Solid Suspension And GAS Dispersion In A Multi-Impeller Three-Phase Stirred Tank With Different Solids

Multi-phase mechanically stirred reactors can be widely applied in petrochemical, pharmaceutical engineering, environmental engineering and other fields with the advantages of convenient operation. Predecessors’ research show that compared with the single impeller, multi-impeller combinations are more suitable for multiphase fluid mixing when the ratio of stirred tank height to diameter is higher than 1.Based on the previous work, the impeller combinations named HEDT+2WHD, HEDT+2WHU, PDT+2WHD are used to investigate the global properties in air-water, air-water-glass beads and air-water-resin particles systems and the ratio of the impeller diameter to the tank diameter D/T is 0.33. We choose glass beads and resin particles as the solid phase with the volumetric solid concentration of 5%,10%,15% and 20%, using an elliptic based cylindrical Perspex stirred tank with the diameter of 0.476 m and the liquid level (H) of 1.66T. By changing the geometric parameters such as impeller combination, rotation speed and flow rate to study the effect of them on the critical dispersion characteristics, critical solid suspension characteristics, gassed power consumption and overall gas holdup in multi-phase systems.The results show that:(1) Critical gas dispersion characteristics:the critical gas dispersion agitation speed, the critical gas dispersion specific power consumption and the complete dispersion agitation speed increase with the increase of the superficial gas velocity Vs. PDT+2WHD requires lower power consumption at critical gas dispersion state than the combinations with HEDT as bottom impeller. Therefore, the combination with PDT is easier to achieve good gas-liquid dispersion. (2) Critical solid suspension characteristics:Both the critical solid suspension agitation speed NJSG and the critical solid suspension specific power consumption PmJSG increase with the increase of both superficial gas velocity VS and solid concentration Cv. At same Cv and VS, the combination with HEDT gets a lower critical solid suspension agitation speed, while the corresponding critical power consumption is higher than the combination with PDT. NJSG and PmJSG in gas-liquid-glass beads system becomes higher than resin particles system, and the difference between two particles decreases gradually with the increase of superficial gas velocity Vs. (3) Gassed power consumption:RPD decreases with the increase of both flow rate and rotation speed, and finally tends to be stable. At a constant rotation speed, Cv has little effect on the RPD. Based on the regression results of NPG, the gassed power number increases slightly with the increase of Cv. When the rotation speed is higher than 8 s-1, RPD in gas-liquid-glass beads system becomes slightly higher than resin particles system. In two-phase and three-phase systems, RPD with different impeller combinations from high to low is HEDT+2WHU, HEDT+2WHD and PDT+2WHD. (4) Overall gas holdup εG:the overall gas holdup increases with the increase of Vs and decreases with the increase of CV. When VS is higher than 0.007 m·s-1, the overall gas holdup in gas-liquid-resin systems is slightly higher than glass beads system. In two-phase system, gas holdup with different impeller combinations from high to low is PDT+2WHD, HEDT+2WHD and HEDT+2WHu. However, in three-phase system the combination from high to low is HEDT+2WHD, HEDT+2WHu and PDT+2WHD. (5) Optimum impeller combination:PDT+2WHD impeller combination is recommended for the gas dispersion in two phase system and the combinations of HEDT+2WHD or HEDT+2WHu are suitable in three phase system with solids. The quantitative regressions of NPG and gas holdup could be applied in the industrial design of multi-phase stirred tanks with multi-impeller operated at similar conditions.