Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 327

Since the integral is undefined at 0 and 1, we take the limits as 0.01 (leading

edge) and 0.99 (trailing edge) so that [A] approaches 1% of the limiting value at both
limits. We then have the following definite integral:

η =

ln 1

.

1
1

( 0.01 ) n − 1
1 − ( 0.99 ) n − 1

−

S t = 4.505 + (6.156)

− n

If we assume that the mass in the adsorption zone is much smaller than the mass in
the saturated zone, the breakthrough time can be obtained:

H B − δ
V ads

t B = ,

where H B is the total height of the adsorbent bed.

E XAMPLE 6.18 B REAKTHROUGH T IME FOR AN A CTIVATED C ARBON A DSORBER

Obtain the breakthrough time for a pollutant in a carbon bed at an air mass velocity of
2.0 kg/s at 1.1 bar and 30 ◦ C with an influent pollutant concentration of 0.008 kg/m 3 .
The Freundlich isotherm parameters for the pollutant are K F = 500 kg/m 3 and n = 2.
The mass transfer coefficient is 50 s − 1 . The bed length is 2 m with a cross-sectional
area of 6 m 2 and a bulk density of 500 kg/m 3 .
Gasdensity, ρ g = P/RT = ( 1.1 )( 29 )/( 303 )( 0.083 ) = 1.3 kg/m 3 . V ads = 2.0 ( 500 ) 0.5
(0.008) 0.5 /( 1.3 )( 500 )( 6 ) = 1.0 × 10 − 3 m/s. Hence

4.595 +

1.0 ln 1 − ( 0.01 ) 1.0

2.0
( 50 )( 6 )( 1.3 )

1

δ = = 0.047 m.

1 − ( 0.99 ) 1.0

Breakthrough time, t B = ( 2.0 − 0.047 )/ 1.0 × 10 − 3 = 1953 s = 0.54 h.

6.3.2.2 Thermal Destruction

Organic compounds that form a large fraction of air pollutants in industrial effluents
can be oxidized in incinerators at high temperatures. These devices are also called
thermal oxidizers or afterburners . Both direct thermal oxidation and catalytic oxida-
tion are practiced in the industry.A schematic of a direct thermal oxidizer for gases is
shown in Figure 6.35. Thermal incineration is also used in destroying organic com-
pounds in a liquid stream or sludge.A rotary kiln incinerator is used for the latter and
a schematic is shown in Figure 6.36.
Cooper and Alley (1994) present the theory and design of a typical incinerator.
Combustion is the chemical process of rapid reaction of oxygen with chemical com-
pounds resulting in heat (Chapter 2). Most fuels are made of C and H, but may also
include other elements (S, P, N, and Cl).Although the exact mechanism of combustion