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200 Lawrence K. Wang et al.

to foul wet scrubber internal components. Lime, Ca(OH) , is less expensive than sodium
2
reagents, but the latter normally offer higher removal efficiency of the pollutants in the
gas stream. If sodium-based solids do form in a wet scrubber, they present a greater dis-
posal problem than calcium-based solids. Therefore, the possible formation of solids in
a wet scrubber must be considered, as well as possible use or disposal of such byprod-
ucts when considering a wet scrubber.
2.1.2. Absorption of Particulate Matter
A typical wet scrubber is a vertical tower in which liquid, normally water, enters from
the top. The polluted gas being treated enters from the bottom of the scrubber so that the
water flows down and the gas flows up. This is the classic “countercurrent” flow scheme.
If the pollutant being removed from the gas stream is PM, no special chemical reagents
are used. Simple water suffices as the absorbent liquor. If a gaseous pollutant is
removed simultaneously with PM removal, the need for a chemical reagent will depend
on the particular gaseous pollutant being controlled.
2.2. Wet Scrubbing Systems

Several methods are available for wet scrubbing. Figure 1 illustrates four common
methods of wet scrubbing. A discussion of these and other methods of wet scrubbing
follows. The method of wet scrubbing chosen to treat a given gaseous pollutant is
always specific to the given pollutant or pollutants present in the gas stream being
treated. A “standard” wet scrubber does not exist.

2.2.1. High-Efficiency Venturi Scrubber
As seen in Fig. 1c, a Venturi scrubber is often a primary control solution. This scrubber
operates at low pH and will remove PM and HCl. Removal efficiency of such a Venturi
scrubber should be 80–95% for particles greater than 2 µm (15).
The Venturi principle states that as gas enters a narrow constriction (the Venturi), the
velocity of the gas increases. At this point of constriction, the absorbent liquor (scrub-
bant) is introduced. The high-velocity gas forces the liquor to atomize into small
droplets, which offer a large total surface area of liquor into which the PM absorbs.
After passing through the Venturi, the gas returns to near original velocity. At this lower
velocity, the scrubbing liquor agglomerates back into the bulk liquid phase, containing
the PM.
Suppliers of Venturi scrubbers commonly provide prefabricated units capable of
treating gaseous streams of up to 80,000 cubic feet per minute (cfm). These units nor-
mally operate at a high pressure drop that increases the power costs of the unit. Such
prefabricated units nevertheless have a considerable initial capital cost advantage when
compared to electrostatic precipitators (ESPs) and fabric filters.

2.2.2. Jet Venturi Scrubber Systems
A slightly different type of Venturi scrubber is the Jet Venturi scrubber. In this type
of scrubber, energy from a flow of pressurized liquid forces a draft to form. This draft
captures PM with an efficiency of greater than 90%. Normal process installations use a
quencher ahead of a Jet Venturi scrubber. However, the Jet Venturi also sometimes accepts
a gas stream directly from a combustion chamber.