Liquid-phase mixing and power consumption in mechanically agitated solid—liquid contactors

Abstract

Liquid-phase mixing has been studied in mechanically agitated contactors of internal diameter 0.57 m and 1.0 m in the presence of solid particles. Tap water was used as the liquid phase and quartz particles were used as the solid phase. The impeller speed was varied within the range 2 – 13.33 rev s⁻¹. The solid loading was varied from 0 to 40 wt.%. Three types of impellers, namely disc turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were employed. The ratio of impeller diameter to vessel diameter was varied within the range 0.25 – 0.58. Mixing time was measured using the transient conductivity technique. Power consumption was measured using a torque table. A new method has been suggested for the measurement of critical impeller speed for solid suspension (N_CS) from mixing-time data. Pitched-blade turbines were found to be more energy efficient for mixing than disc turbines in solid—liquid systems. Further, pitched turbine downflow (PTD) was found to be more efficient than pitched turbine upflow (PTU). The smallest PTD (T/4) impeller requires the least power for suspending solid particles under otherwise similar conditions. For liquid-phase mixing the T/3 PTD impeller was found to be the most energy efficient. A correlation has been developed for the estimation of dimensionless mixing time (Nθ)_CS.