Page 30 - Adsorption Technology & Design, Elsevier (1998)

Adsorbents 27

The ionic nature of most zeolites means that they have a high affinity
for water and other polar molecules such as carbon dioxide and hydrogen
sulphide. However, as the silica-to-alumina ratio is increased the material
can become hydrophobic. Silicalite, a pentasilzeolite, effectively contains
no aluminium and, as with de-aluminized Y-type zeolite, can be used to
remove hydrocarbons from aqueous systems and from humid gases. They
therefore find applications in the removal of volatile organic compounds
from air. As an alternative to activated carbons high silica zeolites have
several advantages. First, they can be used at relatively high humidities.
Carbons take up appreciable quantities of moisture at high humidity
thereby limiting their effectiveness for VOC control. Secondly, zeolites
are inorganic and hence they can be regenerated in air, subject to
flammability considerations. Thirdly, high silica zeolites do not show cata-
lytic activity and problems of heating with ketones do not arise as with
carbons.
Commercially available synthetic zeolites are generally produced via
the following sequence of steps: synthesis, pelletization and calcination.
Synthesis is carried out under hydrothermal conditions, i.e. crystallization
from aqueous systems containing various types of reactant. Gels are
crystallized in closed systems at temperatures which vary between room
temperature and 200~ The time required may vary from a few hours to
several days. The crystals are filtered, washed, ion exchanged (if re-
quired) and then mixed with a suitable clay binder. The pellets are then
formed, usually as spheres or extrudates before being dried and fired to
provide the final product. The binder must provide the maximum resist-
ance to attrition while facilitating the diffusion of adsorbates into the
microporous interior.

2.10 SELECTION OF AN ADSORBENT

The property which is most commonly studied initially is the equilibrium
isotherm(s) for the chemical system to be separated or purified. Equilibrium
data is required for the temperature and pressure ranges of interest and must
be obtained experimentally if it is not available from the adsorbent vendor.
Isotherms for the pure species provide an indication of the suitability of an
adsorbent for a particular separation but care needs to be taken when
interpreting information when more than one species is to be adsorbed. If
the equilibrium data indicates that an adsorbent might be suitable for the
desired separation then it is necessary to determine whether the kinetic
properties are appropriate. Even though most separations are effected
because of the equilibrium effect it is still necessary to ensure that the rate of