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To be attractive commercially, an adsorbent should embody a number of features:
(a) it should have a large internal surface area.
(b) the area should be accessible through pores big enough to admit the molecules to
be adsorbed. It is a bonus if the pores are also small enough to exclude molecules
which it is desired not to adsorb.
(c) the adsorbent should be capable of being easily regenerated.
(d) the adsorbent should not age rapidly, that is lose its adsorptive capacity through
(e) the adbsorbent should be mechanically strong enough to withstand the bulk handling
and vibration that are a feature of any industrial unit.
17.2.1. Molecular sieves
An increase in the use of adsorbers as a means of separation on a large scale is the result of the manufacturers’ skill at developing and producing adsorbents which are tailored for specific tasks. First, by using naturally occurring zeolites and, later, synthesised members of that family of minerals, it has been possible to manufacture a range of adsorbents known collectively as molecular sieves. These have lattice structures composed of tetrahedra of silica and alumina arranged in various ways. The net effect is the formation of a cage-like structure with windows which admit only molecules less than a certain size as shown in Figure 17.1. By using different source materials and different conditions of manufacture, it is possible to produce a range of molecular sieves with access dimensions of 0.3 nm–1 nm. The dimensions are precise for a particular sieve because they derive from the crystal structure of that sieve. Some of the molecules admitted by different molecular sieves are given in Table 17.3 which is taken from the work of BARRER(2). The crystallites of a sieve are about 10 μm in size and are aggregated for commercial use by mixing with a clay binder and extruding as pellets or rolling into spheres. The pelletising creates two other sets of pores, between crystallites and between pellets. Neither may add significantly to the adsorptive surface though each will influence rates of diffusion and pressure drop. It has been estimated by YANG(3) that there are about forty naturally occurring zeolites and that some one hundred and fifty have been synthesised.
The manufacture of molecular sieves has been reviewed in the literature, and particu- larly by BRECK(4), BARRER(5) and ROBERTS(6).
17.2.2. Activated carbon
In some of the earliest recorded examples of adsorption, activated carbon was used as the adsorbent. Naturally occurring carbonaceous materials such as coal, wood, coconut shells or bones are decomposed in an inert atmosphere at a temperature of about 800 K. Because the product will not be porous, it needs additional treatment or activation to generate a system of fine pores. The carbon may be produced in the activated state by treating the raw material with chemicals, such as zinc chloride or phosphoric acid, before carbonising. Alternatively, the carbon from the carbonising stage may be selectively
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