Selective Adsorption of CO₂ and N₂ during Hydrate Formation in Water-Rich Environments

Abstract

Hydrate-based CO2 capture and separation is considered to be an effective technology in reducing the greenhouse effect. To effectively capture CO2 from flue gas, it is crucial to understand the mechanisms underlying the dissolution–hydration reactions of CO2 and N2. However, the selective adsorption of CO2 and N2 during the formation of the CO2 + N2 hydrate complicates the kinetics and alters the gas phase composition and driving force. Unfortunately, limited research has been conducted on the kinetics of the CO2 + N2 dissolution–hydration reactions. To address this gap, this study conducted experiments on hydrate formation from a CO2 + N2 gas mixture in a stirred gas–water system. The main objective was to investigate the kinetic characteristics of CO2 + N2 hydrate formation in water-rich environments, with a primary focus on the selective adsorption of CO2 and N2 during hydrate formation. Analysis of the gas phase composition revealed a gradual decrease in the CO2 content and a corresponding increase in the N2 content as the experiment progressed, indicating the enrichment of CO2 in the hydrate phase. Gas consumption changes indicated a two-step kinetics process for hydrate formation from CO2 + N2 gas mixtures: (1) rapid complexation of CO2 with water during the initial stage to form basic hydrates with empty cavities, and (2) slow adsorption of N2 into the empty cavities (linked cavities) during the later stage. This two-step kinetic process resulted in significant CO2 gas consumption in the initial stage and slower N2 gas consumption throughout the entire hydrate formation process. Notably, lower pressure and higher stirring speed promoted greater CO2 adsorption into hydrate structures, which benefitted the capture of CO2 capture. These findings offer valuable insights for selecting favorable conditions for the application of hydrate-based CO2 capture and CO2/N2 separation.