Common Vapor Treatment
Technologies
Granular Activated Carbon
Vapor-phase carbon adsorption
by granular activated carbon (GAC) is a process by which pollutants are removed
from air by physical adsorption (sticking) onto the large internal surface area
within the activated carbon grains. The
GAC is the most common adsorbent material used for vapor control and is
typically used in packed beds through which the contaminated air flows until
the capacity of the GAC to retain the contaminants at an acceptable outlet
level is exceeded. The capacity of the carbon varies with the concentration of
the contaminants, and generally, the lower the concentration of contaminant in
the inlet stream, the lower the capacity (less mass adsorbed per weight of
carbon) of the carbon for a given outlet concentration. When the acceptable outlet level is exceeded
the bed must be regenerated or replaced.
For most SVE applications the carbon is replaced and the expended GAC is
taken off-site to be disposed of or regenerated[1]
depending upon economic considerations, e.g., transportation versus disposal
cost.
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Figure illustrating two carbon beds being
operated so that adsorber bed one is actively adsorbing while the spent carbon
is being replaced (or regenerated) in bed two.
When the capacity of bed one is exhausted the valve positions are
changed so that the contaminated air flows through bed two while the GAC in bed
one is replaced or regenerated. (Adapted
from the Federal Remediation Technology Roundtable.)
The carbon bed can also be used as a
concentrator of the vapors. Using
vacuum regeneration of the carbon, researchers at the Idaho National
Engineering Laboratory have demonstrated that the vapors can subsequently be
condensed through a specialized process involving the thermodynamic cycle
referred to as the Brayton Cycle. The
condensed contaminants must then be treated and disposed of, but the bed can be
regenerated on-site without the applying steam regeneration. The process adds complexity and cost to the
operation and again an economic decision would be made to determine how best to
deal with the spent carbon.
For a more detail regarding carbon
adsorption and the Brayton Cycle technology, click on the following link to the
Federal Remediation Technologies Roundtable: FRTR Vapor-Phase Carbon
Adsorption.
A technical report summary involving a
site in North Carolina at which GAC was used to control SVE vapors may be
reviewed by clicking on the following link: Granular Activated Carbon.