Activated carbon is the generic term used to describe a family
of carbonaceous adsorbents with a highly crystalline form and
extensively developed internal pore structure.
A wide variety of activated carbon products is available
exhibiting markedly different characteristics depending upon the
raw material and activation technique used in their production.
In selecting an activated carbon, it is important to have a
clear understanding of both the adsorptive and physical
characteristics of the material in order to optimise the
performance capabilities.
What is activated carbon made from ? What makes it activated ?
Activated carbon can be made from many substances containing
a high carbon content such as coal, wood and coconut shells. The
raw material has a very large influence on the characteristics
and performance activated carbon.
Activation refers to the development of the adsorption
properties of carbon. Raw materials such as coal and charcoal do
have some adsorption capacity, but this is greatly enhanced by
the activation process.
Forms of activated carbon
IPEC uses two forms of activated carbon.
- Granular Activated Carbon: irregular shaped particles with
sizes ranging from 0.2 to 5 mm. This type is used in both
liquid and gas phase applications.
- Pelleted Activated Carbon: extruded and cylindrical shaped
with diameters from 0.8 to 5 mm. These are mainly used for
gas phase applications because of their low pressure drop,
high mechanical strength and low dust content.
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granular
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pelleted
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How much surface area does activated carbon have?
Activated carbon has the strongest physical adsorption
forces or the highest volume of adsorbing porosity of any
material known to mankind. It can have a
surface of greater than 1000mē/g.
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What is adsorption ? What makes molecules adsorb on
activated carbon ?
Adsorption is the process where molecules are concentrated on
the surface of the activated carbon.
Adsorption is caused by London Dispersion Forces, a type
of Van der Waals Force which exists between molecules. The force
acts in a similar way to gravitational forces between planets.
London Dispersion Forces are extremely short ranged and
therefore sensitive to the distance between the carbon surface
and the adsorbate molecule. They are also additive, meaning the
adsorption force is the sum of all interactions between all the
atoms. The short range and additive nature of these forces
results in activated carbon having the strongest physical
adsorption forces of any material known to mankind.
- Gas Phase Adsorption: This is a condensation process where
the adsorption forces condense the molecules from the bulk
phase within the pores of the activated carbon. The
driving force for adsorption is the ratio of the partial
pressure and the vapour pressure of the compound.
- Liquid Phase Adsorption: The molecules go from the bulk
phase to being adsorbed in the pores in a semi-liquid state.
The driving force for adsorption is the ratio of the
concentration to the solubility of the compound.
What compounds are adsorbed ? How much adsorbate can the carbon adsorb?
All compounds are adsorbable to some extent. In practice,
activated carbon is used for the adsorption of mainly organic
compounds along with some larger molecular weight inorganic
compounds such as iodine and mercury.The amount that the carbon can adsorb is dependent upon the
type and concentration of the adsorbate. Generally, the
higher the concentration and the larger the molecule, the
greater the amount adsorbed. The typical range experienced
is about 1 to 35 weight percent. That is, one hundred kg of carbon will adsorb 1 to 35
kg of contaminant.
Using activated carbon
In most cases the granular or pelletised activated carbon is
installed in a fixed bed with the liquid or gas passing
through the bed. The compounds to be removed are retained on
the activated carbon.
The carbon is used until exhaustion. It can then be reactivated
(normally off-site), in-situ regenerated or replaced with new
carbon.
- Adsorbers: Most adsorbers are pressure vessels
constructed in carbon steel, stainless steel or plastic. Large
systems for drinking water are often constructed in concrete. In
some cases, a moving or pulsed bed adsorber is employed to
optimise the use of the granular activated carbon.
- Design information: The main factors in the design of an
adsorption system are the carbon consumption (amount of carbon
required to treat the liquid or gas) and the contact time (for a
fixed flow rate, the contact time is directly proportional to
the volume of carbon).
- Treated
activated carbon: The activated carbon used is determined in
relation to the nature of the odourous compound present and can
be supplied in the form of either a standard untreated grade, or
a chemically impregnated type for enhanced loading capacity and
removal efficiency.
How large should the system be?
The size of the system depends on the nature of the
contaminant being removed. The contaminant has an
adsorption potential that depends on the type of compound and
its chemical structure. Some contaminants are strongly
adsorbed; some are not. The more strongly adsorbed (the
higher the adsorption potential) a contamination is, the less
carbon is required to adsorb it.
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