Page 5 - Adsorption Technology & Design, Elsevier (1998)

2 The development of adsorption technology

which have withstood the test of time. It became clear, for example, that the
observed effects were best achieved with porous solids and that adsorption is
the result of interactive forces of physical attraction between the surface of
porous solids and component molecules being removed from the bulk
phase. Thus adsorption is the accumulation of concentration at a surface (as
opposed to absorption which is the accumulation of concentration within the
bulk of a solid or liquid).
The kinetic theory of gases, developed quantitatively and independently
by both Maxwell and Boltzmann in the nineteenth century, with further
developments in the early part of the twentieth century by Knudsen, reveals
that the mass of a gas striking unit area of available surface per unit time is
p(M/2FIRgT) v~, where p is the gas pressure and M is its molecular mass.
As discussed later (Chapter 4), according to the kinetic theory of gases the
rate of adsorption of nitrogen at ambient temperature and 6 bar pressure is
2 x 104 kgm-2s -1. At atmospheric pressure this would translate to
0.33 x 10 4 kg m-2s -1. Ostensibly then, rates of adsorption are extremely
rapid. Even accounting for the fact that adsorbate molecules require
an energy somewhat greater than their heat of liquefaction (q.v.
Chapter 3) the above quoted rates would only be reduced by a factor
exp(--Ea/RgT): if E~, the energy required for adsorption, were
10 kJ mol -~ at ambient temperature and pressure, the rate of adsorp-
tion would be 4.5 x 102 kgm-2s -~. However, observed rates are less
than this by a factor of at least 10 -1~ for several reasons, principally the
resistance offered by mass transfer from the bulk fluid to the surface of the
porous solid and intraparticle diffusion through the porous structure of the
adsorbent. Such transport resistances are discussed more fully in Chapter 4.
Industrial applications of adsorbents became common practice following the
widespread use of charcoal for decolourizing liquids and, in particular, its use in
gas masks during the 1914-18 World War for the protection of military
personnel from poisonous gases. Adsorbents for the drying of gases and
vapours included alumina, bauxite and silica gel; bone char and other carbons
were used for sugar refining and the refining of some oils, fats and waxes;
activated charcoal was employed for the recovery of solvents, the elimination
of odours and the purification of air and industrial gases; fuller's earth and
magnesia were found to be active in adsorbing contaminants of petroleum
fractions and oils, fats and waxes; base exchanging silicates were used for water
treatment while some chars were capable of recovering precious metals.
Finally, some activated carbons were used in medical applications to eliminate
bacteria and other toxins. Equipment for such tasks included both batch and
continuous flow configurations, the important consideration for the design of
which was to ensure adequate contact between adsorbent and fluid containing
the component to be removed (the adsorbate).