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400 Lawrence K.Wang et al.

3.2. Dehumidification
Emission streams can contain both water vapor and VOCs. In a carbon bed, water
vapor competes with VOCs for adsorption sites on the carbon surface. When the
humidity level exceeds 50% (relative humidity) in the emission stream, the efficiency
of the adsorption may be limited for a dilute emission stream. Under conditions when
the concentration of HAP, exceeds 1000 ppmv, relative humidity above 50% can be tol-
erated. Likewise, when the HAP concentration is less than 1000 ppmv, the relative
humidity should be reduced to 50% or less (3).
Generally, dehumidification of an emission stream is accomplished by either cooling–
condensing or by diluting the emission stream. The amount of water vapor in the emis-
sion stream can be lowered by cooling and condensing the water vapor in the emission
stream. Typically, cooling and condensing of the emission stream can be accomplished
by using a shell-and-tube-type heat exchanger. Dilution is another alternative available
for dehumidification. This alternative can be used when the dilution air humidity is
significantly less than the emission stream. The drawback to this alternative is that
it increases the airstream flow, which, in turn, increases the size of the adsorber sys-
tem. As a result, the dilution alternative may not be cost-effective. Another drawback
is that the removal efficiency of the carbon adsorber, which is a constant outlet device,
will be decreased.

3.3. High VOC Reduction
For safety reasons, the designer must consider the reduction of VOC in the air
emission stream. This reduction should be considered when the flammable vapors are
present in emission streams and the VOC and air mixture exceeds 25% of the lower
explosive limit (LEL) for the VOC. This percentage may be raised to a range of 40–50%
of the LEL, if proper monitoring and controls are used. Another reason for considering
VOC reduction is because the heat released during adsorption of the VOC could increase
the bed temperature. For the examples in this handbook, it will be assumed that the
VOC and air mixture will be limited to less than 25% of the LEL. Some of the VOCs
commonly found in emission stream are listed in Table 2.

4. DESIGN AND OPERATION
4.1. Design Data Gathering
As previously described, data are compiled on a HAP Emission Stream Data Form
and the required HAP control is determined by the applicable regulations (7–13,19,20).
The data provide the necessary information to perform the calculations for the required
HAP control. For carbon adsorbers, the size (and purchase cost) of the system depends
on the following parameters:

1. The mass loading of the VOC
2. The volumetric flow rate of the emission stream with VOC
3. The adsorption time
4. The working capacity of the carbon bed
The two most important parameters for sizing and determining the cost of the carbon
adsorption system are mass loading and volumetric flow rate of the VOC. Using mass