Page 322 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

10.6 Control of Volatile Organic Compounds 299

10.6.1 Volatile Organic Compounds Adsorption

As introduced in Part I, low-concentrations VOCs can be physically adsorbed by
activated carbon and other adsorbents. Activated carbon can be produced from a
char or coke material by oxidation with air at 500–600 °C or by oxidation with
boiling nitric acid.
3
Activated carbon adsorption of VOCs works well in the range of 0.05–30 m /s
for gases with VOC concentrations of 20–5000 ppmv. The removal efficiencies are
in the order of 90–98 %. Adsorption of VOCs is also not very sensitive to LFL or
UFL values for the gas.
The adsorption bed will have to be replaced or regenerated when it reaches its
break-through point after a certain time of operation, when the bed becomes sat-
urated. In industry, most operators set the breakthrough value to be the same as that
of the local emission standard. For an activated carbon bed used for adsorption of
VOCs, the regeneration can be done with steam, hot air or hot nitrogen at
400–1000 °C. The VOCs released from the regeneration process are more con-
centrated and can easily be oxidized or recycled for other purposes.
Repeated loading and regeneration reduce the quality of the activated carbon and
it results in lowered adsorption efficiency. In addition, the presence of other species
such as halogenated organics and high moisture contents also reduce the perfor-
mance of the activated carbon bed. For example, carbon bed adsorption is less
suitable for ketones such as acetone, since exothermic polymerization reactions
deactivate the bed and may even result in fires. Eventually, a fresh bed will replace
a deactivated carbon bed and the activated carbon is subjected to disposal.

10.6.2 Oxidation of VOCs

Some air pollutants like VOCs, HC, and CO can be oxidized and converted into
less toxic chemicals. There are three types of VOC oxidation processes, according
to their applicable VOC concentrations.

• Flaring: Flares are usually used for gas streams that have an organic vapor
concentration greater than 2–3 times the LFL.
• Thermal oxidation: Thermal oxidizers are used for contaminated gas streams
that have an organic vapor concentration 25–50 % of the LFL.
• Catalytic oxidation: Catalytic oxidizers are used for gas streams that have VOC
concentrations <25 % of the LFL.