Page 110 - Adsorption Technology & Design, Elsevier (1998)

106 Processes and cycles

through a cylindrical vessel which is lying horizontally. In some applications,
notably cyclic processes which have many changes of pressure and flow
direction, a pre-load is placed on the top of the adsorbent bed to keep the
adsorbent particles restrained. If flow is required to be horizontal through a
bed lying horizontally then it is likely that flow redistributors will be needed
inside the bed to ensure that flow cannot preferentially take place along the
top of the vessel once settlement has occured.
The flow direction for adsorption in a vertical fixed bed is determined not
only by the potential for lifting or fluidizing the bed but also by whether the
feed is a gas or a liquid. For gas and vapour phase applications velocities
which cause crushing of an adsorbent tend to be much higher than those
required to lift a bed and therefore it is convenient to arrange to have the
highest flowrate in the downwards direction through a vertical bed.
For liquid phase applications the buoyancy forces need to be considered
as well. The flow velocity in the upwards direction should normally be
sufficiently low to prevent bed lifting. However, in some applications it is
desirable to allow some bed expansion to occur and so limit the pressure
drop. As the minimum velocity to cause lifting is exceeded, the pressure
drop increases only slightly with further increases in velocity. Too much
expansion, however, can cause the bed to become well mixed. If this were to
occur within a fixed bed then it would resemble the batch process and create
the risk of reduced purity in the product. Other problems caused by high
velocities include abrasion, attrition and erosion. When desirable, expan-
sion is accordingly limited normally to about 10%.
If the liquid contains suspended solids it may be preferable for flow to be
in a downwards direction. In water treatment applications the adsorbent bed
when so used can act as a particulate trap as well as a means of removing
tastes, odours and pollutants. As filtration proceeds the pressure drop
increases and backwashing is therefore required periodically. Water
treatment beds can be either of the gravity or the pressure type. Gravity beds
are similar to concrete sand filters but incorporate provisions for the
addition and removal of adsorbent, usually granular activated carbon
(GAC), and for about 50% expansion during backwashing. The minimum
depth of GAC is around I m and the hydraulic loading is in the range 0.09-
0.27 ma/min/m 2 of bed cross-sectional area. A pressure filter, which takes
fluid in downflow, comprises a lined steel pressure vessel, an adsorbent
support system, drainage, influent and effluent distribution, surface wash
and backwash conveyance systems. The pressure vessel has a higher
hydraulic loading than a gravity vessel, typically in the range
0.09-0.45 m3/min/m 2 but is limited in size to a maximum diameter of 4 m
and length 20 m. If the feed is free from particulate material then flow in
water treatment applications can be upwards. Smaller adsorbent particles