Page 142 - Adsorption Technology & Design, Elsevier (1998)

Processes and cycles 131

A special bed cooling step is not always required. For example, if the
thermal wave caused by the heat of adsorption runs ahead of the mass
transfer zone on the adsorption step then the temperature in the MTZ will
be more or less equal to the feed temperature and may be unaffected by the
temperature used during desorption or regeneration.
The basic TSA cycle clearly consists of four steps, namely (i) adsorption
onto a regenerated solid at 7'1, (ii) a temperature increase to T2 by means of
hot feed, hot purge gas, steam, etc, (iii) desorption with feed or purge fluid
at T2 and (iv) cooling to 7'1 with cold feed or inert purge. Desorption with hot
feed is practicable only if high purity is not required on the subsequent
adsorption step. The loading on the adsorbent in the adsorption step is
improved by operating the bed at the lowest temperature which avoids
condensation. In practice, ambient temperature is often used. The desorp-
tion temperature and the purge flowrate are related. That is, the same extent
of desorption could be achieved either with a low purge flowrate and a high
temperature, or with a high purge flowrate and a low temperature.
The heating and desorption steps must provide sufficient energy to
perform a number of functions. First, the adsorbent, its associated
adsorbate and the containment vessel must be raised to the desorption
temperature. Secondly, the heat of desorption must be provided. Thirdly,
the adsorbent and vessel temperature must be raised to the final regenera-
tion temperature if this needs to be higher than that used for desorption.
Energy will also be required if there are heat losses and if vaporization of
undrained liquid is necessary. TSA cycles can therefore consume large
amounts of energy per unit quantity of adsorbate.
Because beds of adsorbent cannot normally be heated and cooled quickly,
the cycle time of a typical TSA process may range from several hours for a
bulk separation to several days for a purification. Long cycle times inevitably
mean large bed lengths with consequential high adsorbent inventories. Thus
the effective use of the adsorbent is poor if the mass transfer zone length is
short. The times for the adsorption and desorption/regeneration steps must
be equal in a TSA process and the cycle time may be controlled by events in
the desorption part of the cycle. The purge step becomes the limiting step for
situations in which the purge does not have sufficient capacity to remove the
adsorbate as quickly as it can provide the energy for desorption. Purge-
limited desorption/regeneration occurs if the regeneration pressure is
relatively high, the regeneration temperature is relatively low and the
adsorbates are strongly adsorbed. For such situations direct heating with the
purge gas should be employed. If the desorption/regeneration process is
thermally controlled then it is better to employ indirect heating such as
electrical elements or heating coils. The use of heating coils offers a further
advantage in that they can be used to provide indirect cooling as well.