Page 222 - Adsorption Technology & Design, Elsevier (1998)

204 Selected adsorption processes

Examples of the former are the drying of air or natural gas, the removal of
solvent vapours from air streams, the sweetening of natural gas, the removal
of diethylbenzene from aromatics and the purification of liquid organic
compounds. An example of the bulk separation of components by thermal
swing adsorption is the extraction of water from ethanol.

7.5.1 Drying of gases

Removal of low concentrations of water vapour from gases or air is
important for the protection of compressors and also for the care of
electronic equipment. Dewpoints between --30~ and -50~ may be
achieved. Figure 5.18 illustrates a typical two-bed thermal swing process for
the drying of gases. Thus the gas to be dried flows initially through an
adsorption bed where moisture is removed by a porous material suitable for
drying. During the time one bed is adsorbing moisture the other bed is being
regenerated at a higher temperature as described in Section 7.4. The
operation is reversed once regeneration is complete, the second bed now
acting as the adsorber while the first bed is being regenerated.
The choice of adsorbent for the beds depends on the particular drying
application. If only a moderate humidity is required for the process air or
gas, silica gel is to be preferred as the adsorbent as it has a high capacity and
is easily regenerated. If, on the other hand, low dewpoints are required then
a molecular sieve is superior.

7.5.2 Gas sweetening
Natural gas process streams sometimes contain components such as
hydrogen sulphide, mercaptan, carbon dioxide and water vapour. To
remove these undesirable constituents of natural gas (known as sweetening
in the petrochemical industries) a three-bed system can be employed. A
large-pore zeolite is used as adsorbent, the strength of adsorption of the
components to be removed being H20 > H2S > CO2. Because three
adsorbates are present in the feed the breakthrough of each component in an
adsorption bed would display a composite pattern as shown in Figure 7.8.
The most weakly adsorbed component, CO2, breaks through first but as
both H2S and H20 are more strongly adsorbed, some of the CO2 is displaced
from the adsorbent to give a higher gas phase concentration than was
originally present in the feed. As H2S then begins to break through, some
CO2 is re-adsorbed and its gas phase concentration reverts to that of the feed
indicating that the bed is saturated with respect to CO2. Similarly the more
strongly adsorbed water vapour displaces some H2S from the surface until