Visual experimental study of Nanofluids application to promote CO2 absorption in a bubble column

The addition of dispersed-phase nanoparticles in the liquid phase can enhance the gas–liquid transfer process as the suspended nanoparticles affect the transfer process inside the fluid through micro-disturbance or micro-convection effects. In this paper, a high-speed digital camera was used to visualize the bubble behavior of CO2 in pure water and nanofluids to examine the effects of CO2 gas flow rate, nanoparticle solid content and type on the bubble behavior in the fluids. The CO2 absorption performance in three water-based nanofluids were compared in a bubbler. And the mass transfer characteristics during CO2 bubble absorption and the reasons for the enhanced gas–liquid mass transfer effect of nanoparticles were analyzed. The results showed that the presence of nanoparticles affected the formation process of bubbles in the fluid, shortened the bubble detachment time, reduced the detachment diameter, effectively increased the gas–liquid contact area, and improved the bubbles detachment frequency. The system with MCM-41 corresponded to a higher overall mass transfer coefficient. Uncalined MCM-41 contained surfactant which enhanced foaming behavior in water. This prevented the transfer of CO2 to some extent, and the CO2 absorption by uncalined MCM-41/H2O was 5.34% higher than that by pure water. Compared with SiO2 nanoparticles with the same particle size and the same composition, MCM-41 had a higher adsorption capacity and better hydrophilicity due to its larger specific surface area and rich porous structure, which was more favorable to accelerate the collision between nanoparticles and CO2 bubbles to cause micro-convection. Under the condition of 0.1% (mass) solid content, the enhancement of CO2 absorption process by MCM-41 nanoparticles was more significant and improved by 16.9% compared with pure water.