Page 230 - Air Pollution Control Engineering

05_chap_wang.qxd 05/05/2004 3:46 pm Page 209
Wet and Dry Scrubbing 209

Fig. 3. Relationship among Nog, AF, and efficiency.

HAP = HAP [1 − (RE/100)] (14)
o e
The outlet concentration of the pollutant may now be substituted into Eq.(13) to
obtain the depth of packing needed for the specified removal efficiency. A general state-
ment is that a larger value of N yields a higher removal efficiency until the driving
og
force (concentration gradient) is exhausted. At this point, no further transfer of pollutant
between the two phases occurs.
Once the number of transfer units, N , required to meet the removal efficiency
og
requirement is known, the height of each transfer unit, H , may be determined:
og
H = H + (1/AF)H (15)
og G L
where H is the height of the gas transfer unit (ft) and H is the height of liquid transfer
G L
unit (ft).
Based on the packing chosen along with gas and liquid flow rates, generalized cor-
relations to determine H and H are available:
G L
H = [b(3,600 G ) / (L'') ] (Sc ) 0.5 (16)
c
d
G area G
H = Y (L'' / µ '') (Sc ) 0.5 (17)
s
L L L
where b, c, d, Y, and s are empirical packing constants (11) from Tables 1 and 2, L'' is
2
the liquid flow rate (lb/h-ft ), µ '' is the liquid viscosity (lb/ft-hr), Sc is the Schmidt
L G
number for the gas stream (see Table 3), and Sc is the Schmidt number of the liquid
L
stream (see Table 4).