The investigation of hydrodynamic behavior in three-phase external loop airlift reactors is important to their design and scale-up. Pressure difference was used to measure gas holdup and ultrasonic Doppler anemometry was used to measure liquid velocity. Gas holdup

liquid velocity and expansion rate increased with increasing superficial gas velocity. Under the same superficial gas velocity

gas holdup increased with increasing static solid height

and decreased with increasing solid particle diameter.Adding large particles could increase interfacial area

increase gas residual time and intensify mass transfer. Liquid circulation velocity and expansion rate decreased with increasing solid particle diameter and static solid height.Liquid velocity could be controlled in a suitable range by adding particles to improve the performance in airlift loop reactors.