Geoscience Reference
In-Depth Information
Pure Water
x
2
= 0
y
2
2
Z
= packed column height
D
= column diameter
y
1
= inlet gas composition
y
2
= outlet gas composition
x
1
= outlet liquid composition
x
2
= inlet liquid composition
D
Z
D
1
Contaminated gas stream
x
1
*
y
1
= 0.02
FIGURE 18.2
Graphical representation of the packed column. (Adapted from USEPA,
Control of Gaseous
and Particulate Emissions: Self-Instructional Problem Workbook
, EPA 450/2-84-007, U.S. Environmental
Protection Agency Air Pollution Training Institute, Research Triangle Park, NC, 1984, p. 107.)
Given:
Gas mass flow rate = 5000 lb/hr
NH
3
concentration in inlet gas stream = 2.0 mol%
Scrubbing liquid = pure water
Packing type = 1-inch Raschig rings
Packing factor
F
= 160
H
OG
of the column = 2.5 ft
Henry's law constant
m
= 1.20
Density of air = 0.075 lb/ft
3
Density of water = 62.4 lb/ft
3
Viscosity of water = 1.8 cp
Generalized flooding and pressure drop correction graph (USEPA, 1984b, p. 107)
The unit operates at 60% of the flooding gas mass velocity, the actual liquid flow rate is 25% more
than the minimum, and 90% of ammonia is to be collected based on state regulations.
Solution:
What is the number of overall gas transfer units
N
OG
? Remember that the height of pack-
ing
Z
is given by
Z
= (
H
OG
)(
N
OG
)
where
Z
= Height of packing.
H
OG
= Height of an overall gas transfer unit.
N
OG
= Number of transfer units.
Because
H
OG
is given, we only need the value of
N
OG
to calculate
Z
.
N
OG
is a function of both the
liquid and gas flow rates; however, the value is usually available for most air pollution applica-
tions. What is the equilibrium outlet liquid composition (
x
1
) and the outlet gas composition (
y
2
)
for 90% removal? Recall that we need the inlet and outlet concentrations (mole fractions) of both
streams. Calculate the equilibrium outlet concentration
x
1
* at
y
1
= 0.02. According to He n r y's
law,
x
1
* at
y
1
/
m
. The equilibrium outlet liquid composition is needed to calculate the minimum
L
m
/
G
m
, where
Search WWH ::
Custom Search