Adsorption - Principles of Chemical Separations with Environmental Applications - page 196

(a) (b)

Number of cars entering the
lot per time

Number

Number
of
parked
cars

of cars
leaving
the lot
per time

Earl
(Lot empty)

y Lunch time

(Lot almost
full)

Lot full

Close Far

Figure 7.3 Parking lot analogy.

and may be difficult to access. So, Figure 7.3(a), the outflow will increase before the lot
is completely full. The cross-hatched area in Figure 7.3(a) represents the total number of
parked cars in the lot.
Next, imagine a beaker of water containing an organic pollutant to which a large amount
of activated carbon is added. The pollutant will sorb onto, and desorb from, the carbon
particles until the rate of sorption equals the rate of desorption and the system is in
equilibrium. At equilibrium, the concentration of the pollutant in the water will be at
a minimum value. The carbon in equilibrium with this solution will have reached its
adsorptive capacity, and cannot adsorb any more pollutant under the current conditions.
This batch system is one method to measure the equilibrium loading of the sorbent.
Now imagine a column filled with activated carbon through which the aqueous stream
containing the organic pollutant enters at the top. The top layers of the carbon will initially
remove the pollutant from the water, and will do so until that layer of carbon reaches its
capacity. The next section of carbon will then begin to remove the pollutant. If the column is
long enough, one can imagine a mass transfer zone, where all the adsorption is occurring.
Below this zone, the contaminant in the water will have been reduced to its minimum
value. Above the mass transfer zone, the carbon (which is saturated, i.e., at its adsorptive
capacity) is in equilibrium with the influent pollutant concentration and mass transfer is
no longer occurring. The actual height of the mass transfer zone will vary with flowrate.
Typically, you want this zone as small as possible (can you see why?). In the limit of very
low flowrates, the length of the mass transfer zone will reduce to zero and the concentration
profile will propagate as a front. This position is called the stoichiometric front since it
can be calculated from a mass balance (see Problem 7.1).
The mass transfer zone will move down the column with time. When it reaches the
bottom of the column, the effluent pollutant concentration will start to rise above the
minimum value, and this is known as breakthrough . When the very bottom layers of carbon