Ultrasound-assisted gas–liquid mass transfer process in microreactors: The influence of surfactant, channel size and ultrasound frequency

Ultrasound effect on the hydrodynamics and mass transfer behavior of gas–liquid Taylor flow is studied in ultrasonic microreactors with different frequencies (20, 28, 40 kHz) and channel dimensions (0.5 × 0.5, 1.0 × 1.0, 1.5 × 1.5, 2.0 × 2.0 mm2). Upon ultrasound irradiation, intense bubble oscillation is excited on the slug bubble, accompanied by cavitation microstreaming vortices around it. The amplitude of bubble oscillation decreases with the decrease of channel dimension as a result of the confinement effect. Channel dimension of 1.0 × 1.0 mm2 is considered as the critical dimension above which the confinement effect would be eliminated. More intensive bubble oscillation and cavitation microstreaming are observed at lower ultrasound frequency, where more significant mass transfer enhancement is also observed. At frequency of 20 kHz, the overall volumetric mass transfer coefficient is improved by 22 times at the power density of 0.14 W/mL. Adding surfactant (SDS) in the liquid increases the amplitude of bubble oscillation due to the decrease of interfacial tension. Such an increase in the oscillation amplitude enlarges the specific surface area, leading to an increase in the overall volumetric mass transfer coefficient with the increase of surfactant concentration. The detailed effect that how gas–liquid mass transfer would be enhanced by ultrasound is quantified by a mass transfer model, which could predict both the liquid side mass transfer coefficient and the specific surface area satisfactorily.