57
58
Table 4.3.2. Crystallographic information for the CO2 + NH, N2 + NH, and CO2 + N2 + NH systems.
System Lattice Parameter (Å) Structure
CO2 + NH a = 11.96 sI
CO2 (80%) + N2 (20%) + NH a = 11.95 sI
CO2 (60%) + N2 (40%) + NH a = 11.88 sI
CO2 (40%) + N2 (60%) + NH a = 12.24, c = 10.11 (sH)
a = 11.99 (sI) sI-dominant
CO2 (40%) + N2 (60%) + NH a = 12.27, c = 10.14 (sH)
a = 11.99 (sI) sH-dominant
CO2 (20%) + N2 (80%) + NH a = 12.25, c = 10.14 sH
CO2 (10%) + N2 (90%) + NH a = 12.25, c = 10.10 sH
N2 + NH a = 12.23, c = 10.11 sH
The PXRD pattern of the N2 + NH hydrate was indexed using a hexagonal symmetric unit cell (space group P6/mmm) with lattice parameters of a = 12.23 Å and c = 10.11 Å. The CO2 (10%) + N2 (90%) + NH hydrate was also indexed in the hexagonal symmetric unit cell with space group P6/mmm and lattice parameters of a = 12.25 Å and c = 10.10 Å. In addition, for the CO2 (20%) + N2 (80%) + NH system, the PXRD pattern was also indexed with a hexagonal symmetric unit cell (space group P6/mmm) and the lattice parameters were found to be a = 12.25 Å and c = 10.14 Å. From the PXRD patterns above, it was found that the CO2 + N2 gas mixtures with flue gas compositions form sH hydrates in the presence of NH. However, the crystal structure of both the CO2 (60%) + N2 (40%) + NH and CO2 (80%) + N2
(20%) + NH hydrates were found to be cubic sI (space group 3) with a lattice parameter of a = 11.88 Å and 11.95 Å, respectively. As seen in Figure 4.3.4., the CO2 (40%) + N2 (60%) + NH system demonstrated that sI hydrate coexists with sH hydrate, which was also expected from hydrate phase behavior. The lattice parameters for both sI-dominant and sH-dominant hydrates of the CO2 (40%) + N2 (60%) + NH system were shown in Table 4.3.2. It should be noted that the sH-dominant hydrate sample was prepared at the low pressure and temperature range (275.15 K and 4.1 MPa) where its hydrate equilibrium line was slightly shifted to the thermodynamically promoted region as shown in Figure 4.3.1. (b), previous Section 4.3.1. The hydrate sample of the CO2 (40%) + N2 (60%) + NH system prepared at the high pressure and temperature range (280.15 K and 10.0 MPa), where its hydrate equilibrium line is indistinguishable from that of the CO2 (40%) + N2 (60%) system, was found to be sI-dominant. The coexistence of sI and sH hydrates in the CO2 (40%) + N2 (60%) + NH system clearly validates that the structure of the CO2 + N2 + NH hydrates can be transformed from sH into sI as the
59
CO2 concentration increases. Although PXRD results could provide accurate information on the hydrate structure of CO2 + N2 + NH systems, they could not offer direct evidence of CO2 enclathration in the sH hydrates.
Wav enumber (cm-1 )
1180 1220 1260 1300 1340 1380 1420
1380
1380 1276
1274 1276
1380
1276
1380
1274
1380 1276
1380 CO2 (80%) + N2 (20%) + NH
CO2 (60%) + N2 (40%) + NH
CO2 (40%) + N2 (60%) + NH, sI dominant
CO2 (40%) + N2 (60%) +NH, sH dominant
CO2 (20%) + N2 (80%) + NH Pure CO2
Structure I + Structure H
Structure H Structure I
Figure 4.3.5. Raman spectra of the CO2 + N2 + NH hydrates.
60
Raman spectroscopy was used to confirm the hydrate structure and to examine the enclathration of CO2
into the hydrate cages of the CO2 + N2 + NH systems. Although the enclathrated CO2 molecules do not exhibit Raman peak splitting in different cages, the wavenumber shift of the Fermi diad bands occurs as a result of structural transition [66, 75]. As shown in Figure 4.3.5., two Raman peaks from the pure CO2 hydrate, which is known to form sI hydrate [32], were observed at 1276 and 1380 cm-1. On the other hand, the CO2 molecules captured in the CO2 (20%) + N2 (80%) + NH hydrate, which was confirmed to be sH hydrate through PXRD, demonstrated two Raman peaks at 1274 and 1380 cm-1. The CO2 (40%) + N2 (60%) + NH hydrate formed in the low temperature and pressure region (275.15 K and 4.1 MPa) also exhibited two Raman peaks at 1274 and 1380 cm-1. However, the Raman peaks from enclathrated CO2 molecules in the CO2 (40%) + N2 (60%) + NH hydrate at the high temperature and pressure range (280.15 K and 10.0 MPa) as well as those in the other N2-rich hydrates were observed at 1276 and 1380 cm-1. The wavenumber shift (1276 cm-1 ® 1274 cm-1) was attributed to the structural transition (sI ® sH) [94]. In addition, several other peaks from enclathrated NH molecules primarily appearing between 1200 and 1400 cm-1 can also be a good indicator for sH hydrate formation. The experimental results from Raman spectroscopy indicate that CO2 molecules were enclathrated into the cages of sH hydrates in the N2-rich systems and the CO2 (40%) + N2 (60%) was a boundary composition for the structural transition.
Figure 4.3.6. Comparison of CO2 compositions in the hydrate phase at 275.2 K (CO2 + N2 system versus CO2 + N2 + NH system)
CO2 Composition in Feed Gas (%)
0 20 40 60 80 100
CO2 Composition in Hydrate Phase (%)
0 20 40 60 80 100
CO2 + N2 CO2 + N2 + NH
61
To examine preferential CO2 occupation in the hydrate cages depending on hydrate structures and to confirm a boundary composition for the structural change, CO2 compositions in the hydrate phase with respect to CO2 compositions in the feed gas for both the CO2 + N2 and CO2 + N2 + NH systems were measured through a gas chromatograph, and the results were shown in Figure 4.3.6. The CO2
compositions in the hydrate phase for the CO2 + N2 + NH systems were lower than those for the corresponding CO2 + N2 systems in the feed CO2 concentration ranges lower than 40% where sH hydrates can be stable. However, in the sI hydrate formation regions where feed CO2 concentrations are higher than 40%, the CO2 compositions of the CO2 + N2 + NH hydrates were almost identical to those of the corresponding CO2 + N2 hydrates. The difference in the CO2 compositions of the hydrate phase between the CO2 + N2 + NH and CO2 + N2 systems was attributable to the different lattice constructions between sH and sI hydrates even though CO2 was preferentially enclathrated in the hydrate cages for both systems.
A unit cell of the sI hydrate structure consists of 2(512)·6(512’62)·46H2O, while a unit cell of the sH hydrate structure consists of 3(512)·2(435663)·1(51268)·34H2O [32]. In light of the molecular size of CO2, the large 51262 cages of the sI hydrate and the medium 435663 cages of the sH hydrate were more preferentially occupied by CO2 molecules than the small 512 cages of both sI and sH hydrates [94].
Accordingly, due to a larger proportion of occupiable cages for CO2 molecules in sI hydrates, the CO2
compositions of sI hydrates were found to be higher than those of sH hydrates. It should also be noted that a wide variation in the CO2 composition was observed at the structural transition boundary of the CO2 (40%) + N2 (60%) as a result of coexistence of both sH and sI hydrates. Considering the fact that the CO2 molecules could be enclathrated into the sH hydrate cages at temperatures above the freezing point of water, the CH4 - flue gas replacement process for sH hydrate reservoirs would be feasible at milder pressure and temperature conditions compared to sI hydrate reservoirs.
To confirm the structural transformation of the CH4 + NH hydrate after the replacement, 13C NMR spectroscopy was adopted. The cage-dependent chemical shifts of CH4 molecules enclathrated in hydrate lattices can be effectively used to determine the structure type of the gas hydrates and to quantify the distribution of the CH4 molecules over the different types of cages. Figure 4.3.7 (a) presents 13C NMR spectra of the initial CH4 + NH hydrate and the CH4 + NH hydrates replaced with CO2 + N2 in the chemical shift range from 0 to - 8 ppm. The 13C NMR spectrum of the initial CH4 + NH hydrate exhibited two resonance signals at - 4.5 and - 4.9 ppm, which can be attributed to the CH4 molecules enclathrated in the small 512 and medium 435663 cages of sH hydrate, respectively, as reported in previous studies [67, 95]. The hydrates replaced with CO2 + N2 also demonstrated two signals at - 4.5 and - 4.9 ppm, indicating that the structure of the replaced hydrates is still sH.
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Chemical Shift (ppm)
-8 -7 -6 -5 -4 -3 -2 -1 0
-4.5 -4.9
CH4 + NH hydrate
replaced w ith CO2 (10%) + N2 (90%)
replaced w ith CO2 (20%) + N2 (80%)
AM/AS
0.0 0.2 0.4 0.6 0.8 1.0
CH4 + NH,
Replaced with CO2 (10%) + N2 (90%) Replaced with CO2 (20%) + N2 (80%)
(a) (b)
Figure 4.3.7. (a) 13C NMR spectra of the initial CH4 + NH hydrate and the replaced CH4 + CO2 + N2 + NH hydrates; (b) Area ratio of CH4 molecules in the medium 435663 and small 512 cages (AM/AS) before and after replacement.
Using 13C NMR spectroscopy, Shin et al. [41] revealed that initial CH4 + isopentane hydrate (sH) was converted into sI hydrate after replacement by a gas mixture of CO2 (20%) + N2 (80%). However, in the present study, the 13C NMR spectra of the replaced hydrates clearly demonstrate that CH4 + NH - CO2 + N2 replacement did not undergo structural transformation, and thereby isostructural replacement could also occur in sH hydrates when gas mixtures with flue gas compositions were injected. It should be also noted that the structural transition of sH to sI during replacement depends on types of liquid hydrocarbons for large cages of sH and on the compositions of injecting gases for small and medium cages.
Each peak area of the 13C MAS NMR spectra for the CH4 + NH and CH4 + CO2 + N2 + NH hydrates is proportional to the quantities of CH4 molecules enclathrated in each cage of the hydrate structure. In Figure 4.3.7. (b), the area ratios of the CH4 molecules in the medium 435663 and small 512 cages (AM/AS) for the initial CH4 + NH hydrate, the CH4 + NH hydrate replaced with CO2 (10%) + N2 (90%), and the CH4 + NH hydrate replaced with CO2 (20%) + N2 (80%) were found to be 0.69, 0.69, and 0.44, respectively. Due to the preferential occupation of CO2 in the medium 435663 cages and preferential occupation of N2 in the small 512 cages, the area ratio of the CH4 molecules in the medium 435663 and small 512 cages (AM/AS) decreased with an increase in CO2 concentrations. In order to examine the guest distribution and cage occupancy of guest molecules, the relative integrated areas of 13C NMR MAS
63
spectra were combined with the statistical thermodynamic expression, which represents the chemical potential of water in the sH hydrate:
(ℎ) − (ℎ) =
34[3 ln1 − , + 2 ln1 − , + ln(1 − , )
Here, (ℎ) is the chemical potential of the water molecules of a hypothetical empty lattice, and ,
, and denote the fractional occupancies of the small 512, medium 435663, and large 51268 cages, respectively, of sH hydrates. When the gas hydrate is in an equilibrium state with ice, (ℎ) − (ℎ) becomes −∆, where ∆ is the chemical potential of the empty lattice relative to the ice. The value of ∆ used in this study is 1187.5 J/mol for sH hydrates[92, 97]. The cage occupancies of CH4 and NH molecules in the initial CH4 + NH hydrate were found to be , = 0.72, , = 0.74, and
, = 1.00. After being replaced by gas mixtures with flue gas compositions, as summarized in Table 4.3.3., the cage occupancies were found to be , = 0.29, , = 0.29, and , = 1.00 for the CH4 + NH hydrate replaced with CO2 (10%) + N2 (90%), and , = 0.33, , = 0.22, and ,
= 1.00 for CH4 + NH hydrate replaced with CO2 (20%) + N2 (80%). Lower , value and higher
, value were observed for the hydrate replaced by CO2 (20%) + N2 (80%) compared with that replaced by CO2 (10%) + N2 (90%). This result indicates that N2 molecules also participate in replacing CH4 molecules by preferentially attacking the small 512 cages of sH hydrates. Although 13C MAS NMR spectra can provide guest distribution and cage occupancy of CH4, accurate confirmation of the extent of replacement required direct composition measurements using a gas chromatograph in order to verify the influence of the preferential exchange of N2 with CH4 in the small 512 cages. In the previous studies, the extent of replacement for the CH4 - CO2 swapping and CH4 - flue gas swapping occurring in the isostructural systems (sI to sI) was found to be approximately 68% and 79%, respectively[29, 55].
Table 4.3.3. The relative cage occupancies after replacement.
system θS, CH4 θM, CH4 θL, NH
CH4 + NH hydrate 0.72 0.74 1.00
replaced hydrate with CO2 (10%) + N2 (90%) 0.29 0.29 1.00 replaced hydrate with CO2 (20%) + N2 (80%) 0.33 0.22 1.00
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In this study, the extent of replacement for the CH4 + NH - flue gas swapping was approximately 74–
75% depending on the flue gas composition, as shown in Figure. 4.3.8. and Table 4.3.4. Compared to replacement by pure CO2, the presence of N2 molecules in the injecting gas improved the extent of replacement, as confirmed from the 13C NMR results shown in Figure 4.3.7. (a) and (b). However, the extent of replacement by flue gas injection was found to be slightly lower in the isostructural system of sH than that in the isostructural system of sI, due to the differing hydrate cage structures (2S(512)∙6L(51262)∙46H2O for sI and 3S(512)∙2M(435663)∙1L(51268)∙34H2O for sH) and the preferential occupation of each cage by guest molecules. Considering the sizes of each cage and guest molecule, CO2 molecules more preferentially occupy the medium 435663 cages of the sH hydrate and the large 51262 cages of the sI hydrate than the small 512 cages of both sI and sH hydrates. The direct measurements of the hydrate phase compositions demonstrate that CH4 recovery via flue gas injection into the sH hydrates was also effectively achieved without a structural transition as was observed in the isostructural CH4 - flue gas replacement occurring in sI hydrates.
Figure 4.3.8. Compositions in the hydrate phase after replacement.
Gas Composition in Hydrate Phase (%)
0 10 20 30 40 50 60 70 80 90 100
CO2 N2 CH4
CH4 + NH - CO2 (10%) + N2 (90%), this work
CH4 + NH - CO2 (20%) + N2 (80%), this work CH4 - CO2 (10%) + N2 (90%), Lee et al.[29]
CH4 - CO2 (20%) + N2 (80%), Lee et al.[29]
CH4 - CO2, Lee et al.[55]
65 Table 4.3.4. The compositions of hydrate phase after replacement.
initial hydrate injecting gas
replaced hydrates (%)
the extent of replacement
(%) description
CO2 N2 CH4
CH4 CO2 68 ± 2 - 32 ± 2 68 ± 2 Lee et al.[55]
CH4 CO2 (10%) + N2 (90%) 31 ± 1 47 ± 2 21 ± 2 79 ± 2 Lee et al.[29]
CH4 CO2 (20%) + N2 (80%) 48 ± 1 30 ± 2 22 ± 2 78 ± 2 Lee et al.[29]
CH4 + NH CO2 (10%) + N2 (90%) 16 ± 3 58 ± 2 26 ± 1 74 ± 1 this work
CH4 + NH CO2 (20%) + N2 (80%) 30 ± 2 45 ± 2 25 ± 2 75 ± 2 this work
66