Adsorption - Principles of Chemical Separations with Environmental Applications - page 190

Table 7.2 Zeolite molecular-sieve adsorbents [8]. Reproduced with permission
of the American Institute of Chemical Engineers. Copyright c

1987, AIChE. All

rights reserved.

Alternative Channel

Designation name diameter, nm Application

KA 3A 0.29 Drying of various gases

NaA 4A 0.38 CO 2 removal from natural gas

CaX - 0.80 Removal of mercaptans from natural gas

Mordenite - 0.70 I and Kr removal from nuclear off-gases

Silicalite - 0.55 Removal of organics from water

zeolite. By changing the cation type and number, one can modify the selectivity of the
zeolite for a given separation.
3 The effect of the temperature can be substantial in situations involving activated
diffusion.
Many zeolites occur naturally, but the majority of those used commercially have been
synthesized and are designated by a letter or group of letters (Type A, Type Y, Type ZSM,
etc.). The lettering system has evolved empirically and has no relation to the structure.
For example, ZSM stands for Zeolite Sacony Mobil.
Tables 7.2, 7.3, and 7.4 list some zeolites and their environmental applications.

7.6 Sorbent regeneration

In most instances, it is necessary to regenerate the sorbent after each cycle of use. This
step is important to reduce the replacement costs of the sorbent, extend the useful life of
the sorbent and minimize the quantity of solid material that must be discarded. For gases,
there are two methods generally used. In pressure-swing adsorption (PSA), the feed is
introduced at a high pressure to obtain a high degree of solute adsorption. When the
sorbent bed is saturated, the pressure is reduced to remove the sorbed species for recovery
or disposal and regenerate the sorbent. The pressure range will be dictated by cost and the
sorption isotherm. The pressure swing can be accomplished very rapidly. Temperature-
swing adsorption (TSA) uses heat to regenerate the sorbent. The feed is introduced at a low
temperature for sorption. Heat is applied to the saturated bed to provide sufficient thermal
energy to desorb the sorbates and regenerate the sorbent. This temperature swing can
take several hours since it is necessary to heat the entire bed. This procedure is only used
when the bed volumes are small and the components are not damaged by the temperature
change. A combination of increased temperature and reduced pressure may also be used
during sorbent regeneration.