Impeller Selection and Vessel Design

Other high-shear impellers include the tapered blade ChemShear impeller and dispersing disks such as the Cowles impeller. These provide excellent shear, but far less flow than the RDT. They are used primarily in small scale batch applications where dispersion time is not critical. Pitched blade tmbines (PBT) are used when large density differences could lead to a suspension problem. They require higher speed to create the same drop size as the RDT, since they have a lower power number. The flow discharge angle for PBTs varies with Reynolds number and blade angle. 

Impeller size is conveniently specified in terms of the D/T ratio. This helps conceptualization and scale-up. This ratio varies from 0.25 to 0.40 for RDTs and from 0.4 to 0.6 for flow-type hydrofoils and propellers. D/T ratios for retreat curve, glassed steel impellers are larger, usually ranging from 0.5 to 0.8. Vertical placement of the impeller depends on vessel shape and apphcation. For example, for dispersion by continuous addition of a dense phase fluid into a less dense fluid, the impeller should be placed fairly low in the vessel at a clearance C H/4 to H/5, where H is the liquid height. For dispersion of light hquids, it is good practice to place a single impeller between 0.2 C/H 0.5. The subject of impeller type and location with respect to drop suspension was covered in Section 12-6. 

The production of pharmaceuticals and specialty chemicals frequently requires the same vessel and agitation equipment be used for each processing step. Therefore, a gas-liquid dispersion step might require special impellers for that operation. If a liquid-liquid processing step is also required, the equipment chosen for the gas-liquid step will usually be well suited for liquid-liquid dispersion. For such multiuse applications, it is essential to use a variable speed drive. It is common to have to deal with slurries. Care must be taken to ensure adequate mixing during off-loading, so impellers are often located close to the bottom for such applications. 

Multiple impellers are recommended if H/T 1.2 or if Ap 150 kg/m. Assuming a less dense dispersed phase, the second or top impeller often is a hydrofoil placed midway between the RDT and the surface of the liquid. This impeller produces high flow at low power, provides excellent circulation, and complements the flow pattern produced by the RDT. The diameter of the second impeller is usually greater than the RDT, typically D/T 0.45. A good practice is to distribute the total power to f 20% for the hydrofoil and f 80% for the RDT. Since the power number, Np, is known for each turbine, setting the power distribution enables the diameter of the hydrofoil to be determined. The vertical position of the upper turbine must ensure that fluid reaches the lower impeller, but must avoid gas entrainment that could occur if placement is too close to the hquid surface. Flow from a PBT does not complement that from a RDT and is therefore not recommended. Power requirements are discussed in Section 12-7.3. Table 12-6 lists equipment options for different drop sizing objectives (desired result). If ds2 must be less than 30 tim, the use of a stirred tank is not recommended, so other devices are also included in the table. 

Mass transfer among drops is enhanced by repeated coalescence and redispersion. This is very important in hquid-liquid extraction. Disk turbines used in 

---