Page 390 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

12.6 CO 2 Separation by Absorption 369

The rate of reaction for R3 and R4 are, respectively,

r 3 ¼ k OH CO 2 Š OH½ Š ð12:35Þ
½
½
r 4 ¼ k H 2 O CO 2 Š H 2 O½ Š ð12:36Þ
Combination of Eqs. (12.33), (12.35) and (12.36) leads to the overall CO 2
reaction rate by summation of r 1;2 ; r 3 and r 4 :

½
k f1 AmHŠ
¼ þ k OH OH½ Š þ k H 2 O H 2 OŠ ½ CO 2 Š ð12:37Þ
½
r CO 2
1 þ k b1 = k B Bs½ð ŠÞ
Denote the part in the { } bracket as observed reaction rate with respect to
:
CO 2 ; k CO 2
k f1 AmHŠ
½

¼ þ k OH OH½ Š þ k H 2 O H 2 OŠ ð12:38Þ
½
k CO 2
1 þ k b1 = k B Bs½ð ŠÞ
where the first term is called apparent reaction rate constant
½
k f1 AmHŠ
k app ¼ ð12:39Þ
1 þ k b1 = k B Bs½ð ŠÞ

Similar to the denotation of k OH OH½ Š þ k H 2 O H 2 OŠ, we can define the reaction
½
rate constant with respect to amine as
k f1
k AmH ¼ ð12:40Þ
½
ð
1 þ k b1 = k B BsŠÞ
Then Eq. (12.38) becomes,

½
½
½
¼ k AmH AmHŠ þ k OH OH Š þ k H 2 O H 2 OŠ ð12:41Þ
k CO 2
And Eq. (12.37) becomes
½ CO 2 Š ð12:42Þ
r CO 2 ¼ k CO 2
Many researchers have reported k AmH for different amines at different conditions
and some are listed in Table 12.6.

12.6.2.2 CO 2 Absorption Rate

The actual rate of CO 2 absorption into amine is an important indicator of energy
consumption of the solvent. A fast rate of CO 2 absorption minimizes energy use in