Wet Scrubbers for Emission Control - Mathematics and Statistics for the Environment - page 548

Outlet NH 3 gas composition = 0.1 mol%
Air density = 0.0743 lb/ft 3
Molecular weight of air = 29
Henry's law constant m = 0.972
Molecular weight of water = 18
Emission regulation = 0.1% NH 3
Solution: What is the number of overall gas transfer units, N OG ? The number of overall gas trans-
fer units N OG is used in calculating the packing height requirements. This number is a function of
the extent of the desired separation and the magnitude of the driving force through the column
(the displacement of the operating line from the equilibrium line). Calculate the gas molar flow
rate ( G m ) and liquid molar flow rate ( L m ) in lb-mol/ft 2 -hr. Calculate the cross-sectional area of the
tower S in ft 2 :
S = π D 2 /4
where
S = Cross-sectional area of the tower.
D = Diameter of the tower.
S = π D 2 /4 = [π × (3.57) 2 ]/4 = 10.0 ft 2
Calculate gas molar flow rate ( G m ) in lb-mol/ft 2 -hr:
G m = Qp / SM
where
G m = Gas molar flow rate (lb-mol/ft 2 -hr).
Q = Volumetric flow rate of gas stream.
p = Density of air.
S = Cross-sectional area of the tower.
M = Molecular weight of air.
G m = Qp / SM = (1575 × 0.0743)/(10.0 × 29) = 0.404 lb-mol/ft 2 -min = 24.2 lb-mol/ft 2 -hr
Calculate the liquid molar flow rate ( L m ) in lb-mol/ft 2 -hr:
L m = L / M L
where
L m = Liquid molar flow rate (lb-mol/ft 2 -hr).
L = Liquid mass velocity (lb/ft 2 -hr).
M L = Liquid molecular weight.
L m = L / M L = (1000)/(18) = 55.6 lb mol/ft 2 -hr
Calculate the value of mG m / L m , where
m = Henry's law constant.
G m = Gas molar flow rate (lb-mol/ft 2 -hr).
L m = Liquid molar flow rate (lb-mol/ft 2 -hr).
mG m / L m = (0.972)(24.2/55.6) = 0.423