Rapid Nucleation and Formation of CO₂ Hydrate Triggered and Synergized by Tetrafluoroethane: A Kick-Start Effect

The application of hydrate-based technology is restricted by challenges such as high phase equilibrium pressure, long induction time, and slow formation rate, which could be overcome by different promoters. Compared to traditional accelerators such as tetrabutylammonium bromide (TBAB), 1,1,1,2-tetrafluoroethane (R134a) is an effective gas promoter in lower phase equilibrium conditions and easy to separate. The paper proposed a method to trigger and synergize CO2 hydrate rapid nucleation and formation by controlling the trace (3 wt %) of R134a. The effect of different CO2 partial pressures on the induction time, gas consumption, and growth pattern of the CO2 hydrate was investigated. The effect of different R134a and CO2 injection intervals (tau) on the triggering and synergizing of rapid CO2 hydrate nucleation was also studied. The results indicated that the induction time of the CO2 hydrate was reduced to approximately 97.4% of the original duration with the addition of the R134a gas promoter. At a CO2 partial pressure of 2.8 MPa, the gas consumption of the CO2 hydrate reached 0.34 mol, which increased by 47.8% compared to 2.4 MPa. Notably, the induction time of the CO2 hydrate was reduced to 4.5 min when the injection interval (tau) was 30 min. Through an injection interval, R134a acts as a "kick-start" to trigger and synergize the rapid nucleation and formation of CO2 hydrate. This study showed that the injection interval facilitated the first form of the R134a hydrate clusters, providing more nucleation sites and the CO2 molecules' enclathration into the R134a hydrate cluster to form R134a-CO2 hydrate, which further triggers and synergizes the rapid nucleation and formation of CO2 hydrate. Finally, the mechanism of triggering and synergizing rapid nucleation and formation of CO2 by R134a was explained with visual observations and theoretical model diagrams.