Effect of Cyclodextrins on Hydrate Formation Rates

Abstract

Effects of cyclodextrins (CD) on hydrate formation kinetics were investigated. Several kinds of cyclodextrins, such as α-CD, β-CD, β-CD polymers, and β-CD modified with methyl and triacetyl groups were tested as additives for hydrate formation of xenon, with a concentration range of 0−5 wt %. Induction time for hydrate formation was reduced by the addition of any of the CDs; however, no clear relationship between the structure of CDs and the reduction effect was observed. The three-phase equilibrium pressure (H−V−L w) was unaffected by the addition of the CDs, except for β-CDs modified with methyl or triacetyl groups. The hydrate formation rate, which was expressed as a rate constant based on the chemical potential, increased with the addition of α-CD, β-CD, and β-CD polymers. β-CD polymer showed the highest acceleration effect on the xenon hydrate at about 1 wt %; the rate constant was about three times larger than that for pure water. Contrastingly, modified β-CD and dextrin with a straight chain structure reduced the hydrate formation rate. β-CD polymer was subsequently applied as an acceleration additive for the methane hydrate formation process, and it was found that the addition of β-CD polymer increased the methane hydrate formation rate up to five times as compared to that for pure water.

Molecular Mechanisms for Cyclodextrin-Promoted Methane Hydrate Formation in Water

“…To the best of our knowledge, there are only a few scattered works to date on the cyclodextrin-based gas hydrate formation. For example, Kuji et al demonstrated that the methane hydrate formation rate increased by up to five times after the addition of β-cyclodextrin polymer in water. Similar promoting effects were reported by Nozawa et al Nevertheless, it should be noted that the cyclodextrins have been widely used for gas storage in the food industry, which mainly compressed the gas into the interior cavity of cyclodextrin at high pressure to form a gas–cyclodextrin complexation either in liquid solution or in solid powder. − …”

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

“…Natural cyclodextrins have obvious self-associative aggregates in aqueous solutions, and these self-associated aggregates are connected by the hydrogen bond network. , The abnormal solubility of β-CD is because β-CD increases the local water structure around the cavity and molecules . The previous studies indicate that the addition of β-CD at low concentrations (as low as 100 ppm) shortens the induction time for methane hydrate nucleation and promotes hydrate formation. ,, This nucleation promotion may be attributed to the CD-induced reduction in nucleation work, which relates to H-bonding disruption at the gas/water interface generated by CDs . In summary, when hydrophobic β-CD is introduced into the solution, it will change the original hydrogen bond of the water molecules around it, increasing the local water structure and resulting in the self-aggregation of β-CD.…”

“…β-CD has an amphiphilic structure, hydrophilic outside and hydrophobic inside (see the Supporting Information S1 for further details). Previous studies have shown that the addition of natural cyclodextrin reduces the nucleation induction time of hydrate and promotes the formation rate of methane hydrate as well as methane–tetrahydrofuran mixed hydrate. ,− In addition, cyclodextrins do not generate foam during hydrate decomposition, and the combined use of cyclodextrin and SDS can effectively reduce the dosage of SDS. , We chose β-CD over other natural CDs because β-CD may be more hydrophobic. The solubility of β-CD is anomalously 1 order of magnitude lower than those of α-CD and γ-CD .…”

Effect of Eco-Friendly Cyclodextrin on the Kinetics of Mixed Methane–Tetrahydrofuran Hydrate Formation

“…Mohammadi et al reported an increase of 14.63% in dissolution of methane in water in the presence of 0.5 wt % β-cyclodextrin at 274.2 K and low pressures of 3 bar. Kuji et al investigated the effect of α-cyclodextrin, β-cyclodextrin, β-cyclodextrin polymers, and β-cyclodextrin modified with methyl and triacetyl groups on the kinetics of xenon and methane hydrate formation in a stirred tank reactor. 1.0 wt % of β-cyclodextrin polymer was found to be effective in promoting methane hydrate formation at 274.2 K with a high rate constant.…”