Page 424 - Air Pollution Control Engineering

10_chap_wang.qxd 05/05/2004 5:10 pm Page 396
396 Lawrence K.Wang et al.

Three operations are commonly found in most processes involving adsorption: contact,
separation, and regeneration. Initially, the adsorbent comes into contact with the fluid
where the separation by adsorption results. Second, fluid that is not adsorbed is sepa-
rated from the adsorbent. With a gas stream, this operation is completed as the gas
stream passes through the adsorbent bed. Third, the adsorbent is regenerated, removing
the adsorbate from the adsorbent.

1.2. Adsorbents
Commonly used adsorbents for selectively adsorbing certain gaseous constituents
from gas streams include activated carbon, silica gel, alumina, and bauxite. The con-
taminated gaseous constituents should be adequately removed from airstreams for air
pollution control (13–18). Commercially available adsorbents possessing adsorptive
properties exist in great variety. Some of these adsorbents with their industrial uses are
as follows:
• Activated carbon: solvent recovery, elimination of odors, purification of gases
• Alumina: drying of gases, air, and liquids
• Bauxite: treatment of petroleum fractions; drying of gases and liquids
• Bone char: decolorizing of sugar solutions
• Decolorizing adsorbents: decolorizing of oils, fats, and waxes; deodorizing of domestic water
• Fuller’s earth: refining of lube oils and vegetable and animals oils, fats, and waxes
• Magnesia: treatment of gasoline and solvents; removal of metallic impurities from caustic
solutions
• Silica gel: drying and purification of gases
• Strontium sulfate: removal of iron from caustic solutions

1.3. Carbon Adsorption and Desorption
In air pollution control, activated carbon is the most widely used adsorbent and is the
focus of discussion in this chapter. Adsorbents such as silica gel or alumina are less
likely to be used in adsorption systems for air pollution control; therefore, they are not
discussed in this chapter.
Carbon adsorption is a process by which pollutants are selectively adsorbed on the
surface of granular activated carbon beds. It has been shown that activated carbon is
the most suitable adsorbent for the removal of organic vapors. Substantially all organic
vapors in air at ambient temperature can be adsorbed by carbon, regardless of variation in
concentration and humidity. Because the organic adsorbed has practically no vapor
pressure at ambient temperature, the carbon system is particularly adapted to the effi-
cient recovery of the adsorbed organic. As a result, the carbon systems can always be
designed for operation without hazard, because the vapor concentration is always below
the flammable range. At low concentration of organic solvent in the airstream, carbon
systems can efficiently recover organic solvents.
Regeneration is a desorption process by which adsorbed volatile organic compounds
(VOCs) are removed from the carbon beds either by heat desorption at a sufficiently
high temperature (usually using steam) or by vacuum desorption at a sufficiently low
vacuum pressure. Some of the adsorbed organic will remain activated after regenera-
tion. It has been shown that during desorption for carbon regeneration, about 3–5% of
organics desorbed on the virgin activated carbon is absorbed so strongly that it cannot
be desorbed during regeneration (14–18).