Gas Emission Control - Mathematics and Statistics for the Environment - page 460

From Figure 16.8, determine the flooding line from 1.22. The ordinate ε is 0.019. From Equation
16.4, calculate G ′:

05 .

′ = ×××
×× ()

ε PPg

g l c

G

φµ 02

.

F

l

For water, ϕ = 1.0, and the liquid viscosity is equal to 0.0008 Pa-s. For 2-inch Intalox™ saddles,
F = 40 ft 2 /ft 3 or 131 m 2 /m 3 and g c = 9.82 kg/m 3 ⋅sec.

05 .

0 019 . ×××

× (

117 . 1000 982 .

2

G

′ =

= 263 . kg/m ec at flooding ⋅

)

02 .

131 10 . 0 0008 .

Now calculate the actual gas flow rate per unit area:

G ′ operating = ƒ G ′ looding = 0.75 × 2.63 = 1.97 kg/(m 2 -s)

Finally, calculate the tower diameter:

Gasflowrate 102 . kg /min1min/60 sec

1.97 kg/m

×

Towerarea = =

2

G operating ⋅ sec

0.5

Towerdiame ter = 1.13 A = 1.05 m, or at least1m(3.5 ft)

16.3.4.2 Sizing the Packed Tower Absorber Height
The height of a packed tower refers to the depth of packing material needed to accomplish the
required removal efficiency. The more difficult the separations, the larger the packing height
required. For example, a much larger packing height would be required to remove SO 2 than to
remove Cl 2 from an exhaust stream using water as the absorbent. This is because Cl 2 is more soluble
in water than SO 2 . Determining the proper height of packing is important since it affects both the
rate and efficiency of absorption (USEPA, 1981, p. 4-26). The required packing height of the tower
can be expressed as

Z = HTU × NTU (16.9)

where
Z = Height of packing.
HTU = Height of a transfer unit.
NTU = Number of transfer units.

The concept of a transfer unit comes from the operation of tray (tray/plate) tower absorbers.
Discrete stages (trays or plates) of separation occur in tray/plate tower units. These stages can be
visualized as a transfer unit with the number and height of each giving the total tower height.
Although packed columns operate as one continuous separation process, in design terminology it
is treated as if it were broken into discrete sections (height of a transfer unit). The number and the
height of a transfer unit are based on either the gas or liquid phase. Equation 16.9 can be modified
to yield Equation 16.10:

Z = N OG H OG = N OL H OL (16.10)