Page 109 - Adsorption Technology & Design, Elsevier (1998)

Processes and cycles 105

5.3.2 Adsorbent support and flow distribution

Adsorbent particles can be supported in one of two ways in an adsorption
vessel. The first comprises a series of grids with each successively higher
layer having a finer mesh. The second comprises a graded system of inert
particles which may range from ceramic balls down in size to gravel. For
those applications where the adsorbent may have to be removed from the
bottom outlet there may be no support system but the flow distributors may,
as a result, be complex.
At the top of a bed a layer of inert support balls may be used as ballast in
order to prevent movement and hence attrition of the adsorbent. The ballast
needs to be denser and significantly larger than the adsorbent particles and a
retention screen is normally placed on top in order to prevent the ballast
from migrating downwards through the bed. The retention screen cannot be
fixed to the column wall because it must be capable of taking up bed
settlement in cyclic processes. For gas phase applications in which frequent
changes of bed pressure and flow direction occur it is generally necessary to
use a pre-load on the top of the adsorbent bed. This pre-load, which might
take the form of a spring or compressed fibre pad, is used to prevent
movement and automatically allows for settlement.
Intermediate bed supports might be required when the adsorbent is
susceptible to damage by crushing. Intermediate bed supports might also be
used in compound adsorbent systems in which it may be necessary
periodically to change individual adsorbent materials.
Poor fluid flow distribution can be avoided using a variety of techniques.
First, sufficient plenum spaces should be allowed above and below the fixed
bed. Secondly, baffle plates should be fitted when symmetrically placed inlet
and outlet nozzles are used. The baffle plates, which may be solid, slotted or
perforated, should be sufficiently large to ensure that the incoming fluid is
redirected, its momentum is broken and it cannot impinge directly on the
adsorbent particles. If balls and/or gravel are used to further aid distribu-
tion, then screens should be used to surround the baffles. Thirdly, it may be
necessary to use nozzle headers in which flow can enter the bed from several
nozzles along a distribution system. The holes along such a distribution
system may not necessarily be of uniform size.

5.3.3 Flow direction

Fixed bed adsorbers commonly are vertical and cylindrical vessels. While
horizontal vessels are occasionally used, vertical orientation is preferred to
avoid the creation of flow maldistribution when settling of a bed or
movement of particles within it occurs. Flow can be arranged vertically