Page 11 - Adsorption Technology & Design, Elsevier (1998)

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Adsorbents

To be technically effective in a commercial separation process, whether this
be a bulk separation or a purification, an adsorbent material must have a
high internal volume which is accessible to the components being removed
from the fluid. Such a highly porous solid may be carbonaceous or inorganic
in nature, synthetic or naturally occurring, and in certain circumstances may
have true molecular sieving properties. The adsorbent must also have good
mechanical properties such as strength and resistance to attrition and it must
have good kinetic properties, that is, it must be capable of transferring
adsorbing molecules rapidly to the adsorption sites. In most applications the
adsorbent must be regenerated after use and therefore it is desirable that
regeneration can be carried out efficiently and without damage to mechan-
ical and adsorptive properties. The raw materials and methods for produc-
ing adsorbents must ultimately be inexpensive for adsorption to compete
successfully on economic grounds with alternative separation processes.
The high internal surface area of an adsorbent creates the high capacity
needed for a successful separation or purification process. Adsorbents can
be made with internal surface areas which range from about 100 m2/g to over
3000m2/g. For practical applications, however, the range is normally
restricted to about 300-1200 m2/g. For most adsorbents the internal surface
area is created from pores of various size. The structure of an adsorbent is
shown in idealized form in Figure 2.1. Many adsorbent materials, such as
carbons, silica gels and aluminas, are amorphous and contain complex
networks of interconnected micropores, mesopores and macropores. In
contrast, in zeolitic adsorbents the pores or channels have precise