Chemical Engineering May 2023 - 28

used to produce some specialty activatedcarbon
products.
Worldwide, many raw materials are used to
manufacture activated carbon, but the most
industrially significant sources of activated
carbon are coals, ranging from low-rank coals
like lignite, to higher-rank bituminous and anthracite,
as well as biomass-based sources,
such as wood and coconut shell. The respective
raw material used will produce an activated
carbon with some properties inherited
from the raw material, including adsorptive
properties, such as the pore structure (or pore
distribution), as well as physical properties like
inorganic mineral components, termed ash
content, and the hardness of the resulting activated
carbon. The selected carbon source
influences the physical and adsorptive characteristics
of the finished activated carbon
product, making different carbons suitable
for different applications. Coconut shells and
high-rank coals, such as bituminous material,
yield denser, harder activated carbons and
have low ash content with more microporous
pore structures. Wood feedstocks and lowrank
coals, such as lignite, yield softer activated
carbons with more mesoporous and
macroporous structures. The base material
can often have a major impact on carbon performance
for a given application. For example
activated carbons produced from bituminous
Property
Solubility
Effect on liquid-phase
adsorption
TABLE 1. COMMON FACTORS THAT AFFECT ADSORPTION
Property
The higher the solubility, the more
difficult to adsorb
Concentration Wt. % loading on carbon increases
as inlet species concentration
increases
Molecular
weight
Functional
groups
The higher the molecular weight,
the better adsorbed
More complex structures adsorb
more strongly
Effect on vapor-phase
adsorption
coal sources adsorb per- and poly-fluoroalkyl
substances (PFAS) better than carbons produced
to similar specifications from coconut
shell sources.
Activated carbon products can be manufactured
as granular activated carbon (GAC),
in the millimeter particle-size range, as
powdered activated carbon (PAC), typically
consisting of particles falling below 44 micrometers
(μm), or in other forms, such as
pellets and cloth. Figure 2 shows the different
formats and provides specific properties
for each.
Properties and specifications. Specifying
the properties for activated carbon is crucial,
because specifications ensure that the
carbon product will perform as expected
for its intended application, and that it is of
a consistent quality. Because activated carbon
is used in a wide range of industries,
including water treatment, air purification
and food-and-beverage processing, the
specifications that make an optimal carbon
product may vary by application. Table 2
describes several typical specifications for
activated carbon.
Concentration Wt. % loading on carbon increases
as inlet species concentration
increases
Saturation
pressure
Relative
humidity (RH)
Temperature
The closer to saturation pressure,
the more strongly adsorbed
RH above 50% interferes with
adsorption efficiency - lower adsorption
capacity
Higher temperatures result in lower
wt. % loading
Specification
Mesh size
Iodine number
Hardness
Ash content
TABLE 2. TYPICAL ACTIVATED CARBON SPECIFICATIONS
Unit
Description
Mesh
mg/g
%
%
Moisture content %
Apparent density
28
g/cm3
A surrogate measure of the surface area of the carbon, with higher
numbers indicating greater surface area available for adsorption
The size of the carbon particles, measured in the percentage of particles
existing between the U.S. mesh sizes denoted, such as 8x30 or 12x40
A measure of the carbon's resistance to breakage or abrasion, with
higher percentages indicating greater durability
The amount of inorganic material present in the carbon, which can affect
its performance in certain applications
The weight of packed carbon per unit of volume, which can affect its
handling and transportation costs
The amount of water present in the carbon, which can affect its performance
and handling characteristics
How activated carbon is used
The way activated carbon is used highly depends
on the form of carbon chosen. The
most common way activated carbon is used
is in a packed bed, where GAC is
placed in a stationary vessel, and a
stream of liquid or gas flows through
it. This allows the GAC to adsorb
contaminants from the fluid, retaining
them within the GAC particles.
Packed beds of GAC are widely
used in the water-treatment industry
to remove organic compounds
from drinking water, in wastewater
treatment and in liquid chemical processes.
Similarly, for air and industrial-gas
treatment, packed bed systems
are used to remove pollutants,
such as volatile organic compounds
(VOCs) or H2S. Packed bed systems
allow for relatively simple operations
using GAC in both liquid- and vaporphase
applications, and the packed
beds themselves are often designed
with ease of media installation and
removal in mind. Typically, activated
carbon is used as a polishing technology
for relatively low concentrations
in the liquid and gas phase, rather
than used in concentrated streams,
though there are exceptions.
PAC is too fine to be retained
within a packed bed, and instead is
typically added to a liquid stream in
a batch process, in which the carbon
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