Impeller Design

The common impeller designs used for stirred tank reactors are shown in Figure 3.9.

The optimum impeller to tank size ratio for gas or liquid mass transfer depends on the mixing power delivered relative to the gas velocity of the system. At relatively low levels of mixer horsepower, it is seen that relatively large D/T ratios are needed in order to have sufficient impeller pumping capacity to handle the volume of gas passing by the impeller. As the amount of power to the mixer is increased, the optimum D/T is reduced to a relatively small value, approximately 0.15 to 0.2. As the mixer power is increased far beyond that needed to disperse the gas, D/T becomes unimportant because it makes very little difference how the power is distributed. As the impeller diameter is increased, , the maximum shear rate at the same impeller speed is also increased. Thus on scale up, lower average shear rates will be encountered in the stream from the impeller due to lower impeller speeds, but higher maximum shear rates will be encountered due to the higher peripheral velocity of the impeller. The impellers operate at a viscosity range of the fermentation medium as shown in Table 3.9. The impeller spectrum depicting the proportion of flow and shear developed by different impellers is shown in Figure 3.10 (201).

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