Wet Scrubbers for Emission Control - Mathematics and Statistics for the Environment - page 526

Clean gas out

Dirty gas in

Spray
Venturi
throat

Settling
chamber

Water and condensate out

FIGURE 18.1 Venturi wet scrubber. (From USEPA, Control Techniques for Gases and Particulates , U.S.
Environmental Protection Agency, Washington, DC, 1971.)

10 micrometers (µm) in aerodynamic diameter (PM 10 ), and PM less than or equal to 2.5 µm in
aerodynamic diameter (PM 2.5 ). Though capable of some incidental control of volatile organic
compounds (VOCs), generally Venturi scrubbers are limited to controlling PM and high solubil-
ity gases (USEPA, 1992, 1996). Venturi scrubbers operate at extremely high gas and liquid veloci-
ties with a very high pressure drop across the Venturi throat. Venturi scrubbers such as the one
shown in Figure 18.1 are most efficient for removing particulate matter in the size range of 0.5 to
5 µm, which makes them especially effective for the removal of submicron particulates associated
with smoke and fumes.
The Venturi scrubber shown in Figure 18.1 accelerates the waste gas stream to atomize the
scrubbing liquid and to improve gas-liquid contact. In a Venturi scrubber, a “throat” section is
built into the duct that forces the gas stream to accelerate as the duct narrows and then expands. As
the gas enters the Venturi throat, both gas velocity and turbulence increase. Depending upon the
scrubber design, the scrubbing liquid is sprayed into the gas stream before the gas encounters the
Venturi throat, or in the throat, or upwards against the gas flow in the throat. The scrubbing liquid
is then atomized into small droplets by the turbulence in the throat and droplet-particle interaction
is increased. Some designs use supplemental hydraulically or pneumatically atomized sprays to
augment droplet creation. The disadvantage of these designs is that clean liquid feed is required to
avoid clogging (Corbitt, 1990; USEPA, 1998). After the throat section, the mixture decelerates, and
further impacts occur causing the droplets to agglomerate. Once the particles have been captured
by the liquid, the wetted PM and excess liquid droplets are separated from the gas stream by an
entrainment section which usually consists of a cyclonic separator and/or a mist separator (Corbitt,
1990; USEPA, 1998).
Current designs for Venturi scrubbers generally use the vertical downflow of gas through the
Venturi throat and incorporate three features: (1) a wet approach, or flooded-wall entry section, to
avoid a dust buildup at a wet-dry junction; (2) an adjustable Venturi throat to provide for adjust-
ment of the gas velocity and the pressure drop; and (3) a flooded elbow that is located below the
Venturi and ahead of the entrainment separator to reduce wear by abrasive particles. The Venturi
throat is sometimes fitted with a refractory lignin to resist abrasion by dust particles (Perry and
Green, 1984).