Page 223 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 205

finally H20 begins to break through the bed when the H2S gas phase
concentration returns to its feed concentration. These effects, caused by
displacement of a relatively weakly adsorbed component by a more strongly
adsorbed component are sometimes referred to as roll-up effects. The
composite curve in Figure 7.8 thus contains three constant pattern mass
transfer zones (see Chapter 6) separated by two plateau zones. The adsorber
can be designed by utilizing the length of unused bed (LUB) concept (see
Chapter 6). When, as is more usual, the requirement is to remove H2S and
H20 only, the adsorber is operated until the HES starts to break through the
bed. Both the width of the HES mass transfer zone and the length of bed
saturated with H20 should be found when estimating the length of unused
bed. An empirical correlation for the LUB has been published by Chi and
Lee (1973).
Three beds are used for the sweetening of a sour gas stream as depicted
in Figure 7.9. One bed is adsorbing while one of the other beds is being
cooled (following regeneration) and one regenerated. Formation of un-
desirable quantities of COS (by interaction of HES and CO2 on the
adsorbent surface) can be minimized by using CaE§ zeolite
adsorbents rather than the usual Na+-exchanged form. Neither should
the clay adsorbent binder contain traces of iron which would catalyse the
interaction.

7.5.3 Removal of water from volatile organic compounds
Depending on the nature of the process, the organic compound containing
water may be in the form of a vapour or liquid stream. If in the form of an
uncondensed vapour issuing, for example, from a distillation column, water
removal may be accomplished by using two adsorption columns, each
column employed alternately as an adsorber and a regenerator. Further-
more, by judicious choice of operating conditions considerable quantities of
the heat of adsorption may be retained within the adsorbent bed to facilitate
regeneration. When the mixture of volatile organic and water is in liquid
form, a two-column arrangement may still be used but the advantages of
retaining heat within the adsorption bed are lost. The two examples
described are for the removal of water from an alcohol-water mixture with a
composition near to that of the azeotrope. The first example deals with
removal of water from a vapour stream and the second example concerns a
liquid stream.
The adsorbent normally used in both the vapour and the liquid processes
is a zeolite molecular sieve. The particular zeolite used for extracting water
from an ethanol-water liquid or vapour stream is the 3A variety in which
sodium has been replaced by potassium by ion exchange. As the molecular