Acoustic cavitation and ultrasound-assisted nitration process in ultrasonic microreactors: The effects of channel dimension, solvent properties and temperature

Experimental studies on acoustic cavitation and ultrasound-assisted nitration reaction were systematically investigated in two laboratory-built ultrasonic microreactors by tuning the microchannel dimension, solvent properties and temperature. Under ultrasound irradiation, acoustic cavitation microbubbles were generated and underwent violent oscillation in microchannel. With the decrease of channel size, acoustic cavitation was largely confined, and channel size 1 × 1 mm² was recognized as the critical size to eliminate the confinement effect. Acoustic cavitation was also highly dependent on the properties of sonicated liquids. The onset of surface wave oscillation on gas bubble was obviously promoted with decreasing solvent viscosity and surface tension. Additionally, ultrasound-assisted nitration process of toluene was studied in a temperature-controlled ultrasonic microreactor. The effects of channel size as well as liquid properties on ultrasound intensification agreed well with the finding in cavitation research. Under ultrasound power 50 W, toluene conversion was enhanced by 9.9%-36.3% utilizing 50 vol.% ethylene glycol aqueous solution as ultrasound propagation medium, exhibiting ultrasound applicability on intensifying fast reaction processes in microreactors.