Page 42 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN001-13 May 7, 2001 12:29

264 Adsorption (Chemical Engineering)

by heaters within the bed or, more commonly, by purging of reverse-flow regeneration is that the volume of purge
with a hot purge gas. At higher temperatures the adsorp- required to regenerate the bed is reduced, so this mode of
tion equilibrium constant is reduced so that even quite operation is almost always adopted.
strongly adsorbed species can be removed with a compar- Contact between the fluid phase and the solid adsor-
atively small purge gas volume. In a pressure swing pro- bent is generally accomplished in a packed adsorbent bed.
cess desorption is achieved simply by reducing the total A packed bed is simple and relatively inexpensive and
pressure, while purge gas stripping depends on reducing it has good mass transfer characteristics. However, from
the partial pressure by dilution with an inert purge gas. the standpoint of pressure drop, and therefore power con-
This generally requires a rather large purge volume, so sumption, it is relatively inefficient. Such considerations
such a process would normally be used only in special become important when the throughput is large and the
circumstances. “value added” in the process is small. Examples include
Displacement desorption is similar to purge gas strip- volatile organic compound (VOC) removal processes and
ping, except that an adsorbable species is used to dis- desiccant cooling systems. For such systems a “parallel
place the adsorbed component from the bed. The dis- passage” contactor in which the adsorbent is in the form
placing component should be adsorbed somewhat less of a honeycomb, an array of parallel sheets, or a mono-
strongly than the preferentially adsorbed species so that lith, although more expensive in capital cost, proves to be
the adsorption–desorption equilibrium can be shifted by a more economic option. Such adsorbers are commonly
varying the concentration of the desorbent. Such processes configured in the form of a slowly rotating wheel which
run more or less isothermally and offer a useful alter- allows the adsorbent to be exposed alternately to the feed
native to thermal swing processes for strongly adsorbed streams and the regenerant or purge as it rotates. The re-
species when thermal swing would require temperatures generation section is often heated to yield the analog of a
high enough to cause cracking, coking, or rapid aging of traditional thermal swing process.
the adsorbent. Steam stripping, which is widely used in
solvent recovery systems, can be considered a combina-
A. Thermal Swing Processes
tion of displacement desorption and thermal swing. The
advantages and disadvantages of these methods of regen- Cyclic thermal swing processes are widely used for pu-
eration are summarized in Table III. rification operations such as drying or removal of CO 2
Ingeneraldesorptionisnotcarriedtocompletionduring from natural gas. Design of a cyclic adsorption process re-
the regeneration step, so the bed in fact operates between quires knowledge of the dynamic capacity of the bed or the
two partially loaded states. At the end of the desorption breakthrough curve. If mass transfer resistance and/or ax-
cycle the residue of the more strongly adsorbed species is ial dispersion are significant, the dynamic capacity, which
concentrated near the bed outlet. If the same flow direction is determined by the extent to which the mass transfer
were maintained during adsorption this would cause con- front is broadened during passage through the column,
tamination of the raffinate product at the beginning of may be much smaller than the static capacity determined
the next adsorption step. This problem can be avoided from the equilibrium isotherm. If kinetic and equilibrium
by reversing the flow direction. An additional advantage data are available and the system is sufficiently simple to

TABLE III Factors Governing Choice of Regeneration Method
Method Advantages Disadvantages
Thermal swing Good for strongly adsorbed species, Thermal aging of adsorbent; heat loss
since small change in T gives large means inefficiency in energy usage;
change in q ; desorbate can be unsuitable for rapid cycling, so
∗
recovered at high concentration; adsorbent cannot be used with
applicable to both gases and liquids maximum efficiency; in liquid systems,
high latent heat of interstitial liquid
must be added
Pressure swing Good where weakly adsorbed species is Very low pressure may be required;
required in high purity; rapid cycling, mechanical energy more expensive than
efficient use of adsorbent heat; desorbate recovered at low purity
Displacement Good for strongly held species; avoids Product separation and recovery needed
desorption risk of cracking reactions during (choice of desorbent is crucial)
regeneration; avoids thermal aging
of adsorbent