Environmental Engineering Reference
In-Depth Information
step creates the pore structure, and the surface area of the particle is greatly increased.
The activation conditions are varied to obtain the desired pore structure and mechanical
strength.
Activated carbon is the most common adsorbent due to its large surface area per unit
mass (300 to 1,500 m 2
/
g). The surface area per unit volume and the pore-size distribution
vary depending on whether the application is for liquid- or gas-phase feed streams. Larger
pore sizes are used for liquid-phase streams (30-Å-diameter, as opposed to the 10-25-Å-
diameter carbons used for gas-phase feeds) due to the larger size of the sorbates and the
slower diffusion rates for liquids.
The surface of activated carbon is non-polar or only slightly polar as a result of the
surface oxide groups and inorganic impurities. Most other commercially available sor-
bents are polar in nature. This difference has some very useful advantages. Activated
carbon does not adsorb water very well. So, it does not require any pretreatment to
remove water prior to use and is very useful as a selective sorbent for aqueous sys-
tems (aquarium filters!). This property also makes it a useful sorbent for non-polar or
weakly polar sorbates: one environmental application of activated carbon is its use in
canisters on automobile fuel tanks to reduce hydrocarbon emissions (from volatilization).
VOC removal from water or gas streams is another large application. The strength of the
sorbate-sorbent interaction (heat of adsorption) is generally lower for activated carbon.
Desorption and sorbent regeneration are thus easier and require less energy than other
sorbents. Granular activated carbon can be regenerated by heating to oxidize the collected
organic matter. Usually about 5-10% of the original amount is lost, and the regenerated
carbon loses some of its adsorptive capacity. The same process is not economically feasible
for PAC.
Activated carbon is available as 1-3-mm-diameter beads and 2-4-mm-diameter pellets.
It is also available in granular (GAC) and powdered (PAC) forms. The granular form
is often used in beds and columns. The powdered form is more likely to be added to a
stirred-tank reactor, and settled or filtered out after the adsorption process is considered
to be complete.
7.5.2
Activated alumina
High surface area per unit mass alumina, either amorphous or crystalline, which has
been partially or completely dehydrated is termed activated alumina. This material is
very hydrophilic and is often used for the drying or dehydration of gases and liquids.
Environmental applications would include water removal from acid gas or organic sol-
vent streams. Activated alumina is produced by thermal dehydration or activation of
Al 2 O 3 ·
3) to get n close to 0.5. The surface area per unit mass of this ma-
terial is usually in the range of 200-400 m 2
n H 2 O( n
=
1
,
g. The predominant pore diameters are in
the 2-5-nm range. It can be obtained as spheres (1-8-mm diameter), pellets (2-4-mm
diameter), granules, and powder.
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