Molecular Mechanisms for Cyclodextrin-Promoted Methane Hydrate Formation in Water

Abstract

Cyclodextrins were used as environmentally friendly additives to overcome the slow kinetics of hydrate formation that limited the industrial application of gas hydrate technology. Microsecond molecular dynamics simulations were performed to identify the degree and the underlying mechanisms of the cyclodextrin effects on the methane hydrate formation rate by the four-body order parameter and molecular configurations and to clarify the hydrate structure in the presence of cyclodextrin by face-saturated incomplete cage analysis. Overall results indicated that the addition of β-cyclodextrin in water facilitated the hydrate growth rate, promoted the formation of thermodynamically stable hydrate structure, and enhanced the methane storage capacity. It highlighted the role of cyclodextrin to facilitate the formation of water channel bridging hydrate and water (at low ratio of gas to water) and to increase the gas−water interfacial curvature (at high ratio of gas to water) on the accelerated kinetics of hydrate growth. The advantages of the cyclodextrin-promoted gas hydrate approach for gas storage were also highlighted by comparing it with the gas hydrate formation using other additives and the gas−cyclodextrin complexation method.

Size-, Aggregation-, and Oxidization-Dependent Perturbation of Methane Hydrate by Graphene Nanosheets Revealed by Molecular Dynamics Simulations

“…As methane molecules in the aqueous phase were depleted, the rate of hydrate growth was decreased and mainly dependent on the rate of transferring methane molecules from the gas to the aqueous phase. 47 If we decreased the number of methane molecules in the gas phase from 271 to 188, water molecules in the aqueous phase were found to penetrate into the gas phase to open a channel, which resulted in a curved gas−water interface. It has been demonstrated by Walsh et al that the concentration of methane in water increased with the interfacial curvature due to the effective pressure increase in the methane phase as governed by the Young−Laplace equation.…”

“…The numbers of methane and water molecules in the middle aqueous phase were 100 and 1656, respectively. 47 The hydrate phase was represented by a 3 × 3 × 2 supercell of the SI crystal structure to serve as a template for the directional hydrate growth. GNs were positioned in the aqueous phase at 0.6 nm close to the hydrate surface (Figure 1c).…”

“…An elegant solution to these problems is designing delicate inhibitors − and promoters − to regulate formation of gas hydrate. However, because of stochastic and dynamic natures of the gas hydrate evolution, the mode of action and efficacy of additives to influence the hydrate formation is associated with uncertainties.…”

“…By contrast, kinetic hydrate inhibitors (KHIs) can retard the hydrate formation at concentrations below 1.0 wt %, being more economically favorable . KHIs delay the hydrate formation with three possible mechanisms, that is, binding to the nucleus to prevent it from reaching the critical size, adsorbing onto the hydrate surface to inhibit the crystal growth, and structuring water molecules to restrict recrystallization. − Inspired by the anti-icing activity of natural antifreezing proteins (AFPs), which were found to anchor their methyl groups into half-cages at the hydrate surface to prevent guest molecules diffusing into cages, some analogues were synthesized to exhibit similar properties of hydrate inhibition. − For facilitating the hydrate formation, promoters were designed, following one or more of the principles: acting as seeds to promote the hydrate nucleation, lowering surface tension at the gas–liquid interface to enhance the mass transfer from the gas to aqueous phase, , increasing the solubility of gas in the aqueous solution, ,, improving heat transfer efficiency, , and altering morphology of the growing hydrate crystals. ,, …”

Synthesis of Thermosensitive Poly(N-vinylamide) Derivatives Bearing Oligo Ethylene Glycol Chain for Kinetic Hydrate Inhibitor

“…They induce hydrate nuclei (clusters of water and gas) to generate under hydrate-stable temperature and pressure conditions. Hydrate nuclei then grow to the critical size for hydrate crystal growth. , KHI mechanisms have yet to be elucidated, but some theories have been put forward. − Above all, KHI polymers with amide groups show good KHI properties. , Furthermore, KHI polymers usually have thermosensitive behaviors such as lower critical solution temperature (LCST). − The LCST is a property of thermosensitive polymers in aqueous media that has been intensively investigated. − …”