Chemical Engineering August 2014 - 32

Activated
Carbon
Department Editor: Scott Jenkins
orbent materials are used commercially
for bulk separation, as well
as purification of both liquids and
gases. This one-page reference provides
manufacturing and application information
on the most widely used industrial
sorbent material - activated carbon.
Four types of generic sorbents have
dominated industrial adsorption:
activated carbon, zeolites, silica gel
and activated alumina. By worldwide
sales, activated carbon is by far the
most widely used - its manufacture and
sale date back to the 19th century, and
global annual demand for activated
carbon exceeds 1.2 million metric tons.
The most common raw materials for
manufacturing activated carbon are
coal, coconut shells, wood, peat and
petroleum coke.
S
Pore size and surface area
In the context of activated-carbon sorbent
material, the term " activation " refers to a
carefully controlled oxidation of carbon
atoms in the raw material that greatly
expands the material's internal surface
area. The activation process forms a network
of pores that extend from the ones
that naturally occur in the carbonaceous
raw material (Figure 1).
Activation results in a distribution of
pore sizes and shapes that depend on
the nature of the starting material and on
the details of the manufacturing process.
Macroscale pores are greater than 50 nm
in size, while mesoscale pores range from
2-50 nm and microscale pores less than
2 nm wide.
Adsorption
As with other sorbent materials, activated
carbon works when molecules adhere to
its surface in an adsorption process. Adsorption
can be thought of as the accumulation
of gaseous components, or solutes
dissolved in liquids, onto a solid surface.
Adsorption is primarily a physical process
(substances do not undergo chemical
reactions with the adsorbent). If chemical
agents are applied to an adsorbent, they
may react with solutes in a process known
as chemisorption, in which the deposited
substances are chemically altered.
Van der Waals (dipole-dipole) and
London dispersion (induced dipoleinduced
dipole) intermolecular forces are
important in the adsorption phenomenon,
which can be rather complex in practice.
A host of factors influence the fine
details associated with adsorption of molecules
in a sample of gas or liquid onto
activated carbon. Among these factors
are the following:
* Molecular size of the substances to be
removed from the bulk material
* Hydrophilic behavior of the substances
to be removed
* Polarity of the substance to be removed
* Size of interior surface area of the
adsorbent material
* Pore structure of the activated carbon
material (shape, size distribution)
* Solute concentration
* Temperature and pressure
* Composition of the solution or gas
mixture exposed to the adsorbent
* pH value of the solution (for liquidphase)
*
Relative humidity
Chemical activation. In chemical activation,
non-incinerated carbonaceous
material is treated with dehydrating
or oxidizing chemicals and heated to
between 400 and 800˚C. The activating
agents (usually zinc chloride, phosphoric
acid, potassium sulfide or others) are
leached out and recovered. Chemical
The interior surface of activated carbon
is measured and evaluated using the BET
(Brunauer-Emmert-Teller) method. Activated
carbons employed in gas and air treatment
ordinarily have a BET surface area
within the range of 800-1,500 m2/g.
Activated carbons used in water purification
generally have BET surface areas
of between 500 and 1,500 m2/g. In
adsorption, both interior surface and pore
radius distribution play an important role.
Activation processes
Activated carbon manufacture can be
accomplished by either gas activation or
chemical activation.
Gas activation. Gas activation involves
carbonization of the raw material at
400-500˚C to eliminate most of the
volatile matter, followed by partial
gasification at 800-1,000˚C. As the
carbon material is partially gasified (via
the chemical reactions shown below), a
highly porous carbon skeleton is formed
and large internal surface area results. A
mild oxidizing gas, such as steam mixed
with CO2, is used because the intrinsic
surface reaction rate is slower than the
pore diffusion rate. This ensures that
pores will develop uniformly throughout
the material.
C + H20 à 2CO + H2
C + 2H20 à CO2 + 2H2
FIGURE 1. This micrograph shows a particle
of activated carbon, with pores of varying
sizes and shapes
activation is often used on lignin-based
starting materials.
The activation process is carried out
in rotary kilns, multiple hearth furnaces,
shaft or fluidized-bed furnaces, or in
fluidized-bed reactors.
Applications
The most common product forms of
activated carbon include the following
types: extruded (usually in the form on
cylindrical pellets), granular activated
carbon and powder activated carbon (in
specified particle sizes).
Activated carbon finds extensive use
as an adsorbent for the removal of a
wide range of contaminants from liquids
and gases. It is also used to adsorb a
product, such as a solvent, from a process
stream, with the adsorbed product
being subsequently desorbed onsite for
reuse. The use of activated carbon in
liquid-phase applications greatly exceeds
its use in gas-phase applications. The
three largest liquid-phase applications
are treatment of potable water (37% of
total activated carbon used), treatment of
wastewater (21%), and decolorization of
sugar (10%). The three largest gas-phase
applications are air purification (40%),
automotive emissions control (21%), and
solvent vapor recovery (12%).
References
1. Yang, Ralph T., " Adsorbents: Fundamentals
and Applications, " John Wiley & Sons Inc.,
Hoboken, NJ, 2003.
2. Donau Carbon GmbH Inc., Activated Carbon
and its Applications, promotional brochure,
accessed from www.donau-carbon.com, June
2014.
3. Freedonia Group. World Activated Carbon to
2016. Report abstract, April 2012. Accessed
from www.freedoniagroup.com, June 2014.
4. Cameron Carbon Inc. Activated Carbon:
Manufacture, Structure and Properties, white
paper 2006, accessed from
www.cameroncarbon.com, June 2014.
http://www.donau-carbon.com http://www.freedoniagroup.com http://www.cameroncarbon.com
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