Experimental study on gas–liquid–liquid macro-mixing in a stirred tank

In this paper, experimental data on the mixing time of the continuous phase and power consumption of gas–liquid–liquid dispersions in a mechanically agitated baffled tank are presented. The electrical conductivity method is taken for the measurement of mixing time and the shaft-torque method for power consumption measurement. Tap water is used as the continuous phase, and kerosene and air as the dispersed ones. The effects of probe/tracer injection position, agitation speed, type of impeller, clearance of impeller off tank bottom, oil volume fraction, gas holdup and physical properties of the dispersed liquids on the macro-mixing of the gas–liquid–liquid system have been investigated. The phenomenon of gas–liquid–liquid macro-mixing in a stirred tank is largely similar to that of liquid–liquid and gas–liquid stirred tanks. Our experiments indicate that the gas–liquid–liquid macro-mixing can be enhanced at higher gas holdups while damped at low gas holdups. Contrary to gas effect, the dispersed oil phase at low holdups increases the macro-mixing intensity but at higher holdups decreases the macro-mixing intensity of the continuous phase. The experimental results show that axial impellers are more energy efficient for gas–liquid–liquid macro-mixing than radial impellers. A simple correlation is developed for predicting the mixing time in gas–liquid–liquid three-phase systems and satisfactory agreement with experimental data is observed.

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► Mixing time in gas–liquid–liquid and liquid–liquid stirred reactors is determined.
► High gas flow rates enhance macro-mixing while low gas flow rates damp it.
► Low oil holdups increase macro-mixing intensity while higher holdups decrease it.
► Axial impeller is more energy efficient than radial impeller.
► A new mixing time correlation for gas–liquid–liquid stirred reactors is proposed.