Effects of β-Cyclodextrins and Their Aggregates on the Formation of Methane Hydrate

Abstract

The hydrophobic effect of hydrate formation well explains the enhancement effect of a hydrophobic solid surface on hydrate formation and might be extended to the hydrophobic traditional surfactant, amino acids, and oligosaccharides. However, in situ observations are still lacking to support such extension. In this study, we investigated the effect of the addition of 0–1.0 wt % β-cyclodextrins (β-CD) on methane hydrate formation by microscopic observations and in situ Raman quantitative technique. Our hydrate nucleation experiments observed the self-association of β-CD near the gas–liquid interface, confirming that the presence of β-CD aggregates greatly increases (41.8–43.56%) the local methane concentration and that β-CD aggregates may act as hot spots for hydrate nucleation, which is consistent with the description of the hydrophobic effect. In addition, the diffusion coefficient and the concentration gradient distribution of methane in the liquid phase during hydrate growth were measured. The results indicate that the diffusion coefficient of methane increased by 14.47–17.11% in the aggregate-free β-CD solutions. However, the diffusion coefficient of methane in the solution section containing β-CD aggregates is only 0.21–1.96% of that in aggregate-free β-CD solutions. The numerical model based on one-dimensional hydrate growth experiments shows that the equivalent hydrate growth rate (diffusion flux of methane) increases more in the 0.5 wt % β-CD (18.78%) solution than in the 1.0 wt % β-CD solution (2.77%). The excessive β-CD addition may lead to the formation of surplus aggregates, which occupy the whole gas–liquid interface, block the diffusion of methane, and reduce the mass transfer efficiency of methane during hydrate growth. This work not only supports the extension of the hydrophobic effect on amphiphilic surfactants but also may help us better understand the optimal concentration of surfactant additions, while excessive surfactant addition may impair the relative efficiency of the system.