Wet Scrubbers for Emission Control - Mathematics and Statistics for the Environment - page 525

18 Wet Scrubbers for

Emission Control

I durst not laugh for fear of opening my lips and receiving the bad air.

— William Shakespeare ( Julius Caesar )

18.1 INTRODUCTION
How do scrubbers work? To answer this question we need only look to a critical part of Earth's natu-
ral pollution control system—that is, the cleaning of the lower atmosphere by rain. Obviously, this
is most evident by the freshness of the air following a rainstorm. The simplicity of spraying water
into a gas stream to remove a relatively high percentage of contaminants has contributed to scrub-
bers having been used within industry extensively since the early 1900s. Heumann and Subramania
(1997) pointed out that “most pollution control problems are solved by the selection of equipment
based upon two simple questions … (1) Will the equipment meet the pollution control requirements?
And (2) which selection will cost the least?”
How are scrubbing systems capabilities evaluated? They are evaluated based on empirical rela-
tionships, theoretical models, and pilot-scale test data. Two important parameters in the design
and operation of wet scrubbing systems that are a function of the process being controlled are dust
properties and exhaust gas characteristics. Particle size distribution is the most critical parameter
in choosing the most effective scrubber design and determining the overall collection efficiency.
In operation, scrubbers are considered universal control devices because they can control either
or both particulate and gaseous contaminants. There are numerous types of scrubbers, including
wet scrubbers, wet-dry scrubbers, and dry-dry scrubbers. Scrubbers use chemicals to accomplish
contaminant removal; the gaseous contaminants are absorbed or converted to particles and then
wasted or removed from the stream. In this chapter, our focus is on the calculations used for wet
scrubber systems. Much of the information is excerpted from Spellman (1999, 2008) and USEPA
(1984a,b,c).

18.1.1 W et s Crubbers
Wet scrubbers (or collectors) have found widespread use in cleaning contaminated gas streams
(e.g., acid mists, foundry dust emissions, furnace fumes) because of their ability to effectively
remove particulate and gaseous contaminants. Wet scrubbers vary in complexity from simple
spray chambers that remove coarse particles to high-efficiency systems (Venturi types) that
remove fine particles. Whichever system is used, its operation employs the same basic principles
of inertial impingement or impaction and interception of dust particles by droplets of water. The
larger, heavier water droplets are easily separated from the gas by gravity. The solid particles
can then be independently separated from the water, or the water can be otherwise treated before
reuse or discharge. Increasing either the gas velocity or the liquid droplet velocity in a scrubber
increases the efficiency because of the greater number of collisions per unit time. For the ulti-
mate in wet scrubbing, where high collection efficiency is desired, Venturi scrubbers are used.
They are primarily used to control particulate matter (PM), including PM less than or equal to

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