52
53
Table 4.3.1. (a) Hydrate phase equilibrium data for the N2 + NH and CO2 + N2 + NH systems.
N2
CO2 (10%) + N2 (90%)
CO2 (20%) + N2 (80%)
CO2 (40%) + N2 (60%)
CO2 (60%) + N2 (40%)
CO2 (80%) + N2 (20%) T/K p/MPa T/K p/MPa T/K p/MPa T/K p/MPa T/K p/MPa T/K p/MPa 275.0 6.63 275.3 5.73 274.2 4.18 275.3 3.65 274.4 2.47 274.5 2.06 278.3 10.02 276.4 6.78 276.6 6.02 277.1 4.96 275.6 2.92 276.2 2.54 280.5 12.98 278.4 8.84 278.4 7.91 278.6 6.39 277.6 3.78 277.6 3.00 282.7 16.60 280.1 11.45 280.3 10.61 280.0 7.91 279.4 4.82 279.2 3.75 284.4 20.22 281.5 13.66 281.5 12.89 281.4 9.98 280.8 6.00 280.2 4.37
(b) Hydrate phase equilibrium data for the N2 and CO2 + N2 systems.
N2
Lee et al. [66]
CO2 (10%) + N2 (90%) Lee et al.[66]
CO2 (20%) + N2 (80%) Lee et al. [66]
CO2 (40%) + N2 (60%)
CO2 (60%) + N2 (40%)
CO2 (80%) + N2 (20%) 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 16.13 275.0 11.28 274.0 7.26 274.4 3.59 274.4 2.49 274.6 1.84 273.9 17.49 276.4 13.32 275.4 8.23 276.4 4.67 276.3 3.13 276.0 2.18 274.3 18.55 277.5 15.59 276.6 9.42 278.1 6.02 277.6 3.79 277.8 2.82 274.6 19.23 278.7 18.13 278.1 11.28 280.0 8.00 279.2 4.64 279.4 3.52 279.7 20.77 279.5 13.36 281.6 10.20 280.7 5.89 281.0 4.41 280.8 24.51 281.1 16.70
54
As seen in Figure 4.3.1. (a), for the CO2 (20%) + N2 (80%) + NH system, the hydrate equilibrium line was shifted to the more stable region represented by higher temperature at any given pressure or lower pressure at any given temperature. This shift clearly indicates that NH molecules were enclathrated in the large 51268 cages of the sH hydrate, thereby stabilizing the hydrate lattices. When the CO2
concentration increased, the shift of the hydrate equilibrium lines as a result of the NH inclusion did not appear. As shown in Figure 4.3.1. (b), the hydrate equilibrium line of the CO2 (40%) + N2 (60%) + NH system was almost identical to that of the CO2 (40%) + N2 (60%) system at higher pressure and temperature ranges whereas a slight difference was observed in the hydrate equilibrium lines at lower pressure and temperature ranges. In view of the fact that sH hydrates with both large hydrocarbons and small help gases are generally more stable than sI or sII hydrates with only small gas molecules, the result implies that there is a possibility of coexistence of sI and sH hydrates caused by the structural transition of sH into sI in the CO2 (40%) + N2 (60%) + NH system. As seen in Figure 4.3.1. (c), the hydrate equilibrium line of the CO2 (60%) + N2 (40%) + NH system did not exhibit any difference from that of the CO2 (60%) + N2 (40%) system, indicating that the presence of NH affected neither the stability conditions nor the structure of the CO2 (60%) + N2 (40%) system. Therefore, NH molecules are expected to stabilize the CO2 + N2 hydrates in N2-rich systems whereas they are inert in CO2-rich systems. From the shift of hydrate equilibrium lines, it could be suggested that a gas mixtures in the range of flue gas compositions (10-20% CO2) can form sH hydrates in the presence of NH, thereby mitigating the pressure and temperature requirements. Furthermore, it was also found that CO2
concentrations can play an important role in determining the stability of sH hydrates with NH, leading to the structural transition.
For the experimental measurements of the hydrate stability conditions after replacement, the four-phase (H-LW-LHC-V) equilibrium conditions before and after replacement were compared to verify the influence of flue gas injection on the thermodynamic stability of the sH hydrates. Figure 4.3.2. (a) presents endothermic dissociation thermograms of the initial CH4 + NH hydrate and the CH4 + NH hydrate replaced with CO2 (10%) + N2 (90%). As shown in Figure 4.3.2. (b), the equilibrium dissociation temperature at a given pressure was determined from the extremum point of the heat flow changes with respect to temperature (dHF/dT) versus temperature (T) curve considering the pore size distribution of the silica gels. For the replacement experiments, the initial CH4 + NH hydrate was replaced with two different flue gas mixtures: CO2 (10%) + N2 (90%) at 6.30, 11.50, and 16.08 MPa, and CO2 (20%) + N2 (80%) at 4.04, 6.15, and 9.72 MPa.
55
Figure 4.3.2. (a) Dissociation thermograms of the initial CH4 + NH hydrate (8.84 MPa), and that of the CH4 hydrate replaced with CO2 (10%) + N2 (90%) (16.08 MPa) after 72 h; (b) Determination of the equilibrium dissociation temperature considering the pore size distribution.
276 278 280 282 284 286 288 290 292 294 296
H e a t F lo w ( m W )
-40 -30 -20 -10 0 10 20
before replacement (CH4 + NH hydrate)
after replacement (CH4 + CO2 + N2 + NH hydrate)
Temperature (K)
276 278 280 282 284 286 288 290 292 294 296
d H F /d T
-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3
before replacement (CH4 + NH hydrate)
after replacement (CH4 + CO2 + N2 + NH hydrate) equilibrium temperature
(b) (a)
56
The phase equilibrium conditions of the initial CH4 + NH hydrate and the CH4 + NH hydrates replaced with CO2 + N2 at each pressure condition are depicted in Figure 4.3.3. (a) and (b). The equilibrium conditions of the CO2 + N2 and CO2 + N2 + NH hydrates, which correspond to the compositions of each injecting gas used for the replacement procedure, are also included in Figure 4.3.3. for comparison [66, 95, 96]. As seen in Figure 4.3.3. (a) and (b), the phase equilibrium conditions of each replaced hydrate were located similar to those of the corresponding CO2 + N2 + NH hydrates (sH), but very different from those of the corresponding CO2 + N2 hydrates (sI). These shifts in the equilibrium dissociation conditions after replacement indicate that the initial CH4 + NH hydrates were converted into other mixed-gas hydrates through the exchange of guest molecules in the gas hydrate. The shifts observed in the thermodynamic stability of gas hydrates after replacement suggest two possible replacement mechanisms. One is that the initial CH4 + NH hydrate (sH) undergoes structural transformation into CH4 + CO2 + N2 hydrates (sI) in scenarios where limited replacement occurs, and the other is that the initial CH4 + NH hydrate (sH) goes through isostructural conversion into the CH4 + CO2 + N2 + NH hydrates (sH) in scenarios where substantial replacement occurs. Therefore, microscopic analysis should be performed to reveal a possible structural transition and accurate guest distributions before and after replacement in order to fully understand the mechanism of CH4 + NH - flue gas replacement.
Temperature (K)
273 275 277 279 281 283
Pressure (MPa)
0 5 10 15 20
CO2 (20%) + N2 (80%), sI, Lee et al. [66]
CO2 (20%) + N2 (80%) + NH, sH, Lee et al. [96]
CH4 + NH, sH, Lee et al. [95]
CH4 + NH - CO2 (20%) + N2 (80%) replacment
Temperature (K)
273 275 277 279 281 283 285
Pressure (MPa)
0 5 10 15 20 25 30 35
CO2 (10%) + N2 (90%), sI, Lee et al. [66]
CO2 (10%) + N2 (90%) + NH, sH, Lee et al. [96]
CH4 + NH, sH, Lee et al. [95]
CH4 + NH - CO2 (10%) + N2 (90%) replacment
(a) (b)
sI
sH
sI
sH
Figure 4.3.3. Hydrate stability conditions of the CH4 + NH hydrate replaced with (a) CO2 (10%) + N2
(90%) and (b) CO2 (20%) + N2 (80%) compared with those of the initial CH4 + NH, CO2 + N2, and CO2
+ N2 +NH hydrates.
57