Page 111 - Adsorption Technology & Design, Elsevier (1998)

Processes and cycles 107

can be used to obtain higher adsorption rates but care must be taken to
prevent expansion. Without particulate material being present there is no
need for backwashing. Thus the MTZ remains undisturbed and a greater
adsorption efficiency can be maintained.
The expanded bed adsorber with flow in the upwards direction is also
popular in the water industry. Expansion of about 10% separates individual
adsorbent particles and allows suspended solids to pass straight through.
The hydraulic loading is typically around 0.27 ma/min/m 2 but its exact value
depends on the size of the particles.

5.3.4 Drainage and filling in liquid phase adsorption
For the design of a bed used in a liquid phase application which is to be
regenerated using a hot gas it is important to consider the arrangements for
draining after the adsorption step and filling with liquid again after the
regeneration step. Clearly, drainage must be downwards and if the
adsorption step is also downwards then the collected fluid can be added to
the product since it will have left from the cleanest part of the adsorption
bed. Conversely, if the adsorption step has the flow in the upwards direction,
then the drained fluid must be collected and returned to the feed, or
otherwise disposed of. Gravitational flow, sometimes assisted by a 1-2 bar
pressure gradient, is used for drainage. The time of this step could be
significant, perhaps 30 minutes, and even after this period a significant
hold-up of liquid on the adsorbent, perhaps up to 40 cm3/100 g of adsorbent,
might remain in the micro- and macropores of the adsorbent and in the
bridges between adjacent adsorbent particles. In processes in which
regeneration is effected by an increase in temperature this remaining liquid
will consume additional energy when it is vaporized from the bed.
It is preferable in a liquid phase application to refill an adsorbent bed in
the upwards direction because it is easier to sweep out pockets of gas or
vapour and so prevent maldistribution in the proceeding adsorption step.
Consideration must be given to the time required to ensure that the gas and
vapour pockets have been removed completely otherwise there is a risk that
they will contaminate the product in the adsorption step and cause excessive
bed lifting if flow is upwards during the adsorption step.
For those processes in which a second liquid is used to displace the first in
the regeneration step the problem of 'fingering' should be avoided. This
phenomenon arises due to density and viscosity differences at the liquid
-liquid interfaces and can cause columns of one fluid to pass through the
other even in well packed beds. Hydrodynamic instability can be created
when a denser fluid is located above a less dense fluid. Also, if a less viscous
fluid is displacing a more viscous one then any bulge in the interface will tend