Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 324

morning the stability class is D . At 1000 m, then σ y = 70 m and σ z = 30 m.
Hence [A] (1000 m, 0, 0 ) = ( 3700 g/s)/(3.14)(2 m/s)(70 m) ( 30 m ) = 0.280 g/m 3 =

280 mg/m 3 . This is higher than the TLV of benzene (32 mg/m 3 ) .

6.3.2 A IR P OLLUTION C ONTROL
There are two kinds of industrial air pollutants: particulates and vapors (gases). The
control of these two classes of air pollutants is dependent on their mechanism of cap-
tureinspecificreactors.Processesthatremoveparticulatesrelyonphysicalforces,that
is, gravity, impaction, electrical forces, or diffusion to bring about separation. Vapors
and gases are separated using chemical processes such as adsorption, absorption, and
thermal processes. In the selection and design of any method, concepts from reaction
kinetics and mass transfer play important roles. We will describe selected methods
with a view to illustrating the applications of kinetics and mass transfer theories.
Gases and vapors in polluted air streams are treated by chemical methods such as
absorption into a liquid stream (water), adsorption onto a solid adsorbent (activated
carbon, silica, alumina), or by thermal means (incineration, combustion). We shall
discuss each of these with the aim of illustrating the chemical kinetics and mass
transfer aspects.

6.3.2.1 Adsorption

Adsorption is a surface phenomenon as we discussed in Chapter 3. Adsorption on
solids is an effective technology for pollutant removal from water or air. The design
of both water treatment and air treatment is similar. Hence, our discussion in this
instance for air pollution is the same as for water pollution. Adsorption from both
air and water is conducted in large columns packed with powdered sorbent such as
activated carbon. Feed stream containing the pollutant is brought into contact with
a fresh bed of carbon. As the pollutant moves through the bed, it gets adsorbed and
the effluent air stream is free of the pollutant. Once the bed is fully saturated, the
pollutant breaks through and the effluent stream has the same concentration as the
influent feed. The progression of the bed saturation is shown in Figure 6.33. At any
instant in the bed, there are three distinctly discernible regions. At the point where
the pollutant stream enters, the bed is quickly saturated. This is called the saturated
zone . This zone is no longer effective in adsorption.Ahead is a zone where adsorption
is most active. This small band is called the active ( adsorption ) zone . Farther from
this zone is the unused bed where there is no pollutant. Provided the feed rate is
constant, the adsorption zone slowly moves through the bed until it reaches the exit,
at which stage breakthrough of the pollutant is attained. The rate of movement of the
adsorption zone gives us information on the breakthrough time.
Ifthewavefrontisideal,itwillbesharpasshowninFigure6.34.However,because
of axial dispersion (nonideal flow), the wave front will broaden (Figure 6.34). The
velocity of wave front advance can be obtained by determining the capacity of the bed
for a given feed rate G F at a concentration [A] F in the feed stream. A mass balance