Chemical Promoter Performance for CO₂ Hydrate Growth: A Molecular Perspective

Carbon dioxide (CO₂) hydrates, which contain a relatively large amount of captured CO₂ (almost 30 wt % of CO₂ with the balance being water), represent a promising CO₂ sequestration option for climate change mitigation. To facilitate CO₂ storage via hydrates, using chemical additives during hydrate formation might help to expedite formation/growth rates, provided the additives do not reduce the storage capacity. Implementing atomistic molecular dynamics, we study the impact of aziridine, pyrrolidine, and tetrahydrofuran (THF) on the kinetics of CO₂ hydrate growth/dissociation. Our simulations are validated via reproducing experimental data for CO₂ and CO₂ + THF hydrates at selected operating conditions. The simulated results show that both aziridine and pyrrolidine could perform as competent thermodynamic and kinetic promoters. Furthermore, aziridine seems to exceed pyrrolidine and THF in expediting the CO₂ hydrate growth rates under the same conditions. Our analysis unveils direct correlations between the kinetics of CO₂ hydrate growth and a combination of the free energy barrier for desorption of CO₂ from the hydrate surface and the binding free energy of chemical additives adsorbed at the growing hydrate substrate. The detailed thermodynamic analysis conducted in both hydrate and aqueous phases reveals molecular-level mechanisms by which CO₂ hydrate promoters are active, which could help to enable the implementation of CO₂ sequestration in hydrate-bearing reservoirs.