Page 294 - Materials Chemistry, Second Edition

VOC-Laden Air Treatment 277

Example 7.4: Determine the Change-Out (or Regeneration)
Frequency for GAC Adsorbers
Referring to the remediation project described in Example 7.3, the discharge
limit for xylene is 100 ppbV. Determine the service life of the two 55-gal
GAC units.

Solution:
As shown in Example 7.2, the amount of xylene that each drum can
retain before being exhausted is 28.6 lbs. Use Eq. 7.9 to determine the
service life of two drums:

M (2)(28.6 lb)
T = removal = = 88 min < 1.5 h
R removal 0.65 lb/min

Discussion:
1. Although two drums in parallel can provide a sufficient cross-
sectional area for adequate air flow velocity, the relatively high
contaminant concentration makes the service life of the two
55-gal drums unacceptably short.
2. A 55-gal activated-carbon drum normally costs several hundred
dollars. In this example, two drums last less than 90 min. The
labor and disposal costs should also be added, and it makes this
option prohibitive. A GAC system with on-site regeneration or
other treatment alternatives should be considered.

7.2.7 Amount of Carbon Required (On-Site Regeneration)
If the COC concentration of the air stream is high, a GAC system with on-site
regeneration capability would become a more attractive option. The amount
of GAC required for on-site regeneration depends on the mass loading, the
adsorption capacity of GAC, the design service time between two regenera-
tions, and the ratio between the number of GAC units/beds in regeneration
cycle and the number of GAC units/beds in adsorption cycle. It can be deter-
mined by using the following formula [1]:
R removalad  N des 
T
M GAC =  1+  (7.10)
q  N ad 
where
M GAC = total amount of GAC required
T = adsorption time between two consecutive regenerations
ad
N = number of GAC beds in the adsorption phase
ad
N = number of GAC beds in the regeneration (desorption) phase
des