Environmental Engineering Reference
In-Depth Information
submicroscopic pores are produced. Activated carbons originating from dif-
ferent materials vary in their structural properties (e.g., pore size, hardness,
density) and, as a consequence, differ in their ability to sorb and retain vapor-
(and liquid-phase) substances.
1. Hardness and size
Activated carbons vary in hardness depending on materials and processes
used in their production. Hardness is an important use parameter of acti-
vated carbons since they must withstand the impact, compression, and sheer
forces associated with their use. When air moves through a bed of activated
carbon at high flow rates, it causes individual granules of activated carbon
to vibrate. Such vibration may cause fragmentation, decrease in granule size,
and loss of carbon mass from the bed. This may produce voids in thin-bed
adsorption panels and result in reduced sorption efficiency (since air pref-
erentially flows through voids).
Activated carbons are produced in size ranges described by U.S. Sieve
Series standard mesh sizes. An 8-14 mesh size, for example, describes acti-
vated carbon particles with dimensions of 2.36
1.4 mm. Mesh numbers
increase with decreasing granule size. A mesh size of 6-14 is typically spec-
ified for general purpose air cleaning.
Granule size is a major determinant of air cleaning effectiveness. Effec-
tiveness increases with decreasing granule size. As the sorption bed becomes
more tightly packed (as a consequence of smaller granular size), the distance
that a sorbate molecule must travel to come into contact with a sorbent
surface decreases. As a consequence, the transfer rate of vapor to carbon
increases. Though cleaning performance is enhanced when activated carbons
with small granule sizes are employed, such use has the same pressure drop
problems associated with high-efficiency particle filters.
×
2. Adsorbability
The degree of physical attraction between a sorbate and a molecule is
described as adsorbability. It is a direct function of a sorbate's critical tem-
perature and boiling point. Gases such as oxygen (O
2
), nitrogen (N
2
), hydro-
gen (H
2
), carbon monoxide (CO), and methane (CH
4
) have critical tempera-
tures below -50°C and boiling points below -150°C. As a consequence, they
cannot be sorbed at ambient temperatures.
Low boiling point gases/vapors such as ammonia (NH
3
), hydrogen chlo-
ride (HCl), hydrogen sulfide (H
2
S), ethylene (C
2
H
2
), and formaldehyde
(HCHO) have critical temperatures between 0 and 150°C and boiling points
between -100 and 0°C. As a consequence, they are moderately adsorbable.
However, because of poor retention, activated carbons without special
impregnants are not suitable for removal of such gases from air.
Organic vapors that have boiling points >0°C have an increased ten-
dency to be sorbed and retained on activated carbons. These include the
higher aldehydes, ketones, alcohols, organic acids, ethers, esters, alkylben-
zenes, halocarbons, and nitrogen and sulfur compounds.
