Environmental Engineering Reference
In-Depth Information
from the sediment beneath the stream. The chemicals dissipate from the stream by
either chemical reaction or biodegradation; or are removed with the outflow and by
evaporation from the surface. They may also subside into the underlying sediment by
sedimentation with solids.
A similar CSTR box model can also be developed for atmospheric transport of
chemicals (Figure 6.25b). Stationary sources such as industrial plants, waste pits,
and treatment plants provide pollutant inputs to the atmosphere. Mobile sources
such as automobiles also contribute to pollutant influx to the atmosphere. Deposition
of aerosol-associated pollutants removes pollutants from the atmosphere. Advective
flow due to winds constitutes the inflow and outflow from the CSTR. Chemical and
photochemical reactions change concentrations within the box.
Considerawell-mixedboxofdownwindlength
L
,crosswindwidth
W
,andvertical
height
H
. Figure 6.26 depicts the atmosphere of volume
V
LWH
that is homoge-
neously mixed (i.e., no concentration gradients).A large urban atmosphere with good
mixing (turbulence) and a wide distribution of emission sources in the lower atmo-
sphere or a small indoor volume that experiences excellent mixing and ventilation
can be represented by the above scheme. Global atmospheric models rely on the
assumption that the troposphere is a well-mixed CSTR. In some cases, the northern
and southern hemispheres are considered to be two separate reactors with differ-
ent degrees of mixing, but linked via chemical exchange. These models are useful
in obtaining the residence times of important species in the atmosphere (Seinfeld,
1986). For the case in Figure 6.26, a mass balance on species A within the control
volume will give the following:
=
rate of input
=
rate of output
+
rate of reaction
+
rate of accumulation.
V
d
[
]
d
t
.
(6.127)
A
G
[
A
]
0
+
w
v
A
s
[
A
]
∞
+
S
A
A
s
=
G
[
A
]+
w
p
A
s
[
A
]+
w
v
A
s
[
A
]+
r
A
V
+
w
v
[A]
∞
(Air)
H
G
G
S
A
r
A
[A]
0
[A]
(Surface soil)
W
L
FIGURE 6.26
Material balance for a well-mixed box model for an urban atmosphere.
Search WWH ::
Custom Search