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204 Lawrence K. Wang et al.

A quench chamber followed by a Venturi scrubber is often used to control PMs.
This scenario is also possible when controlling acid gases and/or halogenated organic
compounds.
2.3.4. General Remediation Applications
Wet scrubbers are simple to operate compared to other air pollution control options,
making it popular among air pollution engineers. If used to control volatile organic
compounds (VOCs), scrubbing liquor other than water may be required because of sol-
ubility concerns. Such solvents are often proprietary and are always more expensive to
use than water. If VOC concentrations being treated are low, another control step is often
needed to reach the desired removal efficiency, which entails added costs.
With the exception of mercury (Hg), volatile metals will condense at the normal
operating temperature of the typical wet scrubber. Therefore, high-efficiency removal
of heavy metals is possible in a wet scrubber. Unfortunately, the high vapor pressure of
mercury prevents ready condensation of mercury in a wet scrubber. As a result, the
removal efficiency of mercury vapors in wet scrubbers is not established in the literature.
If volumetric flows being treated are low, wet scrubbers do not have high removal
efficiencies. Imparted turbulence in the scrubbing liquor will improve the removal effi-
ciency achieved. Common scrubbing liquors are water, water solutions, and nonvolatile
organic liquids. Two-stage scrubbing systems, first with water and then with an alkaline
solution, are common as acid gas removal efficiency is improved at pH >7.
If PM removal is required, the actual particle size distribution and the required
removal efficiency will determine what type of wet scrubber is used for control purposes.
The various types of wet scrubbers dealt with in this discussion commonly achieve
a removal efficiency of 99.5%. To further improve upon this, as well as to lower the
costs of control operations, wet scrubbers are being developed in tandem with other
technologies, such as ionization.

2.4. Packed Tower (Wet Scrubber) Design
2.4.1. General Design Considerations
The efficiency of an absorption process used to remove a pollutant or pollutants from
an air flow will depend, in part on the following:
1. The solubility of the pollutant(s) in the chosen scrubbing liquor
2. Pollutant(s) concentration in the airstream being treated
3. Temperature and pressure of the system
4. Flow rates of gas and liquid (liquid/air ratio)
5. Gas–liquid contact surface area
6. Stripping efficiency of the liquor and recycling of the solvent
Of the above parameters, the ability to increase gas–liquid contact will always result
in a higher absorption efficiency in a wet scrubber. If the temperature can be reduced
and the liquid-to-air ratio increased, then the absorption efficiency will also be improved
in the scrubber.
The actual design of the tower (diameter, height, depth of packed bed, etc.) will also
depend on the given vapor–liquid equilibrium for the specific pollutant/scrubbing liquor.
Additionally, the type of tower (packed vs tray, etc.) used will affect this equilibrium.