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208 Lawrence K. Wang et al.

Here, the assumption is made that f is the fraction of flooding appropriate for the given
design. Using an f value of 0.6 in this example, the gas stream flow rate, G , for the
area
cross sectional area determined above is
G = f G (9)
area area, f
As also previously mentioned, the normal operating range for fraction of flooding, f, is
0.60–0.75. Therefore, the column (tower) cross-sectional area is

A = G/(3,600G ) (10)
column area
The diameter of the column (packed tower shown in Fig. 1b), D , may now be
column
determined:
D = [(4/π)(A )] 0.5 = 1.13 (A ) 0.5 (11)
column column column
where D is the column (tower) diameter (ft).
column
Now that the tower diameter is known, the height of the packed section, sufficient for
the needed removal efficiency, is determined. This packed height is determined from the
number of theoretical transfer units (NTU), which is multiplied by the height of transfer
unit (HTU).
The HTU is dependent on the solubility of the pollutant being treated in the scrubbing
liquor. Larger HTU values reflect more resistance to mass transfer by the pollutant
into the scrubbing liquor. HTU is given in feet and is expressed as N or N , depend-
og ol
ing on the limiting resistance to mass transfer in the system. In this example, where
a pollutant is being scrubbed from a gaseous stream, the gas film resistance (as
opposed to the liquid film) most likely controls mass transfer. So in this example, N
og
is used.
The height of the column (packed tower) in Fig. 1b is determined by
Ht = N × H (12)
column og og
where Ht is the column (packed tower) height(ft), N is the number of gas transfer
column og
units (based on overall gas film coefficients) (dimensionless), and H is the height of
og
an overall gas transfer unit based on overall gas film coefficients (ft). The actual deter-
mination of N is beyond the scope of this text. Because the solutions here are dilute,
og
the N is determined by
og
N = ln {[(HAP / HAP ) (1 − (1/AF)) + (1/AF)]} / (1 − 1/AF) (13)
og e o
where: HAP is the HAP (hazardous air pollutant) emission stream concentration (ppmv)
e
and HAP is the HAP outlet concentration (ppmv).
o
This is a once-through system, so pure water (pollutant free) is used to scrub in this
system. This makes the above expression possible.
Alternatively, Fig. 3 can be used to graphically determine N . Equation (13) deter-
og
mines the efficiency that will be realized in the scrubber tower. The inlet and outlet
concentration of pollutant is related to the number of transfer units, N , through the
og
absorption factor, AF, as shown in Equation (1).
The removal efficiency (RE) is determined from inlet and desired outlet concentration
of the pollutant: