Absorption - Introduction to Carbon Capture and Sequestration - page 223

We now switch the solvent to 30% MEA while keeping the L/V ratio
constant but changing the number of trays. We fi nd that because 30%
MEA has an extremely low Henry constant at 40°C, the equilibrium line is
essentially horizontal, such that only 5 stages are required for the
absorber ( Figure 5.8.3 ). We fi nd that the stripper operating on 30% MEA
requires only 8 stages, since 30% MEA has an extremely high Henry
constant at 120

C (see Figure 5.8.3 ). Hence, by switching from water to
a 30% solution of MEA while keeping L/V constant, we reduce the num-
ber of stages from 33 to 5 for the absorber, and from 27 to 8 for the
stripper.
We also consider the case in which we switch the solvent to 30%
MEA while keeping the same number of trays, but change the L/V ratio
to achieve the desired separation ( Figure 5.2.6 ). We fi nd that the L/V ratio
for the absorber drops from 3,000 to 11 when we switch from water to
30% MEA, allowing us to reduce the solvent fl ow rate by a factor of 272.
The L/V ratio for the stripper increases from 7,000 to 50,000, allowing us
to use 7 times less vapor in the stripper upon switching from water to
30% MEA ( Figure 5.8.4 ).

°

(a) Adsorber using 30% MEA (b) Stripper using 30% MEA

equilibrium line
maximum mole fraction

minimum mole fraction
operating line

stages

0.14

Bottom 0.50

0.12
0.10

0.40

Top

0.30

0.08

0.06

0.20

0.04

0.10

0.02
0

Bottom

To p

0

0 0.00002 0.00004 0.00006

0 0.00002 0.00004

Mole fraction of CO 2 in a solvent (x) Mole fraction of CO 2 in a solvent (x)

Figure 5.8.3 McCabe-Thiele diagrams for MEA (1)
McCabe-Thiele diagrams for (a) an absorber with 5 stages operating at an L/V ratio of
3,000 and (b) a stripper with 8 stages operating at an L/V ratio of 7,000; both units are
using 30% MEA as the solvent. These columns are operating at the same L/V ratios as
used for the water case shown in Figure 5.8.2. Notice that the equilibrium line in (a) has
a very small slope and essentially coincides with the x-axis.