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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