Chemical Promoter Performance for CO2 Hydrate Growth: A Molecular Perspective.

Abstract

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.