3.2.1. Stability Condition of CO2 + N2 Hydrates
The stability conditions of the CO2 + N2 gas hydrates determined using the three-phase (H-Lw-V) equilibria are fundamental reference data for predicting the pressure and temperature conditions required for the hydrate-based CO2 capture and storage process. The hydrate stability conditions are generally determined using an isochoric (pVT) method. However, Dalmazzone et al. [63] verified that a µ-DSC could be effectively used to measure the H-LW-V equilibrium conditions for gas hydrate systems. In Figure 3.2.1. and Table 3.2.1., the three-phase equilibrium data obtained from the DSC method were compared with those obtained from the pVT method and also with previously reported data [64, 65].
Figure 3.2.1. Comparison of hydrate phase equilibrium conditions (pVT method versus DSC method).
Temperature (K)
272 274 276 278 280 282 284
Pressure (MPa)
1 10
100 N2, pVT
N2, v an Cleef f and Diepen [64]
N2, DSC
CO2 (10%) + N2 (90%), pVT CO2 (10%) + N2 (90%), DSC
CO2 (20%) + N2 (80%), pVT CO2 (20%) + N2 (80%), , DSC CO2, Adisasmito et al. [65]
CO2, pVT
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Table 3.2.1. Hydrate phase equilibrium data for the N2 and CO2 + N2 systems.
N2 CO2 (10%) + N2 (90%) CO2 (20%) + N2 (80%)
pVT DSC pVT DSC pVT DSC
T/K p/MPa T/K p/MPa T/K p/MPa T/K p/MPa T/K p/MPa T/K p/MPa 273.0 16.13 276.1 20.74 275.0 11.28 279.0 19.04 275.4 8.23 277.6 10.51 273.9 17.49 276.6 21.87 276.4 13.32 280.6 23.36 276.6 9.42 279.5 13.44 274.3 18.55 277.5 23.90 277.5 15.59 277.5 15.67 278.1 11.28 280.8 16.53
274.6 19.23 278.7 18.13 279.5 13.36
279.7 20.77 281.1 16.70
280.8 24.51
As shown in Figure 3.2.1., the results obtained from the DSC method were in good agreement with those from the pVT method. These results confirm that the DSC method can be used to determine accurate H-LW-V equilibrium conditions for the CO2 (10%) + N2 (90%), CO2 (20%) + N2 (80%), pure N2, and pure CO2 gas hydrates. Note that the slope of the H-LW-V line for the pure N2 hydrate system is slightly different from those of the H-LW-V lines for the CO2 + N2 hydrate systems, as shown in Figure 3.2.1. This difference implies the structural transition of sII hydrates into sI hydrates due to the inclusion of CO2 in the N2 gas hydrate, considering the fact that pure N2 forms sII hydrate and pure CO2
forms sI hydrate. The experimental results demonstrated that at the given pressures, the onset temperatures that are obtained from the endothermic hydrate dissociation peaks can provide accurate H-LW-V equilibrium conditions, and thus the DSC method can be used as an alternative method for stability condition measurements of the CO2 + N2 hydrates.
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3.2.2. Influence of Replacement on the Stability Condition
In this study, the stability conditions of the CH4 hydrate replaced with flue gas were determined using both the pVT and DSC methods in the presence of porous silica gels. Porous silica gels enhance the gas-water or gas-solid contact area, thereby increasing the conversion of water into hydrate.
Furthermore, natural gas hydrates are generally found in porous sediments such as silt, clay, and sandstones, and therefore, the replacement using porous silica gels is more appropriate than that using bulk water or ice particles for simulating the actual replacement occurring in deep-sea reservoirs.
Although the capillary effect in small pores of silica gels can affect the activity of water, no significant inhibition effect of the 100 nm pores on the hydrate phase equilibrium was observed in the pVT method [66-69].
Figure 3.2.2. (a) Determination of the onset temperature considering the pore size distribution and (b) comparison of the hydrate stability conditions obtained from the DSC method in the porous silica gels (dm = 100 nm) with those from the isochoric (pVT) method in the porous silica gels (dm = 100 nm), the DSC method in the bulk phase, and the thermodynamic model in the bulk phase [CSMGem].
Temperature (K)
260 265 270 275 280 285 290 295 300
Heat Flow (mW)
-50 -40 -30 -20 -10 0 10
dHF/dT
-0.5 -0.3 -0.1 0.1 0.3
Heat Flow dHF/dT
onset temperature
Temperature (K)
272 274 276 278 280 282 284 286 288
Pressure (MPa)
2 4 6 8 10 12
Bulk, CSMGem Silica gel, PVT, Kang et al. [70]
Bulk, DSC Silica gel, DSC
(a) (b)
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The onset temperature in the endothermic thermogram of the DSC experiments using bulk water was determined at the inflection point of the heat flow curve and was taken as the hydrate dissociation equilibrium temperature at a given pressure. However, in the experiment using porous silica gels, it was very difficult to determine the accurate onset temperature because a gradual change in the slope around the expected onset temperature was observed most likely due to the pore size distribution as well as compositional inhomogeneity between individual clathrate particles. Therefore, a new onset temperature that corresponds to the hydrate dissociation equilibrium temperature in the pores with the mean diameter of silica gels should be determined. In order to overcome the inherent difficulties with pore size distribution, in the pVT experiment using the porous silica gels, the maximum inclination of pressure with respect to temperature was chosen as the dissociation equilibrium point with a mean diameter of the silica gel pores [67, 68, 70]. Likewise, in the DSC experiment using the porous silica gels, the new onset temperature was determined at the extremum point of the heat flow changes with respect to the temperature (dHF/dT) versus temperature (T) heating curve considering the pore size distribution of the silica gels, as shown in Figure 3.2.2. (a). As seen in Figure 3.2.2. (b), the dissociation equilibrium conditions obtained from the DSC method considering the pore size distribution were in good agreement with those from the classical isochoric (pVT) method in the porous silica gels, the DSC methods in the bulk phase and the thermodynamic model in the bulk phase [CSMGem]. This result clearly demonstrates that the DSC method can provide reliable H-LW-V equilibrium conditions in the pores of silica gels.
Figure 3.2.3. The dissociation thermograms of the pure CH4 hydrate (8.91 MPa) before the replacement and that of the CH4 hydrate replaced with (a) CO2 (10%) + N2 (90%) (11.54 MPa) and (b) CO2 (20%) + N2 (80%) (8.56 MPa) after 72 h.
Temperature (K)
268 273 278 283 288 293 298
Heat Flow (mW)
-45 -40 -35 -30 -25 -20 -15 -10 -5 0 5
bef ore replacement (pure CH4) af ter replacement (CH4 + CO2 + N2) onset temperature
(b)
Temperature (K)
268 273 278 283 288 293 298
Heat Flow (mW)
-45 -40 -35 -30 -25 -20 -15 -10 -5 0 5
bef ore replacement (pure CH4) af ter replacement (CH4 + CO2 + N2) onset temperature
(a)
25
Figure 3.2.3 indicates the dissociation thermogram of the CH4 hydrate before the replacement and also that of the CH4 hydrate replaced with CO2 (10%) + N2 (90%) and CO2 (20%) + N2 (80%) at 263.2 K.
As shown in Figure 3.2.3., the thermogram after the replacement appears to have a slightly broader peak compared with that of the starting material. The broader thermogram after the replacement indicates compositional inhomogeneity caused by the conversion of the CH4 hydrate into the mixed CH4 + CO2
+ N2 hydrate after the replacement [71, 72]. In this study, the CH4 hydrates were replaced with two flue gas mixtures with CO2 (10%) + N2 (90%) at 11.54, 14.59, and 18.59 MPa, and CO2 (20%) + N2 (80%) at 8.56, 10.75, and 13.66 MPa. At each pressure condition, the hydrate dissociation equilibrium temperatures of the CH4 + CO2 + N2 hydrates after the replacement were obtained from the endothermic dissociation thermograms. In Figure 3.2.4 (a) and (b), the dissociation equilibrium conditions of each replaced hydrate are compared with those of the initial CH4 hydrate and corresponding CO2 + N2 hydrate.
As seen in Figure 3.2.4 (a) and (b), for both cases, the dissociation equilibrium conditions of each replaced hydrate (CH4 + CO2 + N2) are located in close proximity to those of the corresponding CO2 + N2 hydrate. These significant shifts of the dissociation equilibrium conditions after the replacement indicate that the CH4 hydrates readily converted into mixed CH4 + CO2 + N2 hydrates as the CH4-flue gas replacement reaction proceeded and accordingly, the substantial extent of the replacement is achievable through the CH4 - flue gas swapping as confirmed by the direct composition measurements.
Figure 3.2.4. Comparison of the hydrate stability conditions of the CH4 hydrate replaced with (a) CO2
(10%) + N2 (90%) and (b) CO2 (20%) + N2 (80%) with those of the initial CH4 hydrates and CO2 + N2
hydrates
Temperature (K)
272 274 276 278 280 282 284 286
Pressure (MPa)
0 5 10 15 20 25 30
CO2 (10%) + N2 (90%), pVT, Lee et al. [66]
CO2 (10%) + N2 (90%), DSC, Lee et al. [66]
Replaced with CO2 (10%) + N2 (90%) CH4, pVT, Adisasmito et al.[65]
CH4, DSC
(a)
Temperature (K)
272 274 276 278 280 282 284 286
Pressure (MPa)
0 2 4 6 8 10 12 14 16 18 20
CO2 (20%) + N2 (80%), pVT, Lee et al. [66]
CO2 (20%) + N2 (80%), DSC, Lee et al. [66]
Replaced with CO2 (20%) + N2 (80%) CH4, pVT, Adisasmito et al.[65]
CH4, DSC
(b)
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