Absorption - Introduction to Carbon Capture and Sequestration - page 187

0 2 4 6 8 10 12 14

pH

Figure 5.3.3 Concentration of the carbonate species in water as a function of the pH

good thought experiment when vetting ideas like this is to consider the
extremes. What will happen to the solubility of CO 2 if we add a really
strong base like sodium hydroxide (NaOH)? The nice thing about adding
a strong base is that we know it will dissociate completely, meaning in this
case that 1 mole of NaOH will translate to 1 mole each of aqueous Na +
and OH − ions. If we add a large amount of OH − to the system, this will
drive the equilibrium toward bicarbonate. This will improve the mass
transfer driving force of the system, and in turn, increase the uptake of
CO 2 . But there's a problem with using a strong base: the absorption of
CO 2 as bicarbonate is a very exothermic reaction. As a result, the process
will require a signifi cant amount of heat to desorb the CO 2 from the sol-
vent in the regeneration step. That means more energy and more expense,
so a strong base probably wouldn't be a good idea for a traditional
absorber. However, this process is very effi cient in capturing CO 2 even at
very low concentrations, so it isn't a bad idea to use strong bases to cap-
ture CO 2 directly from air [5.5, 5.6]. In one of the next sections we will
discuss the energy associated with the regeneration in more detail.

Amines

The idea that weaker bases might work for carbon capture is not exactly
new. Bottoms used a similar concept in his 1930 patent, where he pro-
posed to use an aqueous solution with amines, like monoethanol amine
(MEA) , as a base. How amines, such as MEA, react depends on which