Extraction - Principles of Chemical Separations with Environmental Applications - page 124

In spray towers the highest rates of mass transfer tend to occur close to the distributor
plates. At small distances above and below the plate the dispersed phase tends to recoalesce
and mass transfer declines significantly, such that it is more effective to add distributors
to redisperse the droplets than to increase the height of the tower. Another solution is to
add packing similar to that used in absorption towers. The packing causes the drops to
coalesce and reform and, thus, reduces the height of each theoretical transfer unit.
The two streams flowing countercurrent creates the possibility of flooding. If the lighter
phase is dispersed and the downward flowrate of the heavier phase becomes too high, the
lighter-phase droplets become held up and exit with the heavy phase through the column
bottom. If the heavier phase is dispersed and the flow of the lighter phase becomes too
high, the heavy-phase droplets become held up on top of the lighter phase and exit the
top of the column. Flooding velocity for the continuous phase as a function of that for the
distributed phase can be estimated from Figure 5.3.

V s , d 2

V s , c +

ρ c

The x -axis is the group

a v µ c

10 4

10 3

10 2

10

10 2 10 3

1 10

2

( )

V

V +

ρ

s,c s,d

c

a

µ

υ

c

Figure 5.3 Flooding velocities in packed extraction towers [1]. L. McCabe, Unit
Operations of Chemical Engineering . Reproduced with permission of McGraw-
Hill companies.