Chemical Engineering May 2023 - 29

and
as
the
FIGURE 2. There are multiple formats for activated carbon products, each with a unique set of properties
particles are dispersed into the fluid and allowed
to adsorb the contaminants. After
the adsorption process, the PAC particles
are filtered out of the fluid, typically using a
mechanical filtration system. PAC can also
be injected into vapor streams and removed
in particulate capture devices, such as baghouses,
a practice used for the removal of
VOCs and dioxins, furan and mercury from
fluegases.
While PAC particles have larger external
surface area than GAC, there is no significant
difference in specific surface area between
GAC and PAC, because the majority
of activated carbon's surface area is internal.
Adsorption kinetics onto activated carbon
are dependent on particle size, and the small
particle size of PAC results in rapid adsorption
kinetics, allowing PAC to adsorb compounds
at a faster rate than larger particles
of carbon.However, there is no difference
in the equilibrium loading of GAC and a
corresponding PAC produced to an equivalent
adsorption
specification.
Indeed,
packed
beds of GAC often result in better
carbon usage rates than PAC. Practically,
GAC used in a packed bed system
often loads to a higher degree than PAC,
adsorbing more contaminants per mass of
carbon used.
Liquid-phase best practices
GAC liquid adsorbers can be used with
single vessels, but designs commonly include
multiple adsorber vessels operating
in series flow, lead-lag configuration, to
allow for the greatest utilization of GAC capacity.
In this configuration, when the lead
vessel has reached its capacity, the lag
vessel is moved into the lead position and
a fresh carbon bed is put online in the lag
position. Typically, at least ten minutes of
empty bed contact time (EBCT) per vessel
are required to account for the kinetics
of adsorption and provide for an adequate
bed life, depending on contaminant inlet
concentrations and treatment objectives.
In general, longer contact times are required
as the carbon particle size increases
fluid viscosity
increases.
Decreasing
the flowrate
through a
given vessel
will reduce
and sharpen
the mass
transfer zone
(MTZ) of the
bed, increasing
carbon loading. Additionally, pretreatment
and operational maintenance (such as
backwashing) may be warranted in some
applications, some of which are detailed in
Table 3.
Vapor-phase best practices
Vapor-phase adsorption applications with
activated carbon differ significantly from liquid-phase
applications in that the required
contact time is often much less in the vapor
phase, due to the kinetic advantages associated
with gas adsorption. For high removal
across a vapor-phase carbon bed, typically
three or more seconds of contact time are
required. Because adsorption is an exothermic
process, and in vapor-phase applications
there is not excess water present to
dissipate heat, there are some guidelines for
TABLE 3. TYPICAL LIQUID PRETREATMENT AND OPERATIONAL GUIDELINES
FOR GAC SYSTEMS
Parameter
Total organic
carbon (TOC)
Total suspended
solids (TSS)
Total dissolved
solids (TDS)
Manganese
Iron
Chlorine (or
other oxidizers)
Oil and grease
Langelier
Saturation Index
(LSI)
Bacteria
Linear velocity
Pretreatment recommendations and operational considerations
* Adsorbable TOC consumes carbon capacity, so lower TOC results in lower
carbon usage rates. GAC can tolerate greater than 1,000+ parts per million
(ppm) TOC, depending on application
* TSS will foul beds and block access to carbon pores
* Upstream filtration of 5-10 μm is recommended
* Backwashing may be required for TSS in the 3-5 mg/L range, otherwise solids
can cause increased ΔP
* Application-dependent, though typical hardness < 100 mg/L for water treatment
*
<0.5 ppm
* <1 ppm
* <5 mg/L
* If not fully dissolved or in solution, remove upstream of GAC
* Do not use GAC to directly treat two-phase systems or free product
containing water
* Avoid corrosive or saturated conditions
* Neutral or negative LSI desired to avoid Ca or Mg buildup
* Backwashing can help mitigate biofouling; caustic solution disinfection recommended
for inoculated carbon beds
* The superficial velocity of the liquid stream as it passes through the carbon
bed is calculated as the volumetric flow rate of the stream divided by the
cross-sectional area of the carbon bed
* The linear velocity should be carefully controlled to prevent channeling (velocity
too low) or excessive pressure drop (velocity too high). Guidelines for linear
velocity in liquid-phase applications typically range from 2 to 9 gal/min/ft2.
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