Page 140 - Adsorption Technology & Design, Elsevier (1998)

Processes and cycles 129

5.8 INCREASE IN TEMPERATURE

Figure 5.14(b) shows schematically the effect of temperature on the
adsorption equilibrium of a single adsorbate. The type I isotherm is used for
illustrative purposes. For any given partial pressure of the adsorbate in the
gas phase (or concentration in the liquid phase) an increase in temperature
leads to a decrease in the quantity adsorbed. Hence, increasing the
temperature from T1 to 7'2 will decrease the equilibrium loading from ql to
q2 assuming that the partial pressure in the gas phase remains constant. The
dependency of the loading on absolute temperature is generally well
described by the van't Hoff equation. Because the most strongly adsorbed
species have the greatest heats of adsorption a relatively large decrease in
loading can be achieved by a relatively modest increase in temperature.
Adsorption isotherms tend to become unfavourable for adsorption at
increased temperature and hence they become more favourable to desorp-
tion. It is therefore generally possible to desorb any species provided that
the temperature is high enough. However, it is important to ensure that the
regeneration temperature does not exceed that required to degrade the
adsorbent or cause it to behave catalytically with the chemicals involved in
the process.
A change in temperature alone is not used in commercial processes
because there is no mechanism for removing the adsorbate from the
adsorption equipment once desorption from the internal pores has occur-
red. Passage of a hot purge gas, or steam, through the equipment to sweep
out the desorbed species is almost always used in conjunction with the
increase in temperature. This is the case for both gas and liquid phase
adsorption processes. For liquid phase separations the liquid is drained from
the adsorption equipment prior to the passage of hot gas.
Figure 5.18 shows one of the simplest practical thermal swing adsorption
cycles based on two beds. The feedstock containing the adsorbate at a partial
pressure pl is passed through the first bed at temperature 7'1. The partial
pressure of the adsorbate in the bed effluent may be zero or very low. When
breakthrough is about to occur this bed is taken off-line and the feed is
switched to the second bed. Simultaneously the first bed is regenerated by
raising its temperature to 7'2 and purging with a hot gas normally in a flow
direction opposite to that used for the adsorption step. This is to ensure that
the outlet end of each bed when used on the next adsorption step (for which
the greatest product purity is required) has the lowest loading of adsorbate
at the end of the regeneration step. The bed which has been regenerated
must now be cooled to temperature 7"l and this can be done by using either
cold inert gas or the feedstock. This flow is normally in the direction for
adsorption.