Page 290 - Materials Chemistry, Second Edition

VOC-Laden Air Treatment 273

(c) The amount of xylene that can be retained by an adsorber before
the GAC becomes exhausted
= (amount of the GAC)(actual adsorption capacity)
= (1,000 lb/unit)(0.193 lb xylene/lb GAC) = 193 lb xylene/unit

Discussion:
1. The adsorption capacity of vapor-phase GAC is typically in the
neighborhood of 0.1 lb/lb (or 0.1 kg/kg), which is much higher
than the adsorption capacity of liquid-phase GAC, typically in
the neighborhood of 0.01 lb/lb.
2. Care should be taken to use matching units for P and q in the
voc
adsorption isotherms.
3. The influent COC concentration in the air stream, not the eff-
luent concentration, should be used in the adsorption isotherms
to determine the adsorption capacity.
4. There are two sets of empirical constants for m-xylene; one should
always check the applicable range for the empirical constants.

7.2.4 Cross-Sectional Area and Height of GAC Adsorbers
To achieve efficient adsorption, the air flow rate through the activated carbon
should be kept as low as possible. The practical design velocity is often ≤60
ft/min, and 100 ft/min is considered as the maximum value. This design
parameter can be used to determine the cross-sectional area of the GAC
adsorbers (A GAC ):

Q
A GAC = (7.4)
Airflowvelocity

where Q is the influent air flow rate. The design height of the adsorber is normally
2 ft or deeper to provide a sufficiently large mass-transfer zone for adsorption.

Example 7.2: Required Cross-Sectional Area of GAC Adsorbers
Referring to the remediation project described in Example 7.1, the 1000-lb
GAC units are out of stock. To avoid delay of remediation, off-the-shelf 55-gal
activated-carbon units are proposed on an interim basis. The type of GAC
in the 55-gal units is the same as that in the 1,000-lb units. The vendor also
provided the following information with regard to the units:

• Diameter of carbon packing bed in each 55-gal drum = 1.5 ft
• Height of carbon packing bed in each 55-gal drum = 3 ft
• Bulk density of the activated carbon = 28 lb/ft 3