The Podbielniak and Alfa Laval centrifugal extractors are essentially continuous differential contactors. The Westfalia and Robatel centrifugal extractors contain discrete mechanical stages, and flow from one to another is effected by spill over discs and skimmers according to usual centrifugal clarifier practice. As the number of discrete stages is increased, the allowable flow rates are proportionately decreased.
Agitated Columns. The size of an extraction column frequently can be estimated from a knowledge of the flow rates and physical properties, combined with some empirical generalizations.
1. The maximum capacity (at zero stirrer speed or pulsation) is directly related to the terminal velocity of the dispersed phase through the minimum physical constriction in the column.
2. The terminal velocity of the dispersed phase droplets is related to the physical properties of the system by the correlation shown in Fig. 15.
3. For many systems, the effect of hindered settling can be approximated by:
where Vv Vd, Vc are the superficial lineal velocities of the drop, dispersed phase, and continuous phase, and h is the holdup.
4. Agitated columns are frequently operated so that the capacity is half what it would be at no agitation (zero rpm or pulsation). Agitation is used to reduce droplet diameter to this equivalent point to increase mass transfer rate and mass transfer area.
5. For sizing purposes, the diameter of the column will be chosen so that the column is operating at 75% of the flood point.
6. The holdup at flooding can be determined by differentiating the equation in criterion #3. Combining this relationship with all the constants leads to the following equation:
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