Chemical Engineering May 2023 - 30

Design requirement/parameter
TABLE
4. TYPICAL VAPOR-PHASE GUIDELINES AND OPERATING CONDITIONS
Description
Particulate matter
and aerosols
* As in liquid-phase applications, GAC will physically filter some particulate
matter, but since backwashing is not typically possible in vaporphase
applications, it is recommended to remove particulate matter
prior to the carbon bed to prevent their buildup and associated issues,
such as channeling
* GAC will not adsorb aerosols in the same way it adsorbs gases
Linear velocity
* The superficial velocity of the airstream as it passes through the
carbon bed is calculated as the volumetric flowrate of the airstream
divided by the cross-sectional area of the carbon bed
* The linear velocity should be carefully controlled to prevent channeling
(velocity too low) or carbon fluidization and vibration (velocity too high).
Guidelines for linear velocity in vapor-phase applications typically
range from 15 to 90 ft/min
Length/diameter
(L /D ) ratio
* The ratio of the bed length (depth) to the bed diameter. L /D is important
because it affects the flow distribution of the air stream through
the carbon bed, which in turn affects the efficiency of the adsorption
process
* In general, a higher L /D ratio leads to better flow distribution and more
efficient adsorption, but also increases pressure drop and energy
requirements
* L /D ratio of 3 to 5 is typically recommended to balance efficiency and
pressure drop considerations. Typically, at least several feet of bed
depth are used
Humidity
* The moisture content of the vapor stream being treated must be considered.
This parameter is important because moisture can compete
with the target species for adsorption sites on the carbon
* Relative humidity (RH) above 50% can negatively impact VOC adsorption
Pressure
drop (ΔP) * Pressure drop is the decrease in pressure that occurs as the vapor
stream passes through the carbon bed. This parameter is a function of
the flowrate of the vapor stream, bed design, carbon type and the gas
properties
* Smaller-sized carbon particles will cause higher pressure drop and
require greater energy to overcome the pressure drop across the bed
reducing thermal risk in these applications.
Typically, inlet concentrations of VOCs exceeding
1,000 ppmv in air are not recommended.
Additionally, some compounds,
such as ketones and aldehydes, can oxidize
on the carbon by atmospheric oxygen,
generating additional heat, and so their inlet
concentrations should be limited. Additional
vapor-phase design and operating considerations
are included in Table 4.
Spent media and reactivation
After activated carbon has reached its capacity
and will no longer adsorb the target
chemical species, it often must be removed
from service. In packed beds using GAC, the
spent GAC is removed from the adsorber
vessel by vacuum or by slurry method, depending
on the vessel design and utilities.
Spent GAC is moved by truck and can be
disposed of by landfill or incineration. A
more environmentally friendly and cost-effective
approach is to reactivate the spent
activated carbon.
Carbon reactivation is a process where
spent,
30
contaminant-loaded GAC is
returned
to a facility with a suitable reactivation
furnace, which operates under similar
high-temperature conditions to physical
carbon-activation furnaces. In the carbon
reactivation furnace, contaminants are desorbed
from the GAC and destroyed, and
the furnace atmosphere facilitates the generation
of new surface area in the GAC. A
downstream abatement process ensures
adequate destruction, control of acid-gas
generation, and particulate-matter removal.
Thus, the carbon reactivation process removes
and destroys contaminants from
GAC, allowing for the GAC to be re-used in
the same or other processes. Reactivation is
typically only practiced on granular materials
with sufficient hardness and density to resist
physical degradation in the aggressive furnace
atmosphere.
To be eligible for carbon reactivation, the
material must undergo a " Carbon Acceptance "
process, where the composition of
the carbon and contaminants are verified
as acceptable. Carbon reactivation has
proven to be effective at removing and destroying
even robust compounds, such as
PFAS. Ref. 1 describes a peer-reviewed
study demonstrating >99.99% destruction
of total PFAS compounds across a carbon
reactivation furnace.
System design and scaleup
System design and performance of GAC adsorbers
is dependent on several factors, but
key details include targeted adsorbate species,
stream composition, inlet concentrations
and outlet concentration goals. There are
several methods that can be used to inform
the design and determine the expected performance
of a larger GAC system, and they
are ranked in the order of increasing accuracy.
Computational models. Technical industry
experts can use computational models to
estimate the performance of carbon in various
applications, inputting key factors, such
as stream properties and composition, selected
model sorbents and so on. These can
be effective for some target species and applications,
but have less value than empirical
data generated using source waters and
chosen sorbents.
Isotherms. Isotherm testing measures the
adsorption capacity of a sorbent by plotting
the mass of adsorbate loaded on carbon
against its concentration at a constant temperature
and is often performed at bench
scale with selected sorbent products and
source waters. An isotherm can inform the
theoretical maximum capacity of a sorbent
for a selected adsorbate. While isotherms
are useful at determining feasibility for an
application, they do not accurately model
full-scale adsorber systems and cannot determine
component breakthrough order
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