Page 145 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

5.1 Adsorption 119

Fig. 5.1 Schematic diagram
of pores in adsorbent
Adsorbate (gas molecules)

Adsorbent
Pores
(Porous media,
e.g. Activated
carbon)

with high surface area and different polarities for different air pollutants. Chemical
treatment is also necessary to further improve its performance. The final properties of
activated carbon depend on the treatment of the activated carbon (e.g., [7]).
Activated carbons can be made in many forms, depending on the need: pellet
and granular forms are common for adsorption beds; fibrous structures are common
for air filters, for example, activated carbon cloth can be used for removal of water
vapor [3].
Most engineered adsorbents are characterized with many pores and consequent
large surface areas and low bulk density. If an adsorbent is porous, then its actual
surface area is much greater than external area.
Figure 5.1 illustrates how adsorbate molecules are adsorbed on the surfaces of
the pores in an adsorbent. The bulk sizes of activated carbon particles can be up to a
few millimeters in form of granules or in micrometers for powder.
For most adsorbents, the sizes of the pores are sufficiently small and the cor-
responding surface area is enormous. The pore diameter of a molecular sieve can be
30 or 40 Angstrom (Å)(1 Å = 0.1 nm). Pore sizes in activated carbon range from
10–10,000 Å in diameter. Those larger than 1,000 Å are called macropores and the
pores smaller than 1,000 Å are micropores. The surface areas of the pores can be
2
500–1,500 m /g of activated carbon [1, 7]. Large amount of internal pores in an
adsorbent greatly reduces its bulk density. Bulk densities of dry granular activated
3
3
carbon are 220–500 kg/m , and those of powders are 340–740 kg/m . The real
3
density of the granular activated carbon itself may be about 2,000 kg/m .

5.1.2 Adsorption Affinity

Adsorbent affinity is the attraction between the adsorbent and the adsorbate and it is
critical to successful adsorption. Affinities can be designed by adjusting the pore
diameters of the adsorbent. For example, silica gel is used for adsorbing water