Environmental Engineering Reference
In-Depth Information
If one knew the average concentration of individual contaminants and
their sorption and retention potentials, service life could be calculated from
Equation 12.4. For toluene alone at an average concentration of 0.20 ppmv,
the service life is predicted to be approximately 9 months. An equivalent
TVOC (total VOC) concentration would be approximately 0.77 mg/m
3
, a
moderate level of VOCs in indoor spaces. Under such use conditions, the
sorbent would require replacement after approximately 9 months of contin-
uous operation. At concentrations of 1.5 mg/m
3
TVOC (a high TVOC con-
centration), sorbent life would be about 4.5 months. If in the initial case the
TVOC level were 0.77 mg/m
3
and flow rate were increased to 200 CFM (to
clean a larger air volume), service life would be approximately 4.5 months.
The service life of thin-bed carbon filters is difficult to determine under
real-world conditions. A variety of approaches have been suggested to deter-
mine when carbon filters applied for odor control should be replaced. These
include (1) detection of odor when the filter is saturated, (2) removal of
sample carbon granules to determine their degree of saturation in the labo-
ratory, and (3) challenge with odoriferous compounds such as isoamyl ace-
tate (banana oil) or wintergreen.
10. Catalytic properties
Activated carbons are often used to remove objectionable gases from emis-
sion sources, indoor air, and other media, based on their catalytic abilities.
Activated carbons have been used in southern California to remove O
3
from
ambient air serving greenhouses and, in some cases, building HVAC sys-
tems. In addition to catalytically destroying O
3
, activated carbons can destroy
other oxidants such as ozonides, peroxides, and hydroperoxides. Activated
carbon filters are used to reduce O
3
emissions from O
3
-generating equipment
such as electrostatic photocopiers and other high-voltage devices.
In the presence of O
2
, activated carbons can catalytically oxidize H
2
S to
elemental sulfur. H
2
S is a common malodorant associated with sewage treat-
ment, oil/gas extraction and refining, and a variety of decomposition pro-
cesses. It is a common contaminant in groundwater supplies used for domes-
tic and industrial purposes, and can be removed from water by activated
carbon filters.
In addition to their natural catalysis of compounds such as O
3
and H
2
S,
activated carbons can be impregnated with catalysts for specific applications.
C. Chemisorption
The large surface area associated with sorbents provides an optimum envi-
ronment for a variety of chemical reactions. To facilitate such reactions,
sorbents are coated or impregnated with selected chemicals that will react
with target substances which come into contact with the chemical impreg-
nants. The process is called chemisorption. It is the process by which con-
taminant levels are determined from gas sampling tube measurements (see
Chapter 9).
