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.