Effect Of Multi-Impeller Combinations On Mass Transfer In Multi-Phase Stirred Tank Reactors

The multi-phase stirred tank reactors are widely used in many industrial processes because of their relatively large contact area among multi-phase, more effective on heat and mass transfer, more flexible operations and wider applicability. Mass transfer coefficient (kLa) is often used to evaluate the gas-liquid mass transfer of aeration systems in multi-phase stirred tank reactors. There are many factors having effects on kLa, including structural factors, operational factors, and physical parameters. Previous research have focused on the effect of temperature on mass transfer in multi-phase stirred reactors. In this experiment, the factors of impellers combinations on mass transfer are studied in gas-liquid stirred reactors and gas-liquid-solid three-phase stirred reactors.This experiment was conducted in a gas-liquid stirred reactor with diameter of 0.3 m and gas-liquid-solid stirred reactors with diameter of 0.476 m and 0.3 m, respectively. Deionized water was used as liquid phase, compressed air was used as gas phase, and glass beads were used as solid phase. Volumetric mass transfer coefficient and relative power demand (RPD) were measured for five kinds of three-layer impeller combinations. The effects of impeller structures, as well as the gas flow rates, agitation speed, diameter of the stirred tank, and solid particles, on RPD and kLa were measured and analyzed. The correlations of RPD and kLa were also obtained in corresponding experimental conditions for each kind of impeller combination. Among five three-layer impeller combinations, two kinds of radial flow impellers HEDT and PDT were used as bottom impellers, while two kinds of axial flow impellers WH and CBY were used as the upper two layers impellers. The five kinds of impeller combinations include HEDT+2WHU, HEDT+2WHD, PDT+2WHD, PDT+CBYWD, and PDT+CBYND.The results show that, firstly, the RPD of five impeller combinations decreases and kLa increases with increasing agitation speed and gas flow rate in a gas-liquid stirred reactor. Given the other conditions, HEDT+2 WHu has the highest RPD among all the five combinations, and PDT+2WHd has the highest kLa. The effect of bottom impeller's type changes RPD significantly, but the two upper impellers'type has little effect on the change of RPD. However, the type of two upper impellers has remarkable effect on kLa, and the variation of width of the two upper impellers has limited effect. Comparing the results in two different diameters of tank, we find that the scale-up the tank reduces RPD and kLa.Secondly, same as the gas-liquid systems, the increase of agitation speed and gas flow rate also lead to the decrease of RPD and increase of of kLa for five impeller combinations in gas-liquid-solid stirred reactors. HEDT+2WHu has the higher kLa and RPD than other impeller combinations. In three-phase system, the type of bottom impeller has dominate effect on RPD, while the addition of solid particles has little effect on RPD. Up-pumping WH impeller is recommended in three-phase systems to increase kLa. Increasing the stirred tank diameter in three-phase system decreases RPD and increases kLa slightly. When the superficial gas velocity is relatively high, increasing the solid concentration causes slight decreasing of kLa except for using HEDT+2WHu.