of the more viscous dispersed phase leads to a higher effective viscosity. Beyond 10%, the increased drag force between the droplets and the continuous phase because of the further increase of effective viscosity modifies the velocities of droplets and leads to weaker turbulence finally (Zhao et al., 2011), which results in an increase of mixing time.
3.2.7. Effect of gas holdup
Since gas is also a dispersed phase, the effect of gas on mixing time is expressed in terms of gas holdup. The gas holdup is measured from the difference in height above the tank bottom between the dispersion and the ungassed liquid. Fig. 8 shows the variation of gas holdup with gas flow rate at different impeller speeds.
As shown in Fig. 8, higher gas flow rate and impeller speed produce larger gas holdup. We focus on low gas flow rates (0– 0.64 L/min), so the gas holdup is relatively small. The effect of gas holdup on mixing time is shown in Fig. 9. The increase in gas holdup resulting from increasing gas flow rate would interrupt the liquid mixing to some extent, i.e., the mixing time increases with the gas holdup at a lower gas flow rate region to QG 1⁄4 0.32 L/ min and then the mixing time decreases with the further increase in gas holdup as shown in Fig. 9.
Similar effect of aeration on the micro-mixing process of liquid phase was observed by Lin and Lee (1997) in a gas–liquid stirred tank. At a lower gas holdup, an increasing amount of energy is spent at the gas–liquid interface with the increaseof jG and correspondingly less energy is available for the liquid-phase mixing. At a higher gas holdup, the intensity of liquid turbulence