Page 95 - Adsorbents fundamentals and applications

80 ACTIVATED CARBON

60
Activated carbon
Cumulative pore volume, cm 3 /100 gm 40 Zeolite 5A
50
Silica gel

30

20

10 Activated alumina
MSC
MSC
0
2 5 10 20 50
Pore diameter, Å
Figure 5.1. Pore-size distributions for activated carbon, silica gel, activated alumina, two
molecular-sieve carbons (MSC), and zeolite 5A (Yang, 1997, with permission).

process is used primarily for gas and vapor adsorption processes. The other acti-
vation process that is also used commercially depends on the action of inorganic
additives to degrade and dehydrate the cellulosic materials and, simultaneously,
to prevent shrinkage during carbonization. This is called chemical activation, usu-
ally used for lignin-based precursors such as sawdust. No pre-carbonization step
is required. Lignin, usually the raw material that is blended with activators such
as phosphoric acid, zinc chloride, potassium sulfide, or potassium thiocyanate,
◦
is carbonized at temperatures between 500 to 900 C. The remaining chemicals
are removed from the carbon by acid or water wash. The product, usually in
powder form, is used for aqueous or gas phase purposes. The inorganic material
contained in activated carbon is measured as ash content, generally in the range
between 2 and 10%. Beside granulated activated carbon (GAC), powdered acti-
vated carbon (PAC) is also used commercially (for example, for dioxin removal
from incinerators by injection of PAC; Yang et al., 1999).
Although the process for manufacturing activated carbon has been largely
empirical, a considerable understanding of the fundamental processes is now
available (e.g., Barton et al., 1999). During carbonization of the precursor raw
material, condensation of polynuclear aromatic compounds and breakage of side-
chain groups occur, resulting in a carbon residue. In the meantime, cross-linking
reactions occur that inhibit the development of a pre-graphite structure. Hence
activated carbon is referred to as non-graphitizing, as opposed to graphitizing. The
graphitizing type applies to pitches derived from petroleum, coal tar, and aromatic