Gas Dispersion And Solid Suspension In A Three-Phase Stirred Tank With Multiple Impellers

Mixing is a common unit operation in both the laboratory and industrial applications such as chemical industry, metallurgy, pharmaceutical, biological engineering and so on. In recent years, more studies are focused on the multi-impeller stirred tanks with the development of large-scale mixing devices. Previous studies have shown that the impeller combination with radial flow impeller as the bottom impeller and the axial flow impellers as the top and the middle impellers can have good performance in gas dispersion. In this work, the impeller combination named PDT+2CBY is used to research the gas dispersion and solid suspension in a three-phase stirred tank. The ratio of the impeller diameter to the tank diameter D/T of PDT+2CBY is 0.33, which has been optimized in a gas-liquid two-phase stirred tank by previous researchers.Two types of gas-liquid-solid three-phase systems are involved in this work. Firstly, the critial solid suspension charateristics, the gassed power consumption and the overall gas holdup using the PDT+2CBY were measured in the air-water-glass bead (p=2500 kg·m-3) system. The results show that:1. Critical solid suspension characteristics:either the critical solid suspension agitation speed NJSG or the critical solid suspension specific power consumption PmJSG increases with the increase of both superficial gas velocity Vs and solid concentration Cv.2. Gassed power consumption:with the increase of both flow rate and rotating speed, RPD decreases and finally tends to be stable. At a constant rotating speed, RPD changes less than 10% with solid concentration Cv.3. Overall gas holdup:the overall gas holdup increases with the increase of both superficial gas velocity Vs and the specific power consumption PTm, and decreases with the increase of solid concentration Cv at the same Vs and PTm.Secondly, similar studies are carried out in the air-water-resin particles (p=1300 kg·m-3) system. The results show that:1. Critical solid suspension characteristics:both NJSG and PmJSG in the air-water-resin particles system are less than those in the air-water-glass beads system at different Vs. The difference of NJSGand PmJSG between the two systems at small superficial gas velocity like Vs=0.0074 m·s-1 is larger than those for higher superficial gas velocities like Vs=0.0152 or 0.0237 m·s-1. In the air-water-resin particles system, the solid concentration Cv has less effect on both ΔNJSG/NJS and ΔPmJSG/Pmjs than that in the air-water-glass bead system.2. Gassed power consumption:the particle characteristics have little or no effect on the relative power consumption RPD. Both Cv and Flg have larger effect on RPD in the air-water-resin particles system than that in the air-water-glass bead system. The influence of Fr on RPD has little difference between the two systems.3. Overall gas holdup:the overall gas holdup in the air-water-resin particles system is larger than that in the air-water-glass bead system for given superficial gas velocity Vs and specific power consumption PTm. The correlations of the critical solid suspension characteristics, the power consumption and the overall gas holdup in different three-phase systems were obtained and could be applied as references for the industrial designs and applications of three-phase stirred tanks at similar conditions.