Adsorption - Principles of Chemical Separations with Environmental Applications - page 197

reach their adsorptive capacity and the mass transfer zone disappears, the effluent pollutant
concentration will reach the inlet concentration and the column is said to be exhausted .
Adsorbent columns can be used until they are exhausted by placing them in series. As
one column is exhausted, it is taken off line and the adsorbent is replaced or regenerated.
The column can then be placed below the second column, and the process is continued.
If only one column were to be used, it would have to be regenerated sometime before
exhaustion, depending on the allowable effluent pollutant concentration. Placing columns
in parallel is also a possibility, so that breakthrough in one column will not significantly
affect the effluent quality.

7.8.1 The mass transfer zone and breakthrough

We can use two methods to visualize the effect of the mass transfer zone (MTZ) on the
adsorption column operation. In one method, we measure the solute concentration in the
fluid phase at the column exit (position is fixed, time is a variable).
Figure 7.4 shows a typical result. The effluent solute concentration is at some minimum
level ( C d ) for a period of time until the concentration starts to rise as the MTZ arrives
at the column exit. When the concentration rises to the maximum allowable effluent
solute concentration ( C bt ), breakthrough is said to occur. Column loading is defined as
the amount of sorbed material in the bed at the breakthrough divided by the total weight
of sorbent in the bed. If the bed were continued in operation past time t bt , the effluent
solute concentration would continue to rise until the entire bed was completely loaded and
the effluent solute concentration equaled the feed solute concentration ( C f ). This situation
corresponds to column exhaustion. Note that an analogous plot could be made using total
fluid volume processed instead of time as a variable.
The second method is to plot the solute concentration in the fluid phase as function
of position (distance, L , from inlet) in the column for a fixed time that is less than t d .
Figure 7.5 shows a typical plot.

C f

Effluent solute
concentration

C bt

C d

t d t bt t s t e

Time
Figure 7.4 Effluent solute concentration vs time.