Environmental Engineering Reference
In-Depth Information
Figure 12.17
Thin-bed activated carbon panel.
dry air at constant temperature and pressure over a bed of sorbent previously
saturated with a specific vapor. Retentivity is expressed as the ratio of the
weight of the retained vapor to the weight of the sorbent. The retentivity
ratio is always less than the adsorption capacity. This indicates that sorbents
have a higher capacity to sorb vapors than to retain them. Though activated
carbons have a relatively high adsorption capacity for water vapor, retentiv-
ity is low. Because of this low water vapor retentivity, sorbed gases and
vapors flowing through an activated carbon bed will cause sorbed water to
leave the sorbent and progressively reduce its sorptive capacity for water
vapor. Activated carbons are poor sorbents for low-molecular-weight sub-
stances such as HCHO and ethylene (C 2 H 2 ) because they are poorly retained.
5. Carbon beds and filters
Gas/vapor air cleaning in industrial and indoor applications is often accom-
plished by passing contaminated air through a bed of activated carbon. Car-
bon beds used in solvent recovery and industrial air cleaning have depths of
circa 1 to 2 meters (3 to 6'). In indoor applications, such bed depths are
impractical. As a consequence, thin-bed panels with bed depths of 25 to 30 mm
(1 to 1.25”) are used ( Figure 12.17 ) . Because of severe pressure drop problems
with such thin-bed filters, they are usually inserted in a module with multiple
V configurations, much like that in an extended-surface pleated filter.
Increasingly, activated carbons are being introduced into pleated
extended-surface fiber filters. Dry-processed carbon, composite-based
adsorption filters utilize very fine activated particles evenly dispersed
throughout the filter mat. Like other extended-surface media filters, they
have the advantage of low pressure drops and can be used in many appli-
cations without expensive filter modules and thin-bed panel filters.
6. Gas/vapor removal in carbon beds
Gas/vapor-phase contaminants are removed in a distinct pattern of adsorp-
tion waves ( Figure 12.18 ) . As air moves through an activated carbon sorbent
 
Search WWH ::




Custom Search