Civil Engineering Reference
In-Depth Information
Q
L
volumetric flow rate, gpm (L / sec)
K
5.0
10
7
, constant to convert emissions rate to lb / hr
K
3.6
10
6
, constant to convert emissions rate to kg / hr
Emissions of the compound are compared with local air quality standards to determine
if treatment of the off-gas is necessary. Site-specific factors that may impact the eval-
uation include:
Projected ground-level concentrations
•
Proximity to human habitation
•
Treatment plant worker exposure
•
Local air quality
•
Local meteorological conditions
•
The methods for off-gas treatment include:
Vapor-phase carbon adsorption
•
Thermal destruction
•
Catalytic incineration
•
Ozone destruction with ultraviolet (UV) radiation
•
UV photooxidation
•
Carbon Adsorption
The most common method to treat the off-gas is by vapor-phase
carbon adsorption. The off-gas is usually processed by a heating step to remove hu-
midity before being applied to the carbon units. Parameters that influence the design
of the carbon adsorption system include the airflow rate, influent concentrations of
VOCs, and degree of VOC removal by the packed tower unit. The carbon usage rate
for vapor-phase treatment of VOCs is generally less than half that for removing VOCs
in liquid-phase granular activated carbon (GAC) units. During vapor-phase carbon
adsorption, the contaminants are transferred from one phase to another. As with liquid-
phase carbon absorption, the exhausted carbon is either regenerated on-site or replaced
(see Chapter 17).
Photocatalytic Oxidation
Photocatalytic oxidation, which is still an experimental
technology for treating off-gas, utilizes ultraviolet (UV) light to oxidize VOCs. Studies
to date have utilized the process to treat the off-gas downstream of a packed tower
and in a closed-loop air-stripping process (CLASP). Chlorinated ethylenes, such as
trichloroethylene (TCE), are more susceptible to photocatalytic oxidation than are chlo-
rinated ethanes, such as trichloroethane (TCA). Water vapor was found to significantly
inhibit the VOC destruction rate for TCE.
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For this reason, photocatalytic oxidation
units should be placed downstream of a dehumidifier. The main benefit of operating
gas-phase photocatalytic oxidation in a closed-loop process is absorption of the end
products into the water flow, which eliminates the release of off-gases to the atmo-
sphere and need for an absorbent, such as granular activated carbon.
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