Absorption - Introduction to Carbon Capture and Sequestration - page 188

HO

NH 2

monoethanol amine

Chemical Formula: C 2 H 7 NO

ones we are using. To better grasp this concept, let's do some basic
review on the structure of amines.
Amines are compound derivatives of ammonia (NH 3 ), which consists
of a central nitrogen atom attached to one lone pair of electrons and up
to three functional groups, or R-groups. R-groups consist of a non-
hydrogen substituent, such as a methyl (—CH 3 ) or other alkane group.
Amines are classifi ed as either primary, secondary, or tertiary, depending
on the number of R-groups attached to each molecule. The number and
size of the R-groups that an amine has will infl uence the molecule's prop-
erties and ability to react with other compounds. This will be important
as we decide what type of amines we can “tune” to make better solvents
for carbon absorption.

H

R 3 H

R 1 N

R 1 R 1

N N

H R 2 R 2

Primary
amine

Secondary
amine

Tertiary
amine

The amines are weak bases relative to NaOH. As we have seen, 1 mole
of NaOH will completely dissociate and give us 1 mole each of aqueous
Na + and OH − ions. For amines, we can represent equilibrium in solution as:

RNH 3 + HO −

H 2 O + RNH 2 +

As an example, a 1 molar MEA solution has a pH of approxi-
mately 11.7 (compared to a pH of 14 for 1 molar NaOH). Because amines
are weak bases, they infl uence the solubility of CO 2 by shifting the equi-
librium toward the formation of carbonate ions. The process is slightly
different depending on the structure of the amine. The reaction of CO 2
with primary and secondary amines, shown in Figure 5.3.4 , involves the