Environmental Engineering Reference
In-Depth Information
phase. The additional removal mechanism by solvent extraction has been known to
effectively increase the rate of removal from the aqueous phase. This process of flota-
tion and extraction using air bubbles into an organic solvent layer is termed
solvent
sublation
. The process of solvent sublation has been investigated in bubble column
reactors (see Valsaraj, 1994; Valsaraj and Thibodeaux, 1987 for details). It has been
shown to be effective in removing several organic compounds and metal ions.
E
XAMPLE
6.11 R
EACTOR
S
IZING FOR A
D
ESIRED
R
EMOVAL IN
A
IR
S
TRIPPING
1,2-Dichloroethane (DCA) from a contaminated groundwater is to be removed using
air stripping. Obtain the size of a bubble column reactor required for 80% removal of
DCA from 10,000 gallons per day of groundwater using a 1-ft-diameter column.
The air-water partition constant
K
aw
for DCA is 0.056.
Q
L
=
10,000 gpd
=
0.027 m
3
/min. Consider a ratio of
Q
g
/Q
L
=
100. Hence
Q
g
=
2.7 m
3
/min. Since the
column radius
r
c
=
0.152 m,
A
c
= π
r
c
=
0.0729 m
2
. Hence the superficial gas velo-
city
u
g
=
Q
g
/A
c
=
0.617 m/s. This parameter appears in the correlation for
ε
g
and
k
l
a
.
Although several correlations are available for bubble column reactors, we choose the
one recommended by Shah et al. (1982):
ε
g
(
1
− ε
g
)
4
=
0.2
α
u
g
(gD
c
)
1
/
2
and
a
=
1
3
D
c
α
1
/
8
1
/
12
1
/
2
1
/
10
1.13
g
β
β
ε
,
where
α =
gD
c
ρ
L
/
σ
and
β =
gD
c
/
ν
L
. The different parameters are
g
=
9.8 m
2
/s,
1000 kg/m
3
,
10
−
6
m
2
/s, and
ρ
L
=
ν
L
=
1
×
σ =
0.072 N/m. Using these parameters,
ε
g
/(
1
− ε
g
)
4
=
2.087.
A trial and error solution gives
ε
g
=
0.35. Hence, we obtain
a
=
525 m
−
1
. This
gives
a/
ε
g
=
1500
=
6
/D
b
, and the average bubble diameter in the column is
D
b
=
0.004 m. Since the aqueous-phase diffusivity of DCA is 9.9
×
10
−
10
m
2
/s (see the
Wilke-Chang correlation—Reid et al., 1987), we obtain
k
w
a
=
0.076 s
−
1
. As before
1
/K
w
a
=
(
1
/k
w
a)
+
(
1
/k
g
aK
aw
)
, where
k
w
and
k
g
are individual phase mass trans-
fer coefficients. For predominantly liquid-phase-controlled chemicals (such as DCA),
K
w
a
=
k
w
a
. Thus for the given conditions
S
=
5.6. If the desired removal in the
CSTR is
R
CSTR
=
0.80, we have
φ
=
1
−
exp
φ
H
s
)
=
0.71, and
φ
H
s
=
1.24. Since
φ
H
s
=
K
w
aV
w
/K
aw
Q
g
, we obtain
V
w
=
0.033 m
3
. Therefore, the height of the reactor
is
H
s
=
V
w
/A
c
=
0.45 m.
If back mixing is insignificant, the bubble column will behave as a plug-flow reactor.
In such a case, for
R
PFR
=
0.80,
φ
H
s
=
0.34, which gives
V
w
=
0.011 m
3
, and hence
H
s
=
0.15 m. Thus if axial back mixing is avoided in the bubble column, a high degree
of removal can be obtained in small reactors. In reality, this is rarely achieved because
at large air rates the bubbles are larger (low
a
values) and the axial dispersion increases.
6.2.2.2
Oxidation Reactor
In this section, we will discuss the application of a redox process for wastewater
treatment. Consider the oxidation of organic compounds by ozone. Ozone (O
3
)isan
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