High driving force
At higher pressure (5 MPa), when the stirring was turned on, spiral agitation can induce two-way convection, dual-agitation and continuous interfacial impact35, and these effects cause methane to be dissolved into liquid phase quickly, and this excellent mass transfer leads to fast hydrate formation within minutes as seen in Figure 6(b). During stage I, hydrates were generated rapidly and massively, which were carried upwards from liquid phase at the bottom by spiral blades (Figure 6(b)-(c)), but lots of unconverted interstitial water is enclosed in hydrates. With the proceeds of hydrate formation, the liquid level is almost disappeared in about 30 min as presented in Figure 6(c), while methane uptake still remains a large rate at this time. Because the difference in hydrate morphologies and conversion would produce different hindrance to the spiral blades, the torque of the stirring rod was measured during hydrate formation, which was shown in Figure 7. Clearly, the torque is low and almost keeps constant at stage I, so it can be speculated that hydrates are relatively soft due to the existence of interstitial water. As hydrate growth goes on, the surface of hydrates is gradually hardened, and this hinders the continuous conversion of interstitial water. However, with the hardening of hydrate surface, large hydrate chunks are gradually crushed into small ones due to the effect of spiral agitation, and this further improves gas-liquid contact and mass transfer, causing a large methane uptake rate at this stage. It is clear from Figure 7 that the torque increases linearly with the increase in the conversion in the middle period of stage I, and this is obviously caused by the hardening of hydrate surfaces. In a word, large driving force causes fast hydrate growth kinetics, although surface hardening hinders inner mass transfer, it is improved by hydrate crush induced by spiral agitation, and this causes methane uptake keep a relatively high rate for a long period (50-60 min) in stage I, giving rise to a large conversion of ca. 50% in this period. Additionally, when nano-promoters were introduced, enhanced hydrate formation was observed as seen in Figure 3. It was reported that the nano-promoters could affect the morphology of hydrates by increasing their pore texture25, and the improved porosity increases mass transfer of methane to the interior of hydrates, causing local methane uptake rate to be higher than that in pure water.
When the stirring was turned off in stage II, methane uptake rate in pure water decreases rapidly, and it maintains a relatively low value for a long time. The duration of each stage is summarized in Figure 8, and it can be seen that stage II lasts for nearly 10 h at the inclined angle of 35° and 45°. Although the conversion of interstitial water to hydrates is extremely slow due to the limitation of mass transfer, a large amount of methane is also stored in hydrates in this stage. In comparison, it only takes less than 5 h for the nano-promoter systems to get a similar effect, demonstrating the excellent promotion effect of nano-promoters. It is worth noting that there is no secondary uptake stage in -SO3-@PSNS, indicating that the conversion is relatively sufficient in stage I due to high driving force, and the t 85 that corresponds to the time of 85% water-to-hydrate conversion is less than 172 min in -SO3-@PSNS systems (see in table S2).