Page 101 - Adsorption Technology & Design, Elsevier (1998)

98 Processes and cycles

MTZ, the adsorbent will be in equilibrium with the feed and unable to
adsorb further adsorbate molecules. Downstream of the MTZ, the
adsorbent will not have been in contact with any adsorbate molecules and
therefore, despite having the capability of doing so, will also be unable to
adsorb adsorbate molecules. Thus, if the time selected for progress of the
MTZ through the bed is long the bed will be large and it will contain a large
inventory of expensive adsorbent. In addition the pressure drop will be
proportionately large.
(2) Any time up to breakthrough it is practicable to take the adsorbent
bed off-line. Therefore, in order to have a continuous stream of product it is
necessary to have more than one bed of adsorbent in the overall adsorption
equipment. The regeneration time for the second bed must not be longer
than the time to reach breakthrough of the adsorbate during adsorption in
the first bed. In practice more than two beds are often used which introduces
the need for complex pipe and valve arrangements together with a control
system.
(3) Adsorption is always an exothermic process (see Section 3.1) and
desorption can therefore be effected by raising the temperature of the
adsorbent. In thermal regeneration, or thermal swing, processes it is difficult
to heat and cool large beds of highly porous adsorbent materials quickly
because the heat transfer processes are not especially good. Poor heat
transfer leads to long heating and cooling times which thereby creates the
need for large beds. A further disadvantage of poor heat transfer can
manifest itself in a rise in the temperature of the bed in or near to the MTZ
due to the exothermic nature of the adsorption process. Since the loading of
an adsorbate is reduced by increasing the temperature of the adsorbent, the
performance of the bed will become inferior and the product purity may
become poorer if the bed cannot be kept cool near the exit end.
(4) Despite the apparent simplicity of fixed beds they are difficult to
design accurately because the progress of the MTZ introduces time into the
design equations. To solve the problem rigorously it is necessary, in most
practical applications, to solve sets of partial differential equations which
describe the mass and heat transfer phenomena. Several short-cut design
techniques exist but they can vary considerably in their accuracy. The
uncertainties which arise, and the simplifications which are often required,
inevitably introduce conservatism into the bed sizing calculations. In turn,
this leads to equipment sizes and adsorbent inventories being larger than the
minimum requirements.

The main advantage of moving bed processes is that the adsorbent can be
regenerated as soon as its role in the adsorption step has been completed.
Thus, in theory at least, the inventory of adsorbent can be kept to a