Page 425 - Air Pollution Control Engineering

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Gas Phase Activated Carbon Adsorption 397

Vapor adsorption by activated carbon occurs in two stages. These stages are described
as the adsorption and saturation stages:
• Adsorption stage: During this initial stage, the carbon rapidly and completely adsorbs
the vapor, but a stage is reached in which the carbon continues to adsorb but at
decreasing rate.
• Saturation stage: During the process, a point is reached when vapor concentration leaving
the carbon equals that of the inlet. This means that carbon is saturated and it has adsorbed
the maximum amount of vapor at a given temperature and pressure. This saturation value
is different for each vapor and carbon.
The carbon’s saturation value must be determined experimentally. Dry air, which is
saturated with a selected vapor or gas, is passed through the carbon at a known airflow
rate and a known carbon weight. With constant pressure and temperature, the air is
passed through the carbon. The weight of the carbon gradually increases as a result of
the adsorption, and, finally, the carbon ceases to increase in weight. When this point is
reached, the carbon is considered saturated with the adsorbate.
As previously discussed, activated carbon adsorbs all of the usual solvent vapors
that have a low boiling temperature. As a result, it can be used to recover practically
any single solvent or any combination of low-boiling solvents. Physical adsorption
is limited to vapors that have a higher molecular weight than the normal components
of air. Practically speaking, gases with molecular weight over 45 can be removed
by physical adsorption. Most solvents used exceed this limit, except for methanol
(10,12).
Retention capacity and breakpoint are important characteristics of activated carbon
when considering activated carbon for air pollution control. The retention capacity of
an activated carbon is expressed as the ratio of the weight of the adsorbate retained to
the weight of the carbon. After initial saturation of an activated carbon with a selected
absorbate, the retention capacity of activated carbon is the amount of this selected adsor-
bate that the carbon retains when pure air is passed through the carbon at a constant
temperature and pressure. The retention capacity represents the weight of the particular
gas or vapor that the carbon can completely retain.
The breakpoint of an activated carbon represents an adsorption stage when the
retentive capacity of the carbon is reached. Adsorption is 100% initially when an air
vapor mixture is passed over carbon, but as time passes, the retention capacity of the
carbon is reached. As a result, traces of the vapor begin to appear, which is described
as the breakpoint. Beyond the breakpoint, the removal efficiency of the carbon
decreases rapidly. As the flow continues to pass over the carbon, an additional amount
of vapor is adsorbed, but the amounts of vapor in the exit air increases and eventually
equals that in the inlet, at which time the carbon is saturated at the particular operating
conditions.
Carbon adsorption is an exothermic process. An exothermic process is a physico-
chemical process during which heat is liberated and the temperature of the adsorbent
bed increases. As a result, it may be necessary to provide cooling for the carbon bed.

2. Adsorption Theory
A variety of theories have been set forth to explain the phenomenon of selective
adsorption of certain vapors or gases, but the exact mechanism is still being disputed.