Calibration of Raman Quantification Factors of Guest Molecules in Gas

Article pubs.acs.org/jced

Calibration of Raman Quantification Factors of Guest Molecules in Gas Hydrates and Their Application to Gas Exchange Processes Involving N2 Junfeng Qin and Werner F. Kuhs* GZG Abt. Kristallographie, Georg-August-Universität Göttingen, 37077 Göttingen, Germany ABSTRACT: Methane-dominated natural gas hydrate deposits have been considered as a potential hydrocarbon resource and as long-term storage reservoirs for the anthropogenic greenhouse gas CO2 via CH4−CO2−N2 replacement in gas hydrates. In this study, N2-hydrates of structure type I (sI) were formed, characterized, and quantified in terms of N2 cage occupancies using synchrotron X-ray diffraction. Pure sI CH4- and N2hydrates with known cage occupancies were used to calibrate the relative Raman quantification factors (F-factors) of N2 to its H2O framework and to CH4 in sI hydrate phase. The F-factors of CO2/CH4, CO2/H2O, and CH4/H2O in the hydrate cavities were corrected for the presence of ice Ih. Using these empirical ratios of F-factors, the absolute cage occupancies, the bulk guest composition, and hydration number of gas hydrates containing CH4, CO2, N2, and C2H6 molecules can now be determined by Raman spectroscopy without additional thermodynamic assumptions. In this way, one can gain insight into details of the gas composition in mixed hydrates, for example, during the N2-assisted CH4−CO2 exchange reaction, as well as into the preference of certain gas species for small or large cages.

■

INTRODUCTION Gas hydrates are nonstoichiometric crystalline compounds, which consist of a framework of hydrogen-bonded water molecules encaging small guest molecules. Natural gas hydrates (NGHs), mainly containing methane as guest, are found worldwide as pore space fillings in sedimentary matrices of continental margins as well as in and below permafrost regions, given a sufficiently low temperature and high pressure (fugacity) to guarantee gas hydrate stability. NGHs generally crystallize into cubic structure I (sI), cubic structure II (sII), and hexagonal structure H (sH).1 Pure CH4- and CO2-hydrates are usually found to be sI, with a unit cell containing 46 water molecules organized into two small cages (SCs), 512, and six large cages (LCs), 51262. N2-hydrates, on the other hand, are known to form sII at low temperatures and moderate pressures.2−4 Considerable efforts have been made to develop efficient methods to extract hydrocarbons from hydrate-bearing sediments, mostly based on promoting the in situ dissociation of gas hydrates, such as by depressurization, heating, or sometimes by the addition of chemical inhibitors; unfortunately, the decomposition of NGHs produces sand and other fine-grained sediments as well as water, leading to a mechanical instability of the reservoir and technical malfunction.5 In the spring 2013, approximately 12 000 m3 of methane were successfully produced from the first offshore test at Nankai Trough, Japan, using depressurization, but the test was terminated after 6 days because of uncontrollable sand production.6 Thus, methane production from NGH deposits has proven to be a considerable challenge for which, aside from engineering issues, economic, environmental, and safety concerns are raised. © 2014 American Chemical Society

An alternative conceptual approach to recover CH4 from NGHs is by injecting CO2, which has two advantages. First, the CH4 recovery is coupled with the long-term sequestration of the greenhouse gas CO2, as was first proposed by Ohgaki et al. on the basis of the preferable guest distributions in the equilibrated CH4−CO2−water−hydrate system;7 CH4−CO2 replacement in gas hydrates is thermodynamically favorable because CO2-hydrates are more stable than CH4-hydrates at temperatures below 283 K. Second, water remains bound in the solid state by the formation of CO2 hydrates. The volume change from CH4-hydrates to CO2-hydrates is small because of its identical crystalline structure (sI) with very similar lattice constants. Consequently, the stability of the original sea floor or permafrost area will be less affected upon methane recovery; indeed, experimental results show that hydrate-bearing sediments remained mechanically stable during and after displacement.8,9 Laboratory-scale studies have verified the successful release of CH4 from synthetic or natural gas hydrates and the formation of CO2-hydrates caused by CO2 addition in condensed gas, liquid, supercritical phase, or CO2-in-water emulsion, see Table 1.10−25 As it can also be seen in Table 1, the reported recovery ratios from the CO2−CH4 substitution experiments vary widely; performed under various conditions and using different analyzing methods, such as mass balance Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: July 3, 2014 Accepted: August 29, 2014 Published: September 12, 2014 369

dx.doi.org/10.1021/je500613y | J. Chem. Eng. Data 2015, 60, 369−375

Journal of Chemical & Engineering Data

Article

Table 1. Replacement Studies of CH4 in sI CH4 Hydrate Substituted by CO2 and CO2 + N2 Mixturea starting fluid

hydrate formation

P/MPa

0.97CO2 + 0.03CH4 CO2 CO2 CO2

liquid + gas

3.85 to 4.34 2.9 2 6

274.6 to 274.1 271 274 270

800

13

80 12 19

> 80 82b,c 50

CO2 flow

gas

3 0.8

278 253

∼ 150 20

∼ 100b < 10b

water, stirring

CH4-hydrates (MHs) MHs MHs powdered MHs (5 μm to 50 μm) powdered MHs (100 μm to 250 μm) MHs

CO2

gas

MHs

CO2

liquid

271.2 to 275.2 273.2

110 to 145 307

15 to 17

water, stirring

3.1 to 3.34 3.6

water, stirring

MHs

CO2 CO2 CH4→CO2 CO2 CO2 CO2:H2O CH4→CO2 Salt water→ CO2

gas liquid gas liquid liquid emulsion gas supercritical

273.2 273.2 273.2 277.2 281.2 281.2 274 281.2 275.2/283.2 274.7 275.2/280.2 273.5

285 260 280 330 96 96 115 44/76 46/73 230 359/683 96

27c 30c 31c 59c 19 24 to 27 30 ± 13c,d 40.7/37.5 3.4/10.7 27 28/35 40 to 70c

10

MB

water, stirring

11 12 13

Raman Raman NMR

14

Raman

water powdered ice D2O ice (5 μm to 50 μm) powdered ice

15

MB, Raman MB, Raman MB, Raman

18 19

MRI, MB MB

brine water

sandstone quartz sand

20 21

Raman MB, GC

ice ( 24 24

∼ 67 85

sand

MHs

CO2 0.2CO2 + 0.8N2 CO2 0.22CO2 + 0.78N2 0.2CO2 + 0.8N2 0.4CO2 + 0.6N2

gas gas

3/3.5 5

273.2/277.2 273.2

784/507 190

17.4/17.6 36.9

gas

15

273.2

90/100

85

gas

7

277.2 to 280.2

2.7 to 6

5 to 30

MHs MHs MHs MHs

MHs/NMHs sand

MHs

a

MB, NMR, MRI, and GC represent material balance, nuclear magnetic resonance, magnetic resonance imaging, and gas chromatography, respectively. “ A→CO2” means the replacement cell started with “A” atmosphere and then applied CO2 flow. bFluid pressure is lower than the dissociation pressure of CH4-hydrates. cxCH4/(xCH4 + xCO2) was given in the replaced CH4 -hydrates. dThe error is the standard deviation of five measurements.

and may improve the overall economic balance of the operation; however, the achievable CH4-recovery and carbonstorage ratios via a combined CO2−N2−replacement in CH4hydrates and, in particular, the evolution of CO2 and N2 cage occupancies during the exchange remains unclear. Further experimental work under well-defined conditions is needed to remove the observed inconsistencies in order to establish the details of this exchange process. Raman scattering studies are likely to be of prime importance in this context. Raman spectroscopy is a simple, inexpensive, nondestructive and easily accessible method which has been extensively applied in studies of gas hydrates. The vibrational modes of the encaged molecules reflect their local environment, which in a number of cases can be used to distinguish molecules located in the small and large cages. Raman scattering was previously used in a qualitative and semiquantitative manner, such as chemical and structural identifications, or the determination of the relative population of CH4 molecules in the LCs and SCs.30,31 The absolute cage occupancies of CH4 in hydrate cavities can be derived from the Raman peak intensities by using thermody-

calculation and Raman and NMR spectroscopy, results from approximately 13% up to 100% were obtained. More recently, Park et al. proposed that a 20 (in molar percentage) CO2 + 80N2 gas mixture, similar to the composition of flue gas, is more efficient than the injection of pure CO2 for recovering CH4 from sI CH4-hydrates; a significantly increased CH4 recovery ratio of up to 85% was observed.26 They presumed that the addition of N2 molecules considerably promoted the gas exchange as CH4 molecules in the SCs and LCs are preferentially replaced by the smaller N2 and the larger CO2 molecules, respectively. It should be noted, however, that the reports of the achievable CH4 recovery yields under different experimental conditions (unfortunately sometimes given with insufficient detail) vary over a large range, see Table 1.26−29 In summary, over the past decade, the view of NGHs has partly moved from just a new source of gas to a potential storage medium for CO2 to the extent that CO2 sequestration is now frequently considered concomitantly with methane recovery. This approach would reduce some of the abovementioned concerns with mechanical stability of the sea-floor 370



Article

Table 2. Formation Conditions, Absolute Cage Occupancies and Hydration Numbers of sI N2-, CH4-, and CO2-Hydrate Useda hydrates

P/MPa

T/K

time/day

θS/%

θL/%

N2 *CH4 *CO2

109.3 6 3

268.2 268.2 268

11 21 21

98.0 87.1 ± 0.5 68.5 ± 0.4

111.9 ± 0.8 100.0 ± 0.5 98.4 ± 0.3

D

ice/% 2.4 10 3

R

ice/%

3±2 5±4 5±4

a Gas hydrates marked with and asterisk (∗) were also studied in Qin and Kuhs.33 The amount of ice Ih is given as mass fraction. The superscripts “D” and “R” represent diffraction and Raman, respectively.

procedure suggested by Kuhs et al.3 Once loaded and sealed, N2 gas was immediately purged into the reactor through a capillary tube linked to the gas-handling system and an airdriven compressor (SITEC Inc.), and gradually increased to the target pressure (110 MPa) within 2 min. The pressure was monitored by an Ashcroft pressure transducer with an accuracy of 0.7 MPa calibrated by a Heise gauge; the precision of the pressure reading is 0.1 MPa. During the first day, the pressure drop was very large, mounting up to around 7 MPa. The pressure was set back manually to the target pressure and was stable at (109.3 ± 0.2) MPa after 3 days; the uncertainty given reflects the variation of the system pressure during the hydrate formation. The temperature was controlled by a cryostat bath (NESLAB) within 0.1 K from the set-point. The formation conditions are listed in Table 2. At the end of the formation the cell was quenched in liquid N2, the pressure was released, and N2-hydrates were immediately retrieved and stored in liquid N2. The experimental setup and procedure to prepare CH4hydrates is given in Staykova et al.36 and Qin and Kuhs.33 Raman Spectroscopy. Raman measurements were made on a LabRAM HR800 (Horiba Jobin Yvon) confocal Raman spectrometer equipped with a 600 grooves/mm grating. Ar+ laser emitting a wavelength of 488 nm with an output power of 20 mW was used in 180° geometry. The laser was focused on the sample via a 100 μm confocal hole and 50× long working distance objective. The spectral resolution was 2.2 cm−1. Widerange spectra from (0 to 4000) cm−1 were acquired two times with an exposure time of 30 s. Samples were kept at 113 ± 0.1 K by a cooling stage (Linkam THMS600) using evaporating liquid N2 at ambient pressure. To calibrate the Raman cross section ratios, pure sI N2- and CH4-hydrates were placed into the cooling chamber; then samples were measured one after the other; 8 pairs of spectra were measured. Raman spectra were processed by the “dmfit” software.37 The same spectral region and Gaussian/Lorentzian ratio were employed to integrate each peak. Peak positions were calibrated against three emission lines of a mercury lamp (in air) by acquiring data simultaneously with the sample. The estimated spectral uncertainty obtained from the reproducibility of peak position is better than ±0.5 cm−1. The estimated statistical uncertainty of the intensity measurements is negligibly small for our purposes; intensity variations may however arise from variable amounts of ice present in the illuminated spot as discussed below. X-ray Diffraction. The absolute cage occupancies of CH4hydrates were determined by high-resolution powder diffraction data measured on ID31 at ESRF, Grenoble.33 The crystalline structure of sI N2-hydrates was determined by synchrotron X-ray diffraction at beamline ID15B at ESRF, Grenoble. The synchrotron radiation wavelength at ID15B was 0.14007 Å calibrated with LaB6 standard powder from NIST. The powdered gas hydrate samples were filled in a Kapton capillary (0.8 mm) and placed on a cryojet cooling stage where

namic constraints from the van der Waals and Platteeuw model.32 In our previous study, we proposed a calibration protocol to establish Raman spectroscopy as a stand-alone quantitative tool for studying gas hydrates without any additional thermodynamic assumptions.33 The molar concentration of a certain species a (xa) in gas hydrates can be obtained from the following equation, xa =

Aa /(σaηa) n ∑i = 1 Ai /(ση i i)

=

Aa /Fa n ∑i = 1 Ai /Fi

(1)

where Ai refers to the integrated peak area (intensity) of a Raman active peak of species i, σi is the Raman scattering cross section, ηi is the instrumental efficiency, and Fi is the Raman quantification factor (F-factor), and i stands for a given species in the mixture of n constituents. Obviously, this equation requires that σ and η or F values of all species in gas hydrates are known to obtain xa. The instrumental efficiencies of different species (ηi) can be assumed to be identical when pure gas hydrates with the same structure type are placed in the same chamber and acquired at the same time or immediately one after the other, and the empirical F-factor ratio of different components can be derived by comparing their specific peaks. Using the empirical ratios of Raman scattering cross sections of guests and host framework calibrated in gas hydrates, rather than taking the available data calibrated in the fluid phase,34 in addition to the bulk guest composition, the absolute cage filling and the hydration number of the pure or mixed gas hydrates containing CH4, CO2, and C2H6 can be directly determined by Raman spectra. It is known that gas hydrates can be inhomogeneous in their guest cage occupancy31 and in their distribution of relic ice or water,35 making the calibration more difficult than in a homogeneous fluid phase. Considering the suggested addition of N2 gas into the CH4− CO2−exchange reaction, we formed sI N2-hydrates and calibrated the relative Raman quantification factors of N2 to CH4 and N2 with respect to the H2O framework following our previous work.33 With these empirical quantification factors, we are able to determine the bulk guest composition and the absolute cage occupancies, and gain insight into guest evolution in cages during CH 4−C 2H 6−CO 2−N2 -exchange in gas hydrates. For the first time we address explicitly the possible presence of relic ice Ih or water which turns out to be problematic for establishing relative quantification factors.

■

EXPERIMENTAL SECTION Sample Preparation. Gas hydrates were synthesized from spherical ice Ih particles which were prepared by spraying distilled water into liquid N2. Gases were supplied by Air Liquide with the labeled molar fraction purities of 0.99995 (CH4), 0.99998 (CO2) and 0.99999 (N2). Ice particles with diameters of tens of micrometers were transferred to an Al-vial and inserted into a precooled highpressure cell by liquid N2 to form sI N2-hydrates following a 371



Article

the temperature was set to 90 K. The phase identification was conducted using the program GSAS.38

■

RESULTS AND DISCUSSION In this work, pure sI N2-hydrates were formed to calibrate the Raman quantification factor for this structure type. The preparation follows the experimental procedures of Kuhs et al.,3 in which the formation of the metastable phase sI and the coexistence of sI and sII in N2-hydrates at pressures higher than several hundred bars were observed. Figure 1 shows the

Figure 2. Typical Raman spectra of ice Ih and sI N2- and CH4hydrates measured at 113 K.

stretching-vibrational mode of CH4 molecules encaged into the LCs and SCs,30 which has been elucidated in Qin and Kuhs.33 The relative Raman quantification factor of N2 molecules with respect to the H2O host framework is obtained by FN/FNH =

AN × 46 ANH × (6 × θNL + 2 × θNS)

(2)

where AN refers to the intensity of N2 peak in the range of (2300 to 2350) cm−1, AHN is the peak area of the O−H stretching bands in N2-hydrates within (2830 to 3600) cm−1, 46, 6, and 2 represent the number of H2O molecules and the large and small cages per unit cell of sI gas hydrate, respectively. As shown in Figure 2, the O−H stretching intensities originating from gas hydrates and from H2O ice Ih cannot be disentangled. In our earlier work we had assumed that all points were measured at locations free of ice Ih. This ad hoc assumption can be removed by quantifying the amount of ice Ih in the gas hydrate sample. Using X-ray diffraction data the space-averaged mass fraction of ice can be obtained. This ice fraction (see Table 2) may be assumed to be present in each sample position studied by Raman scattering. The resulting contribution from gas hydrates (AH) can then be calculated by ρ AH = AHT − x ice × ice × AHT ρH (3)

Figure 1. Synchrotron X-ray diffraction pattern of N2-hydrates fitted with a sI crystallographic model. The top and bottom black bars represent the Bragg peak positions of sI N2-hydrate and ice Ih, respectively. The bottom line corresponds to the difference between the observed and calculated pattern.

synchrotron X-ray diffraction pattern of N2-hydrates. A fullpattern crystallographic structure refinement yields the following composition: sI N2-hydrates 0.976 in mass fraction, 0.024 unreacted ice Ih, and no detectable sII N2-hydrates. The absolute cage occupancies and the hydration number n of sI N2hydrates are given in Table 2. The large cage filling of nitrogen (θNL) exceeding 100% means that the LCs are partly doubly occupied. More details can be found in Kuhs et al.3 and Chazallon and Kuhs.4 Raman peaks of N2 and CH4 molecules trapped in sI N2- and CH4-hydrates are shown in Figure 2. One band at (2323.4 ± 0.1) cm−1, with a full width at half-maximum of 3.5 ± 0.2 cm−1, is assigned to the N−N stretching-vibrational mode of N2 molecules. A Raman peak splitting of N2 molecules encaged in the LCs and SCs was observed in the high-pressure study of sII N2-hydrates, in which a broad peak around 2324.5 cm−1 was tentatively resolved into three peaks and assigned to N2 in the LCs and doubly occupying the LCs and SCs, respectively.39 Yet, at lower pressures the N2-vibration in the SCs and LCs cannot be resolved even with enhanced spectral resolution.40 The peak position of the N−N stretching mode in sI N2hydrates is very close to that in sII N2-hydrates. This indicates that Raman frequencies of N2 in N2-hydrates are not very sensitive to pressure, temperature, and structure type, which is consistent with the observation from neutron inelastic scattering that N2 molecules are only weakly coupled to the hydrate lattice.41 For methane hydrate two peaks located at 2901.3 and 2013.0 cm−1 arise from the C−H symmetric

where AHT is the O−H peak areas contributed from gas hydrates and ice, xice is the fraction of H2O in ice Ih compared to the total H2O in the illuminated volume, ρice/ρH = 1.13, is the relative density of H2O molecules in ice and gas hydrates. However, it should be noted that the averaged ice fraction determined by diffraction may not faithfully represent the ice within the illuminated volume (∼1 μm in diameter and tens of micrometers in depth) of each Raman measurement;42 some local variations can be expected due to the formation from ice.35 Thus, the Raman spectrum with the highest intensity ratio of guest to H2O host cages, acquired from the N2-hydrate sample with the ice mass fraction of ∼0.03 determined by diffraction, is assumed to be obtained from an ice-free spot. Furthermore, the guest cage occupancies of this spot are assumed to be identical to their space-averaged diffraction results (Table 2). With these assumptions the maximum peak ratio AN/AHN was substituted into eq 2 to calculate the relative F-factor of N2 to H2O framework, FN/FHN. The calibrated value is (0.130 + 0.006), shown in Table 3. FN/FHN can now be used 372



Article

formed under moderate pressures show nearly fully occupied LCs (θML ≈ 100%). In the absence of X-ray diffraction data or for samples with a high mass fraction of ice, the abundance of ice Ih of each measured spot can then be directly derived from eqs 5 and 3,

Table 3. Relative Raman Quantitative Factors of Guest to Guest and Guest to H2O Host Lattices Calculated by Considering the Presence of Icea A/AH

F-factor

no.

average

max.

N2 to H2O ML to H2O MS to H2O (CO2)ν+ to H2O (CO2)ν‑ to H2O N2 to MS (CO2)ν+ to MS

8 16 16 8

0.0236(8) 0.125(6) 0.039(2) 0.040(2)

0.0245 0.1325 0.0424 0.044

0.130 1.016 1.121 0.279

0.006 0.053 0.058 0.014

0.95(6) 1.03(7) 0.25(2)

8

0.020(1)

0.022

0.140 + 0.007

0.12(1)

0.104 0.255

0.26(2)

8 8

this work

Qin and Kuhs33

Raman peak

+ + + +

H = AM

H AML × 46 × FM θML × 6 × FML

(5)

In Figure 2 it can be seen that H2O stretching bands of sI N2hydrates are similar to those in sI CH4-hydrates and in ice Ih. A close similarity in the spectra of sI CH4-, CO2-, and C2H6hydrates within the complex O−H and O−D stretching regions was previously observed.33 On the basis of these observations, Raman spectra of the host lattices can be assumed to be largely independent of the encaged guest species. Thus, the Raman peak area of the H2O host framework can be used to a good approximation as an internal calibrant for the amount of water in the host lattices. A scaling factor, CNM, was introduced to obtain the same amount of H2O host molecules in the focal volume of sI CH4- and N2-hydrate particles,

a

Notation: no. is the number of Raman measurements. MS and ML represent CH4 in the LCs and SCs. The ν+ and ν‑ bands of CO2 are the peaks at ∼1379.4 and 1275.5 cm−1, respectively. The uncertainty behind “+” is estimated by assuming the spectra with the observed maximum A/AH.

H C NM = ANH /AM

to determine the total cage occupancy of N2 molecules in sI N2hydrates,

(6)

where AHN and AHM refer to the peak area of the O−H stretching bands of H2O in N2- and CH4-hydrates within (2830 to 3600) cm−1, respectively. The H2O peak area contributed from ice Ih was subtracted using the above-mentioned method. Using this scaling factor the intensities of guests in the same structure can be directly compared. Despite the approximate 11.7 cm−1 difference in Raman shift between the C−H stretchingvibrational mode of CH4 molecules in the LCs and SCs, the difference in their Raman cross sections is only several percent;33 because of this and the observation that the Raman peaks of N2 trapped into the LCs and SCs cannot even be separated, the Raman cross sections of N2 in the LCs and SCs are assumed to be identical. Given the insensitivity of the N−N stretching modes to various environments, this appears to be a well justified assumptionin contrast to the CH4 case. As a result, the ratio of the Raman quantification factors of N2 to CH4 can be calculated by,

6 × θL + 2 × θS θT = (4) 8 assuming that this relative F-factor does not change with the population of the trapped guest molecules, in particular for the case of doubly occupied cages. The previously reported results of FCO2/FH and FCH4/FH were corrected for the presence of ice in the same way. The uncertainties were estimated by assuming that the ice content (in mass fraction) at the measured spot is 0.03 for CO2- and N2-hydrates, and 0.05 for CH4-hydrates, instead of the previously assumed ice-free situation.33 The number of Raman measurements for each sample is shown in Table 3. The CH4-hydrate sample recovered from the same formation experiment was measured together with N2- and CO2-hydrates. The listed number for CH4-hydrates is the total number of these two separate measurements. As can be seen in Table 3, the corrected F-factors are higher than those in Qin and Kuhs,33 slightly beyond the uncertainty of the earlier data as a result of the neglected existence of ice. Using these newly calibrated values, the content of ice Ih in each illuminated spot can be determined; thus the distribution of ice in gas hydrates with known guest cage occupancies could be estimated from Raman data using eqs 2 and 3. The results obtained for N2-, CO2-, and CH4-hydrates are indeed comparable with the bulk fraction of ice in gas hydrates obtained from diffraction, see Table 2. This suggests, that X-ray diffraction can be used to determine the average abundance of ice in gas hydrates. In case this space-average ice concentration is assumed for each individual Raman spectrum, some variation in the determination of guest cage occupancies is expected; to obtain representative results, a sufficiently large number (at least ∼10) of Raman measurements are needed. It should be noted that, for gas hydrates containing a higher mass fraction of ice (like >0.15), due to the inhomogeneous distribution of ice and the fact that visual selection of spots for the measurements is often guided by the optimization of the signal for the Raman active peak, the diffraction results may be distinctly higher than the average ice content derived from the limited Raman spectra. It is well established that synthetic and natural CH4-hydrates

⎛ AML × θNL A × θNS ⎞ + MS FN/FMS = AN /⎜ ⎟ /C NM θMS ⎝ θML × σML /σMS ⎠ (7)

where, σML/σMS is the cross section ratio of CH4 in the LCs to SCs, AML and AMS are the integrated peak areas of CH4 in the LCs and SCs obtained by the multiple-peak fitting including peaks of CH4 and H2O in the range (2830 to 3600) cm−1. The calibrated Raman quantification factor FN/FMS ratio of N2 to CH4 in sI hydrate lattices is 0.104, see Table 3, which is 21% smaller than that in the fluid phase.34 A similar significant difference between the F-factors calibrated in the hydrate phase and in the fluid phase for the C−H stretching bands of C2H6 and the upper Fermi diad band (ν+) of CO2 was reported.33 These results indicate that directly taking the available Raman cross sections of guest molecules calibrated in the fluid phase, without further calibration in the hydrate phase, likely introduces considerable bias in estimated guest composition. The F-factors of the deuterated C2H6-hydrates given in Qin and Kuhs33 are not corrected because of the absence of X-ray diffraction data. 373



■

Article

(8) Espinoza, D. N.; Santamarina, J. C. P-wave monitoring of hydrate-bearing sand during CH4−CO2 replacement. Int. J. Greenhouse Gas Control 2011, 5 (4), 1031−1038. (9) Hyodo, M.; Li, Y.; Yoneda, J.; Nakata, Y.; Yoshimoto, N.; Kajiyama, S.; Nishimura, A.; Song, Y. A comparative analysis of the mechanical behavior of carbon dioxide and methane hydrate-bearing sediments. Am. Mineral. 2014, 99 (1), 178−183. (10) Hirohama, S.; Shimoyama, Y.; Wakabayashi, A.; Tatsuta, S.; Nishida, N. Conversion of CH4-hydrate to CO2-hydrate in liquid CO2. J. Chem. Eng. Jpn. 1996, 29 (6), 1014−1020. (11) Uchida, T.; Takeya, S.; Ebinuma, T.; Narita, H. Replacing methane with CO2 in clathrate hydrate: Observations using Raman spectroscopy. In Proceedings of the 5th International Conference on Greenhouse Gas Control Technologies, Cairns, Australia, 2001. (12) Komai, T.; Kawamura, T.; Kang, S.; Nagashima, K.; Yamamoto, Y. In situ observation of gas hydrate behaviour under high pressure by Raman spectroscopy. J. Phys.: Condens. Matter 2002, 14 (44), 11395− 11400. (13) Lee, H.; Seo, Y.; Seo, Y. T.; Moudrakovski, I. L.; Ripmeester, J. A. Recovering methane from solid methane hydrate with carbon dioxide. Angew. Chem., Int. Ed. 2003, 42 (41), 5048−5051. (14) Yoon, J. H.; Kawamura, T.; Yamamoto, Y.; Komai, T. Transformation of methane hydrate to carbon dioxide hydrate: In situ Raman spectroscopic observations. J. Phys. Chem. A 2004, 108 (23), 5057−5059. (15) Ota, M.; Abe, Y.; Watanabe, M.; Smith, R. L.; Inomata, H. Methane recovery from methane hydrate using pressurized CO2. Fluid Phase Equilib. 2005, 228553−559. (16) Ota, M.; Morohashi, K.; Abe, Y.; Watanabe, M.; Smith, R. L.; Inomata, H. Replacement of CH4 in the hydrate by use of liquid CO2. Energy Convers. Manage. 2005, 46 (11−12), 1680−1691. (17) Ota, M.; Saito, T.; Aida, T.; Watanabe, M.; Sato, Y.; Smith, R. L.; Inomata, H. Macro and microscopic CH4−CO2 replacement in CH4 hydrate under pressurized CO2. AIChE J. 2007, 53 (10), 2715− 2721. (18) Kvamme, B.; Graue, A.; Buanes, T.; Kumetsoua, T.; Ersland, G. Storage of CO2 in natural gas hydrate reservoirs and the effect of hydrate as an extra sealing in cold aquifers. Int. J. Greenhouse Gas Control 2007, 1 (2), 236−246. (19) Zhou, X. T.; Fan, S. S.; Liang, D. Q.; Du, J. W. Replacement of methane from quartz sand-bearing hydrate with carbon dioxide-inwater emulsion. Energy Fuels 2008, 22 (3), 1759−1764. (20) Schicks, J. M.; Luzi, M.; Beeskow-Strauch, B. The conversion process of hydrocarbon hydrates into CO2 hydrates and vice versa: Thermodynamic considerations. J. Phys. Chem. A 2011, 115 (46), 13324−13331. (21) Deusner, C.; Bigalke, N.; Kossel, E.; Haeckel, M. Methane production from gas hydrate deposits through injection of supercritical CO2. Energies 2012, 5 (7), 2112−2140. (22) Yuan, Q.; Sun, C.-Y.; Yang, X.; Ma, P.-C.; Ma, Z.-W.; Liu, B.; Ma, Q.-L.; Yang, L.-Y.; Chen, G.-J. Recovery of methane from hydrate reservoir with gaseous carbon dioxide using a three-dimensional middle-size reactor. Energy 2012, 40 (1), 47−58. (23) Yuan, Q.; Sun, C.-Y.; Liu, B.; Wang, X.; Ma, Z.-W.; Ma, Q.-L.; Yang, L.-Y.; Chen, G.-J.; Li, Q.-P.; Li, S.; Zhang, K. Methane recovery from natural gas hydrate in porous sediment using pressurized liquid CO2. Energy Convers. Manage. 2013, 67257−264. (24) Beeskow-Strauch, B.; Schicks, J. M. The driving forces of guest substitution in gas hydrates−A laser Raman study on CH4-CO2 exchange in the presence of impurities. Energies 2012, 5 (2), 420−437. (25) Lee, S.; Park, S.; Lee, Y.; Seo, Y. Thermodynamic and 13C NMR spectroscopic verification of methane−carbon dioxide replacement in natural gas hydrates. Chem. Eng. J. 2013, 225636−640. (26) Park, Y.; Kim, D. Y.; Lee, J. W.; Huh, D. G.; Park, K. P.; Lee, J.; Lee, H. Sequestering carbon dioxide into complex structures of naturally occurring gas hydrates. Proc. Natl. Acad. Sci. U.S.A. 2006, 103 (34), 12690−12694. (27) Liu, B.; Pan, H.; Wang, X.; Li, F.; Sun, C.; Chen, G. Evaluation of different CH4−CO2 replacement processes in hydrate-bearing

CONCLUSIONS The relative Raman quantification factors of N2 to CH4 and N2 to H2O host lattices were calibrated by pure sI N2- and CH4hydrates with known cage occupancies. Quantitative information on cage occupancies and guest composition of mixed gas hydrates containing CH4, C2H6, CO2, and N2 can now be determined by Raman scattering using our experimentally established F-factors. On the other hand, the concentration of existing ice within the illuminated volume of a one gas-hydrate particle can also be estimated. Our results will be helpful for a better understanding of the mechanisms of the exchange process of gas hydrates with a condensed CO2 or CO2−N2 mixture by quantifying the partitioning of certain gas species into different hydrate cavities. This is important in order to optimize the conditions of gas exchange processes, largely occurring in the solid state, to achieve the maximum ratios of methane recovery and carbon dioxide storage. The established F-factors may also prove useful for other Raman studies in chemical engineering and geology.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 551 3933891. Fax: +49 551 399521. Funding

This research was supported by the German Ministry of Education and Research (BMBF) within the SUGAR-II research initiative by the Grant 03G0819B (TP B2-3). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. Andrzej Falenty (Göttingen), Mathias Jansen (Göttingen), and Stefanie Stracke (Göttingen) for experimental help. We are also thankful for beam-time and support from ESRF/Grenoble concerning the synchrotron powder diffraction experiments on ID31 and ID15B.

■

REFERENCES

(1) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases, 3rd ed.; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2008. (2) Lundgaard, L.; Möllerup, J. Calculation of phase-diagrams of gashydrates. Fluid Phase Equilib. 1992, 76141−149. (3) Kuhs, W. F.; Chazallon, B.; Radaelli, P. G.; Pauer, F. Cage occupancy and compressibility of deuterated N2-clathrate hydrate by neutron diffraction. J. Inclusion Phenom. Mol. Recognit. Chem. 1997, 29 (1), 65−77. (4) Chazallon, B.; Kuhs, W. F. In situ structural properties of N2-, O2-, and air-clathrates by neutron diffraction. J. Chem. Phys. 2002, 117 (1), 308−320. (5) Moridis, G. J.; Collett, T. S.; Boswell, R.; Kurihara, M.; Reagan, M. T.; Koh, C.; Sloan, E. D. Toward production from gas hydrates: Current status, assessment of resources, and simulation-based evaluation of technology and potential. SPE Reservoir Eval. Eng. 2009, 12 (5), 745−771. (6) Boswell, R. Japan completes first offshore methane hydrate production testMethane successfully produced from deepwater hydrate layers. Fire in the Ice 2013, 13, 1−2, http://www.netl.doe.gov/ research/oil-and-gas/methane-hydrates/fire-in-the-ice. (7) Ohgaki, K.; Takano, K.; Sangawa, H.; Matsubara, T.; Nakano, S. Methane exploitation by carbon dioxide from gas hydratesPhase equilibria for CO2−CH4 mixed hydrate system. J. Chem. Eng. Jpn. 1996, 29 (3), 478−483. 374



Article

sediments by measuring P-wave velocity. Energies 2013, 6 (12), 6242− 6254. (28) Koh, D.-Y.; Kang, H.; Kim, D.-O.; Park, J.; Cha, M.; Lee, H. Recovery of methane from gas hydrates intercalated within natural sediments using CO2 and a CO2/N2 gas mixture. ChemSusChem 2012, 5 (8), 1443−1448. (29) Masuda, Y.; Maruta, H.; Naganawa, S.; Amikawa, K.; Nagao, J.; Hironori, H.; Konno, Y. Methane recovery from hydrate-bearing sediments by a N2−CO2 gas mixture injection: Experimental investigation on CO2−CH4 exchange ratio. In Proceedings of the 7th International Conference on Gas Hydrates, Edinburgh, Scotland, United Kingdom, 2011. (30) Sum, A. K.; Burruss, R. C.; Sloan, E. D. Measurement of clathrate hydrates via Raman spectroscopy. J. Phys. Chem. B 1997, 101 (38), 7371−7377. (31) Uchida, T.; Hirano, T.; Ebinuma, T.; Narita, H.; Gohara, K.; Mae, S.; Matsumoto, R. Raman spectroscopic determination of hydration number of methane hydrates. AlChE J. 1999, 45 (12), 2641−2645. (32) van der Waals, J. H.; Platteeuw, J. C. Clathrate solutions. Adv. Chem. Phys. 1959, 21−57. (33) Qin, J.; Kuhs, W. F. Quantitative analysis of gas hydrates using Raman spectroscopy. AIChE J. 2013, 59 (6), 2155−2167. (34) Burke, E. A. J. Raman microspectrometry of fluid inclusions. Lithos 2001, 55 (1−4), 139−158. (35) Kuhs, W. F.; Staykova, D. K.; Salamatin, A. N. Formation of methane hydrate from polydisperse ice powders. J. Phys. Chem. B 2006, 110 (26), 13283−13295. (36) Staykova, D. K.; Kuhs, W. F.; Salamatin, A. N.; Hansen, T. Formation of porous gas hydrates from ice powders: Diffraction experiments and multistage model. J. Phys. Chem. B 2003, 107 (37), 10299−10311. (37) Massiot, D.; Fayon, F.; Capron, M.; King, I.; Le Calve, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z. H.; Hoatson, G. Modelling one- and two-dimensional solid-state NMR spectra. Magn. Reson. Chem. 2002, 40 (1), 70−76. (38) Larson, A. C.; Von Dreele, R. B. General structure analysis system (GSAS); Los Alamos National Laboratory: Los Alamos, NM, 2004. (39) Sasaki, S.; Hori, S.; Kume, T.; Shimizu, H. Microscopic observation and in situ Raman scattering studies on high-pressure phase transformations of a synthetic nitrogen hydrate. J. Chem. Phys. 2003, 118 (17), 7892−7897. (40) Champagnon, B.; Panczer, G.; Chazallon, B.; Arnaud, L.; Duval, P.; Lipenkov, V. Nitrogen and oxygen guest molecules in clathrate hydrates: Different sites revealed by Raman spectroscopy. J. Raman Spectrosc. 1997, 28 (9), 711−715. (41) Chazallon, B.; Itoh, H.; Koza, M.; Kuhs, W. F.; Schober, H. Anharmonicity and guest−host coupling in clathrate hydrates. Phys. Chem. Chem. Phys. 2002, 4 (19), 4809−4816. (42) Murshed, M. M.; Kuhs, W. F. Kinetic studies of methane− ethane mixed gas hydrates by neutron diffraction and Raman spectroscopy. J. Phys. Chem. B 2009, 113 (15), 5172−5180.

375


Calibration of Raman Quantification Factors of Guest Molecules in Gas

Recommend Documents