Page 19 - Adsorbents - fundamentals and applications

4 INTRODUCTORY REMARKS

Some other reported figures are (according to 2001 demand) zeolites ($1,070
million), silica gel ($71 million), activated alumina ($63 million), and clays ($16
million) (Chemical Engineering, February 2000, p. 59).
Activated carbon has been used as an all-purpose sorbent. It is “hydropho-
bic.” Its precedent, charcoal, was first used in the sugar industry in England in
1794 to decolorize sugar syrup. The major development of activated carbon took
place during World War I, for use in filters to remove chemical agents from air.
The commercial activated carbon has taken its present form since the 1930’s
(Jankowska et al., 1991). Silica gel and activated alumina are used mainly as
desiccants, although many modified forms are available for special purification
applications. Synthetic zeolites, the youngest type among the four, were invented
by Milton in 1959 (Milton, 1959). The zeolites that are in commercial use today
are mainly the types in Milton’s invention, i.e., types A, X, and Y. It is remark-
able that most of the $100 million annual sales of zeolites and the businesses
associated with the zeolites are generated by a single invention. Zeolites are used
for their special adsorption properties due to their unique surface chemistries and
crystalline pore structures. It should be noted, however, that a sizable portion of
the commercial zeolites is used for ion exchange and as catalysts.
Polymeric resins are used increasing use in potable water purification, because
for some organics they can remove to lower concentration levels than activated
carbon does. Acid-treated clays and pillared clays are used for treatments of
edible and mineral oils.
Table 1.1 shows examples of commercial applications of these sorbents. Both
bulk separation and purification processes are given. Here bulk separation is
defined (by Keller, 1983) as having the concentration of the adsorbed component
above 10 wt % in the feed. For purification, the concentration of the adsorbed
component is generally <2 wt % in the feed. The liquid-phase bulk separations
that use the zeolites listed in Table 1.1 are accomplished with the simulated mov-
ing bed process. Not included in Table 1.1 are many liquid-phase bioseparations

Table 1.1. Examples of commercial adsorption processes and sorbents used
Separation Adsorbent

Gas Bulk Separations
Normal paraffins/isoparaffins, aromatics Zeolite
Zeolite
N 2 /O 2
Carbon molecular sieve
O 2 /N 2
Activated carbon followed by zeolite (in
CO, CH 4 ,CO 2 ,N 2 ,Ar, NH 3 /H 2
layered beds)
Hydrocarbons/vent streams Activated carbon
H 2 O/ethanol Zeolite (3A)
Chromatographic analytical separations Wide range of inorganic and polymer
resin agents