Environmental Engineering Reference
In-Depth Information
further processing to meet the current permit standards. The liquid stream, once treated
to remove radioactivity, is then discarded.
Anew permit with the state of New Mexico will require this liquid stream to meet
the domestic water supply limit for dissolved NO
3
of 10 mg
L measured as
N
(in order
to comply with the federal
Clean Water Act
and the
New Mexico Water Quality Act
). A
nitric-acid fractionation (distillation column) is to be installed to recover these nitrates
from the discard stream prior to disposal. The column (Figure 4.22) will include a total
condenser and partial reboiler and will operate with a reflux ratio of 0.4.
A 1.0 L
/
min stream ofa2M(22.3 wt% acid) HNO
3
solution will be fed to a preheater
prior to entering the distillation column. The preheater will raise the temperature of
the feed to just below its bubble point. The stream which is to be disposed will be
treated to less than 0.007 M HNO
3
(450 ppm). The bottoms from the column must be
concentrated to 12 M (62.9 wt% acid) HNO
3
before it can be reused in the plutonium
stabilization process. An azeotrope exists at 15.6 M HNO
3
(45 wt%). Find the number
of equilibrium stages in the column, assuming the overall efficiency is about 0.7.
Solution:
The equilibrium data are given in Figure 4.23. Note that the mole fractions are H
2
O,
not HNO
3
. The concentrations given above are converted to mole fractions of H
2
O
using the mass flowrates listed in the summary sheet from LANL (Figure 4.24). For
example, the mole fraction of water in the feed,
x
F
,isfound:
/
52
.
932 kg H
2
O
hr
kgmol
18
01 kg
=
2
.
94 kgmol H
2
O
/
hr
.
15
.
155 kg HNO
3
hr
kgmol
63
01 kg
=
0
.
24 kgmol HNO
3
/
hr
.
.
2
.
94
∴
x
F
=
24
=
0
.
92
.
2
.
94
+
0
.
Similarly,
x
D
=
1, and
x
B
=
0.67.
Feed line
One point on the feed line is
y
=
x
=
x
D
. Since the feed is a saturated liquid near its
bubble point,
q
=
1 and the slope of the feed line is infinity.
Top operating line
One point on the top operating line is
y
=
x
=
x
D
. The slope is
L
/
D
R
D
R
D
+
0
.
4
1
=
1
=
1
=
0
.
29
.
L
/
D
+
0
.
4
+
Bottom operating line
A point on the bottom operating line is
y
0.67. The line also goes through
the point where the top operating line and the feed line cross. Alternatively, the slope
of the bottom operating line could have been found as:
=
x
=
x
B
=
L
+
F
L
+
F
L
/
D
+
F
/
D
0
.
4
+
3
.
179
/
2
.
442
=
D
=
=
=
1
.
21
.
V
L
+
L
/
D
+
1
0
.
4
+
1
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