Extraction flooding velocities

No simple correlation is available to predict the flooding velocities in extraction columns, and hence the column diameter needed. The more specialised texts should be consulted to obtain guidance on the appropriate method to use for a particular problem see Treybal (1980), Perry et al. (1997) and Humphrey and Keller (1997). 

Figure 53 Flooding velocities in packed extraction towers [1]. L. McCabe, Unit Operations of Chemical Engineering. Reproduced with permission of McGraw-HiU companies.

Estimate the flooding velocity for an extraction column packed with 15-mm spheres and operating with water and toluene at SO C. Toluene is dispersed and has a flow rate twice that of the water phase. 

The characteristic velocity k is a function of droplet size, density difference, viscosity, etc. Thus, the holdup tends to increase either as the superficial flow velocities Uc and Ud are increased or as the characteristic velocity is reduced (e.g., by increasing agitation). A point is eventually reached where the increase in holdup becomes unstable (typically when = 0.3-0.4). This phenomenon is known as flooding, and it imposes a limit on the flow rates and agitation levels that can be used in countercurrent extraction processes. 

Most column extraction processes operate with a solvent of lower density than the aqueous phase and discrete solvent drops are usually allowed to ascend in the continuous aqueous medium, with an interface near the top of the column. Aqueous phase enters just below the interface and solvent phase is fed via the first distributor at the base of the column. Flooding of the column takes place at certain critical flow rates. This phenomenon arises when, for example, the flow rate of the dispersed phase is unduly increased with a constant flow of continuous phase. The additional column hold-up of dispersed phase leaves less space for continuous phase and therefore the linear velocity of the continuous phase is increased. A tendency to drag the dispersed phase droplets in the direction of the continuous phase thus arises. When the flooding point is reached, the dispersed phase is discharged along with the continuous phase and counter-current flow ceases. 

Flooding" of an extraction column commonly occurs when the superficial velocity of the continuous phase exceeds the terminal velocity of the dispersed droplets, assumed to be the lighter phase introduced at the bottom. The droplets will then cease to rise and will ultimately coalesce into a pool at... 

Chapter 7 introduces for the first time the basic fluid dynamics principles of packed columns for liquid/liquid extraction. The previously mentioned SBD model for gas/liquid systems is transferable to liquid/liquid systems. The method used to calculate the gas velocity at flooding point of the disperse and continuous phases will be explained by means of some numerical examples. 

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