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398 Lawrence K.Wang et al.

In 1916, Langmuir proposed that adsorption is the result of a chemical combination of
the gas with the free valence of atoms on the surface of the solid in the monomolecular
layer (11). Another theory proposed that the adsorbents exerted strong attractive forces,
resulting in the formation of many adsorbed layers. Pressure is applied to the lower lay-
ers by the higher layers and the attractive force on the surface. Other investigators have
shown that adsorption is the result of the liquefaction of the gas and its retention by
capillary action in the exceedingly fine pores of the adsorbing solid. Mostly likely,
adsorption is superimposed. For example, the adsorption power of activated charcoal is
mainly the result of molecular capillary condensation, whereas the adsorption power of
silica gel is mainly the result of capillary condensation. However, it must be noted that
the method of preparing the solid adsorbent and the nature of the gas or vapor will affect
the adsorption power of the solid adsorbent.
The adsorption isotherms for a carbon represent the equilibrium adsorption capacity
of carbon. The isotherms relate the amount of VOC adsorbed (adsorbate) to the equilib-
rium pressure (or concentration) at constant temperature. Typically for activated carbon,
as the molecular weight of the adsorbate increases, the adsorption capacity of the acti-
vated carbon increases. Additionally, the chemical characteristics of the compound can
affect the adsorption. Unsaturated compounds and cyclic compounds are more com-
pletely adsorbed than either saturated compounds or linear compounds. The adsorp-
tion of capacity of a carbon virtually, for all adsorbates, is enhanced at lower operat-
ing temperatures and at higher VOC concentrations. The vapor pressure of the VOC
also influences the adsorption capacity. VOCs with lower vapor pressures are more eas-
ily adsorbed than those with higher vapor pressures. For VOCs, the vapor pressure is
inversely proportional to the molecular weight of the compound. Thus, the heavier VOCs
will tend to be more easily adsorbed than the lighter VOCs. This characteristic is not
true for very heavy volatile compounds; hence, carbon adsorption is not recommended
for compounds with molecular weights above 130 lb/lb-mol.
At equilibrium, there are several factors that determine the quantity of hazardous air
pollution (HAP) in a gas stream that is adsorbed on activated carbon. These factors
include the adsorption temperature and pressure, the specific compound being adsorbed,
and the carbon characteristics (e.g., pore size and structure). The equilibrium adsorp-
tivity defines these relationships. For a given constant temperature, a relationship exists
between the mass of adsorbate (i.e., HAP) per unit weight of adsorbent (i.e., carbon) and
the partial pressure of HAP in the gas stream. Adsorption isotherms are developed and
fitted to a power curve using

W = k(P ) m (1)
e partial
where W is the equilibrium adsorptivity (lb adsorbate/ lb adsorbent), P is the partial
e partial
pressure of the HAP in the emission stream, and k and m are empirical parameters.
The partial pressure is calculated as
6
P = (HAP )(14.696 × 10 ) psia (2)
partial e
The Freundlich equation, for example, is this type of equation because it is only valid
for a specified adsorbate partial pressure range and curves are fitted for the equation. In
Eq. (1), the equilibrium adsorptivity, W , represents the maximum amount of adsorbate
e