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Available online at www.sciencedirect.com

Fluid Phase Equilibria 261 (2007) 407–413

Dissociation studies of CH4–C2H6 and CH4–CO2 binary gas hydrates

Laura J. Rovetto, Kristen E. Bowler, Laura L. Stadterman, Steven F. Dec,Carolyn A. Koh, E. Dendy Sloan Jr. ∗

Center for Hydrate Research, Department of Chemical Engineering, Colorado School of Mines, Golden, CO 80401, USA

Received 12 May 2007; received in revised form 2 August 2007; accepted 2 August 2007Available online 14 August 2007

bstract

The objective of this work was to enhance understanding of gas hydrate dissociation, and provide information to help evaluate gas hydraterocesses, such as ocean CO2 sequestration and CH4 recovery from natural gas hydrates in deep sea sediments. The composition and dissociation

ehavior of CH4 + C2H6 and CH4 + CO2 structure I binary gas hydrates were determined using NMR spectral analysis. Single-pulse excitation wassed in combination with magic-angle spinning (MAS). Time-resolved decomposition experiments were carried out on sealed, pressurized samplesf both binary gas hydrate systems. Decomposition profiles have been obtained for each case. At the experimental conditions of this work, the gasydrate composition did not change after the dissociation experiments. These results suggest that the unit cell decomposes as a single entity.

2007 Elsevier B.V. All rights reserved.

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eywords: Methane; Ethane; Carbon dioxide; Clathrate hydrate; NMR

. Introduction

Clathrate hydrates are solid crystalline inclusion compoundsonsisting of a hydrogen-bonded network of polyhedral wateravities, which host small guest molecules (<0.9 nm). Gasydrates typically occur at low temperatures and elevated pres-ures, and are generally classified into three different crystalypes, known as structures, I, II and H, which differ from eachther in the number and size of cavities present in the unit cell.he pentagonal dodecahedron 512 cavity (constructed of 12 pen-

agonal faces) is present in all three of these structures. In sI andII this cavity is accompanied by polyhedral 51262 and 51264 cav-ties, respectively (formed by pentagonal and hexagonal faces).tructure H comprises two other cavities in addition to the 512

avity, the 435663 and the 51268 cavities [1–3].Natural gas hydrates occurring in the deep sea and per-

afrost are recognized as a potential large energy resource, dueo their significant abundance in geological settings [4] (on therder of 1–5 × 1015 m3 at STP [5]). The recovery of the nat-

ral gas encaged into gas hydrate structures has been recentlyested with successful results [6]. Natural gas hydrates contain-ng mainly methane (which is an effective greenhouse gas) are

∗ Corresponding author. Tel.: +1 303 523 7788; fax: +1 303 273 3730.E-mail address: esloan@mines.edu (E.D. Sloan Jr.).

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378-3812/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.fluid.2007.08.003

lso identified as a possible environmental hazard; therefore theissociation of hydrates could have a significant impact on thenvironment [4–7].

On the other hand, as a by-product of combustion processes,he world CO2 gas production is in constant increase; beinghe main factor contributing to the greenhouse effect and globallimate change [8]. In order to reduce the CO2 concentration inhe atmosphere, CO2 gas hydrate formation (by injecting liquidarbon dioxide in the deep ocean) is under investigation as anossible solution for carbon dioxide sequestration [9].

A combined mechanism for CH4 recovery from naturaleposits and simultaneous CO2 sequestration in the remainingas hydrate structures appears to be an attractive technology10,11]. To convert this potential technology into a viable pro-ess, thermodynamic information alone is not sufficient [12];inetics studies must also be performed to provide the nec-ssary information to evaluate the potential of this combinedechnology.

The thermodynamics of gas hydrates is reasonably well estab-ished [1] but studies of kinetic processes, especially at the

icroscopic level, are more restricted. Several techniques, suchs X-ray [13], neutron diffraction [14–17], NMR and Raman

pectroscopy [18,19] have been used to study the molecular-cale hydrate formation and decomposition processes.

Hydrate structure and guest distribution in the hydrate cagesave been studied using time-resolved Raman spectroscopy in

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08 L.J. Rovetto et al. / Fluid Pha

ydrate formation experiments; formation of the 51262 cage ofI methane hydrate was found to be the rate limiting step [19].MR spectroscopy has been applied to study the formation andecomposition of xenon sI hydrate, using hyperpolarized 129Xe.he relative occupancy of the small 512 cage of sI Xe hydrateas found to be higher during the initial formation stage and

herefore considered as a precursor of hydrate formation [20].as chromatography, Raman and X-ray diffraction were used to

how that during the formation of the binary methane–propaneas hydrate, the propane molecules were preferentially encagedn the hydrate crystals over methane, at propane concentrationselow 10 vol% [21]. These results for gas hydrate formationhow that hydrate cavities may have different formation rates.

In NMR decomposition experiments of hyperpolarized29Xe, the xenon sI hydrate dissociation showed no preferen-ial rate of dissociation of either cage [20]. A more recent studyas used time-resolved 13C magic-angle spinning (MAS) NMRo study the decomposition of methane sI hydrate [22], in whichoth the large and small cages filled with methane gas were foundo decompose at the same rate. The methane and xenon sI hydrateecomposition studies suggest that sI hydrate unit cells of theseimple gas hydrates decompose as a single entity, at least on theime scale of the NMR measurements. The decomposition of theinary ethane–methane sI gas hydrate was also recently studiedsing NMR spectroscopy [23]. The observed faster decompo-ition rate of the large cavity occupied by ethane molecules isxplained by a simple model, which takes into account the frac-ional occupancy of the small cavities by methane moleculesi.e., function of hydrate composition). Unit cells with emptymall cages are less stable than those fully occupied by guestolecules, giving an apparently faster decomposition rate of the

arge cage; such behavior was more evident at higher subcooling.Studies of methane replacement by carbon dioxide have been

eported applying magnetic resonance imaging. The dynamicsf the gas exchange was monitored in sandstone porous sam-les, and methane gas production was reported to occur withimultaneous hydrate conversion to carbon dioxide gas hydrate24].

In this work, 13C MAS NMR spectroscopy was used to studyhe molecular-scale decomposition of binary CH4 + C2H6 andH4 + CO2 hydrate systems, with the intention of providingseful information for gas recovery and sequestration processes.

. Experimental

.1. Materials

Enriched 13C gases were used for the 13C NMR studies;his technique provides information only on 13C atoms present.3CH4 and 13CO2 (both 99% purity) were purchased from Cam-ridge Isotope Laboratory and used to form methane–carbonioxide gas hydrates. Enriched 13CH3CH3 (99% purity) wasupplied by Isotec and used to form the methane–ethane

ydrates. In our experiments all the components (13CH4,3CH3CH3, 13CO2) have the same number of 13C present onheir structure because 13C is enriched at only one carbon site.eionized water was used without any further purification.

uawt

uilibria 261 (2007) 407–413

.2. Experimental procedure

.2.1. Hydrate sample preparationFine ice particles (250–500 �m) were used as the starting

aterial for gas hydrate formation. Approximately 0.1 cm3 ofranular ice was placed into a 1.0 cm3 pyrex glass tube andressurized with the gas mixture (CH4 + C2H6 or CH4 + CO2),etween 3.5 and 5 bar above its calculated equilibrium pressure.he tube filled with ice particles plus the gas mixture was thenlaced in a temperature controlled bath at 266 K for 2 days.he bath temperature was later increased to 273 K (usually for–14 days) to allow full conversion of ice particles into gasydrate. After gas hydrates were formed, the free gas present inhe glass tube was removed by using a vacuum pump, keeping theydrate portion submersed in the bath at approximately 266 Knd then, immediately transferred to a liquid nitrogen bath tovoid the decomposition of the sample. Finally, the bottom of theube containing the gas hydrate was sealed using a flame torchhile immersed in liquid nitrogen giving as a result a closed

ystem glass bulb containing the hydrate sample. The sealingrocedure may cause the decomposition of some small amountf hydrate, therefore the bulb was placed in a freezer at 253 Kor at least 2 days in order to reestablish the equilibrium of theystem. Any change in composition during the sample sealingrocess is negligible for the purpose of this study; the overallomposition actually produced was determined using 13C MASMR spectroscopy after this procedure.

.2.2. NMR spectroscopyAll 13C MAS NMR spectra were recorded on a Chemagnetics

nfinity 400 NMR spectrometer operating at 100.5 MHz for 13C.roton decoupling fields of 50 kHz and MAS speeds of aboutkHz were used.

For both binary hydrate samples, single-pulse excitationxperiments (90◦ pulses of 5 �s) and various pulse delays,epending on the spin–lattice time (T1), were used to recordully relaxed spectra at various temperatures. Time-resolved3C MAS NMR spectra were recorded with single-pulse excita-ion (45◦ pulses of 2.5 �s for CH4 + C2H6 samples; 67◦ pulsesf 3.7 �s for CH4 + CO2 samples) and various pulse delays inhe hydrate dissociation experiments. Under the conditions ofhe time-resolved 13C MAS NMR experiment with an � pulse45◦ for CH4 + C2H6 samples; 67◦ for CH4 + CO2) and pulseepetition rate of TR the magnetization M(TR) is given by [25]

(TR) = Meq1 − e−TR/T1

1 − cos(α) e−TR/T1(1)

eq is the equilibrium magnetization. M(TR) reached its steady-tate value after about five pulse repetitions. Spin–latticeelaxation times (T1) were measured using a standardnversion–recovery pulse sequence [26] and are summarized inable 1 for both systems.

The methylene carbon resonance line of adamantane was

sed as an external chemical shift standard and was assignedvalue of 38.83 ppm [22]. The spectrometer was equippedith Chemagnetics solid-state MAS speed and temperature con-

rollers.

L.J. Rovetto et al. / Fluid Phase Equilibria 261 (2007) 407–413 409

Table 1Spin–lattice relaxation times of sI gas hydrates

Site T (K) T1a (s)

CH4–C2H6 binary gas hydrate system [23]CH4 gas phase 253–270 0.17 ± 0.01C2H4 gas phase 253–270 0.8 ± 0.2

CH4 sI small cage 253–270 4.5 ± 0.2C2H6 sI large cage 253–270 5.9 ± 0.3

CH4–CO2 binary gas hydrate systemCH4 gas phase 253–268 0.07 ± 0.01CO2 gas phase 253–268 0.99 ± 0.06

CH4 sI small cage 253–268 1.63 ± 0.17CH4 sI large cage 253–268 1.15 ± 0.02CO2 sI hydrate phaseb 253–268 1.29 ± 0.13

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Fig. 2. Relative integrated intensities of 13C resonance lines vs. time, obtainedfrom a thermally activated CH4–C2H6 sI hydrate decomposition experiment(11.7 ± 3.5% CH4 and 88.3 ± 4.4% C2H6 gas phase composition). A total of150 single-scan spectra were collected over a period of time of 475 s; a 45◦ee

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a Average over temperature range specified.b Small and large cage occupancy are not distinguishable.

Temperature calibration at the position of the sample has beenescribed elsewhere [22]. Thermal activation of the samplesas achieved using a temperature-jump method by increasing

he set temperature of the temperature controller; single-pulsepectra were recorded during this temperature change over aertain period of time. The sample reached the final temperaturef the temperature-jump, TJ, in about 40 s. Temperature gradi-nts across the sample are negligible due to the small sampleize. Details regarding each thermally activated decompositionxperiment are provided in the text and figure captions.

. Results and discussion

.1. CH4–C2H6 system

Fig. 1 shows a typical 13C MAS NMR spectra of CH4 + C2H6I hydrate (gas phase 11.7 ± 3.5% methane and 88.3 ± 4.4%

thane; hydrate phase 6.5 ± 0.9% methane and 93.5 ± 1.3%thane), initially at equilibrium (bottom of figure) and after par-ial decomposition (top of figure). Four different 13C isotropic

ig. 1. 100.5 MHz 13C MAS NMR spectra of a CH4–C2H6 sI hydrate at equi-ibrium conditions and partially decomposed. A 90◦ excitation pulse of 5 �sas applied, with a pulse delay of 5 s and acquisition time equals 0.1024 s. The

pectrum corresponds to the sum of 16 scans.

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xcitation pulse of 2.5 �s was used with a pulse delay of 3 s and acquisition timequals to 0.1638 s. �T = 2 K; TJ = 273 K.

esonance lines appear in each spectrum. The hydrate phase3C resonance line at about −3.7 ppm corresponds to CH4 inhe small cage of sI hydrate, whereas the 13C resonance line atbout −10.4 ppm corresponds to CH4 in the gas phase. The 13Cesonance line at 7.8 ppm is due to C2H6 in the large cage of sIydrate and that at 3.8 ppm is due C2H6 in the gas phase [27,28].MR peaks represent only those small and large cages that areccupied by a guest molecule.

The effect of raising the temperature of the sI sample yieldshe expected result; the relative intensities of CH4 and C2H6 inhe hydrate phase decrease while those of CH4 and C2H6 in theas phase increase in this closed system. The change of the inten-ities of each 13C resonance lines reflects the change in vapornd hydrate compositions; such compositions were calculatedy normalizing the integrated peak area of every componentresent in each phase.

The decomposition process was thermally activated byncreasing the temperature in a 2◦ temperature ramp at time t = 0.he integrated relative intensities I (corrected according to Eq.

1)), obtained from the time-resolved 13C MAS NMR spectrauring the decomposition experiment are plotted as a functionf time in Fig. 2.

During the first 40 s the sample approaches TJ (273 K) andome hydrate starts to decompose. In Fig. 2 the decomposi-ion profile shows that between 0 and 210 s the CH4–C2H6ydrate decomposes at its fastest rate. After approximately 210 she intensity of all CH4 and C2H6

13C NMR resonance linespproaches a constant value at which point the partial hydrateecomposition process has ended; significant changes were notbserved any longer at the final temperature (TJ).

Although the pressure of the system can not be directlyeasured, by taking into account the final gas composition at

emperature TJ (273 K), the equilibrium pressure reached for theystem can be predicted using the software CSMGem [29] ands equal to 4.9 bar.

410 L.J. Rovetto et al. / Fluid Phase Eq

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ig. 3. Relative integrated intensities of C2H6 sI large cage and CH4 sI smallage of data from Fig. 2, between 0 and 210 s. �T = 2 K; TJ = 273 K.

The period of time where the hydrate decomposes at its fastestate (between 0 and 210 s) was expanded upon and analyzed ashown in Fig. 3. The hydrate decomposition rate was determineds the rate of intensity decrease and obtained from the lineart of the data in Fig. 3. The hydrate decomposition and gaselease rates from the linear fit are summarized in Table 2. In sIas hydrates there are six 51262 large cages per two 512 smallages (ratio large/small = 3). In our analysis we divided the areaf large cages by a factor of three to remove the large cageegeneracy, resulting in the relative cage occupancy. The ratio,n a per cage basis, of the C2H6 sI large cage decomposition rate0.00043/3 cage−1 s−1) to the CH4 sI small cage decompositionate (0.00003 cage−1 s−1) is approximately equal to five.

During this decomposition step, 10% of the hydrate phaseresent in the system dissociates; however the composition ofhe phases (gas hydrate with 5.0 ± 2.1% methane, 94.9 ± 2.5%thane and gas phase with 12.5 ± 2.8% methane, 87.5 ± 4.9%thane) remains the same within experimental error. Based onhe NMR spectra, ethane only occupied the large cages while

ethane occupied the small cages. After normalizing the ethaneontent to reflect that there are three times as many large cagesersus small cages in sI, the calculated relative ratio of ethaneo methane concentration in the hydrate was approximately five.or stoichiometric C2H6–CH4 gas hydrate (75% C2H6 and 25%H4) the composition ratio C2H6/CH4 is equal to one. However

n this case, due to the low CH4 concentration, it is not surprising

hat the C2H6/CH4 gas hydrate concentration shows a higheralue than the stoichiometric sI methane–ethane gas hydrate.t this concentration, a big fraction of small cages are empty;

ssuming fully occupancy of large cages by ethane it is estimated

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able 2inear fit values to the initial sI gas hydrate decomposition and gas release rate for the

ate)

Gas release rate (dI/dt)(intensity 104 s−1)

Hydra(inten

H4 0.3 ± 0.1 −0.3

2H6 4.3 ± 0.2 −4.2

uilibria 261 (2007) 407–413

hat only 21% of the small cages were occupied by methaneCSMGem [29] predict 0.95 fractional occupancy of large cagesy ethane and only 0.13 fractional occupancy of small cages byethane).The C2H6/CH4 gas hydrate composition on a per cage basis

emained unchanged (∼5) after the partial dissociation causedy the temperature ramp, indicating that unit cells were decom-osing as a single entity, with no preferential cage dissociationbserved.

Under these conditions the computed ratio of (C2H6(large cage)/H4(small cage)) cage dissociation rates obtained from Fig. 3 ispproximately 5 and proportional to the bulk C2H6/CH4 gasydrate composition of this sample; however this is not a gen-ral result since the decomposition rate ratio is strong functionf TJ. Our previous work [23] shows that the observed decompo-ition rate ratio expressed as (C2H6(large cage)/CH4(small cage)) is aunction of the fractional occupancy of the gas hydrate. For thisinary system with gas composition 11.7 ± 3.5% methane and8.3 ± 4.4% ethane, at temperature ramps at lower TJ (273 K),he unit cells with more empty small cages are hypothesizedo dissociate first, hence the experimentally observed decom-osition rate ratio would be higher than the bulk concentrationf the hydrate. Through successive temperature tamps (as TJs increased), the observed decomposition rate ratio wouldecrease. At some TJ, as shown in this work, the observedecomposition rate ratio is equivalent to the concentration ofhe hydrate. At temperature ramps to a higher TJ the observedecomposition rate ratio would decrease to values lower thanhose of the bulk sample [23].

.2. CH4–CO2 system

Due to the molecular size of CO2 molecules, both cavitiesf sI gas hydrate can be occupied by CO2 molecules, but withreferential occupancy of the 51262 large cavity (in this case thearge cage to molecule diameter ratio is closer to its optimumalue [1]). The 13C resonance signal for CO2 hydrate with-ut MAS shows a motionally averaged powder pattern betweenbout 100.2 and 188.5 ppm due to the anisotropic rotation ofO2 molecules entrapped in the asymmetrical 51262 large cav-

ty of sI gas hydrate [30]. When the small cavities 512 of sIas hydrate (with pseudo-spherical symmetry) are occupied byO2 molecules, their motion gives a relatively sharp peak at23.1 ppm superimposed on the CO2 large cage powder pattern.

uch information cannot be obtained by performing single-pulse3C MAS experiments because the isotropic resonance lines ofO2 in the large and small cages of sI and in the gas phase areot resolved. As a result of spinning the sample, the 13C reso-

CH4–C2H6 system (ratio of C2H6 large cage to CH4 small cage decomposition

te dissociation rate (dI/dt)sity 104 s−1)

Hydrate dissociation (1/3)large/small cage ratio

± 0.2 (small cage) ∼5± 0.3 (large cage)

L.J. Rovetto et al. / Fluid Phase Eq

Fig. 4. 100.5 MHz 13C MAS NMR spectra of a CH4–CO2 sI hydrate at equilib-ras

nsil

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tseoeintrate of CH4 hydrate content is equal to the dissociation rate of

ium conditions and partially decomposed. A 90◦ excitation pulse of 5 �s waspplied, with a pulse delay of 5 s and acquisition time equals 0.04096 s. Thepectrum corresponds to the sum of 16 scans.

ance lines of CO2 are modulated resulting in the appearance ofpinning side bands due to CO2 in the hydrate phase at approx-mately 207, 180, 153, 98 and 70 ppm as well as the isotropicine at 125.6 ppm (see Fig. 4).

A typical single-pulse 13C MAS spectrum for the binaryI CH4–CO2 gas hydrate (gas phase 13.0 ± 1.0% methanend 87.0 ± 1.0% carbon dioxide; hydrate phase 12.3 ± 1.1%ethane and 87.7 ± 1.1% carbon dioxide) is shown in Fig. 4;

t equilibrium (bottom of figure) and after partial decomposi-ion (top of figure). The 13C resonance lines corresponding to theH4 region are located at equivalent positions as the CH4–C2H6

ystem, i.e., CH4 in the small and large cage of sI hydrate at −3.6

ig. 5. Relative integrated intensities of 13C resonance lines vs. time, obtainedrom a thermally activated CH4–CO2 sI hydrate decomposition experiment12.3 ± 1.1% CH4 and 87.7 ± 1.1% CO2 gas phase composition). A total of50 single-scan spectra were collected over a period of time of 385 s; a 67◦xcitation pulse of 3.7 �s was used with a pulse delay of 1.5 s and acquisitionime equals 0.04096 s. �T = 2 K; TJ = 270 K.

C

b

Foioetl

uilibria 261 (2007) 407–413 411

nd −5.9 ppm, respectively, and the gas phase at −10.3 ppm.he 13C resonance lines corresponding to the CO2 region ares assigned above and labeled in the figure. When the samples partially decomposed, an increase in the relative intensity ofhe peaks assigned to the gas phase is observed (−10.3 ppm forH4 and 125.9 ppm for CO2); with a simultaneous decrease in

he intensities of the peaks corresponding to the hydrate phase.For the decomposition study, the sample was thermally acti-

ated following the same procedure applied for the CH4–C2H6ystem. Fig. 5 shows the system response to a 2◦ temperatureamp; the integrated relative intensities I (corrected according toq. (1)) are plotted versus the time length of the experiment. The

otal relative intensity of the CO2 gas hydrate phase is obtainedy summing the integrated relative intensity of the 13C isotropicesonance line at 125.6 ppm and all its observable spinning sideands during the time-resolved 13C MAS NMR experiment (cor-esponding to the 13C resonance lines at 180, 153 and 98 ppmn this case).

The CO2 gas phase integrated relative intensity is obtainedy deconvolution of the overlapped peaks corresponding to CO2n the gas phase at 125.3 ppm and the isotropic resonance linef CO2 gas hydrate at 125.6 ppm.

From the data plotted in Fig. 5, very little hydrate is observedo decompose over the time of this experiment; the rate of inten-ity change is practically constant over the duration of thisxperiment. Similar constant decomposition profile has beenbserved in different samples and successive decompositionxperiments with 2◦ temperature ramps, below and above thece point. Based on the linear fit of the data in Fig. 5, there iso observable difference on the CH4 and CO2 gas release dueo the decomposition of the hydrate; moreover the dissociation

O2 hydrate content (within experimental error).In order to obtain more conclusive information, the same

inary sI CH4–CO2 gas hydrate sample was subsequently ther-

ig. 6. Relative integrated intensities of CO2 and CH4 in the gas phase vs. time,btained from a thermally activated CH4–CO2 sI hydrate decomposition exper-ment (12.3 ± 1.1% CH4 and 87.7 ± 1.1% CO2 gas phase composition). A totalf 500 single-scan spectra were collected over a period of time of 769 s; a 67◦xcitation pulse of 3.7 �s was applied with a pulse delay of 1.5 s and acquisitionime equals 0.04096 s. �T = 7 K; TJ = 275.5 K. Gas release rates obtained by ainear fit of data from 0 to 230 s.

412 L.J. Rovetto et al. / Fluid Phase Equilibria 261 (2007) 407–413

Table 3Linear fit values to the initial sI gas hydrate decomposition and gas release rates for the CH4–CO2 system (ratio of CO2 to CH4 gas release rate)

Gas release rate (dI/dt)(intensity 104 s−1)

Hydrate dissociation rate (dI/dt) (intensity 104 s−1) Gas release CO2/CH4

rate ratio

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ally activated with a 7◦ temperature ramp. Under a moreevere system perturbation (i.e., a larger temperature ramp withJ = 275.5 K), the decomposition profile exhibits a more signif-

cant change in the relative intensities between 0 and 230 s (seeigs. 6 and 7); after approximately 230 s the intensities of all the3C NMR resonance lines reach a constant value, meaning thathe decomposition process ceases.

Based on the gas composition of the sample after the disso-iation experiment at temperature TJ, the equilibrium pressure

s predicted to be approximately 17 bar using CSMGem [29].

Fig. 6, depicts the changes of integrated relative intensities Icorrected according to Eq. (1)) of the CH4 and CO2 present inhe gas phase as a function of time. From the linear fit of the data

ig. 7. (a) Relative integrated intensities of CO2 and CH4 in the hydrate phase vs.ime, obtained from a thermally activated CH4–CO2 sI hydrate decompositionxperiment (12.3 ± 1.1% CH4 and 87.7 ± 1.1% CO2 gas phase composition).T = 7 K; TJ = 275.5 K. (b) Relative integrated intensities of CO2 gas hydrate

nd CH4 in the small and large cage of sI hydrate data between 0 and 230 s, froma). �T = 7 K; TJ = 275.5 K. Decomposition rates obtained by a linear fit of datarom 0 to 230 s.

otiwdi

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4

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etween 0 and 230 s, the rate of change of the relative intensityf CO2 gas is one order of magnitude larger than that of theH4 gas (see Table 3). After 230 s the relative intensities of bothomponents present in the gas phase reach a constant value; gaselease due to gas hydrate dissociation has ended.

Consistent with this information, Fig. 7 shows the change onhe integrated relative intensities of the CO2 and CH4 hydratehase as function of time during the 7◦ temperature ramp. Fig. 7aepicts the total length of the experiment whereas Fig. 7b focusesn the changes observed in the integrated relative intensity ofhe gas hydrate phase between 0 and 230 s; a larger decreasen the relative intensity of the CO2 gas in the hydrate phaseas observed (see Table 3) and complemented the informationepicted in Fig. 6. After 230 s there were no observable changesn the relative intensities of the hydrate components.

After the 7◦ temperature ramp, the amount of hydrate phaseresent in the system dropped from 74 to 32%; however theelative composition of the hydrate phase remained constant87.3 ± 0.6% CO2 and 12.7 ± 0.6% CH4). Moreover, the CH4age occupancy large/small ratio was determined before andfter the decomposition process and the value of 0.33 ± 0.02 alsoemained unchanged. These results suggest that at these condi-ions the unit cell decompose as an entity, without preferentialissociation of hydrate cages.

Additionally, the linear fit from the gas release data in Fig. 6,ives a decomposition rate ratio for CO2/CH4 of approximately; a similar value to the CO2/CH4 gas hydrate composition.t these experimental conditions the CO2/CH4 gas release rate

atio is proportional to the relative concentration of the guestolecules (CO2/CH4) in the gas hydrate phase.

. Conclusions

Thermally activated decomposition of CH4–C2H6 (gas com-osition 11.7 ± 3.5% CH4) and CH4–CO2 (gas composition3.0 ± 1.0% CH4) sI gas hydrates have been monitored usingime-resolved 13C MAS NMR spectroscopy.

Under the experimental conditions used in this work, the gasydrate concentration did not change as a consequence of theissociation process when the temperature ramp �T = 2 K waspplied. In addition, the decomposition of the sI 51262 large cageoccupied by C2H6 molecules) relative to the sI 512 small cageoccupied by CH4 molecules) was found to be equal to the bulkas hydrate composition ratio at TJ = 273 K.

Similar results were obtained from the CH4–CO2 sI system

t TJ = 270 K, the gas hydrate concentration remains the samewithin the experimental error) after the dissociation experi-ent. For this particular case, a more severe thermal activation

�T = 7 K) than for the CH4–C2H6 system was required, in order

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L.J. Rovetto et al. / Fluid Pha

o be able to obtain the dissociation rates. The different responsef the CH4–CO2 system under a temperature ramp �T = 2 K isnclear at this moment; additional experiments are under wayn order to further elucidate this point. Also at these conditions,he ratio of CO2 gas release rate relative to CH4 gas release wasound to be proportional to the gas hydrate concentration ratio.

Overall, under the experimental conditions applied in thisork, no changes were observed in the overall gas hydrate com-osition due to the dissociation process. The results from thisork indicate that each unit cell decomposes as a single entity;

hese findings are in agreement with previous reports on Xe andH4 pure gas hydrates and the CH4–C2H6 binary system.

cknowledgement

The authors gratefully acknowledge the financial supportrovided by the National Science Foundation, research grantTS-0419204.

eferences

[1] E.D. Sloan, C.A. Koh, Clathrate Hydrates of Natural Gases, third ed., Taylor& Francis, CRC Press, 2007.

[2] E.D. Sloan, Nature 426 (2003) 353–359.[3] C.A. Koh, Chem. Soc. Rev. 31 (3) (2002) 157–167.[4] M.D. Max (Ed.), Natural Gas Hydrate in Oceanic and Permafrost Environ-

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