Environmental Engineering Reference
In-Depth Information
concentration of CO
2
that we wish to achieve in the fl ue gas exhaust. We
know from Section 4.2 that, although it would be nice to capture 100%
of the CO
2
in the absorption column, it is relatively expensive to separate
the last 10%. We can capture 90% of the CO
2
with reasonable effi ciency,
say, if our coal-fi red power plant is bringing in 70 x 10
8
g of CO
2
per day.
The mass balance will tell us the concentration of CO
2
in the solvent at
different plates in the absorber and at the bottom,
x
sat
CO
2
.
Let us now start by looking at a single plate P (see
Figure 5.2.6
).
Henry's law relates the solvent concentration
x
CO
2
(P) to the gas concen-
tration
y
CO
2
(P) on this plate. More colloquially, Henry's law tells us that in
theory all froths behind a weir are described by a point on the equilibrium
line. The mass balance equation relates the solvent concentration
x
CO
2
(P)
in the liquid descending from a plate to the concentration in the gas
ascending from the plate below,
y
CO
2
(P-1). The operating line, then,
assures us that mass is conserved between any two plates. A geometric
construct from this graph shows the progress of CO
2
absorption as the
gas moves through the absorber. CO
2
entering plate P has mole fraction
y
CO
2
(P-1). It achieves thermodynamic equilibrium with the froth above
plate P, then exits plate P with
y
CO
2
(P). Graphically we form a triangle,
which is called a stage, representing a plate in our absorber. We can
Figure 5.2.6
Mass balance and thermodynamic equilibrium on a plate
This
animation can be viewed at
: http://www.worldscientifi c.com/worldscibooks/
10.1142/p911#t=suppl
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