Page 149 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

5.1 Adsorption 123

Table 5.3 Coconut-based activated carbon reported [14]
3
2
3
AC type Density (g/cm ) Surface area (m /g) Pore volume (cm /g)
A 0.38–0.44 1,500–1,625 0.9–1
B 0.44 1,270 0.7
C 0.41 1,090 0.94
D 0.45 1,098 0.57
E 0.43 1,240 0.65

5.1.3.2 Freundlich Isotherm

Freundlich isotherm is also the empirical model and is described as

1
M eq ¼ K F C n ð5:6Þ
where K F and n are constants for a specific adsorbate–adsorbent system at certain
condition. A logarithm conversion on both side transforms this equation into a
linear one as

1
log M eq ¼ log K F þ log C ð5:7Þ
10
10
10
n
Again this linear relationship allows the coefficients to be determined by linear
regression of a few experimental data points. M eq ¼ K F when C ¼ 1, and the slope
of the straight line is 1=n.
Although both Langmuir and Freundlich models are empirical, there are some
differences. The Langmuir isotherm is a model with some theoretical analyses and
assumptions. It assumes reversible adsorption and desorption of the adsorbate
molecules. The Freundlich isotherm is an empirical model without assumption. In
general, the Langmuir isotherm works well for typical single component and high
adsorbate concentration. The Freundlich isotherm can be used for mixtures of
compounds and it agrees well with experimental data.
Freundlich is more relevant to air emission studies where air pollutants are
diluted. With a ¼ log K F and b ¼ 1=n, Eq. (5.7) becomes
10
log M eq ¼ a þ blog C ð5:8Þ
10
10
For a greater accuracy, Yaws et al. [16]refined the model with one more term
2
log M eq;g=100 g ¼ a þ blog C ppmv þ d log C ppmv ð5:9Þ
10
10
10
The values of a, b,and d are pollutant specific. Experimental data obtained using
243 VOCs adsorbed using activated carbon are available in the literature [16]. Some
of them are listed in Table 5.4 for training purpose only in this book. Users are
reminded that the units have to match on both sides of the above equation. In order