Environmental Engineering Reference
In-Depth Information
The calculation can also be performed using the Kremser equation:
ln
{
[1
−
(500
/
750)][0
.
1
−
0
/
0
.
01
−
0]
+
(500
/
700)
}
N
=
=
3
.
4
.
ln(750
/
500)
Rounding up, it is again determined that
four equilibrium stages
are needed.
[Note that this analysis would be appropriate for the case of a more concentrated H
2
S
stream entering the column, as long as the equilibrium relationship still holds at that
concentration.]
0.2
0.18
0.16
0.14
0.12
Operating line
0.1
4
0.08
0.06
3
Equilibrium line
0.04
2
0.02
1
0
0
2
4
6
8
10
12
14
16
18
20
x
H
2
S
(× 10
5
)
Figure 3.31
McCabe-Thiele graphical method for determining the number of
equilibrium stages, Example 3.6.
3.7
Minimum number of stages
Another important concept is the calculation of the minimum number of equilibrium stages
to perform a given separation. This is a theoretical concept since it occurs when one or
both flowrates in a countercurrent cascade are infinite. It is useful to calculate since the
actual number of stages will be larger (i.e., this is a limit). An analytical equation can be
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