Environmental Engineering Reference
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Figure 5.8.1 The absorption and stripping column
The absorption column: L = liquid fl ow rate, V A , V S = gas fl ow rate in the absorption/
stripping unit, x , y represent the mole fraction of CO 2 in all the variables, the super-
script “ A ” refers to the absorption column. For the stripping unit, we will use the
superscript “ S ”:
x A , x S = CO 2 in liquid entering the absorber/stripper
x A , x S = CO 2 in liquid at stage N in the absorber/stripper
y N + 1 , y N + 1 = CO 2 in gas entering the absorber/stripper
y A , y S = CO 2 in gas leaving the absorber/stripper
Note that the labeling of the stage is different from that in the previous section, here the
top of the column is stage 1.
Henry constants for these two solvents. Henry constants describe the
partial pressure of a solute (in this case CO 2 ) which is in equilibrium with
a liquid; a smaller Henry constant indicates that the liquid is more
effective at absorbing CO 2 . At 40°C, the Henry constant for CO 2 in water
is 2,397 atm; the Henry constant for CO 2 in 30% MEA is 10.1 atm [5.16].
Hence, for a given partial pressure of CO 2 in equilibrium with a liquid at
40°C, a 30% MEA solution will contain over two orders of magnitude
more CO 2 compared to water alone. As a result, a 30% MEA solution can
capture a signifi cantly larger amount of CO 2 .
Additionally, it is important to understand how the Henry constant
changes with temperature. When the temperature of pure water is
increased from 40°C to 120°C, its Henry constant increases by a factor
of 5 while the Henry constant of a 30% MEA solution increases by a
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