Encyclopedia of Environmental Science and Engineering, Volume I and II

1220 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS

The equilibrium concentrations have been determined for

most commercially available adsorbents and typical pollutants

and are presented as either Langmuir or BET isotherms. In

general such equations take the form

C f Cis ( )ig*

(13)

The generation term is excluded, as it is assumed that chemi-

cal reaction is not taking place in the system. To develop an

expression for the net rate of accumulation of the mass of Ai

in the column, that is, the rate expression for the adsorption

process, one also needs the function of the concentration of

Ai in the fluid phase.

Considering a differential column segment and writing the

continuity relationship for pollutant Ai in each phase for this

differential section, one gets for the solid phase in this section.

1

1

M

P

C

t

k C C

a

s

is

( ) g i( )

  *



  









(14)

where:

kg: is the gas phase mass transfer coefficient in units

of (time – 1).

Ma: is the molecular weight of A 1.

Ps: is the averaged density of the solids.

: is the fractional voidage in the bed.

For the gas phase in this differential segment,













 

C

t

V

C

z

if if k C Cg i( g*)

(15)

where:

Vz: is the superficial velocity of the fluid.

These partial differential equations (13)–(15) may be

solved simultaneously by numerical analysis using difference

formulas to approximate the partial derivatives. In such a way

the breakthrough curves of hazardous organic vapors may be

predicted for a given adsorbent.

Smoothed computerized results were plotted on Figure 7

for five different compounds having Langmuir type behavior

on activated carbon under the same hypothetical operating

conditions. If one wishes to attain a 90% removal of certain

organic vapor, one could easily see from Figure 7 that diethyl

ether requires the shortest re-cycle time and methyl isobutyl

ketone the longest among the five materials on the graph.

Properties of Adsorbents

Figures 8 and 9 are adsorption isotherms for activated

carbon with nitrous oxide and carbon dioxide respectively.

A more sophisticated correlation of adsorption data is pre-

sented in Figures 10–12 for pure CO, C 2 H 4 and CO 2 gases.

Here (RT/Vs)ln fs/fg is plotted versus Ns (in which:—gas

constant, T—temperature, K, Vs—molar volume of adsor-

bate, cc/mole, fs and fg—fugacites of adsorbate and gas

and N—amount of gas adsorbed, g—moles/gm. adsorbent.

Hydrocarbons and SO 3 adsorb readily on activated carbon.

SO 2 has a maximum retention of 10 wt.% on carbon at

20 C, 760 torr. Ozone decomposes to oxygen on carbon

(Ray and Box, 1950).

Figure 13 has comparable results plotted for CO 2 adsorp-

tion on silica gel. Activated carbon has significantly better

equilibrium properties than does silica gel (vis Figure 9 vs.

Figure 13).

Other results for activated carbon and zeolites may be found

in the book by Strauss (1968). Basic facts about adsorption

properties of activated charcoal, system types and components

and applications are discussed by Lee (1970). He tabulated

data on the air purification applications for inexpensive, non-

regenerative, thin bed adsorbers and for regenerative systems,

and discusses the design of a solvent vapor recovery system.

I. Diethyl ether

II. Acetone

III. Carbon disulfide

IV. MEK

V. Methyl isobutyl

ketone

V

IV

III

III

II

I

0 50

0.5

1.0

t^100150

X=cc

0

FIGURE 7 Break through curves for various compounds at 20C and 1 atm with

C 0  0.00548 mole/liter.

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