Mixing impeller spacing

With reference to Figure 9.2, observe that the power demand for multiple radial impellers spaced at one or more impeller diameters apart is additive, but at closer spacing, power is greater than the sum of each impeller. This is due to flow interference. As mentioned earlier, the total power demand for multiple mixed-flow impellers is complex. 

Wake-mixing loss. This loss is from the impeller blades, and it causes a wake in the vaneless space behind the rotor. It is minimized in a diffuser, which is symmetric around the axis of rotation. 

Stirred tanks are often used for gas-liquid reactions. The usual geometry is for the liquid to enter at the top of the reactor and to leave at the bottom. The gas enters through a sparge ring underneath the impeller and leaves through the vapor space at the top of the reactor. A simple but effective way of modeling this and many similar situations is to assume perfect mixing within each phase. 

In the case of the full bubble column, the bottom gas phase mixed region is coincident with the impeller, and no significant gas mixing occurs below the impeller, in the lower region of the tank, as shown in Fig. 5.125. The dashed arrows in Fig. 5.125 and Fig. 5.126 represent mass transfer interchange between the gas space and liquid volumes of the tank. The liquid circulation, which is not shown, is identical to Fig. 5.124. 

Impeller size relative to the size of the tank is critical as well. If the ratio of impeller diameter D to tank diameter T is too large (Z)/r is > 0.7), mixing efficiency will decrease as the space between the impeller and the tank wall will be too small to allow a strong axial flow due to obstruction of the recirculation path (21). More intense mixing at this point would require an increase in impeller speed, but this may be compromised by limitations imposed by impeller blade thickness and angle. If P/Pis too small, the impeller will not be able to generate an adequate flow rate in the tank. 

While very little information is available on gas-phase backmixing in a mechanically agitated CSTR, the extent of mixing increases with the impeller speed. If the reaction depends on the partial pressure of gas, an increased backmixing decreases the driving force for the absorption rate. An increase in impeller speed also increases fcLaL, kgaL, etc. As a consequence, it is likely that an optimum impeller speed exists that gives maximum space-time yield (kmol/m3 s) and this optimum speed probably lies in the vicinity of critical impeller speed. 

Related Calculations. Impeller power requirements are relatively independent of mixing-tank diameter. However, the power numbers shown in Fig. 12.1 assume fully baffled conditions, which for a cylindrical tank would require four equally spaced (at 90°) vertical plate-type baffles. The baffles should extend the full height of the vertical wall (i.e., the straight side) of the tank and should be one-twelfth to one-tenth the tank diameter in width. 

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