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).