Air Pollution Control Equipment Selection Guide, 2nd Edition

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Adsorption Devices

2.1 DEVICE TYPE

Adsorption devices consist of adsorptive media, either static or mobile, in a contain-

ing vessel through which the gas and its contaminants are passed. The contaminants

are adsorbed onto and into pores in the adsorbing media.

2.2 TYPICAL APPLICATIONS AND USES

Adsorbers are most commonly used for solvent recovery; control of hydrocarbon

emissions from storage tanks, transfer facilities, printing operations; and similar

processes where volatile hydrocarbons are present. Activated carbon types are also

used to control sulfurous odor, such as that from sewage treatment plants. Special

impregnated carbons are used to chemically react with the contaminant once it is

adsorbed, thereby extending the carbon life. Where the hydrocarbon has recovery

value, adsorbers are often used after process vents, evaporators, or distillation col-

umns to polish the emission down to regulatory limits. They are also used on process

vents in lieu of thermal oxidizers.

Regenerative adsorbers are generally not used where the contaminant is not eco-

nomically recoverable or the desorption process has a low yield. For example, cases

where adding steam to desorb the carbon results in an unusable water mixture tend

to make adsorption less attractive.

Drum type units are often attached to process tanks to control hydrocarbon

breathing or ll venting losses. The gas ow rates are typically low, and these drum

type units can be applied very economically.

Filter type units are used in ventilation systems for hospitals, clean rooms, audito-

riums, bus stations, loading docks, and other environments where adsorbable hydro-

carbons may be present.

2.3 OPERATING PRINCIPLES

Gas adsorption is the physical capturing of contaminant gas molecules onto or into

the surface of a suitable solid adsorbent, such as activate carbon, zeolite, diatoma-

ceous earth, clays, or other porous media. The gas molecule is physically trapped

by the pore openings in the medium and accumulates over time until the medium

saturates and can hold no more. In some devices, the medium is desorbed in place

through the application of a gas such as nitrogen, or steam, to drive the contaminant

from the pore openings of the medium. In others, the medium itself is directed to

a device where thermal energy (heat) is applied to desorb and recover the medium.

Adsorption is basically a pore surface and size phenomenon. The size of the gas

molecule dictates the pore size of the required adsorbent, and the bulk pore area

2 Air Pollution Control Equipment Selection Guide

of the adsorbent per unit volume determines the amount of adsorbent required to

control the specic pollutant. Adsorbents exhibit certain physical characteristics

with respect to pore size. These characteristics are generally called macropores and

micropores as shown in Figure2.1. As dened by the word prexes, macropores are

large pore openings and micropores are small pore openings. In practice, adsorbents

exhibit a mixture of both. The volume of adsorbent required is controlled by the

contaminant gas rate, and the amount of time allowed before breakthrough is per-

mitted to occur. Breakthrough occurs when the pores are effectively lled with the

contaminants or interfering compounds.

The process of activating activated carbon is basically one of opening up its pores.

The carbon can be acid washed then carefully heated in a reducing atmosphere, or it

can be otherwise treated to open the available pores.

Various adsorbents reect known pore sizes and exhibit specic areas per unit

volume. Application engineers have developed adsorption isotherms for various

pollutants as they relate to specic adsorbent types. In the family of activated

carbons, for example, there are dozens of different carbon types (peanut shell

based, coconut shell based, mineral carbon based, etc.), each exemplifying spe-

cic pore size and area characteristics. The adsorption isotherms are used to

Area available

to both

adsorbates

and solvent.

Area available

only to

solvent and

smaller

adsorbate.

Area

available

only to

solvent.

FIGURE2.1 Macropores and micropores (Barnebey Sutcliffe Corp.).