Wet Scrubbers for Emission Control - Mathematics and Statistics for the Environment - page 540

• The gaseous pollutant diffuses from the bulk area of the gas phase to the gas-liquid
interface.
• The gas moves (transfers) across the interface to the liquid phase. This step occurs extremely
rapidly once the gas molecules (pollutants) arrive at the interface area.
• The gas diffuses into the bulk area of the liquid, thus making room for additional gas mol-
ecules to be absorbed. The rate of absorption (mass transfer of the pollutant from the gas
phase to the liquid phase) depends on the diffusion rates of the pollutant in the gas phase
(first step) and in the liquid phase (third step).

To enhance gas diffusion and, therefore, absorption, the steps include the following:

• Provide a large interfacial contact area between the gas and liquid phases.
• Provide good mixing of the gas and liquid phases (turbulence).
• Allow suficient residence, or contact, time between the phases for adsorption to occur.

18.4 ASSORTED VENTURI SCRUBBER EXAMPLE CALCULATIONS
In this section, we provide several example calculations environmental engineers would be expected
to perform with regard to problems dealing with Venturi scrubber design, scrubber overall collection
efficiency, scrubber plan review, spray tower, packed tower review, and tower height and diameter.

18.4.1 C alCulations For s Crubber d esign oF a v enturi s Crubber
■ EXAMPLE 18.5
Problem : Calculate the throat area of a Venturi scrubber to operate at specified collection efficiency
(USEPA, 1984b, p. 77).

Given:
Volumetric flow rate of process gas stream = 11,040 acfm (at 68°F)
Density of dust = 187 lb/ft 3
Liquid-to-gas ratio = 2 gal/1000 ft 3
Average particle size = 3.2 µm (1.05 × 10 -5 ft)
Water droplet size = 48 µm (1.575 × 10 -4 ft)
Scrubber coefficient k = 0.14
Required collection efficiency = 98%
Viscosity of gas = 1.23 × 10 -5 lb/ft-s
Cunningham correction factor = 1.0

Solution : Calculate the inertial impaction parameter (Ψ) from Johnstone's equation, which describes
the collection efficiency of a Venturi scrubber (USEPA, 1984a, p. 9-11):

η= −− (

(

)

)

1exp kQ LG Ψ

where
η = Fractional collection efficiency.
k = Correlation coefficient, the value of which depends on the system geometry and operating
conditions (typically 0.1 to 0.2 acf/gal).
Q L / Q G = Liquid-to-gas ratio (gal/1000 acf).
Ψ = Inertial impaction parameter: