Page 42 - Adsorbents fundamentals and applications

TEMPERATURE SWING ADSORPTION AND PRESSURE SWING ADSORPTION 27

For surface diffusion, (ε AV ) and (ε BV ) may be assumed to be the heats of
adsorption (Chen and Yang, 1992). For diffusion in zeolite, the derivation is also
valid, provided that the activation energies for diffusion are used for (ε AV ) and
(ε BV ). In contrast to surface diffusion, in which the activation energy is always
lower than the heat of adsorption, the activation energy for diffusion in zeolite can
be higher than heat of adsorption because the repulsive forces between the diffus-
ing molecule and the aperture or pore surfaces can influence the diffusion process.
From the experimental values of λ A and ε AV the value for ε AA can be obtained
from Eq. 3.41. Likewise, the value of ε BV can be obtained form λ B . Finally, the
interaction energies between unlike molecules, ε AB or ε BA , can be calculated by
assuming the mixing rule:

ε AB = ε BA = (ε AA ε BB ) 1/2 (3.45)

Equations 3.34–3.37 give the main-term and cross-term diffusivities. The
only needed data are the pure component diffusivities and their concentration
dependence (which yield the interaction energies). This model has been used
successfully for predicting the mixed nitrogen/oxygen diffusivities in carbon
molecular sieve (Chen and Yang, 1992; Chen et al., 1994). A prediction compari-
son with experimental data for diffusion of nitrogen/oxygen mixture in molecular
sieve carbon will be given in Chapter 5 (under molecular sieve carbon) (Chen
et al., 1994).
A molecular dynamics approach can also be used to predict mixed gas dif-
fusivities in microporous materials, at the expense of computation cost (e.g.,
Qureshi and Wei, 1990; Chitra and Yashonath, 1995; Trout et al., 1997; Snurr
and Karger, 1997). The empirical correlation of Vignes (1966) for binary dif-
fusivities in liquid solutions and also metallic alloys has been used extensively
for calculating binary diffusivities, using the Maxwell–Stefan formalism for flux
equations (e.g., Krishna, 1990).

3.2. TEMPERATURE SWING ADSORPTION AND PRESSURE SWING
ADSORPTION
Because the sorbent needs to be regenerated for most commercial applications,
adsorption processes are necessarily cyclic. A number of cyclic adsorption pro-
cesses are available, depending on the way the sorbent is regenerated. These pro-
cesses have been discussed extensively elsewhere (e.g., Yang, 1987; Humphrey
and Keller, 1997).
Adsorptive gas separation processes can be divided into two types: bulk sep-
aration and purification. The former involves adsorption of a significant fraction,
10% by weight or more from a gas stream according to Keller’s definition (Keller,
1983). Whereas in purification, <10% (usually <2%) by weight of a gas stream
is adsorbed. Such a differentiation is useful because, in general, unique process
cycles are used for different types of separation.
For purification, temperature swing adsorption (TSA) is generally the process
of choice. For bulk separation, pressure swing adsorption (PSA) is more suitable.