Packed Towers
Packed towers are gas absorption devices that utilize internal media of a variety of
types to enhance the mass transfer of gases into an absorbing liquid. Please also see
lament/mesh scrubbers (Chapter 9), which share many of the same design and use
characteristics of the packed tower.
For both air pollution control and recovery of process gases, packed towers are one
of the most common mass transfer devices in current use.
They are used for control of soluble gases such as halide acids (such as HF and
HCl) and to remove soluble organic compounds such as alcohols and aldehydes.
When the scrubbing solution is charged with an oxidant such as sodium hypochlo-
rite, they are used to control sulde odors from wastewater treatment facilities and
rendering plants. They are used to absorb and concentrate acids for recovery. When
gases and aerosols are both present, the packed tower is frequently used ahead of
aerosol collectors such as berbeds and wet electrostatic precipitators (WESPs).
Packed towers are also used as gas coolers and condensers (Please see Chapter 22
on energy recovery). They sometimes are used after a hot gas quencher to act as a gas
cooler. Some are tted with ceramic packing that can resist temperature extremes.
When tted ahead of a Venturi scrubber to function as a water vapor condenser/
absorber, the packed tower becomes a critical part of a ux force condensation sys-
tem for particulate control. The tower in this case acts as both an acid gas absorber
and a direct contact vapor condenser.
They are also used after Venturi scrubbers on medical waste incinerators to con-
trol acid gases such as HCl.
To control the combined vent gases from semiconductor manufacturing, large
packed towers are used. Called house scrubbers, they clean the small concentrations
of acid gases usually using pH control and neutralization with caustic. In contrast,
the same industry uses small packed towers at specic tools in a point-of-use con-
guration. The point-of-use scrubbers are designed to treat the specic emissions
source and often vent into a combined ventilation system, eventually leading to a
house scrubber. The emissions are effectively double scrubbed before the carrying
gas is released to atmosphere.
Pulp and paper mills often use packed towers for bleach plant applications to control
chlorine and chlorine dioxide where bers or chemical scaling is minimal. Fluidized
bed type scrubbers are used in cases where bers or scaling are known challenges.

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