Adsorption - Principles of Chemical Separations with Environmental Applications - page 191

Table 7.3 Environmental applications of zeolites [9]. Reproduced with permission of the
American Institute of Chemical Engineers. Copyright c 1999, AIChE. All rights

reserved.

Application Zeolites used Advantages

Selective catalytic
reduction of NO x

Copper ZSM-5;
mordenite

Good for high dust applications
Extended temperature range
Cheaper
Higher selectivity
Mordenite is particularly stable in acid
streams

Lean NO x Copper, cobalt ZSM-5;

beta

Uses fuel hydrocarbons as reductants
No ammonia
No special handling
Cheaper

Lean-burn
(oxygen-rich)
diesel-engine NO x
removal

Copper, cobalt ZSM-5;
beta

More effective than three-way catalytic
converter for NO x

Removal of N 2 O Cobalt, copper ZSM-5;

mordenite; ferrierite;
beta; ZSM-11

N 2 O decomposes over zeolites at higher
temperatures (400 ◦ C)

VOC removal in dilute,
high-volume, humid
streams

High-silica,
hydrophobic zeolites

Effective where carbon is not
Systems available from several vendors

VOC removal during
automotive cold starts

High-silica,
hydrophobic medium-
and large-pore zeolites

Achieved 35-70% reduction

Two additional methods which can shift equilibrium conditions to favor desorption are
purge stripping and displacement, as described by Ruthven [5].

In a thermal swing cycle, the bed may be purged either with feed gas or with an inert gas. The
inert purge has the advantage in that the theoretical purge volume required to clean the bed
is reduced. Since the bed is generally also heated by the purge, however, the purge require-
ment may in practice be determined by the heat balance, not by equilibrium considerations.
The theoretical advantage may not, therefore, be realizable and purging with hot feed may
prove more economical. Isothermal purge gas stripping is seldom economical since an inordi-
nately large purge volume would be required. However, displacement desorption, in which a
competitively adsorbed species is used to displace the strongly adsorbed feed component, is
commonly employed for systems in which thermal swing operating is precluded, for example
by the reactivity of the sorbate. Since the displacement cycle requires the displacing agent to
be recovered and recycled, it is used generally only when simpler cycles are impractical. Steam
stripping, which is commonly used for regeneration of activated carbon beds, may be regarded
as a combined displacement/thermal swing operation.