Page 14 - Adsorption Technology & Design, Elsevier (1998)

Adsorbents 11

separation applications. Most are manufactured but a few, such as some
zeolites, occur naturally. Each material has its own characteristics such as
porosity, pore structure and nature of its adsorbing surfaces. Each or all of
these properties can play a role in the separation process. The extent of the
ability of an adsorbent to separate molecule A from molecule B is known as
its selectivity. The separation factor provides a numerical value for
selectivity and is defined as follows:
XdYi
= (2.1)
XjlYj
Here, Xi and Yi are strictly the equilibrium mole fractions of component i in
the adsorbed and fluid phases, respectively. In practice, the units of X and Y
can be altered to suit the system under study, bearing in mind that it is
important in comparative studies for a, to remain non-dimensional. For
example, Xj could represent the loading of component j on the adsorbent in
units of mg/g, rather than mole fraction. Selectivity may manifest itself in
one or a number of ways in any particular separation process.

(1) Differences may exist in the thermodynamic equilibria for each
adsorbate-adsorbent interaction; this is often known as the equil-
ibrium effect.
(2) Differences may exist in the rates at which different adsorbates
travel into the internal structure of the adsorbent; this is often
known as the kinetic effect.
(3) Pore openings may be too small to allow penetration by one or
more of the adsorbates; this is known as the molecular sieving
effect and can be considered to be an extreme case of the kinetic
effect.
(4) Differences may exist in the rate at which different adsorbates can
be desorbed from the adsorbent; this is generally known as the
desorption effect.

Equilibrium separation factors depend upon the nature of the adsorbate-
adsorbent interactions, that is, on whether the surface is polar, non-polar,
hydrophilic, hydrophobic, etc. and on the process conditions such as
temperature, pressure and concentration. Kinetic separations are generally,
but not exclusively, possible only with molecular sieve adsorbents such as
zeolites and carbon sieves. The kinetic selectivity in this case is largely
determined by the ratio of micropore diffusivities of the components being
separated. For a useful separation to be based on kinetics the size of the
adsorbent micropores must be comparable with the dimensions of the
diffusing adsorbate molecules.