Low driving force
Compared with high pressure, hydrate growth is more complicated under lower pressure (3.8 MPa), which is divided into three stages. Regardless of pure water or two nano-promoter systems, methane uptake is stable at a rate of about 100 mmol/min for about 1 h after the stirring was turned on, demonstrating that hydrate growth kinetics is governed by spiral agitation in stage I. In this stage, hydrate growth kinetics is sluggish due to low driving force, thus a large amount of interstitial water is enclosed by hydrates, which cannot be shaped up, causing the effect of nano-promoters doesn’t work, and the conversion is only about 20%. As the proceeds of hydrate growth, its surface is gradually hardened, and then it can be broken into smaller pieces on account of continuous stirring, and then the gas-liquid contact and mass transfer are greatly improved, so the uptake rate drastically increases in stage II. Interestingly, the conversion is close to 50% at the end of stage II, which is approximately equivalent to that in stage I at 5 MPa. Obviously, stage I at high pressure is divided into two stages when pressure is low due to slow hydrate growth kinetics. In contrast, hydrate growth kinetics is fast at high pressure, so the hardening and crush of hydrates is fast, causing only one stage occur during stirring. Subsequently, the interstitial water is suffered severely by mass transfer limitation, causing the declination of conversion efficiency, and then the promotion effect induced by spiral agitation disappears, so methane uptake rate almost unchanged after turning off agitation. In stage III, inner interstitial water is difficult to permeate outward due to the limitation of hydrate shells, so hydrates mainly grow inward with the diffusion of methane inward, and this leads to lots of interstitial water remains unconverted in pure water due to low driving force, causing low final conversion (<50%).
Comparably, the scenarios in nano-promoter systems are significantly improved, especially for -SO3-@PSNS systems, and the unique “secondary uptake” phenomenon increases the final conversion to a large extent. The excellent ductility and porosity of hydrates formed in -SO3-@PSNS systems are very similar to that in amino acid system9,37. The increased ductility of hydrates reduced their hardness, making large hydrate chunks to be easily broken into small ones, thus alleviating the restrictions of mass transfer. Additionally, the rich porous properties improve the permeability of hydrates, this is beneficial to the conversion of inner interstitial water, and with the inward growth of hydrates, micro cracks would occur due to space constraint, which further increases mass transfer and the conversion of interstitial water. However, no obvious “secondary uptake” phenomenon was observed in -COO-@PSNS systems. Considering that methane uptake remains a relatively large rate after turning off agitation in -COO-@PSNS systems, it can be speculated that the porosity of hydrates formed in -COO-@PSNS systems is also excellent, while the ductility should be poor. Moreover, the conversion of interstitial water in -COO-@PSNS systems strongly depends on the effect of previous agitation, causing it to be more sensitive to the inclined angle. In addition, it is worth to note that the reaction time of stage III in pure water is short at 3.8 MPa, while it is greatly prolonged in the presence of nano-promoters as presented in Figure 8, and it is precisely the extension of stage III that causes the final conversion to be satisfied. In a word, the effect of nano-promoters under low driving force only works after the formation of hydrates, and the presence of nano-promoters made up the low conversion under the conditions.
Ultimately, hydrate growth depends on mass transfer and reaction kinetics. A schematic diagram of hydrate growth in different systems is given in Figure 9. The reaction kinetics is strong under high driving force, thus hydrate growth is mainly limited by mass transfer in this situation. In the initial stage (stage I) of hydrate formation, the mass transfer that enhanced by the spiral agitation accelerates hydrate growth, resulting in quickly hardening of hydrate surface, and this causes large hydrate chunks to be easily crushed into small ones by spiral agitation and further increases the mass transfer. When the stirring is turned off (stage II), hydrates continue to grow inward, but part of interstitial water remains unconverted due to the limitation of mass transfer. However, this scenario is improved by the addition of nano-promoters, which increase the porosity of hydrates, giving rise to enhanced mass transfer and large conversion. In comparison, although mass transfer is enhanced by spiral agitation, hydrate growth kinetics is poor under low driving force, so hydrate growth is mainly limited by reaction kinetics in this situation, and this causes low conversion in the initial stage (stage I), where hydrates are difficult to be harden and crushed. With further reaction, hydrate chunks gradually harden and then are broken into small ones by spiral stirring. At this time, the gas-liquid contact and mass transfer are both greatly intensified, resulting in larger methane uptake rate in stage II. After the stirring is turned off (stage III), interstitial water is continuously converted at an extremely slow rate, and this is improved by nano-promoters, which can increase hydrate porosity. Moreover, hydrates ductility is also improved in -SO3-@PSNS systems, this induces micro cracks, which further enhances mass transfer and gives rise to the “secondary uptake” phenomenon, ensuring excellent methane storage capacity under mild conditions.