Absorption - Introduction to Carbon Capture and Sequestration - page 204

which shows that, similarly to diffusion limitation, reaction limitation gives
an effective mass transport coeffi cient that causes a shift of the equilib-
rium line in the McCabe-Thiele diagram ( Figure 5.3.1 ).
Additional constraints
Now we also consider some common-sense limitations. For example, as
mentioned previously, the solvents we focus on in absorption need to be
transported from the bottom of the absorber to the top of the stripper.
This implies that we have to use energy to move large solvent quantities
around, so naturally we would prefer a solvent with a low viscosity; a
polymer gel that can take up CO 2 may sound great, but in the end it sim-
ply won't work with certain types of designs.
Absorption relies on gas molecules moving from the solvent to gas
phase. CO 2 in its gas phase will reach the liquid-gas interface in the froth,
where it will dissolve in the solvent with a certain rate of mass-transfer.
Clever engineering can help us increase the interfacial area by several
orders of magnitude, but cannot eliminate the problem of mass transport
rates. Even if a potential solvent has great solubility, we will still encoun-
ter problems if its transfer is very slow. If we need to have extremely slow
fl uxes of gas and liquid in order to give the CO 2 molecules enough time
to move from the gas phase to the liquid phase, it would completely nul-
lify the positive effect of the solubility.
There are additional constraints specifi c to using amine solvents in
our absorbers. So far we have looked at fl ue gasses as binary mixtures
of CO 2 and N 2 . But in reality fl ues gasses contain many other compo-
nents, such as O 2 or SO x . It is well known that these molecules react with
amines, which causes our solvent to degenerate over time; eventually we
have to change out our solvent. Amines also react with carbon steel, so
we need to use expensive hydrated steel in our absorber design. Another
issue is that, to the best of our knowledge, nobody has published much
about the potential consequences should amines like MEA escape from
these absorber/strippers, either to the air or to the water. Without know-
ing the toxicology of your solvent, you are at risk to create a situation like
we saw with MTBE in gasoline, for example, where, if deployed on a
large scale, secondary environmental effects could pose a serious
problem.