Adsorption - Principles of Chemical Separations with Environmental Applications - page 186

on geometric factors (sieving); and, (c) differences in diffusion rates from the fluid
to solid surface, usually the intra-particle diffusion rate (kinetic). Most sorbents are
based on (a).
4 The method of sorbent regeneration (see below).
5 The sorbent bed volume. This value includes the portion of the bed which is not com-
pletely saturated when the sorption step is stopped as well as the void space in the
bed. The mass transfer rates (transport processes) for sorption have a large influence on
the bed size and dimensions. Some design approaches are based on sizing the unused
portion of the sorbent bed, as discussed in Section 7.8.
6 Sorbent deactivation. Components in the feed stream can strongly adsorb and/or react
with the sorbent and substantially reduce its effective lifetime.
7 Cost. Less expensive sorbents that can be easily regenerated are preferred.
The type of sorbent that will work for a given application will be chosen by con-
sideration of these factors. Once the sorbent type is chosen, the particle size and shape
need to be considered. These factors affect both the pressure drop in the sorbent bed and
the mass transfer rate from the fluid to the sorbent surface. As general rules of thumb,
the pressure drop through the bed increases as the particle size decreases. In contrast, the
mass transfer rate usually increases as the particle size decreases. The factors do not vary
to the same degree with particle size so some balance needs to be made in the chosen
design [6].

7.5 Various sorbents

Table 7.1 lists the typical sorbents used; their uses as well as strengths and weaknesses.
The four major commercial adsorbents are the following: zeolite molecular sieves (zms),
activated alumina, silica gel, and activated carbon. The surfaces of activated alumina
and most molecular-sieve zeolites are hydrophilic, and will preferentially adsorb water
over organic molecules. Silicalite, which is a hydrophobic zeolite, is the main exception.
Activated carbon, on the other hand, preferentially adsorbs organic and non-polar or
weakly polar compounds over water. The surface of silica gel is somewhere in between
these limits and has affinity for both water and organics. Detailed information about each
of these classes of adsorbents can be found in Refs. [1, 4, 6, 7].

7.5.1 Activated carbon

Activated carbon is prepared by (a) heating organic materials such as wood, coal char,
almond, coconut, or walnut shells, as well as vinyl copolymers or recycled tires in a
stoichiometric O 2 -deficient atmosphere, and (b) activating the product by exposure to a
mild oxidizing gas (CO 2 or steam, for example) at a high temperature. The activation