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.