Page 69 - Adsorption Technology & Design, Elsevier (1998)

4

Rates of adsorption of

gases and vapours by

porous media

4.1 INTRINSIC RATES OF ADSORPTION AND TRANSPORT
EFFECTS

If a clean non-porous adsorbent surface were exposed to a gas or vapour
adsorbate and the fluid boundary layer adjacent to the surface offered no
resistance to transport of gas to the surface, then the rate of adsorption
would be extremely rapid. The classical kinetic theory of gases may be
invoked to assess the order of magnitude of such intrinsic adsorption rates.
Physical adsorption entails attractive van der Waal's forces between
adsorbate and adsorbent (outlined in Section 3.1) and the energy barrier
which molecules have to overcome in order that adsorption may occur is
usually very small. The energy released when physical adsorption occurs is
close to the heat of liquefaction of the adsorbate gas, while for desorption to
take place an approximately similar quantity of heat has to be absorbed.
Thus, the processes of adsorption and desorption are reversible and non-
activated. This is in contrast to chemisorption when moderately strong
chemical bonds are formed and molecules have to have sufficient energy
(the activation energy) to surmount the energy barrier and fall into the lower
potential well of the chemisorbed state. From the kinetic theory of gases an
upper limit may be placed on the mass of a gas striking unit area of surface