Page 147 - Adsorption Technology & Design, Elsevier (1998)

136 Design procedures

capable of effecting the required separation. Then it is necessary to consider
the effect that the other adsorbates in a multicomponent feedstock may
have on the equilibrium of each component, and the method by which the
adsorbent is to be regenerated if it is not going to be discarded after the
adsorption step.
(2) Adsorbate-adsorbent kinetic relationships (as described in Chapter
4), again including appropriate interaction data for multicomponent systems;
separations are rarely controlled by equilibrium considerations alone and
therefore it is necessary to determine whether or not the selected adsorbent
has the requisite kinetic properties. The rate of adsorption will determine
the size of the equipment for those separations which do not have extremely
fast kinetics, i.e. those which cannot be described as being equilibrium
controlled.
(3) Heat of adsorption as a function of the operating conditions including
the composition; whether the process can be considered to be isothermal or
not depends on the magnitude of the heat of adsorption per mol and the
concentration(s) of the adsorbate(s) in the feedstock. The design process
using rigorous methods is simplified considerably if the heat released on
adsorption is low. Even so, energy balance calculations may still be required
if desorption, or regeneration of the adsorbent is to be carried out at
elevated temperature.
(4) Hydrodynamic data; such data are required to determine pressure
gradients and to evaluate the importance of dispersion in the design process.
(5) Physical property data; basic information required over the ranges of
temperature, pressure and composition to be encountered in the process
includes the density, viscosity, thermal conductivity, specific heat and
molecular diffusivities of fluids together with the specific heat of the
adsorbent and the bulk voidage and bulk density of the adsorbent bed. Many
more properties of the system may need to be obtained if rigorous
approaches to the design problem are adopted.

6.2 STAGEWISE CONTACTING

The design principles of stagewise contacting may be illustrated by use of a
simple pollution control example shown schematically in Figure 6.1.
Consider a volume V m 3 of water which contains an impurity at a very dilute
concentration of Co kg/m 3 which is to be reduced to a concentration of
cf kg/m 3 by adsorption onto M kg of granular activated carbon (GAC).