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Effect of Impeller Types on Mixing Time in Stirred Tanks

Mixing time; flow pattern, and power number of a vessel depend on design and type of its impeller. That is, impeller effectiveness has often been evaluated via mixing time. In order to investigate the effect of impeller type on the mixing time, some experiments have been done for different types of impellers in a stirred vessel with a diameter of 175 mm. The mixing time measurements are carried out by means of electrical conductivity variations technique in five different rotational speeds (in the range of 4002000- rpm) for the following agitators: a two-bladed propeller, a three-bladed propeller, a four-bladed turbine, and a six-bladed turbine. Water is used as the main fluid; besides, 7cc NaCl solution with 200000 ppm concentration is injected as the tracer in a specific position. The mixing time is defined as the period between release of tracer and the time when the concentration of the tracer reaches 95% of the final concentration in the tank. By changing the type of impellers, the mixing time of different impellers have been evaluated. Indeed, the impeller giving the shortest mixing time is introduced as the best mixing impeller for the governed experimental conditions. It has been shown that increasing the rotational speed leads to a considerable decrease in mixing time and an increase in turbulency. Finally, by installing some baffles in the tank diminishing of vortexes is observed; consequently, mixing time decreases.