Page 287 - Materials Chemistry, Second Edition

270 Practical Design Calculations for Groundwater and Soil Remediation

beyond this point, the breakthrough point will be reached and the effluent
concentration will rise sharply. Eventually, the activated carbon in service
would be saturated, exhausted, or spent when most of the sites are occupied.
The spent carbon needs to be regenerated or disposed of.
Two pretreatment processes of the influent air are often required to opti-
mize the performance of GAC systems. The first is cooling, and the other
is dehumidification. Adsorption of VOCs is generally exothermic, which is
favored by lower temperatures. As a rule of thumb, the waste air stream
needs to be cooled down below 130°F. On the other hand, water vapor will
compete with VOCs in the waste air stream for available adsorption sites.
The relative humidity of the waste air stream generally should be reduced to
50% or less. Taking the off-gas stream from an air stripper as an example, it
is typically saturated with water. The air stream may need to be cooled down
(e.g., using chiller water) to condense out the moisture and then heated up
to some extent (e.g., using an electrical heater) to raise its relative humidity if
activated carbon is used to remove the VOCs before discharge.

7.2.2 Sizing Criteria for Granular Activated Carbon

Two common types of vapor-phase activated-carbon systems are (1) canister
systems with off-site regeneration and (2) multiple-bed systems with on-site
batch regeneration (while some of the adsorption units are in the adsorption
cycle and the others are in the regeneration cycle).
Sizing of the GAC systems depends primarily on the following parameters:

• Volumetric flow rate of VOC-laden gas stream
• Concentration or mass loading of VOCs
• Adsorption capacity of the GAC
• Design GAC regeneration frequency

The design flow rate affects the sizing of the cross-sectional area of the
GAC unit, the fan and motor, and the duct. The other three parameters (i.e.,
mass loading, GAC adsorption capacity, and regeneration frequency) affect
the size and the number of the units as well as the amount of GAC required
for a specific project. Design principles for vapor-phase activated-carbon sys-
tems are basically similar to those for liquid-phase activated-carbon systems,
as discussed in Section 6.3.

7.2.3 Adsorption Isotherm and Adsorption Capacity

The adsorption capacity of GAC depends on the characteristics of GAC, char-
acteristics of VOCs and their concentration, temperature, and the presence of
other species competing for adsorption. At a given temperature, a relation-
ship exists between the mass of the VOC adsorbed per unit mass GAC to the