66
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Figure 4.3.9. and Table 4.3.5. present the DHd values of both the CO2 + N2 and CO2 + N2 + NH hydrates with respect to the feed CO2 compositions. The DHd values of pure N2 hydrate (sII), CO2 (10%) + N2
(90%) hydrate (sI), and CO2 (20%) + N2 (80%) hydrate (sI) were reported to be 51.3 ± 0.1, 50.1 ± 0.4, and 51.7 ± 0.3 kJ/mol gas, respectively [66]. In this study, the DHd values of the CO2 (40%) + N2 (60%) hydrate, CO2 (60%) + N2 (40%) hydrate, and CO2 (80%) + N2 (20%) hydrate, which were found to be sI hydrates, were measured to be 53.6 ± 0.1, 54.6 ± 0.2, and 55.6 ± 0.3 kJ/mol gas, respectively.
In the presence of NH, the DHd values of the N2 + NH hydrate, CO2 (10%) + N2 (90%) + NH hydrate, and CO2 (20%) + N2 (80%) + NH hydrate, which were revealed to be sH hydrates, were found to be 57.4 ± 0.1, 60.6 ± 0.3, and 61.4 ± 0.3 kJ/mol gas, respectively. In the isostructural systems of each sI and sH hydrate, the DHd values of the CO2 + N2 and CO2 + N2 + NH hydrates increased with an increase of CO2 concentration. However, the CO2 (40%) + N2 (60%) + NH system showed a relatively wide variation in the DHd value (57.3 ± 2.9 kJ/mol gas) and also demonstrated abrupt and discontinuous behavior in the DHd value compared to neighboring ones, indicating the coexistence of sI and sH hydrates caused by the structural transition in the CO2 (40%) + N2 (60%) + NH system. For the CO2
(60%) + N2 (40%) + NH and CO2 (80%) + N2 (20%) + NH hydrates, the DHd values were found to be 54.5 ± 0.2, and 55.4 ± 0.2 kJ/mol gas, respectively. These values were in good agreement with those of the CO2 (60%) + N2 (40%) and CO2 (80%) + N2 (20%) hydrates, respectively.
Table 4.3.5. The dissociation enthalpies of the CO2 + N2 systems and CO2 + N2 + NH systems.
Composition ΔHd
(kJ/mole gas) Hydration number Structure Description
N2 51.3 ± 0.1 6.19 sII Lee et al. [66]
CO2 (10%) + N2 (90%) 50.1 ± 0.4 6.00 sI Lee et al. [66]
CO2 (20%) + N2 (80%) 51.7 ± 0.3 6.08 sI Lee et al.[66]
CO2 (40%) + N2 (60%) 53.6 ± 0.1 6.16 sI This work
CO2 (60%) + N2 (40%) 54.6 ± 0.2 6.20 sI This work
CO2 (80%) + N2 (20%) 55.6 ± 0.3 6.22 sI This work
CO2 57.1 ± 0.2 6.3 sI Lee et al.[66]
N2 + NH 57.4 ± 0.1 7.06 sH This work
CO2 (10%) + N2 (90%) + NH 60.6 ± 0.3 7.01 sH This work
CO2 (20%) + N2 (80%) + NH 61.4 ± 0.3 6.99 sH This work
CO2 (40%) + N2 (60%) + NH 57.3 ± 2.9 6.16 - 6.99 sI & sH This work
CO2 (60%) + N2 (40%) + NH 54.5 ± 0.2 6.19 sI This work
CO2 (80%) + N2 (20%) + NH 55.4 ± 0.2 6.22 sI This work
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The experimental results indicate that the DHd values for the CO2 + N2 + NH systems can also provide information on the structural transition of sH to sI hydrates with respect to the CO2 concentration in the feed gas. The higher DHd values of sH hydrates for the CO2 + N2 + NH systems than those of sI hydrates for the corresponding CO2 + N2 hydrates and the abrupt DHd value change at the feed CO2 concentration of 40% should be considered for estimating and predicting the amount of heat required for hydrate formation/dissociation in an actual replacement processes.
Figure 4.3.10. Dissociation enthalpies of gas hydrates before and after replacement in the structure- transitional and isostructural systems.
D H (kJ/mol guest)
50 55 60 65 70 75 80
CH4 + NH, sH
CH4 + NH, sH
CH4 + NH, sH CO2, sI, Lee et al.[55]
CO2, sI, Lee et al.[55]
CH4, sI, Lee et al.[55]
replaced with CO2 (10%) + N2 (90%), sH
replaced with CO2 (20%) + N2 (80%), sH replaced with CO2, sI
replaced with CO2, sI, Lee et al.[55]
CO2 (10%) + N2 (90%) + NH , sH, Lee et al.[96]
CO2 (20%) + N2 (80%) + NH , sH, Lee et al.[96]
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Figure 4.3.10. and Table 4.3.6. present the influence of replacement on the dissociation enthalpies of the structure-transitional system (CH4 + NH - CO2 replacement, sH → sI) and the isostructural systems (CH4 - CO2 replacement, sI → sI and CH4 + NH - flue gas replacement, sH→ sH). In the replacement system accompanying a structural transition, the DHd value of the initial CH4 + NH hydrates (sH) was found to be 72.0 ± 0.1 kJ/mole guest, and that of the CH4 + NH hydrate replaced with CO2 (sI) was 56.2
± 0.3 kJ/mole guest. This DHd value of the CH4 + NH hydrate replaced with CO2, whose components are expected to be CH4 and CO2 in the hydrate phase, was almost identical to that of the CH4 hydrate replaced with CO2 (55.2 ± 0.2 kJ/mole guest) in the sI-isostructural system. The slight difference in DHd
values between these two systems can be attributed to the higher CO2 composition in the hydrate phase after replacement, because the CH4 + NH - CO2 replacement accompanying a structural transition (sH
→ sI) facilitated greater extent of replacement than CH4 - CO2 replacement with an isostructural system.
In the sH-isostructural system, the DHd value of the CH4 + NH hydrate replaced with CO2 (10%) + N2
(90%) was found to be 63.0 ± 0.3 kJ/mole guest, and that of the CH4 + NH hydrate replaced with CO2
(20%) + N2 (80%) was found to be 63.3 ± 0.2 kJ/mole guest. As shown in Figure 4.3.10. and Table 4.3.6,. the DHd value of the replaced hydrate in the structure-transitional system was significantly lower than that of the initial CH4 + NH hydrate and even slightly lower than that of pure CO2 hydrate.
Interestingly, the DHd values of the replaced hydrates in both structure-transitional and isostructural systems became similar to those of the hydrates formed from the corresponding injecting gases or injecting gases with NH. These changes in the DHd values indicate that the initial hydrates were converted into the mixed gas hydrates with relatively low CH4 content. However, the DHd values of the replaced hydrates in the isostructural replacement did not change as remarkably as in the structure- transitional replacement. Furthermore, in the isostructural replacement, the DHd value of each replaced hydrate was located between that of each initial hydrate and that of each hydrate formed from the injecting gases or injecting gases with NH. As the DHd value is generally a function of both hydrate structure and cage occupation of guest molecules, the DHd values before and after replacement can be effectively used to predict the structural transition and to estimate the extent of replacement. This experimental study on the dissociation enthalpies of the gas hydrates provides further insights into the dissociation behavior and thermodynamic stability according to the structural system during CH4 - flue gas replacement.
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Table 4.3.6. The dissociation enthalpy changes after replacement in multi-structural system and isostructural system.
system component
DHd (kJ/mole
guest)
structure
structure- transitional replacement
initial CH4 + NH 72.0 ± 0.1 sH
replaced with CO2 CH4 + CO2 56.2 ± 0.3 sI
(structure transformed)
injecting gas CO2 [55] 57.1 ± 0.1 sI
isostructural replacement
initial CH4 [55] 54.1 ± 0.2 sI
replaced with CO2 CH4 + CO2 [55] 55.2 ± 0.2 sI
injecting gas CO2 [55] 57.1 ± 0.1 sI
initial CH4 + NH 72.0 ± 0.1 sH
replaced with CO2 (10%) + N2 (90%) CH4 + CO2 + N2 + NH 63.0 ± 0.3 sH
injecting gas CO2 (10%) + N2 (90%) + NH [96] 60.6 ± 0.3 sH
initial CH4 + NH 72.0 ± 0.1 sH
replaced with CO2 (20%) + N2 (80%) CH4 + CO2 + N2 + NH 63.3 ± 0.2 sH
injecting gas CO2 (20%) + N2 (80%) + NH [96] 61.4 ± 0.3 sH
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