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206 Lawrence K. Wang et al.

In this example, there are no heat effects caused by absorption in the tower and both
the airstream and liquid stream are dilute solutions. Flow rates are constant and the equi-
librium curve is linear. The needed data for this design are available in the literature
(1–5,11–17).
A material balance determines the scrubbing liquor required flow, based on the liquid-
to-gas ratio determined from the equilibrium curve. The absorption factor is widely
accepted to range from 1.25 to 2.0 for best economics in a scrubber design project. The
absorption factor determines the liquid–gas molar flow rates (10–17). For this example,
an absorption factor of 1.6 is used.

L = (AF)(m)(G ) (1)
mol mol
where AF is the absorption factor (explained earlier), L is the liquid (absorbent)
mol
flow rate (lb-mol/h), G is the gas flow rate (lb-mol/h), and, m is the slope of the
mol
equilibrium curve.
Note that the value of m is temperature dependent for the given system (1,4,5). Other
systems are defined elsewhere (1,3,6,14).
At the assumed value of AF, Eq. (1) yields

L = (1.6)(m)(G ) (2)
mol mol
Defining the gaseous stream flow rate in scfm to be Q , it follows that
e
G = 0.155 Q (3)
mol e
where Q is the emission stream flow rate (scfm).
e
Now, L can be converted to gpm:
mol
L = [L × MW × (1/D ) × 7.48] / 60 (4)
gal mol solvent L
where MW is the molecular weight of the scrubbing liquor (solvent), L is the
slovent gal
3
liquid (solvent) flow (gpm), and D is the density of the liquid (solvent) (lb/ft ).
L
The factor 7.48 is used to convert cubic feet to gallons. When water is used as the
3
solvent, then D is equal to 62.43 lb/ft and the MW is equal to 18 lb/lb-mol. Then,
L solvent
Eq. (4) yields
L = 0.036 L (5)
gal mol
2.4.3. Packed Tower (Wet Scrubber) Sizing
Once the gas and liquid streams entering and leaving the packed tower are identified
along with pollutant and solvent concentrations, the flow rates are calculated and oper-
ational conditions determined. These data combined with the type of packing used will
determine the actual size of the tower. The tower size must be sufficient to accept the
gas and liquid flows without excessive head loss.
The determination of the tower (see Fig. 1b) diameter has traditionally been based on
an approach to flooding. Normal operating range to achieve maximum efficiency has
been to use 60–75% of the flooding rate for tower sizing purposes. (Note: With flood-
ing, the upward flow of gas through the tower impedes the downward flow of liquid. The
actual point of flooding is somewhat arbitrary in definition.) A common correlation to
determine the tower diameter is given in Fig. 2.