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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
 
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