Page 71 - Adsorption Technology & Design, Elsevier (1998)

68 Rates of adsorption of gases and vapours by porous media

4.2 TRANSPORT PROCESSES IN POROUS SOLIDS

We consider each of the possible resistances to rapid adsorption which are
listed above and examine their significance and magnitude. Interparticle
(external to particles) transport resistance occurs in series with the
intraparticle (within particles) transport resistances, enumerated 2, 3 and 4,
which, if each were present, would be in parallel.

Interparticle mass and heat transport
4.2.1
Rates at which mass and heat are transported between a flowing bulk fluid
phase and the exterior surface of the adsorbent particles are limited by a
relatively thin layer of comparatively stagnant fluid immediately adjacent
to, and completely enveloping, each of the solid particles. Such layers can be
thought of as films of stagnant fluid or, as described by a more elaborate
theory, in terms of so-called mass and heat transfer boundary layers. For the
purposes of application to adsorption either of these concepts can be
accommodated by defining the mass or heat flux from fluid to solid as the
product of a lumped coefficient and a linear driving force. These mass and
heat transfer coefficients for the respective interparticle processes subsume
the thickness of the film or boundary layer and relevant fluid properties
while the driving force is simply the difference in concentration (mass
transport) or temperature (heat transport) between bulk fluid and exterior
surface of the particle. The rate of mass transport of a specific component
from bulk fluid to the exterior surface of a particle, expressed as the number
of moles of the component transferred per unit adsorbent bed volume per
unit time, is
R = kap (Cg- c) (4.2)
where k is the mass transfer coefficient and a is the exterior surface area of a
solid particle per unit volume. The dimensions of k are thus LT -1 or, in SI
units, m s -1. An analogous equation describes the rate of heat transfer
Rh = hap ( T- Tg) (4.3)

where T is the temperature at the exterior surface of the solid, Tg the bulk
fluid temperature and h is the heat transfer coefficient which will have units
(SI system) W m -2 K -1 if the rate of heat transfer is based on unit volume of
adsorbent bed. Whereas equation (4.2) indicates mass transfer during
adsorption from bulk fluid to surface (Cg > c), because adsorption is
exothermic (see Section 3.1) heat is released from within the pellet and is
transferred from the particle exterior surface to the bulk fluid (T > Tg).
Boundary layer theory predicts different values of coefficients for the