VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1217
pollution producing process. To analyze the concentrations
of pollutant it still remains to make component material
balances of n − 1 species within the system (for which n
components exist). For separation processes an additional
phase equation is usually required for transfer of pollutants
between the rich and lean phases.
The mass balance on a particular species may be found
for component A by examining the imaginary stationary dif-
ferential element of thickness dz. Assuming plug flow we
may derive an expression for cA:Accum. Net Input Generation
rate of rate of rate of
A A A(6)cA—molar concentration
Fa—flux of A at position Z, moles A flowing/(time)
(cs. area) vcA
RA—production rate of component A, by chemical or
nuclear reaction, moles A formed/(time) (vol.)
v—average fluid velocity.Also, rA is usually some empirical function of cA such as kcAn
for an irreversible decomposition reaction of nth order.Gas AdsorptionAdsorption is the process by which a solid surface attracts
fluid phase molecules and forms a chemical or physical bond
with them. The mechanism of adsorption includes:1) diffusion of the pollutant from bulk gas to the
external surface of the particles,
2) migration of the adsorbate molecules from the
external surface of the absorbent to the surface of
the pores within each particle,
3) adsorption of the pollutant to active sites on the
pore.The attraction for a specific gas phase component will depend
on properties such as the concentration of the gas phase com-
ponent, the total surface area of absorbent, the temperature,
polarity of the component and adsorbent, and similar prop-
erties of competing gas molecules. Adsorption is used to
concentrate (30–50 fold) or store pollutants until they can
be recovered or destroyed in the most economical manner.
Adsorption is an exothermic process. The heat of adsorp-
tion for chemical adsorption is higher than that for physical
adsorption. In the former, if the amount of pollutants to be
removed large, it is necessary to remove the heat of adsorp-
tion, since the concentration of adsorbed gas decreases with
increasing temperature at a given equilibrium pressure. For
chemical adsorption, properties which affect reaction kinet-
ics will also come into play (see section on Gas Reaction).
Activated carbon, silicon, aluminum oxides, and molecular
sieves make up the majority of commercially significant adsor-
bents. Activated carbon is the least affected by humidity and
physically adsorbs nonpolar compounds since it has no greatelectrical charge itself. The adsorption rate of activated carbon
can be increased with chemical impregnation. For instance,
activated carbon impregnated with oxides of copper and chro-
mium are found very useful to remove the hydrogen sulfide in
gas streams where oxygen is not present (Lovett and Cunniff,
1974). Alumina and silica materials preferentially adsorb polar
compounds. Molecular sieves have greater capture efficiencies
than activated carbons but they often have a lower retention
efficiency and are considerably more expensive.
The ease of adsorbent regeneration depends on the mag-
nitude of the force holding the pollutants on the surface of
adsorbent. The usual methods for the adsorbent regeneration
include stripping (steam or hot inert gas), thermal desorp-
tion, vacuum desorption, thermal swing cycle, pressure
swing cycle, purge gas stripping, and in situ oxidation.
In many respects the equilibrium adsorption characteris-
tics of a gas or vapor upon a solid resemble the equilibrium
solubility of a gas in a liquid. For simple systems, a single
curve can be drawn of the solute concentration in the solid
phase as a function of solute concentration or partial pressure
in the fluid phase. Each such curve usually holds at only one
specific temperature, and hence is known as an isotherm. Five
types of commonly recognized isotherms are shown by the
curves in Figure 4. There are three commonly used mathemat-
ical expressions to describe vapor or gas adsorption equilib-
rium: the Langmuir, the Brunauer-Emmett-Teller (BET), and
the Freundlich isotherm. The Langmuir isotherm applies to
adsorption on completely homogeneous surfaces, with neg-
ligible interaction between adsorbed molecules. It might be
surmised that these limitations correspond to a constant heat
of adsorption. The Freundlich isotherm can be derived by
assuming a logarithmic decrease in heat of adsorption with
fraction of coverage. Gas adsorption is an unsteady state pro-
cess. The curve of effluent concentration as a function of time
is commonly referred to as the break-through curve. It usually
has an S shape. The break-through curve may be steep or rela-
tively flat, depending on the rate of adsorption, the adsorption
isotherm, the fluid velocity, the inlet concentration, and the
column length. The time at which the break-through curve
first begins to rise appreciably is called breakpoint.
The design of an adsorption column requires prediction
of the breakthrough curve, and thus the length of the adsorp-
tion cycle between elutions of the beds, given a bed of certain
length and equilibrium data. Because of the different forms
of equilibrium relationship encountered, and the unsteady
nature of the process, prediction of the solute break-through
curve can be quite difficult. At present, detailed design of
adsorption columns is still highly dependent on pilot scale
evaluations of simulated or real systems.
Before discussing the method of predicting the break-
through curves, one should consider the isotherm. For Langmuir
isotherm (Langmuir, 1917), if it is assumed that A 1 reacts with
an unoccupied site X 0 to form adsorbed component X 1 ,A X X 1kk 0 111�
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