Gas Emission Control - Mathematics and Statistics for the Environment - page 467

YmX
YmX

−
−

mG
L

+

mG
L

1 2 m m

m

ln 1 −

2 2 m

N =

p

L
mG

m
m

ln

0030
0 003

.
.

−
−

42 735
204

. × . 42 735

204

. × .

+

ln 1 −

0

N =

p

204
42 735

ln

. × .

= 394theoreticallplates .

Assuming that the overall plate efficiency is 70%, the actual number of plates is

Actual number of plates = 3.94/0.7 = 5.6 (or 6) plates

The height of the tower is given by

Z = N p + Top height

where N p is the tray spacing. The top height is the distance (freeboard) over the top plate that allows
the gas-vapor mixture to separate. This distance is usually the same as the tray spacing.

Z = (6 plates × 0.53 m) + 0.53 m = 3.71 m

Note that this height is approximately the same as that predicted for the packed tower in Example
16.4. This is logical since both the packed and plate towers are efficient gas absorption devices.
However, due to many assumptions, no generalization should be made.

16.4 ADSORPTION
Adsorption is a mass transfer process that involves passing a stream of effluent gas through the
surface of prepared porous solids ( adsorbents ). The surfaces of the porous solid substance attract
and hold the gas ( adsorbate ) by either physical or chemical adsorption. Adsorption occurs on the
internal surfaces of the particles (USEPA, 1981, p. 5-1). In physical adsorption (a readily reversible
process), a gas molecule adheres to the surface of the solid because of an imbalance of electron
distribution. In chemical adsorption (not readily reversible), once the gas molecule adheres to the
surface, it reacts chemically with it.
Several materials possess adsorptive properties. These materials include activated carbon, alu-
mina, bone char, magnesia, silica gel, molecular sieves, strontium sulfate, and others. The most
important adsorbent for air pollution control is activated charcoal. The surface area of activated
charcoal will preferentially adsorb hydrocarbon vapors and odorous organic compounds from an
airstream.
In an adsorption system (in contrast to the absorption system where the collected contaminant is
continuously removed by flowing liquid), the collected contaminant remains in the adsorption bed.
The most common adsorption system is the fixed-bed adsorber, which can be contained in either a
vertical or horizontal cylindrical shell. The adsorbent (usually activated carbon) is arranged in beds
or trays in layers about 0.5 inch thick. Multiple-beds may be used. In multiple-bed systems, one or
more beds are adsorbing vapors, while the other bed is being regenerated.