Environmental Engineering Reference
In-Depth Information
tions. In closely packed thin-bed activated carbon filters, the half-life of a
contaminant (time required to reduce the concentration by 50%) flowing
though it is 0.01 seconds. In theory, four half-lives would be required to
achieve 90% contaminant removal and require a minimum bed depth of 10
to 13 mm (0.4 to 0.5”).
9. Service life
When contaminant breakthrough (breakpoint) occurs, the sorbent is becom-
ing saturated and needs to be replaced. It is difficult to know with certainty
when the breakpoint is reached. The service life of a thin-bed filter can be
calculated using the following equation:
6.43 10
6
SW
EQMC
(
)
T
=
---------------------------------
(12.4)
where T = service life, hr
S = fractional saturation of sorbent (retentivity)
W = sorbent weight, lb (kg)
E = fractional sorption efficiency
Q = air flow rate, CFM (L/sec)
M = average molecular weight of sorbate
C = average vapor concentration, ppmv
Applying this equation to the removal of toluene (the most common and
abundant indoor contaminant) by a small room air cleaner, let us make the
following assumptions:
E = 0.95
C = 0.20 ppmv
Q = 100 CFM (47.5 L/sec)
S = 0.30
W = 6 lb (2.27 kg)
then
10
6
6.43
×
×
0.30
×
6
T
=
------------------------------------------------------
0.95
×
100
×
92
×
0.20
=
6621 hr
=
276 days
