INTRODUCTION
As
people pay more attention to environmental matters, the use of
conventional fossil energy continues to decline, while clean energy,
such as low-carbon natural gas, has been becoming popular. The question
that follows is how to efficiently store and transport natural gas,
especially for the remote areas lack of natural gas resource. Gas
hydrates are non-stoichiometric compounds composed of water molecules
and gas molecules under low-temperature and high-pressure conditions,
where gas molecules are packed in the crystal lattices that consist of
hydrogen-bonded water molecules1. The high storage
capacity of about 170 V/V and extraordinary security of hydrates (can be
long-term stored under 253.15 K and atmospheric pressure) endow it with
great potential in natural gas storage and
transportation2–4. This gives rise to a new
technology, the so-called solidified natural gas (SNG),which was
reported to be one of the most promising alternative for conventional
technologies, such as liquefied natural (LNG) and compressed natural gas
(CNG)5. However, it is difficult to generate another
new phase (crystal nucleus) in a system, so hydrate nucleation generally
requires a persistent induction period, which has great randomness and
uncertainty especially when the driving force is feeble, and this
stochastic nature of hydrate nucleation impedes the application of the
SNG technology6. Meanwhile, hydrate growth kinetics is
also sluggish, this would cause the storage capacity of natural gas in
hydrates to be unsatisfactory7. Therefore, it is of
significant importance to enhance hydrate nucleation and growth
kinetics, while large storage capacity is also accessibility.
During
the past few decades, surfactants have been widely used to improve
hydrate formation by enhancing mass transfer and creating favorable
morphology for sustained hydrate formation8–13, such
as sodium dodecyl sulfonate (SDSN), sodium dodecyl sulfate (SDS), sodium
dodecyl benzene sulfonate (SDBS), etc. Among these surfactants, SDS
produces the most efficient promotion, which has been viewed as the most
potential promoter9,11. It was reported that hydrate
formation process could be shorten to 30 min by producing porous
hydrates in SDS systems14, and considerable methane
storage capacity of 138 V/V can be obtained with the addition of 1000
ppm SDS15. However, the severe “climbing wall
effect” during hydrate formation in SDS systems could decrease gas
storage density16, and substantial foam usually
produces during hydrate dissociation17, both of these
two drawbacks are detrimental to its practical application. Another
class of kinetics additive is amino acid18–21, which
promises to be comparable to SDS.
Amino acid can significantly improve
methane hydrate formation, and the absence of foam during degassing
makes them attract attention22,23. However, amino
acids are perishable, and its cost is relatively high, although the
dosage of amino acids has been controlled to extremely low (0.3
mmol/L)24, the question of economic feasibility for
their large-scale industrial application is still open. Therefore, the
design and preparation of high-efficiency promoters that could produce
similar or better promotion as SDS and amino acids are required, while
the defects of them should also be overcome. In our previous
work25–27, -SO3-(similar as the hydrophilic group of SDS) coated nano polymers
(-SO3-@PSNS) had been successfully
prepared, and methane storage capacity of 59 V/V-142 V/V can be obtained
in -SO3-@PSNS systems, while there was
no foam generated during hydrate dissociation. Since the preparation
method is simple and cost is low,
-SO3-@PSNS can be treated as one kind
of potential alternative of SDS and amino acids.
In addition, take the excellent nature of mass and heat transfer into
account, the promotion approach of mechanical stirring has been
extensively applied, and enhanced hydrate nucleation and growth kinetics
were commonly reported. Hydrate induction time and growth time are 230
min and 390 min separately under mild condition (5
MPa)28, and the induction time even reduces to 2.33
min when pressure increases up to 10 MPa22. However,
mass transfer would be significantly hindered when hydrate slurry is
formed29, so poor later performance in mechanical
systems usually occurs. To overcome the drawback, the synergistic
promotion of mechanical and chemical approaches appears, giving rise to
improved hydrate nucleation and growth kinetics30–33.
Hydrate
induction time can be controlled within 10 min under the combined action
of SDS (300 ppm) and spraying32. By introducing
leucine into a stirring system, Veluswamy et al.22also founded that the stochasticity of hydrate nucleation significantly
decreases. Moreover, hydrate growth rate was reported to increase
approximately 4-fold with the presence of SDS (144.2 ppm) in a
gas-inducing agitation reactor34.
Recently, we proposed a novel spiral-agitated
reactor35, and fast hydrate nucleation was observed in
pure water. Hydrate induction time is less than 4 min at 3.5 MPa and 60
rpm, suggesting excellent promotion of spiral agitation on hydrate
nucleation, and this also guarantees fast hydrate growth rate in the
original agitation stage.
Clearly,
compared with conventional stirring approaches, spiral agitation
performs much better on enhancing hydrate nucleation and original growth
kinetics under mild conditions (low-pressure and low-revolving speed).
Therefore, it can be anticipated that more enhanced hydrate formation
kinetics would be feasible under mild conditions by introducing chemical
additives into the spiral-agitation systems. Excitingly, when amino
acids were introduced24, hydrate formation kinetics
was significantly improved, and it was found that methane storage
capacity in 2 mmo/L L-methionine system is up to 145.97 V/V under an
extremely mild condition of 3.8 MPa, 275.15 K and 60 rpm.
Ultimately, the synergy of spiral agitation and chemical additives sheds
light on high-efficiency hydrate formation, and this may pave the way on
the application of SNG technology. However, due to the drawback of SDS
and amino acids, economical and stable promoters are required, which
also should cooperate well with spiral agitation.
In this work, two nano-promoters
were prepared by fixing -SO3- and
-COO- groups on the surface of polystyrene
nanospheres, respectively. The synergy of the nano-promoters and spiral
agitation on hydrate formation was evaluated. It is worth to highlight
that when nano-promoters were introduced into the spiral-agitated
reactor, enhanced hydrate formation kinetics was obtained even under
extremely mild conditions, which is superior over the synergy of spiral
agitation and amino acids.