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HOW ARE PERC EMISSIONS CONTROLLED?
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Primary
Control Systems |
Use refrigerated
condensers, carbon adsorbers, and polymeric vapor adsorbers to reduce the perc vapor
concentration in the drum to 8,600 parts per million volume (ppmv) or less. |
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Refrigerated
Condensers |
Refrigerated condensers are
operated throughout the drying cycle. During this cycle, perc-laden air is continually
cooled and recirculated through the condenser. The condenser recovers both perc and water
vapor from the air stream. This mixture is separated by means of a decanter or water
separator. The perc recovered by the water separator then goes to a solvent storage tank.
During the cool-down step of the drying cycle, the air stream is no longer reheated,
thereby allowing the refrigerated condenser to cool the air stream further and recover
more perc. At the end of the cool-down step, the temperature of the air exiting the
refrigerated condenser should be equal to or less than 45ºF. At 45ºF, the perc
concentration in the drum is approximately 8,600 ppmv. |
Carbon
Adsorbers |
Mainly used on transfer and
vented dry-to-dry machines and as secondary control devices in conjunction with
refrigerated condensers, carbon adsorbers use activated carbon to collect or adsorb perc
from a perc-laden air stream. Perc adsorbed in the bed is later removed by desorbing, or
stripping the carbon with steam or hot air. The solvent vaporized by the steam or hot air
is recovered by a condenser downstream.
A carbon adsorber must
be desorbed and maintained properly to be an effective control device; otherwise it will
be saturated, and perc vapors will be emitted directly into the air. Carbon beds desorbed
by steam produce waste water which must be treated as hazardous (i.e., hauled away by a
licensed hazardous waste hauler or treated by a waste water elimination system). Carbon
beds desorbed by hot air eliminate this problem. Carbon adsorbers can effectively reduce a
perc-laden air stream to 300 ppmv or less. |
Polymeric
Vapor
Adsorber |
Polymeric vapor adsorbers are
also called temporary vapor storage, which include a synthetic polymeric adsorbent bed.
Polymeric beds are desorbed by heating when the next load is being placed into the
machine. |
Secondary
Control Systems |
Generally consist of
a carbon adsorber which operates in series after the refrigerated condenser on a
closed-loop machine to reduce fugitive emissions of perc and to further reduce perc
concentration in the drum to 300 ppmv or less. Secondary control systems are activated at
the end of the cool-down step before the machine is opened. These devices normally route
perc vapor from the drum, button and lint traps and still through the adsorber which
strips the perc vapors from the air. The air is then recirculated back through the drum
and the traps to pick up more residual perc vapors. Most secondary control systems use hot
air or heating coils to desorb perc from the carbon beds. |
Fugitive
Control Systems |
Activate
whenever the main door is opened to route perc vapors through a carbon adsorber (not
required by AQMD Rule 1421). |
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