Chemical Engineering July 2017 - 36

Cabot
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
< 2 nm
FIGURE 3. The distribution of pore volumes
differs depending on the starting
material for the activated carbon
2-50 nm
Pore diameter
Washing. Activated carbon may be
acid-washed to reduce ash content and
remove soluble impurities, such as iron.
Varying levels of increased purity are required
for some applications, including
double-layer capacitors, pharmaceuticals
and food and beverages.
Sizing. Granular products may be
sieved to specific particle size ranges.
Common mesh-size
specifications
range from 20 x 40 to 4 x 8 mesh (approximately
0.4 mm x 0.8 mm to 2 mm
x 5 mm). Extrudates are produced with
diameters ranging from 0.8 to 5 mm and
length-to-diameter ratios between about
1:1 to 4:1. Granule and extrudate sizes
are specified to balance mass transfer
and pressure drop. Powder products
are often ground in roller, hammer or
jet mills to achieve a target particle-size
distribution (PSD), often specified by the
terms d5, d50 and d90, which are measures
of particle diameters. D50 (median
particle size) can range from 5 to about
20 micrometers (µm). Smaller particles
improve mass transport, but can lead
to filterability challenges. Tight control of
PSD can optimize the balance.
Shaping. Powdered activated carbons
may be formulated with binders
and shaped into cylinders, tubes, honeycombs
and other extruded forms to
optimize surface-to-volume ratio and
pressure drop for various applications.
Granules and extrudates may also be
formed into filter blocks and plates using
polymeric binders.
Impregnation. Some contaminants,
such as formaldehyde, elemental mercury
and hydrogen sulfide are not adsorbed
on activated carbon. Chemical
impregnation can add functionality to the
carbon sorbent to catalyze a reaction or
promote chemisorption. Activated carbon
manufacturers add functionality for a
34
> 50 nm
Bituminous
Lignite
Wood
Peat
range of applications. Silver is commonly
used to disinfect water. A more specialized
example is impregnation with metal
salts and amines for military gasmasks.
On a larger scale, impregnation is used
to promote the oxidation of mercury in
coal-fired utility fluegas applications.
Products for enhanced sulfur removal
are impregnated with potassium iodide
or sodium hydroxide. Another example
of impregnation by end-users is the deposition
of precious metals on activated
carbon to form heterogeneous catalysts.
Re-activation. Some granular and extruded
products can be re-activated
and recycled after use to minimize cost
and environmental impact. In this process,
spent carbon loaded with organic
contaminants is thermally treated in a
process similar to steam activation. Reactivation
desorbs and destroys volatile
contaminants and restores much of the
original pore volume and adsorptive capacity.
Typical product loss during reactivation
is 5-15%, requiring makeup
with virgin material.
Adsorption fundamentals
Activated carbon sorbents remove low
concentrations of chemicals (adsorbates)
from a fluid (liquid or gas) by adsorption,
the process of accumulation of materials
onto a solid surface. Adsorption occurs
within the activated carbon pore structure
by two distinct mechanisms: physical
and chemical adsorption.
Physical adsorption. Molecules are
attached to the carbon surface by van
der Waals attractive forces. These intermolecular
forces are very weak and
diminish with increasing distance between
the carbon surface and the adsorbate
molecule. Because the weak
attractive forces are greatly dependent
on distance, physical adsorption occurs
primarily within pores that have a radius
only a few times greater than the molecular
diameter of the adsorbed molecule.
Pores that are smaller than the size of
the impurity molecule are inaccessible
and do not participate in the adsorption
process. Pores that are significantly
larger than the adsorbate molecule are
not as effective at adsorption because
the attractive force diminishes as the
distance between the pore surface and
the adsorbate is increased.
Adsorption occurs after the impurity
molecule has diffused into the carbon
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Pore volume, mL/g
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Chemical Engineering July 2017

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