A 13C NMR spectrum of the CH4 molecules enclathrated in the CH4 + NH hydrate has already been presented in Figure 4.2.2. However, the 13C NMR spectra of all guest molecules including CH4 and NH before and after the replacement are presented in Figure 4.2.5. The 13C NMR spectrum of the CH4 + NH hydrate before the replacement exhibited four resonance peaks at 36.6, 29.9, 28.9, and 8.4 ppm for the NH molecules enclathrated in the large 51268 cages of sH hydrates as well as two resonance peaks at - 4.5 and - 4.9 ppm for the CH4 molecules enclathrated in the small 512 cages. It should be noted that there were also very small resonance peaks that could be assigned to unclathrated NH molecules;
however, two were not pronounced due to their very close appearance and merging with neighboring major resonance peaks. After the replacement, resonance peaks corresponding to the CH4 molecules appeared at - 4.3 and - 6.6 ppm, which were identical to those of the sI CH4 hydrate. Four resonance peaks at 36.6, 30.5, 28.8, and 8.9 ppm represent the unclathrated NH molecules that were excluded from the initial sH hydrate after the structural transition from sH to sI. In Figure 4.2.5., the intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) of sI was 0.96 after the replacement, which also confirmed preferential occupation of CO2 molecules in the large 51262 cages of sI hydrate.
Figure 4.2.5. 13C NMR spectra of all guest molecules, including CH4 and NH, before and after replacement.
Chemical Shift (ppm)
-10 0
10 20
30 40
50
36.6 29.9 28.9 8.4 -4.5 -4.9
* *
*
* *
*
-4.3 -6.6
8.9
30.5
36.6 30.5 28.8 8.9
before the replacement
after the replacement
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Figure 4.2.6. Comparison of the extent of the replacement (sI
→ sI vs. sH → sI).
For the isostructural replacement with a CO2 injection into the CH4 hydrate (sI to sI), it was reported that CO2 could replace approximately 68% of CH4 in the sI structure [23, 55, 57]. However, in this study, when CO2 was injected into the sH (CH4 + NH) hydrate, a direct composition measurement using a gas chromatograph showed that the concentration of CO2 retrieved from the dissociation of the hydrate phase after the replacement was 88 ± 2% (Figure 4.2.6.). This significantly higher extent of the replacement for the initial sH hydrate can be attributed to an increased CO2 occupancy in the cages of the sI hydrate during the restructuring process of the gas hydrate structure from sH into sI. As clearly depicted in Figure 4.2.5., the initial sH (CH4 + NH) hydrate was transformed into sI after the CH4 - CO2
replacement, and the intensity ratio of the CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) was 0.96. It should be noted that in the previous study, the intensity ratio of CH4 molecules in the large 51262 cages and small 512 cages (IL/IS) for the isostructural replacement (sI to sI) was 1.44, which corresponds to 68% replacement. The lowered intensity ratio of 0.96 after the replacement that accompanied the structural transformation indicates that the CO2 occupancy in the large 51262 cages was more significantly increased than that in the small 512 cages under the hydrate dissociation and subsequent reconfiguration of the crystal structure, which leads to the re-establishment process of the
sI -> sI
Extent of Replacment (%)
0 20 40 60 80 100
sH -> sI
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guest distribution in the hydrate structure. In the isostructural replacement (sI to sI), recent MRI and DSC studies demonstrated that the initial CH4 hydrate was not dissociating into the liquid water or ice and then forming the CH4 + CO2 hydrate through only a partial breakup of the cage structures and subsequent exchange of CH4 and CO2 molecules [27, 29, 55]. However, the structural transformation during the CH4 - CO2 replacement that occurred in sH hydrates involves the dissociation of the sH hydrate and reformation of the sI gas hydrate, and alters cage filling characteristics of guest molecules and guest distributions in each cage; thus, it results in enhanced CH4 recovery via CO2 injection into sH hydrates.
Figure 4.2.7. Heat flows in the blank run, the CH4/CO2 replacement, and the (CH4 + NH)/CO2
replacement at 274.2 K.
Time (min)
0 1 2 3 4 5
Heat Flow (mW)
-30 -20 -10 0 10 20
CH4 - CO2 replacement CH4 + NH - CO2 replacement blank run
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In this study, the heat flow involved in the CH4 - CO2 replacement that occurs in sH hydrates was monitored using a HP μ-DSC in order to confirm the dissociation and reformation process of the gas hydrate structures during the replacement. For the CH4 - CO2 replacement in sH hydrates, quick evacuation of the vapor phase of the CH4 + NH hydrate should be followed with an injection of pre- cooled CO2. Any possible hydrate dissociation during the evacuation and injection must be minimized for accurate quantification of the heat flow involved in the replacement process. As depicted in Figure 4.2.7., the heat flow changes in the CH4 gas evacuation and CO2 injection process in the presence of the initial sI CH4 gas hydrate, which is referred to as ‘CH4 - CO2 replacement’, was comparable with that in the absence of the gas hydrate, which is referred to as a ‘blank run’, which indicates that the gas hydrate dissociation during the gas evacuation and injection process for the replacement is almost negligible and that there is no significant gas hydrate dissociation and reformation during the isostructural replacement as reported in previous studies [29, 55]. However, a large exothermic peak after the appearance of a significant endothermic peak was observed in the CH4 + NH - CO2 replacement, which indicates that the initial sH hydrate dissociation was followed by the formation of sI hydrate as the replacement reaction proceeded. The relatively smaller area for the endothermic peak than that for the exothermic peak can be attributed to the fact that a substantial portion of the endothermic peak from the sH hydrate dissociation was offset by the immediate appearance of the exothermic peak from the subsequent formation of the sI hydrate structure. The DSC result also supports that the structural transformation can occur during the CH4 – CO2 replacement in sH hydrates.
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