis introduced into a system of gas hydrates via bubbling, an in-stant formation of hydrates of injected gas is likely to occur.

ConclusionsThis study examined preliminary characteristics of the CH4

CO2 exchange phenomenon. Several noteworthy observations areas follows: (i) the induction times of hydrate formation were gen-erally shorter for CO2 than CH4, by as much as by a factor of four,with or without sediments; (ii) the hydrate formation data showthat when a secondary gas was injected into a system containingpreformed hydrates, the entering gas formed the hydrate phaseinstantly (within minutes); (iii) CO2 hydrates formed in a systemthat already contained CH4 hydrates were found to be morestable, whereas CH4 hydrates formed in a system consisting ofCO2 hydrates as hosts were initially stable, but CH4 gas in hydratesquickly exchanged with free CO2 gas to form more stableCO2 hydrates; (iv) the hydratememory effect was noted during the3 runs performed to form sand-hosted gas hydrates. In all 5 runs,even though the system was depressurized, left over a week atroom temperature, and ushed with nitrogen gas in betweenruns, the system still exhibited the memory effect. These resultscontradict those previously reported in the literature. In sum-mary, the observed fast CO2 hydrate formation from free CO2 gasin the presence of preformed CH4 hydrate indicate the feasibilityof developing a CO2 sequestration scheme using natural CH4 hy-drate reservoirs. Our ongoing and planned work includes runsthat will quantify the extent of uptake and liberation of gasesthrough the CH4CO2 exchange as a function of key hydrate pa-rameters and at the micrometer scale using X-ray computed mi-crotomography.

AcknowledgementsThe authors thank the Ofce of Vice-President of Research

(OVPR) at Stony BrookUniversity for providing funds for thework.The work was partially supported by the Program Developmentfunds at Brookhaven National Laboratory.

References(1) Sulston, J.; Rumsby, M.; Green, N. Environ. Resour. Econ. 2013, 55, 469. doi:10.

1007/s10640-013-9681-8.(2) Boswell, R.; Collett, T. S. Energy Environ. Sci. 2011, 4 (4), 1206. doi:10.1039/

C0EE00203H.(3) Collett, T.; Bahk, J.-J.; Baker, R.; Boswell, R.; Divins, D.; Frye,M.; Goldberg, D.;

Huseb, J.; Koh, C.; Malone, M.; Morell, M.; Myers, G.; Shipp, C.; Torres, M.J. Chem. Eng. Data 2015, 60 (2), 319. doi:10.1021/je500604h.

(4) Collett, T.; Bahk, J.-J.; Frye, M.; Goldberg, D.; Husebo, J.; Koh, C.; Malone, M.;Shipp, C.; Torres, M.; Myers, G.; Divins, D.; Morell, M. 2013; p Medium: ED.

(5) Priest, J. A.; Clayton, C. R. I.; Rees, E. V. L. Mar. Petrol. Geol. 2014, 58, 187.doi:10.1016/j.marpetgeo.2014.05.008.

(6) Ota, M.; Morohashi, K.; Abe, Y.; Watanabe, M.; Smith, R. L.; Inomata, H.Energy Convers. Manage. 2005, 46 (1112), 1680.

(7) Ohgaki, K.; Takano, K.; Moritoki, M. Kagaku Kogaku Ronbunshu 1994, 20 (1),121. doi:10.1252/kakoronbunshu.20.121.

(8) Komatsu, H.; Ota, M.; Smith, R. L., Jr.; Inomata, H. J. Taiwan Inst. Chem. Eng.2013, 44 (4), 517. doi:10.1016/j.jtice.2013.03.010.

(9) Jadhawar, P.; Mohammadi, A. H.; Yang, J.; Tohidi, B. Subsurface carbon dioxidestorage through clathrate hydrate formation; In Advances in the Geological Storage

Table 2. Induction timemeasuredwhen hydrates were rst sighted inthe Jerguson cell.

Induction time (h)

Run #Hostsediments

Gas additionsequence Charge 1 Charge 2

1 No CH4/CO2 96 Instant2 No CO2/CH4 22a 23 Ottawa sand CH4/CO2 24

of Carbon Dioxide. Nato Science Series: IV: Earth and Environmental Sciences, 2006;Vol. 65, pp. 111126.

(10) Graue, A.; Kvamme, B.; Baldwin, B. A.; Stevens, J.; Howard, J.; Aspenes, E.;Ersland, G.; Husebo, J.; Zornes, D. SPE J. 2008, 146.

(11) Baldwin, B. A.; Stevens, J.; Howard, J. J.; Graue, A.; Kvamme, B.; Aspenes, E.;Ersland, G.; Husebo, J.; Zornes, D. R. Magn. Reson. Imaging 2009, 27, 720.doi:10.1016/j.mri.2008.11.011.

(12) White, R.; Kumagai, T. Platts Gas Daily 2011, 28, 206.(13) Schoderbek, D.; Farrell, H.; Hester, K.; Howard, J.; Raterman, K.;

Silpngarmlert, S.; Martin, K. L.; Smith, B.; Klein, P. ConocoPhilips Company,2013.

(14) Brewer, P. G.; Peltzer, E. T.; Walz, P. M.; Coward, E. K.; Stern, L. A.;Kirby, S. H.; Pinkston, J. Energy Fuels 2014, 28 (11), 7061. doi:10.1021/ef501430h.

(15) Kester, D. R.; Duedall, I. W.; Connors, D. N.; Pytkowic, R. M. Limnol. Oceanogr.1967, 12 (1), 176. doi:10.4319/lo.1967.12.1.0176.

(16) Horvat, K.; Kerkar, P.; Jones, K.; Mahajan, D. Energies 2012, 5, 2248. doi:10.3390/en5072248.

(17) Santamarina, J. C.; Ruppel, C. In Proceedings of the 6th International ConferenceonGasHydrates (ICGH 2008), Vancouver, British Columbia, Canada, July 610, 2008;Vancouver, British Columbia, Canada, 2008.

(18) U.S. Silica Company. Material Safety Data Sheet Silica Sand and Ground Silica.(19) Colorado School of Mines. Software; Available from http://hydrates.mines.

edu/CHR/Software.html.

(20) Bishnoi, P. R.; Natarajan, V. Fluid Phase Equilib. 1996, 117 (12), 168. doi:10.1016/0378-3812(95)02950-8.

(21) Wu, Q.; Zhang, B. J. Nat. Gas Chem. 2010, 19 (4), 446. doi:10.1016/S1003-9953(09)60086-4.

(22) Clennell, M. B.; Hovland, M.; Booth, J. S.; Henry, P.; Winters, W. J. J. Geophys.Res.: Solid Earth 1999, 104 (B10), 22985. doi:10.1029/1999JB900175.

(23) Uchida, T.; Ikeda, I. Y.; Takeya, S.; Kamata, Y.; Ohmura, R.; Nagao, J.;Zatsepina, O. Y.; Buffett, B. A. ChemPhysChem 2005, 6 (4), 646. doi:10.1002/cphc.200400364.

(24) Handa, Y. P.; Stupin, D. Y. J. Phys. Chem. 1992, 96 (21), 8599. doi:10.1021/j100200a071.

(25) Jensen, L.; Thomsen, K.; von Solms, N. Chem. Eng. Sci. 2008, 63 (12), 3069.doi:10.1016/j.ces.2008.03.006.

(26) He, Y.; Rudolph, E. S. J.; Zitha, P. L. J.; Golombok, M. Fuel 2011, 90 (1), 272.doi:10.1016/j.fuel.2010.09.032.

(27) Golombok, M.; Ineke, E.; Luzardo, J. C. R.; He, Y. Y.; Zitha, P. Environ. Chem.Lett. 2009, 7 (4), 325. doi:10.1007/s10311-008-0173-y.

(28) Zhang, B. Y.; Wu, Q.; Gao, X.; Liu, C. L.; Ye, Y. G. Int. J. Environ. Pollut. 2013, 53(34), 201. doi:10.1504/IJEP.2013.059913.

(29) Sloan, E. D.; Koh, C. A. Clathrate hydrates of natural gases, 3rd ed.; CRC Press:Boca Raton, FL, 2008.

(30) Mazloum, S.; Yang, J.; Chapoy, A.; Tohidi, B. In Proceedings of the 7th InternationalConference on Gas Hydrates, Edinburgh, Scothland, United Kingdom, Edinburgh,Scothland, United Kingdom, 2011.

(31) Qasim, M.; Kvamme, B.; Baig, K. Int. J. Geol. 2011, 5 (2), 48.

Pagination not final (cite DOI) / Pagination provisoire (citer le DOI)Horvat and Mahajan 9

Published by NRC Research Press

Can.

J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.c

om b

y SU

NY

AT

STO

NY

BRO

OK

on

06/1

4/15

For p

erso

nal u

se o

nly.

ArticleIntroductionMaterials and methodsGasesArtificial seawaterHydrate former unit descriptionModes of unit operationGas analysis

Results and discussionRun 1: Baseline run with no host, CH4 followed by CO2 chargeRun 2: Baseline run with no host, CO2 followed by CH4 chargeRun 3: CH4 followed by CO2 charge hosted in Ottawa sandRun 4: CO2 followed by CH4 charge hosted in Ottawa sandRun 5: CH4 followed by CO2 charge hosted in Ottawa sandComparison of data from runs

Conclusions

AcknowledgementsReferences

/GrayImageMinResolutionPolicy /OK/ConvertImagesToIndexed true/MaxSubsetPct 99/Binding /Left/PreserveDICMYKValues false/GrayImageMinDownsampleDepth 2/MonoImageMinResolution 1200/sRGBProfile (sRGB IEC61966-2.1)/AntiAliasColorImages false/GrayImageDepth -1/PreserveFlatness true/CompressPages true/GrayImageMinResolution 150/CalCMYKProfile (U.S. Web Coated \050SWOP\051 v2)/PDFXBleedBoxToTrimBoxOffset [0.00.00.00.0]/AutoFilterGrayImages true/EncodeColorImages true/AlwaysEmbed []/EndPage -1/DownsampleColorImages true/ASCII85EncodePages false/PreserveEPSInfo false/PDFXTrimBoxToMediaBoxOffset [0.00.00.00.0]/CompatibilityLevel 1.3/MonoImageResolution 600/NeverEmbed [/Arial-Black/Arial-BlackItalic/Arial-BoldItalicMT/Arial-BoldMT/Arial-ItalicMT/ArialMT/ArialNarrow/ArialNarrow-Bold/ArialNarrow-BoldItalic/ArialNarrow-Italic/ArialUnicodeMS/CenturyGothic/CenturyGothic-Bold/CenturyGothic-BoldItalic/CenturyGothic-Italic/CourierNewPS-BoldItalicMT/CourierNewPS-BoldMT/CourierNewPS-ItalicMT/CourierNewPSMT/Georgia/Georgia-Bold/Georgia-BoldItalic/Georgia-Italic/Impact/LucidaConsole/Tahoma/Tahoma-Bold/TimesNewRomanMT-ExtraBold/TimesNewRomanPS-BoldItalicMT/TimesNewRomanPS-BoldMT/TimesNewRomanPS-ItalicMT/TimesNewRomanPSMT/Trebuchet-BoldItalic/TrebuchetMS/TrebuchetMS-Bold/TrebuchetMS-Italic/Verdana/Verdana-Bold/Verdana-BoldItalic/Verdana-Italic]/CannotEmbedFontPolicy /Warning/PreserveOPIComments false/AutoPositionEPSFiles true/JPEG2000GrayACSImageDict >/PDFXOutputIntentProfile ()/EmbedJobOptions true/JPEG2000ColorACSImageDict >/MonoImageDownsampleType /Average/DetectBlends true/EmitDSCWarnings false/ColorImageDownsampleType /Average/EncodeGrayImages true/AutoFilterColorImages true/DownsampleGrayImages true/GrayImageDict >/AntiAliasMonoImages false/GrayImageAutoFilterStrategy /JPEG/GrayACSImageDict >/ColorImageAutoFilterStrategy /JPEG/ColorImageMinResolutionPolicy /OK/ColorImageResolution 300/PDFXRegistryName ()/MonoImageFilter /CCITTFaxEncode/CalGrayProfile (Gray Gamma 2.2)/ColorImageMinDownsampleDepth 1/PDFXTrapped /False/DetectCurves 0.1/ColorImageDepth -1/JPEG2000GrayImageDict >/TransferFunctionInfo /Preserve/ColorImageFilter /DCTEncode/PDFX3Check false/ParseICCProfilesInComments true/DSCReportingLevel 0/ColorACSImageDict >/PDFXOutputConditionIdentifier ()/PDFXCompliantPDFOnly false/AllowTransparency false/UsePrologue false/PreserveCopyPage true/StartPage 1/MonoImageDownsampleThreshold 1.0/GrayImageDownsampleThreshold 1.0/CheckCompliance [/None]/CreateJDFFile false/PDFXSetBleedBoxToMediaBox true/EmbedOpenType false/OPM 0/PreserveOverprintSettings false/UCRandBGInfo /Remove/ColorImageDownsampleThreshold 1.0/MonoImageDict >/GrayImageDownsampleType /Average/Description >/CropMonoImages true/DefaultRenderingIntent /RelativeColorimeteric/PreserveHalftoneInfo false/ColorImageDict >/CropGrayImages true/PDFXOutputCondition ()/SubsetFonts true/EncodeMonoImages true/CropColorImages true/PDFXNoTrimBoxError true>>setdistillerparams>setpagedevice