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12.6 CO 2 Separation by Absorption 373

carbon atom. For example, 2-Amino-2-methyl-1-propanol (AMP) is a sterically
hindered primary amine and 2-piperidineethanol (PE) is a sterically hindered sec-
ondary amines.

12.6.3 Non-amine-Based Chemical Absorption

12.6.3.1 Sodium Hydroxide-Based Chemical Absorption

The chemical reactions between CO 2 and sodium hydroxide can be described as
follows:

CO 2 þ H 2 O ! H 2 CO 3 ð12:49Þ

H 2 CO 3 þ NaOH ! NaHCO 3 þ H 2 O ð12:50Þ
NaHCO 3 þ NaOH ! Na 2 CO 3 þ H 2 O ð12:51Þ

Water and CO 2 first react to form carbonic acid, which reacts with sodium
hydroxide to produce bicarbonate (NaHCO 3 ). Bicarbonate also reacts with sodium
hydroxide to produce a more stable product of carbonate. There are NaHCO 3 and
Na 2 CO 3 in the final product. Their relative quantity depends on the pH of the liquid
in the last two reactions.
Regeneration of NaOH is usually achieved by adding lime (CaO) into the final
product converting Na 2 CO 3 to CaCO 3 , followed by calcination at 870 °C or higher
to release the pure CO 2 for storage of other applications. The corresponding
chemical reactions are

CaO þ H 2 O ! CaðOHÞ ð12:52Þ
2
CaðOHÞ þ Na 2 CO 3 ! 2NaOH þ CaCO 3 ð12:53Þ
2
CaCO 3 ! CaO þ CO 2 ð12:54Þ
The calcination reaction Eq. (12.54) is energy intensive and the corresponding
enthalpy of reaction is about DH R ¼ 250 kJ=mol-CO 2 . It makes this process eco-
nomically challenging. Alternatively, sodium hydroxide can be regenerated using
sodium trititanate (Na 2 O 3TiO 2 ), which requires half the energy for regeneration
of NaOH.