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Wet and Dry Scrubbing 205

Such data are often not available for all pollutants encountered in industry today. If
data are available, empirical data will always be superior to theoretical data for design
purposes. If such empirical data are unavailable, a similar type of pollutant having
available data, with an added safety factor built into the design, should be used to model
the system.

2.4.2. Packed Tower (Wet Scrubber) Design Variables
As an in-depth analysis of design methods for all types of absorption tower is beyond
the scope of this discussion, a design for a typical, common wet scrubber is given here.
The example is a packed tower wet scrubber, as shown in Fig. 1b. This type of tower is
commonly found in air pollution control installations. The configuration used is some-
what simplified. The tower is packed with 2 in. ceramic Raschig Rings (note: 1 in. =
2.54 cm) and the scrubbing liquor (absorbent) used is water. The water is sprayed from
top and the slurry is collected at the bottom. The scrubbing liquor spray system is
described as a once-through process with no recirculation. It should be noted that in a
field installation, this once-through method has the consequence of sending a large flow
of water to a treatment facility. This example is applicable for either organic or inorganic
air pollutant control (1–3,14–17).
In any absorption process, possible removal efficiency is controlled by the concen-
tration gradient of the pollutant being treated between the gas and the liquid phases.
As previously defined, this concentration gradient is the driving force to mass trans-
fer between the phases. Therefore, the solubility of the given pollutant in the gas and
liquid phases will determine the equilibrium concentration of the pollutant in the
given example.
If a pollutant is readily soluble in the scrubbing liquor, the slope m of the equilibrium
curve is low. There is an inverse relationship between m and driving force; the smaller
the slope, the more readily the pollutant will dissolve into the scrubbing liquor. This rep-
resents a high-driving-force system. The size of the tower in such a system will be
minimal, as mass transfer (absorption) between the phases occurs readily. If the slope
is relatively large, approx 50 or more, this represents limited solubility of pollutant in
the scrubbing liquor. For absorption to occur with limited driving force, the contact time
between the phases must be extended, so the needed tower size will increase. A high
liquid-to-gas ratio requirement is also indicated by the limited solubility of the pollutant
in the scrubbing liquor if high removal efficiency of the pollutant is desired. As a prac-
tical rule of thumb, if m > 50, a removal efficiency of the pollutant of 99% will most
likely not be practical.
In normal circumstances for an air pollution wet scrubber design control project,
the inlet concentration of pollutant, gas flow rate, temperature, and pressure are fixed.
The removal efficiency (outlet concentration of pollutant) is also normally specified
and the available scrubbing liquor is known. The challenge of the design is to determine
the scrubber tower diameter, the depth of packed section, and the needed scrubbing
liquor flow rate to accomplish the specified outlet concentration of pollutant. The total
height of the tower will then be determined based on these results. A further consideration
is that the total head loss through the tower will directly impact the cost of operating the
scrubber system.