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chemical reaction (e.g., HCl being absorbed into and then reacting with lime-based
slurry of CaCl ), then the process is a chemical absorption process. In dry scrubbing, an
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alkaline reagent is injected into the gas stream while preventing the gas from being
saturated with water vapor.
Although the most common name for such a unit operation installation is a scrubber
or absorber, other names commonly used to reference such installations in industry are
spray towers and packed or plate columns. It should be noted that the latter three unit oper-
ations may operate slightly differently from the wet and dry scrubbers (absorbers) defined
here. These terms are mentioned here, because they are sometimes used interchangeably
with mass transfer unit operations.
1.2. Wet Scrubbing or Wet Absorption

The physical criteria in designing a wet scrubber are simple:
1. Use a liquid for absorption that offers a high solubility of the pollutant in the gas stream
being treated
2. Maximize gas–liquid contact surfaces
When both conditions are met, the pollutant will readily diffuse out of the gas phase and
be absorbed into the liquid phase.
Theoretically, absorption of a pollutant in a gas phase into a contacting liquid phase
occurs when the liquid contains less than the equilibrium concentration of the pollutant.
In other words, the pollutant in the gas phase must have some solubility in the liquid
phase. For absorption into the liquid phase to occur, the maximum concentration of the
same pollutant in the liquid phase must be avoided initially. This is because the con-
centration difference across the phase boundary is the driving force for absorption to
occur between the two phases. Additionally, absorption (mass transfer) from gas into
liquid (or vice versa) is dependent on the physical properties of the gas–liquid matrix
(e.g., diffusivity, viscosity, density) as well as the conditions of the scrubber system (e.g.,
temperature, pressure, gas and liquid mass flow rates). Absorption of a pollutant is
enhanced by lower temperatures, greater liquid–gas contact surfaces, higher liquid–gas
ratios, and higher concentration of the pollutant in the gas phase (or, alternately, lower
concentration of the pollutant in the liquid phase). In some instances, elevated pressures
are used to give added driving force of the pollutant into the liquid stream as well (1–21).
Wet scrubbers are often the technology of choice if high removal efficiencies of acid
*
gases are required. An HCl removal efficiency greater than 99% is easy to obtain in
wet scrubbers. SO is a more difficult pollutant to wet scrub; traditionally, wet scrubber
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designs call for 90–95% removal efficiency for SO . Scrubber designs have been chal-
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lenged by new regulations regarding SO removal efficiency. In 1998, the US EPA
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(Environmental Protection Agency) instituted new air pollution regulations known as
NESHAP (National Emission Standards for Hazardous Air Pollutants) that call for the
retrofitting of existing SO wet scrubbers to achieve 98% removal efficiency and for all
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new wet scrubbers used to control SO emissions to achieve 99% removal efficiency.
2
* This efficiency is possible only for HCl that is not in aerosol form. Possible formation of aerosols when
scrubbing HCl must always be accounted for, as such aerosols will not be treated in a wet scrubber. The presence
of HCl in aerosol form will form a distinctive white plume when exiting the stack of a wet scrubber.