Encyclopedia of Environmental Science and Engineering, Volume I and II

VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS 1221

“Activated carbon filters were used to concentrate

atmospheric mixtures of acrolein, methyl sulfide, and

n-propyl mercaptan. Removal efficiency and carbon capac-

ity for each of the odor compounds were investigated

using two different carbones, Cliffchar (4–10 mesh) and

Barnebey–Cheney (C-4). A closed system was devised to

establish a known atmospheric odor concentration for each

filter run. Solvent extraction techniques were employed to

desorb and recover the odor compounds from the carbon

filters. All quantitative analyses were conducted with gas

liquid chromatography utilizing the hydrogen flame ion-

ization detector. The removal studies conducted indicate

that the efficiency of removal of a carbon filter is essen-

tially 100% up to the point of filter breakthrough. This

breakthrough point is governed by the filter’s capacity for

a particular compound. This study indicated that the filter

capacity is dependent both on the type of carbon employed

and the particular odor compound adsorbed. Solvent recov-

ery of the odor compounds from the carbons varied from 0

to 4.5% for the mercaptan up to 96 to 98% for acrolein. Per

cent recovery was found to vary for a given odor compound

with different carbons and for a given carbon with different

odor pollutants.” (Brooman and Edgerley, 1966.)

Gas Absorption

Adsorption is a diffusional process that involves the transfer

of molecules from the gas phase to the liquid phase because

of the contaminant concentration gradient between the two

phases. Adsorption of any species occurs either at the sur-

face of the liquid film surrounding the packing or at the

bubble surface when the gas is the dispersed phase. When

a gas containing soluble components is brought into contact

with a liquid phase an exchange of the soluble components

will occur until equilibrium in a batch system or steady state

in a flow system is attained. Adsorption may involve only a

simple physical solubility step or may be followed by chem-

ical reaction for more effective performance. The latter is

usually used for flue gas desulfurization and denitrification.

Rates of adsorption depend on the solubility of the gas. At

equilibrium, for gaseous species of low or moderate solu-

bility, the partial pressure of the component is related to its

liquid mole fraction according to Henry’s law,

pi  HXi

where H is Henry’s constant. Both partial pressures and mole

fraction may be related to concentration

pi  CigRT and Xi  Cil/C 1.

At constant temperature, Cig  H′Cil, where H′  H/CtRT. If

the gas is highly soluble in the liquid, H will be small. The

solubility of the gas is affected by the concentration of ions

in the solution at the interface. Van Krevelen and Hoftijzer

(1948) proposed an empirical equation to correct the effect

of concentration of ions on Henry’s constant.

The rate of mass transfer is proportional to both the

interfacial area and the concentration driving force. The

proportional constant is known as the mass transfer coeffi-

cient. Because material does not accumulate at the interface

(Figure 14) the flux in each phase must be the same. Thus

the rate of transfer per unit area is

Ni  jg(Cig  CigI)  hL(CiLI  CiL).

10 20 30 40 50

50

100

150

200

250

300

p IN ATMOSPHERES

p/s (s IN g PER g) 20°

20°

20°

45°

45°

67°

67°

FIGURE 8 The system nitrous oxide-carbon.

McBain and Britton: J. Am. Chem. Soc. 52, 2217 (1930)

(Fig. 14).

0 10

10

20

30

40

50

60

70

80

20 30 40 50 60 70

Saturation in cc S.T.P. per q

Pressure in cm Hg

151.5°

80° 30° 0°C

FIGURE 9 Absorption isotherms for the system

CO 2 -carbon (note that tc  31 C).

C022_001_r03.indd 1221 11/18/2005 2:32:48 PM