Enhanced Hydrate Formation
Hydrate nucleate kinetics can be greatly improved by spiral agitation, resulting in fast hydrate nucleation. It can be seen from Table S2 and S3 that hydrate nucleates rapidly within 5 min in pure water (5 MPa), and the longest induction period is only 7.3 min even under mild condition (3.8 MPa). The nucleation kinetics is further strengthened by nano-promoters, especially under mild conditions, and hydrate induction time is less than 5 min in both of the nano-promoter systems. In addition, hydrate growth kinetics is also enhanced due to the improvement of mass transfer induced by spiral agitation, giving rise to large gas storage capacity. Methane storage capacity in different systems is given in Figure 1, and it is clear that large methane uptake is accessible even in pure water, where hydrate growth kinetics strongly depends on driving force. The storage capacity is close to 70 V/V at 3.8 MPa, while it goes up to 139.54 V/V and 144.54 V/V at 5 MPa for the inclined angle of 35° and 45°, respectively, and water-to-hydrate conversion is more than 85%. Besides, it is worth noting that the convection 35 of liquid phase inner the spiral-agitated reactor intensifies with the increase in the inclined angle, so the mass transfer between gas and liquid is enhanced at large inclined angle, leading to the increase in conversion, but when the driving force was feeble, this effect was inconspicuous.
It is worth to note that the conversion increases by adding -COO-@PSNS, but the effect also depends on the driving force and inclined angle. Apparently, the conversion increases with the increase in inclined angle at 3.8 MPa, and the maximum gas storage capacity of 141.33 V/V is obtained at 45°, but the promotion of COO-@PSNS greatly degenerates at high pressure (5 MPa) at the same inclined angle, where the conversion is only 40.35 %. What is more noteworthy is that hydrate growth kinetics in -SO3-@PSNS systems is excellent even under mild condition (3.8 MPa), and the conversion is up to 97.29 % at 5 MPa and 35°, and large conversion (>85 %) can also be achieved for low pressure (3.8 MPa). In conclusion, the nucleation and growth kinetics of hydrates are enhanced by spiral agitation, making it possible to achieve admirable conversion (85%) in pure water, and this enhancement is further improved by nano-promoters, causing larger gas storage capacity. Additionally, compared with -COO-@PSNS, -SO3-@PSNS performs better on the promotion, especially under mild conditions.
In order to clarify the promotion mechanisms of spiral agitation on hydrate growth kinetics in different systems, the rate of methane uptake was evaluated as shown in Figures 3 and 4, and it is clear that methane uptake rate is severely affected by driving force. The uptake rate in pure water is relatively stable during the stirring period at 5 MPa, which almost maintains a constant rate of 400 mmol/min, but it decreases rapidly after the agitation was turned off, demonstrating the excellent promotion of spiral agitation on hydrate growth kinetics. When nano-promoters are introduced, hydrate growth kinetics is significantly strengthened, and methane uptake rate increases up to 800 mmol/min. Moreover, the uptake rate decreases more slowly than that in pure water after turning off the agitation, this means that nano-promoters improve hydrate formation in static conditions.
In contrast, due to mass transfer limitation, hydrate growth kinetics is relatively feeble at low pressure (3.8 MPa), resulting in a lower uptake rate, as presented in Figure 4. The variation of methane uptake rate in the nano-promoter systems is similar to that in pure water at stirring stage, methane uptake keeps a constant rate of 100 mmol/min originally, and then it dramatically increases. Apparently, during the initial stage of hydrate formation, hydrate growth kinetics is controlled by spiral agitation, and the effect of nano-promoters is negligible. However, when the stirring was turned off, the scenario changes completely. As can be seen in Figure 4, methane uptake rate in pure water is very low during hydrate static formation stage, but it subsequently increases sharply at about 150 min in -SO3-@PSNS systems, and the rate peak even reaches 100 mmol/min. Moreover, this phenomenon lasts for nearly 300 min and is named as ”secondary uptake” stage, which should contribute to the large methane storage capacity in -SO3-@PSNS systems, and the occurrence of this phenomenon should be closely related to the characteristics of hydrates. Although there is no obvious phenomenon of “secondary uptake” in -COO-@PSNS systems, methane uptake rate decreases more slowly than that in pure water at the inclined angle of 35° and 45°, and this prolongs the period of methane uptake, so large water-to-hydrate conversion is also obtained in -COO-@PSNS systems.