Page 112 - Adsorption Technology & Design, Elsevier (1998)

108 Processes and cycles

to grow because the resistance to flow is less and the less viscous fluid will
continue to intrude. For those situations in which the less dense fluid is the
upper fluid, or the more viscous fluid is the displacing fluid, then flow
instabilities are likely to become corrected.

5.3.5 Number of beds
The factors which determine the number and arrangement of fixed beds
include total feed flowrate, allowable pressure drop, other energy demands,
the length of the mass transfer zone, the method of adsorbent regeneration
and the capital investment. In order to achieve a steady flow of product most
applications include at least two beds such that one is in the adsorption mode
while the other is in the regeneration mode (if regeneration of the adsorbent
is being carried out in situ). For liquid phase applications more than one bed
in the adsorption mode can be operated in parallel, in series or in
combination. A single bed would be used in the adsorption step with a
relatively low total flowrate and a short MTZ length (which would produce a
sharp breakthrough curve). Multiple beds in parallel would be used with a
relatively high total flowrate and a short MTZ length while multiple beds in
series would be used if the MTZ were long. Hence, for high flowrates and
large MTZ lengths the choice is likely to be multiple beds in series and
parallel. Similar principles apply for gas phase separations. For processes in
which regeneration is effected by a reduction in pressure, multiple bed
systems are used to gain other processing advantages such as a reduction in
overall energy demand by equalizing the pressure between beds of high and
low pressure.

Fixed bed pulsed processes (chromatographic processes)
5.3.6
Strictly, a chromatographic process requires the adsorbent to be contained
within a fixed packed bed and the mixture which is to be separated is
introduced as a pulse into a flowing stream of a carrier fluid. Separation of
the components in the feed then occurs as the pulse of feed passes through
the column if the repetitive steps of adsorption and desorption are different
in nature (equilibrium and/or rate) for each adsorbate.

5.4 MOVING BED PROCESSES

Two general possibilities exist to have the adsorbent in motion. In the first
the adsorbent particles move relative to the walls of the containing vessel
and in the second the particles remain in a fixed position relative to the walls