Page 172 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

146 5 Principles for Gas Separation

Combine Eq. (5.52) and (5.53), we have:

N ¼ k x aV x i x ss Þ ¼ L x ss þ kx ss ð5:54Þ
ð
1 x ss
For cases where x ss 1; 1 x ss 1, then

k y aV y i y ss Þ ¼ Ly ss þ ky ss ð5:55Þ
ð
Solving this equation we can get the steady state concentration of the target gas
in the liquid phase

x ss k x aV
¼ ð5:56Þ
x i k x aV þ L þ k

This equation indicates that there are three factors that affect the steady state

absorption ratio, which is defined as x ss =x i , and they are k x aV; L and k. They stand
for the effects of interfacial mass transfer, liquid flow rate, and kinetic rate of
chemical reaction, respectively. Practically, it is challenging to determine the mole
fraction at the interface, x i ; although it could be estimated by extensive theoretical
analysis.

5.2.4.1 Enhanced Absorption Factor, e
A more practical approach to this problem is to employ an enhanced absorption
factor, e.It isdefined as the ratio of extra amount of target gases absorbed into the
liquid by chemical absorption to that by physical absorptions.
x 0
e ¼ ð5:57Þ
x
0
where x stands for the extra absorption resulted from chemical absorption. The
theoretical enhanced absorption factor could be very high, but the actual value
depends on the design and operation of the tower. Then, with chemical absorption
considered, Eq. (5.19) becomes

0
yG y 1 G 1 ¼ x þ xð ÞL x 1 L 1 ¼ x 1 þ eÞL x 1 L 1 ð5:58Þ
ð
It indicates that the amount of liquid flow rate is decreased by a factor of 1 þ eÞ:
ð
Then all results obtained by the analysis for physical absorption can be applied to

chemical absorption, by multiply L with a factor of 1 þ eÞ. For example, Eq. (5.21)
ð
leads to