Control of Volatile Organic Compounds Emissions from Manufacturing

stream downward through a fixed carbon bed, Granular carbon is usually
favored because it is not easily entrained in the exh,aust stream.
Figure 3-7 is a schematic of a typical fixed bed, regenerative
carbon adsorption system. The process offgases are filtered and cooled (1)
to mi ni mize bed contami nation and maximize adsorpti or1 ef f ici ency. The
offgas is directed through the porous activated carbon bed (2) where
adsorption of the organics progresses until the activated carbon bed is
"saturated". When the bed is completely saturated, the organic will
"breakthroughu the bed with the exhaust gas and the inlet gases must
then be routed to an alternate bed. The saturated bed is then regenerated
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to remove the adsorbed material.
Low-pressure steam (3) is usually used to heat the carbon bed
duri ng the regeneration cycl e, dri vi ng off the adsorbed organi cs , which
are usual ly recovered by condensing the vapors (4) and separating them
from the steam condensate by decanting or di stil lation (5). The adsorption/
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regeneration cycle can be repeated numerous times, but eventually the
carbon loses its adsorption activity and must be replaced. The carbon
can sometimes be reactivated by recharri ng.
3.2.2.1 Adsorber Control Efficiency. The ef f ici ency of an adsorption
unit depends on the properties of the carbon and the adsorbate, and on
the conditions under which they contact. Lower temperatures aid the
adsorption process, while higher temperatures reduce the adsorbent's
~a~acity.41Removal efficiencies of 95 to 99 percent are achieved by
we1 I-designed and we1 1-operated units.42
3.2.2.2 Applicability of Adsorbers. Adsorbers effectively control
streams with dilute concentrations of organics. In fact, to prevent
excessive temperatures within the bed due to the heat of adsorption,
f nlet concentrations of organics are usual ly limited to about 0.5 to
1 percent.40 The maximum practical inlet concentration is about 1 percent,
or 10,000 ppmv.43 Higher concentrations are frequently handled by
allowing some condensate to remain from the regeneration process to
remove the heat generated during adsorption, ~lso, 'the inlet stream can
be diluted by use of a condenser or addition of air lor nitrogen upstream
of the adsorber. If the organic is reactive or oxygen is present in the
vent stream, then additional precautions may be necessary to safeguard
the adsorption system.
3-28
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