3.2. N 2 -assisted CH 4 hydrate dissociation and replacement
3.2.2. Kinetics of the CH 4 hydrate dissociation induced by gaseous N 2 injection
To examine the effect of the temperature and N2 injecting pressure (PN2) on the dissociation behavior of the CH4 hydrate, in situ GC measurements were conducted at two different PN2 (4.0 and 8.0 MPa) and three different temperatures (268.8, 274.2, and 278.2 K), as shown in Fig. 3.2.1. As hydrate dissociation accompanies the release of the entrapped guest gas molecules, an increase in the system pressure is a critical indicator in detecting whether the gas hydrates dissociate or not. As shown in Figs.
3.2.1(a) and (b), for PN2 = 4.0 MPa and 8.0 MPa at 278.2 K, the pressure of the system gradually increased immediately after the injection of the N2 gas and finally reached saturation, indicating that the dissociation of the CH4 hydrate was initiated by the N2 gas injection. When there was no further increase in the pressure, the vapor phase of the reactor was immediately evacuated, and the reactor was heated up to 293.2 K, but no residual hydrate phase was observed. This shows that the stabilization of the pressure in Figs. 3.2.1(a) and (b) was caused by the full decomposition of the CH4 hydrate. The CH4
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composition in the vapor phase also increased over time and reached saturation after the full decomposition of the CH4 hydrate.
The full decomposition of the CH4 hydrate was also observed in the case of PN2 = 4.0 MPa and 8.0 MPa at 274.2 K, as shown in Figs. 3.2.1 (c) and (d). The pressure and the CH4 composition in the vapor phase converged into approximately 5.7 MPa and 35% for PN2 = 4.0 MPa, respectively, and approximately 9.4 MPa and 21% for PN2 = 8.0 MPa, respectively, at both 274.2 K and 278.2 K, indicating that the N2
gas injection at both 274.2 K and 278.2 K resulted in the full decomposition of the CH4 hydrate in the closed system. However, it should be noted that at 268.8 K, the pressure and the CH4 composition in the vapor phase did not reach these values after the N2 gas injection. As an increase in the system pressure in a closed system is a good indicator of the dissociation of gas hydrates, as mentioned earlier, the conversion to a relatively lower system pressure and CH4 composition in the vapor phase implied the incomplete dissociation of the CH4 hydrate and the consequent existence of the residual hydrate phase in the reactor.
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Fig. 3.2.1 Changes in pressure and CH4 composition in the vapor phase for PN2 = 4.0 MPa and PN2 = 8.0 MPa at 268.8 K, 274.2 K, and 278.2 K
To examine the quantitative dissociation behavior of the CH4 hydrate by N2 gas injection, the normalized residual amounts of CH4 in the hydrate phase in each case were obtained based on the results from the in situ GC measurements (Fig. 3.2.2). The changes in the normalized residual CH4 in the hydrate phase under each N2 injecting condition showed that the dissociation behavior of the CH4
hydrate varied depending on the temperature and the PN2 conditions. The dissociation rate of the CH4
hydrate was accelerated with an increase in temperature, and the termination of the CH4 hydrate dissociation was faster at PN2 = 4.0 MPa than at PN2 = 8.0 MPa (4 h [PN2 = 4.0 MPa] and 6 h [PN2 = 8.0 MPa] at 278.2 K; 10 h [PN2 = 4.0 MPa] and 30 h [PN2 = 8.0 MPa] at 274.2 K). The faster kinetics of the CH4 hydrate dissociation induced by the N2 gas injection were observed at a higher temperature and a
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lower PN2. Conversely, the CH4 captured in the hydrate phase was not completely released at 268.8 K even after a long time, and 22% and 39% of the initial CH4 in the hydrate phase was still trapped in the hydrate cages for PN2 = 4.0 MPa and PN2 = 8.0 MPa, respectively. This indicates that a higher extent of CH4 production could be obtained at a lower PN2 when incomplete hydrate dissociation occurred at the N2-induced CH4 production. Kang et al. already reported incomplete hydrate dissociation through air injection in a closed system and demonstrated that the dissociation of the CH4 hydrate in a high-pressure reactor through the injection of air and air + CO2 mixtures was terminated when the vapor phase composition reached a certain CH4 concentration, which is called the critical methane concentration (CMC)24.
Fig. 3.2.2. Changes in the normalized amount of residual CH4 in the hydrate phase for PN2 = 4.0 MPa and PN2 = 8.0 MPa at 268.8 K, 274.2 K, and 278.2 K
The dissociation of gas hydrates is caused by the thermodynamic instability of the system, as shown in the conventional methods for NGH production, such as depressurization and thermal stimulation. As inhibitor injection is also closely related to the thermodynamic instability of the system, the retention of the thermodynamic driving force for hydrate dissociation is the core issue to avoid the decline in the rate and extent of hydrate dissociation. In the case of gaseous N2 injection, hydrate dissociation is triggered by the disequilibrium state of the vapor and hydrate phases. As shown in Fig. 3.2.1, the
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thermodynamic driving force for hydrate dissociation could not be maintained during hydrate melting in the closed system due to an increase in the pressure and CH4 composition in the vapor phase.
Fig. 3.2.3 illustrates the changes in the thermodynamic environment and the paths that CH4 hydrates go through during dissociation at each temperature for PN2 = 4.0 MPa and PN2 = 8.0 MPa. As previously mentioned, at both 274.2 K and 278.2 K, the pressure and the CH4 composition in the vapor phase converged into 5.7 MPa and 35%, and 9.4 MPa and 21% for PN2 = 4.0 MPa and PN2 = 8.0 MPa, respectively, after the full decomposition of the CH4 hydrate. As shown in Fig. 3.2.3 (a), in the case of PN2 = 4.0 MPa at 274.2 K, the dissociation of the CH4 hydrate was initiated immediately after N2 gas injection (red solid circle), and the pressure and the gas composition in the vapor phase at that point were 4.0 MPa and 100% N2, respectively. The pressure and the CH4 composition in the vapor phase gradually increased because of hydrate dissociation, and the changes were terminated by the full decomposition of the hydrate phase (red solid diamond: 5.7 MPa and CH4 (35%) + N2 (65%)). However, this point was still located below the equilibrium pressure of the CH4 (35%) + N2 (65%) hydrate (blue solid diamond) at 274.2 K, indicating that the possibility of full decomposition of the CH4 hydrate by N2 gas injection could be predicted from the hydrate phase equilibria. Therefore, at both 274.2 K and 278.2 K, the CH4 hydrates for PN2 = 4.0 MPa and PN2 = 8.0 MPa were expected to be fully decomposed from the H β Lw β V equilibria of the CH4 + N2 hydrates. This prediction is consistent with the results presented in Fig. 3.2.1. However, at 268.8 K (PN2 = 4.0 MPa), the equilibrium pressure (4.3 MPa, blue empty diamond) of the CH4 (35%) + N2 (65%) hydrate was lower than 5.7 MPa, as shown in Fig. 3.2.3 (a). This implies that the CH4 hydrate at 268.8 K might not be fully dissociated by N2 gas injection and that a certain fraction of the hydrate phase might eventually remain. As shown in Fig. 3.2.1(e), the terminal pressure and vapor phase composition were 5.1 MPa and CH4 (30%) + N2 (70%), respectively, and 30% CH4 was considered the CMC at 268.8 K for PN2 = 4.0 MPa with our experimental setup. Note that in the case of PN2 = 8.0 MPa, at 268.8 K, the N2 gas injecting pressure (8.0 MPa) was initially higher than the equilibrium pressure (6.2 MPa) of the CH4 (21%) + N2 (79%) hydrate (Fig. 3.2.3 (b)), and the CH4 composition in the vapor phase slightly and gradually increased without a significant increase in pressure, as shown in Fig. 3.2.1(f).
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Fig. 3.2.3. The three-phase equilibria of CH4 + N2 hydrates and traces of pressure changes after N2 gas injection for (a) PN2 = 4.0 MPa and (b) PN2 = 8.0 MPa
The guest composition and saturation of the residual hydrate phase 48 h after the N2 gas injection (PN2
= 4.0 MPa and 8.0 MPa) at 268.8 K are shown in Fig. 3.2.4. The saturation of the residual hydrate (Sr) was estimated using the following equation:
ππ = ππ
πππΆβ 100
where Nr is the number of moles of the residual CH4 in the final hydrate phase, NT is the number of moles of the total CH4 in the initial hydrate phase, and C is the mole fraction of CH4 in the hydrate phase obtained using GC after dissociating the residual hydrate phase. For simplicity, the total cage occupancy of the CH4 hydrate was assumed to be equal to that of the CH4 + N2 hydrate. As shown in Fig. 3.2.4, a remarkable fraction of N2 in the residual hydrate was observed for both PN2 = 4.0 MPa and PN2 = 8.0 MPa, indicating that N2 participated in the hydrate lattices as a hydrate guest during the hydrate dissociation process. This is not surprising because, with the residual hydrate phase, the compositions of the vapor and hydrate phases would be rearranged to achieve a new three-phase equilibrium of the CH4 + N2 hydrate at a given thermodynamic circumstance, and thus, N2 should be included in the hydrate phase. The saturations of the residual hydrates were 41% and 77%, and the N2 compositions in the residual hydrate phase were 47% and 49% for PN2 = 4.0 MPa and PN2 = 8.0 MPa, respectively. This indicates that as PN2 increased, hydrate dissociation declined sharply, whereas the extent of N2 storage in the hydrate lattices increased.
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Fig. 3.2.4. Guest composition and saturation of the residual hydrate phase for PN2 = 4.0 MPa and PN2 = 8.0 MPa at 268.8 K after 48 h