Applications of Chemical Kinetics in Environmental Systems - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 330

The reaction zone in the incinerator can be modeled as a plug-flow reactor. The
overall destruction efficiency is given by

[ A ] exit

e − k τ res . η = 1 −

] feed =

1 − (6.158)

[

A

E XAMPLE 6.19 I NCINERATOR FOR A IR P OLLUTION C ONTROL

At what air velocity should a toluene/air mixture be introduced in a 20-ft incinerator to
obtain 99.99% efficiency at a temperature of 1300 ◦ F?
T = 1300 ◦ F = ( 5 / 9 )( 1300 ) + 255.4 = 977 K.FromTable6.11, A = 2.3 × 10 13 s − 1
and E a = 236 kJ/mol. Hence k = 2.3 × 10 13 exp [− 236 /( 8.31 × 10 − 3 )( 977 ) ]=
5.3 s − 1 . Hence E = 1 − exp ( − k τ res ) = 0.9999, k τ res = 9.21, τ res = 1.74 s, and since
τ res = Z s /V g ,wehave V g = 11.5 ft/s.

6.3.3 A TMOSPHERIC P ROCESSES
In this section, we shall analyze gas- and liquid-phase chemical reactions in relation
to various atmospheric processes. These applications involve the use of the principles
of chemical kinetics and transport that we discussed in Chapter 5 and also in previous
sections of this chapter.

6.3.3.1 Reactions in Aqueous Droplets

Acid rain is a problem in some industrialized nations (Hutterman, 1994). It has been
particularly severe in parts of Western Europe and the northeastern United States.
Similar issues have become prominent in several developing nations (e.g., India,
China). In India, acid rain has defaced one of the cherished monuments of the world,
the Taj Mahal. Its exquisite marble sculpture is eroding due to uncontrolled SO 2
emissions from local industries. In Germany, the beautiful Black Forest is severely
affected by acid rain. In the United States, the forests of the northeast are known to
be affected by acid precipitation involving pollutants from the region's industrial belt
(BrickerandRice,1993).Table6.8listsspecificpollutantsandaverageconcentrations
in the northeastern United States. pH values as low as 4 with high concentrations of
SO 2 4 and NO 3 have been observed in parts of the northeastern United States.
Oxides of sulfur and nitrogen released to the atmosphere from fossil fuel com-
bustion form strong acids by combining with the atmospheric moisture. These acids
include H 2 SO 4 and HNO 3 . They react with strong bases (mainly NH 3 and CaCO 3 )
and also associate with atmospheric dust (aerosols). Below cloud scavenging by rain
and fog wash these particles to the surface with a low pH. This is called acid rain .
In the absence of strong acids in the atmosphere, the pH of rainwater is set by the
dissolution of CO 2 to form carbonic acid (H 2 CO 3 ) . The equilibrium is

H + + HCO 3 . CO 2 + H 2 O H 2 CO 3 −→ (6.159)