Geoscience Reference
In-Depth Information
Calculate the value of (
y
1
- mx
1
)/(
y
2
- mx
2
), where
y
1
= Inlet gas mole fraction.
m
= Henry's law constant.
x
1
= Outlet liquid mole fraction.
y
2
= Outlet gas mole fraction.
x
2
= Inlet liquid mole fraction.
(
y
1
-
mx
1
)/(
y
2
-
mx
2
) = [0.02 - (0.972)(0)]/[0.001 - (0.972)(0)] = 20.0
Determine the value of
N
OG
and use the values of (
y
1
- mx
2
)/(
y
2
-
mx
2
) and
mG
m
/
L
m
to find the value
of
N
OG
:
N
OG
= 4.3
What is the height of an overall gas transfer unit
H
OG
? The height of an overall gas transfer unit
H
OG
is also used in calculating packing height requirements.
H
OG
values in air pollution are almost
always based on experience.
H
OG
is a strong function of the solvent viscosity and difficulty of sepa-
ration, increasing with increasing values of both. Calculate the gas mass velocity (
G
) in lb/ft
2
-hr:
G
=
pQ
/
S
where
G
= Gas mass velocity (lb/ft
2
-hr).
p
= Density of air.
S
= Cross-sectional area of the tower.
G = pQ
/
S
= (1575 × 0.0743)/10.0 = 11.7 lb/ft
2
-min = 702 lb/ft
2
-hr
The
H
OG
value is 2.2 ft.
What is the required packed column height (
Z
) in ft?
Z
= (
N
OG
)(
H
OG
)
where
Z
= Height of packing.
H
OG
= Height of an overall gas transfer unit.
N
OG
= Number of transfer units.
Z = N
OG
×
H
OG
= 4.3 × 2.2 = 9.46 ft
Compare the packed column height of 8 ft specified by Pollution Unlimited, Inc., to the height cal-
culated above. What is our recommendation? Disapproval, because the calculated height (9.46 ft) is
higher than that (8 ft) proposed by the company.
18.4.4 p
aCKed
C
olumn
h
eight
and
d
iameter
C
alCulations
■
EXAMPLE 18.10
Problem:
A packed column is designed to absorb ammonia from a gas stream. Given the operating
conditions and type of packing below (see Figure 18.2), calculate the height of packing and column
diameter (USEPA, 1984b, p. 106).
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