Impact of antifreeze and promoter on clathrate hydrate nucleation and growth

Abstract

A deeper understanding of the nucleation and growth mechanisms involved in carbon dioxide (CO₂)/methane (CH₄) gas hydrate formation has significant implications for natural gas exploitation, as well as for greenhouse gas capture, storage, and transportation. To enhance CO₂/CH₄ gas uptake, introducing chemical additives into the solution can effectively accelerate the hydrate formation process. In this study, we employ molecular dynamics to examine how antifreeze molecules like ammonia (NH₃) and methanol (CH₃OH), along with and without the thermodynamic promoter tetrahydrofuran (THF), influence the kinetics of CO₂/CH₄ gas hydrate nucleation and growth stages. We find that nucleation does not occur in the presence of antifreeze at 100 % CO₂ gas in solution, while it does occur with gas mixtures containing 75 % and 50 % CO₂ within simulation time period. The results demonstrate that hydrate nucleation begins with the formation of the small 5¹² cages and a greater number of these small cages are formed compared to the large 6²5¹² and 6⁴5¹² cages. Furthermore, in the presence of THF or a combination of antifreeze and THF, cage formation increases as CO₂ concentration in the gas mixtures reduces. The concentration of CO₂ in gas mixture significant influences the cages formation when both an antifreeze and a promoter are present. The result display that in the presence of both antifreeze and promoter reduces the induction time and enhances the nucleation rate of the gas hydrate formation compared to the presence of an antifreeze alone. The finding supports experimental evidence that more CH₄ molecules are trapped in the 5¹² cages when both antifreeze and THF are present, compared to hydrate formation without antifreeze. Furthermore, the study highlights the influence of temperature, pressure, and antifreeze molecule concentration on gas hydrate formation. The results indicate that increasing temperature reduces the hydrate formation during the initial formation stage but promotes the trapping of more gas molecules inside the cages during the growth phase Additionally, higher antifreeze concentrations suppress both gas hydrate formation and gas uptake capacity. At elevated temperatures (270 K and 50 MPa), the formation of a greater number of 5¹² and 6⁴5¹² cages is observed, with THF molecules predominantly occupying the 6⁴5¹² cages, suggesting the formation of THF hydrates. The finding reveals the effect of thermodynamic conditions and chemical additives in gas hydrate formation processes.