Air cleaning - Indoor Environmental Quality - page 390

Figure 12.18 Adsorption patterns in an activated carbon bed. (From Turk, A.C.,
Adsorption, in Air Pollution , 3rd ed., Vol. IV, Stern, A.C., Ed., Academic Press, New
York, 329, 1977. With permission.)
bed, concentrations of contaminants in the airstream fall rapidly to zero at
some finite distance downstream. As sorption continues, carbon granules
near the face gradually become completely saturated. In the saturation zone,
a dynamic equilibrium becomes established between saturated sorbent and
in-coming contaminants. Downstream of this saturation or equilibrium zone,
carbon granules are actively sorbing influent gases/vapors. This is described
as the mass transfer zone. It is an area of the bed between zero concentration
and complete saturation. It moves progressively from one face of the bed to
the other with time. It can be described over time by curves a-g. When the
bed reaches the condition described by curve e, breakthrough occurs, and
the bed has reached the end of its useful life. Carbon in the bed must now
be replaced or reactivated.
7. Reactivation and replacement
In industrial and commercial applications, saturated sorbents are reactivated
once they become saturated. They are reactivated (desorbed) by passing low-
temperature steam or hot air through the bed. In most indoor applications,
thin-bed filters are replaced with new filters when they become saturated. They
are often discarded but may be returned to the manufacturer for reactivation.
8. Residence time
Because of the limited bed depth associated with thin-bed filters used in
indoor applications, residence time of air moving through such filters is a
critical factor in their performance. It would take approximately 0.08 seconds
to traverse a 25 mm (1”) thin-bed filter under standard air velocity condi-