Page 253 - Air Pollution Control Engineering

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232 Lawrence K. Wang et al.

Then the abscissa (ABS) is determined from Eq. (6):

ABS = (L/G) (D / D ) 0.5 (6)
G L
ABS = (17,410 / 13,200) (0.071/62.18) 0.5 = 0.045
Using Fig. 2, ABS = 0.045, so the ORD for flooding is equal to 0.15. If 2-in. ceramic
Raschig rings are used to pack the column, the packing constants are (11)
a = 28 and e = 0.74
Also,
2
g = 32.2 ft/s and µ = 0.85 cP at 85°F, from ref. 1
c L
Then, substituting in Eq. (8),
3
G = {[(ORD) D D g )] / [(a/e )(µ 0.2 )]} 0.5 (8)
area, f G L c L
0.2
3
G = {(0.15)(0.071) (62.18)(32.2) / [(28 / 0.74 ) (0.85 )]} 0.5
area, f
2
G = 0.56 lb/s-ft (at flooding)
area, f
If the fraction of flooding f = 0.6, then Eq. (9) becomes
2
G = fG = (0.6)(0.56 lb/s-ft ) (9)
area area, f
G = 0.34 lb/s-ft 2
area
Use Eq (10) to find the area of the column:
A = G/(3600 G ) (10)
column area
A = 13,200 / [(3600)(0.34)]
column
A = 10.8 ft 2
column
The column diameter is determined as follows:
D = 1.13 (A ) 0.5 = 1.13 (10.8) 0.5 (11)
column column
D = 3.7 using 4 ft
column
Example 6
A step-by-step procedure to determine column (tower) height and packed depth of a wet
scrubber (see Fig. 1b) follows.
Solution
A. Using Eq. (13) and Eq. (14) to determine N (number of gas transfer units) as well as
og
Fig. 3,
HAP concentration in the emission stream (HAP ) = ppmv
e
With the efficiency, RE, of the wet scrubber, HAP , and Eq. (14),
e
HAP = HAP [1 − (RE/100)] (14)
o e
Then the outlet concentration can be determined:
HAP = ppmv
o
N = ln { [ (HAP / HAP ) (1 − (1/AF)) + (1/AF) ] } / (1−1/AF) (13)
og e o
N =
og
Using Fig. 3, determine
HAP / HAP =
e o