Impacts of Stirring Rate on CO2-in-H2O Emulsion Synthesis and Stability for Enhanced CO2 Hydrate Formation Kinetics

With the global demand for carbon reduction, hydrate-based CO2 sequestration evolves as a promising technology for offshore CO2 storage. However, the slow kinetics of CO2 hydrate formation, primarily due to the limited contact area and mass-transfer resistance between immiscible liquid CO2 and the H2O phases under high pressures, remain a critical bottleneck for its practical application. To this end, we proposed a CO2-in-water (C/W) emulsion system by employing a nonionic surfactant, alkyl polyglucoside (APG), using simple high-speed stirring. We systematically examined how the degree of mixing, i.e., stirring rate, impacts the stability of the C/W emulsion and the subsequent CO2 hydrate formation kinetics. Using a full-visual reactor coupled with an in-house developed phase identification algorithm, C/W emulsification and demulsification kinetics were unveiled. C/W emulsion stability under various high stirring rates and APG concentrations was examined. We further demonstrate the superior CO2 gas uptake and rapid CO2 hydrate formation kinetics realized by the synthesized stable C/W emulsion. Our results suggest that increasing the stirring rate substantially speeds up the C/W emulsification synthesis process, achieving only 9 s at an optimal stirring rate of 1600 rpm and 3.00 wt% APG. The stability of C/W emulsion is further enhanced under high shearing rate, with complete demulsification time reaching 5 h. Furthermore, increasing the stirring rate from 800 to 1600 rpm not only improves the C/W emulsion stability but also results in shortened induction time, high CO2 hydrate formation kinetics, and high CO2 final uptake (maximum gas uptake 167.7 v/v with a water conversion of 84.5%). The experimental results provide direct evidence on how the stirring rate affects C/W emulsion preparation and stability. The findings also shed light on a promising emulsion-based method for effective CO2 hydrate formation kinetics promotion, which can be adopted in a series of CO2 hydrate-based applications.