Chemical Engineering July 2017 - 35

Cabot
Activated carbon sorbents are tailored
for specific applications mainly based on
pore size and pore volume requirements.
Porosity and other parameters are controlled
by the following: 1) raw material
selection; 2) activation process conditions;
and 3) post-processing
steps.
Depending on the application, activated
carbon may be in the form of powder
(PAC), granule (GAC) or extrudate (EAC).
All three forms are available in a range of
particle sizes.
Raw materials
Almost any carbon-containing material
can be used to produce activated
carbon. In practice, economics and
target product properties are the determining
factors in the selection of raw
materials. The base raw material has a
significant impact on the final product
properties, including pore size distribution
and volume, hardness and purity.
Most commercial activated carbons
are manufactured from the following
raw materials:
* Coal (anthracite, bituminous, subbituminous,
lignite)
* Coconut shell
* Wood
Some types of activated carbon are
produced from less conventional raw
materials, such as peat, olive stones,
fruit pits, petroleum coke, pitch, synthetic
polymers, scrap tires and waste
cellulose materials.
Raw materials may undergo pre-processing
steps to control size, form and
other properties. They may be crushed,
milled, briquetted or mixed with binders
and extruded prior to activation.
Activation processes
Activated carbons are manufactured via
one of two processes: steam activation
at high temperature or chemical activation
using a strong dehydrating agent.
Steam activation is the most commonly
used method for activated carbon
production. It is performed in rotary
kilns, shaft kilns, multi-hearth furnaces
or fluidized beds, and proceeds through
the following steps:
Drying. Activated carbon raw materials
in commercial use contain residual
moisture that must be removed before
activation can take place.
Devolatilization. Volatile organic compounds
(VOCs) are formed by cracking
reactions in the raw material at temperatures
between 100 and 400°C.
Charring. Higher-molecular-weight organic
materials are converted to a carbonaceous
char residue at temperatures
of 400-600°C. At this point, incipient
porosity and increased internal surface
area begin to form.
Activation. Activation is generally conducted
in a steam atmosphere at temperatures
between 700 and 1,050°C,
depending on the pore structure of the
carbon being produced. The desired reaction
is shown in Equation (1).
C + H2O ➞ CO + H2
(1)
This reaction gasifies portions of the
solid carbon to create pore volume.
Typically, about half of the carbonaceous
char material entering the activation
step will be reacted away to create
the desired internal pore structure. The
diagrams in Figure 2 depict the development
of porosity during steam activation.
Chemical activation is used to produce
carbons with pore structures and
compositions that are somewhat different
from those in steam-activated carbons.
For example, chemically activated
wood has higher mesoporosity and
higher oxygen content than a steamactivated
carbon. The chemical activation
process consists of mixing raw biomass
with a strong dehydrating agent
and heating to about 400 to 700°C. The
dehydrating agent (typically phosphoric
acid or zinc chloride) extracts the moisture
from the raw material and fixes the
volatile component of the biomass while
the activation occurs. The degree of
activation is determined by the ratio of
raw material to dehydrating agent and
by the heating time and temperature.
After activation, the product is extracted
to yield a highly porous activated carbon
product and the the dehydrating agent
is recovered.
Chemical activation with potassium
hydroxide is of great recent interest because
it can produce highly microporous
carbons with specific surface areas
at or beyond the theoretical value for
graphene (about 2,600 m2/g).
Post-processing
Following activation, activated carbon
sorbents may undergo a series of postprocessing
steps, including the following:
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JULY 2017
33
A
B
C
D
FIGURE 2. This series of diagrams illustrates
pore development during steam
activation
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Chemical Engineering July 2017

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