Chemical Engineering July 2017 - 39

and micropores, where adsorption
takes place. Figure 3 compares the
pore distribution of four different activated
carbons from a variety of raw
materials and activation processes.
Surface area can be determined
the
by nitrogen adsorption
using
BET (Brunauer, Emmet, Teller)
method. Pore size distribution can
be quantified by examining the adsorption
and desorption of nitrogen,
carbon dioxide and other adsorbates.
Mercury porosimetry is well
suited to measure macropore and
large mesopore volume. However,
it is more common and convenient
to measure and rank activated carbon
performance using specific adsorbates
that mimic the application.
Examples include iodine adsorption
to assess small pore capacity and
dye molecule adsorption (methylene
blue, bromophenol blue, and so on)
to assess medium-sized pores. In
some cases, the characterization
test is directly related to the final application,
as in the case of molasses
decolorizing efficiency as a predictor
of performance in sugar applications,
and butane working capacity
as a performance metric for carbons
used to control automobile gasoline
vapor emissions.
Particle size and distribution impact
performance in both PAC and
GAC applications. For PAC, size and
size distribution correlate to masstransfer
resistance and filterability.
In batch applications, a fine particle
size provides rapid adsorption, but
also a high pressure drop and slow
filtration when removing the sorbent.
The best balance of performance is
often found by narrowing the size
distribution. In GAC and extrudate
packed-bed applications, size is
again related to mass-transfer resistance
and pressure drop.
Durability is of particular concern
for GAC and EAC forms. Particles
must be able to resist damage and
fines formation during transport, column
loading and use - especially if
the application involves column backwashing.
Activated carbons for gold
extraction have some of the most
stringent durability requirements. Durable
particles are also required to
minimize losses during re-activation.
High purity is critical in some applications.
Food, beverage and potable
water applications require low levels
of extractables. Pharmaceutical activated
carbons must have ultra-high
purity and full traceability. Emerging
uses in double-layer capacitors and
other electrochemical applications
depend on high purity for extended
cycle life.
Application types
Activated carbon sorbents are used
in two broad application classes:
vapor and liquid purification. Within
each class are examples of two
types of fluid-sorbent contacting.
These are PAC dosing and GAC/
EAC packed columns. Beyond purification,
activated carbons are used in
a number of specialized applications.
In PAC dosing systems, activated
carbon particles are injected into the
contaminated fluid, dispersed within
the fluid for an appropriate contact
time, and then removed by sedimentation
or filtration. PAC dosing may
be used in batch or continuous injection
systems. PAC contact times
range from 0.05 to 2 seconds in gasphase
systems and 1 to 60 minutes
in liquid-phase batch applications. In
municipal water treatment, PAC may
be added continuously as a slurry or
powder and then can be removed
by flocculation, sedimentation and
filtration. In coal-fired utility mercury
removal, PAC is injected into the
fluegas through a distribution lance
and removed in the electrostatic precipitator
or fabric filter.
In packed-bed systems, fluid flows
through a static bed of GAC or EAC.
As the contaminant concentration
in the fluid decreases, the loading
on the carbon increases, creating
a concentration gradient along the
column. The mass-transfer zone is
defined by the gradient between
the inlet concentration existing
in
the fully loaded bed and the outlet
concentration. Breakthrough occurs
when the mass-transfer zone travels
to the exit of the column. Packedbed
systems are sized based on
either calculated or experimentally
determined isotherms, contact time
based on estimated or measured kinetics
and mass transfer, and presCHEMICAL
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