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Gas Phase Activated Carbon Adsorption 399

Table 1
Parameters for Selected Adsorption Isotherms a
Adsorption Isotherm Range of
Temperature parameters isotherm b
Adsorbate (ºF) k m (psia)
1. Benzene 77 0.597 0.176 0.0001–0.05
2. Chlorobenzene 77 1.05 0.188 0.0001–0.01
3. Cyclohexane 100 0.508 0.210 0.0001–0.05
4. Dichloroethane 77 0.976 0.281 0.0001–0.04
5. Phenol 104 0.855 0.153 0.0001–0.03
6. Trichloroethane 77 1.06 0.161 0.0001–0.04
7. Vinyl chloride 100 0.20 0.477 0.0001–0.05
8. m-Xylene 77 0.708 0.113 0.0001–0.001
77 0.527 0.0703 0.001–0.05
9. Acrylonitrile 100 0.935 0.424 0.0001–0.015
10. Acetone 100 0.412 0.389 0.0001–0.05
11. Toluene 77 0.551 0.110 0.0001–0.05
Note: Each isotherm is of the form: W = kP . (See text for definition of terms).
m
a
e
Data are for adsorption on Calgon-type “BPL” carbon (4 × 10 mesh).
b Equations should not be extrapolated outside of these ranges
Source: US EPA.

the carbon can retain at a given temperature and partial pressure. When designing a car-
bon bed system, the system must be such that equilibrium of the carbon with adsorbate
is not reached, because this would result in excessive emissions and bed breakthrough.
Usually, the carbon beds are operated to be taken off-line when the HAP concentration
in the bed reaches about 50% of the equilibrium. As a result, the actual bed capacity is
less than the equilibrium capacity. This actual capacity of carbon system is commonly
referred to as the effective or working capacity (W ). Generally, the working capacity is
c
50% less than the equilibrium capacity. Adsorption isotherm parameters for selected
organic compounds are presented in Table 1. If no information is available on the work-
ing capacity (W ), it is common practice to use 50% of the equilibrium adsorptivity (W )
c e
as the default value. If no information is available on W or W , the default value of 0.100
e c
can be used for W .
c
3. CARBON ADSORPTION PRETREATMENT
Depending on the HAP influent characteristics, three possible pretreatments are cool-
ing, dehumidification, and high VOC reduction, which may or may not be needed, prior
to the gaseous-phase carbon adsorption.
3.1. Cooling
Lower temperatures provide for a more favorable condition for adsorption of
VOCs. When emission stream temperatures are significantly higher than 130ºF, a
heat exchanger may be used to lower the temperature of the emission stream to
130ºF or less.