Environmental Engineering Reference
In-Depth Information
factor of 3237.2 with the same temperature increase. This demonstrates
that the Henry constant of a 30% MEA solution is signifi cantly more tem-
perature sensitive, which means that temperature swings are effective at
controlling whether the material absorbs or releases CO
2
. This “switch-
like” behavior for CO
2
absorption as a function of temperature is neces-
sary so that we can effectively capture the CO
2
in the absorber and then
subsequently release it in the stripper.
With this physical understanding of how amines change the Henry's
law behavior of water, we are prepared to perform McCabe-Thiele analy-
ses for the two systems. Going back to our absorber/stripper design
diagram, one of the parameters which we may set within reasonable
limits is the mole fraction of CO
2
in the solvent entering the absorption
column, which is identical to that exiting the stripping column (
x
0
A
x
0
S
).
We estimate this value to be 3 × 10
−
6
based on the equilibrium line/
Henry's constant for water
.
We assume that the carrier gas in the stripping column is water vapor.
The water vapor which exits the re-boiler and enters the stripping column
has a negligible amount of CO
2
(
y
N
+
1
=
0). We can now solve for the mole
fraction of solute in the solvent exiting the absorption column using a mass
balance. This is most easily done using mole ratios, defi ned as follows:
=
A
A
A
y
y
x
A
A
A
N
+
1
1
0
Y
=
,
Y
=
, and
X
=
N
+
1
1
0
A
A
A
1
−
y
1
−
y
1
−
x
N
+
1
1
0
We can now solve for
X
N
using a mole balance:
A
x
V
A
A
A
A
N
X
=
=
X
+
Y
−
Y
N
0
N
+
1
1
N
L
1
−
x
From here, we can get the mole fraction of the CO
2
in both the
absorber and stripper columns: x
A
5.35 x 10
−
5
. Similarly, we can
construct a mole balance for the stripper column. Eventually, this yields:
x
S
=
=
S
y
V
S
S
S
S
1
Y
=
=
Y
+
X
−
X
1
N
+
1
0
n
S
L
1
−
y
1
We assume L/V is constant in our system. The L/V ratio is an adjustable
design parameter. We manually adjusted the L/V ratio until we have
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