Kinetics studies of CO2 hydrate formation in the presence of l-methionine coupled with multi-walled carbon nanotubes

In recent years, gas hydrate technology has been considered to be a promising method for CO2 capture and transport. Nevertheless, the development of the gas hydrate technology is hindered by the slow hydrate formation. In this work, the effects of concentration, initial pressure and gas-liquid ratio on the CO2 hydrate formation kinetics in the compound system of environmentally friendly additive l-methionine (L-met) coupled with low-dose multi-walled carbon nanotubes (MWCNTs) were investigated for the first time. The results showed that L-met coupled with MWCNTs could effectively promote the CO2 hydrate formation, and the promotion effect was related to the MWCNTs concentration. At 4.0 MPa, compared to the 0.1250 wt% L-met single system, the compound system with the MWCNTs concentration of 0.0270 wt%∼0.0720 wt% increased the CO2 gas consumption (Gsum) by an average of 6.37 %, and the induction time (tin) of CO2 hydrate formation were shortened by an average of 36.03 %. The optimal concentration of MWCNTs was 0.0450 wt%. Meanwhile, the increase of the pressure and gas-liquid ratio could effectively accelerate the kinetics of CO2 hydrate formation. The promotion effect of the pressure increasing was better than that of the gas-liquid ratio increasing. The pressure increasing could result in the maximum increase of 39.75 %, 122.83 %, and 69.85 % in Gsum, initial gas consumption rate (N10), and the gas to hydrate conversion (GTH), respectively; and the highest reduction of tin was 83.72 %. Furthermore, the hydrate morphological observations revealed that CO2 hydrate initially nucleated in the inner of L-met solution, and the hydrate particles in the compound system of L-met coupled with the MWCNTs were porous snowflakes. The wall climbing effect induced by capillary forces was further explained, and the presence of L-met enhanced the mass transfer capacity of CO2 hydrates. The practical use of fast hydrate formation technology for CO2 capture and sequestration has been facilitated by this work.