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Nonhalogenated
VOC's:
Acetone, Styrene, etc.
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Nonhalogenated VOC's in
Air Emissions / Off-Gases
Three technologies that are most commonly
used to treat nonhalogenated VOC's in air emissions/off-gases
are:
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Carbon Adsorption
Carbon adsorption is a remediation technology
in which pollutants are removed from air by physical adsorption
onto the carbon grain. Carbon is "activated" for this purpose
by processing the carbon to create porous particles with a large
internal surface area (300 to 2,500 square meters per gram of
carbon) that attracts and adsorbs organic molecules as well
as certain metal and other inorganic molecules.
Commercial grades of activated carbon are
available for specific use in vapour-phase applications. The
granular form of activated carbon is typically used in packed
beds through which the contaminated air flows until the concentration
of contaminants in the effluent from the carbon bed exceeds
an acceptable level. Granular activated carbon systems typically
consist of one or more vessels filled with carbon connected
in series and/or parallel operating under atmospheric, negative,
or positive pressure. The carbon can then be regenerated in
place, regenerated at an off-site regeneration facility, or
disposed of, depending upon economic considerations.
Catalytic Oxidation
Catalytic oxidation is an alternative
for the treatment of VOC's in air streams resulting from remedial
operations. VOC's are thermally destroyed at temperatures typically
ranging from 200°C to 600°C by using a solid catalyst.
First, the contaminated air is directly preheated
(electrically or, more frequently, using natural gas or propane)
to reach a temperature necessary to initiate the catalytic oxidation
of the VOC's. Then the preheated VOC-laden air is passed through
a bed of solid catalysts where the VOC's are rapidly oxidised.
In most cases, the process can be enhanced to reduce auxiliary
fuel costs by using an air-to-air heat exchanger to transfer
heat from the exhaust gases to the incoming contaminated air.
Typically, about 50% of the heat of the exhaust gases is
recovered. Depending on VOC concentrations, the recovered heat
may be sufficient to sustain oxidation without additional fuel.
Catalyst systems used to oxidise VOC's typically use metal oxides
such as nickel oxide, copper oxide, manganese dioxide, or chromium
oxide. Metals such as platinum and palladium may also be used.
However, in a majority of remedial applications, non-precious
metals (e.g., nickel, copper, or chromium) are used. Most commercially
available catalysts are proprietary.
Thermal Oxidation
Thermal oxidation equipment is used for
destroying contaminants in the exhaust gas from air strippers
and SVE systems.
Thermal oxidation units are typically single
chamber, refractory-lined oxidisers equipped with a propane
or natural gas burner and a stack. Lightweight ceramic blanket
refractory is used because many of these units are mounted on
skids or trailers. Thermal oxidisers are often equipped with
heat exchangers where combustion gas is used to preheat the
incoming contaminated gas. Operating temperatures range from
700°C to 1000°C, and gas residence times are typically
1 second or less.
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