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MAINTENANCE
1) The aluminium oxide fluidising medium, not being degradable,will only require replacement
when losses occur due to attrition and spillage, or contamination with inert pigments, fillers or
acidic by products from the cleaning process. Fluidising medium lost through entrainment in
the exhaust may be collected in a cyclone trap.
Should the fluidised bath be stored for long periods of time under damp or humid conditions,
moisture may be absorbed by the alumina which is hydroscopic. To avoid violent fluidisation
which occurs when damp alumina is heated above 100°C, operate the bath for a period of
approximately 8 hours at 90°C prior to operation at elevated temperatures.
NEVER ADD COLD OR DAMP ALUMINA TO A HOT BATH AS THIS WILL ALSO CAUSE
VIOLENT FLUIDISATION WHICH CAN BE DANGEROUS.
Allow the bath to cool then add the fresh alumina. If this fresh alumina is a large proportion of
the charge then dry the whole charge as above.
The fluidised bath may be emptied via the drain chute. The blanking cap should be removed
whilst the bed is unfluidised, a suitable container positioned under the chute and the bed
fluidised, causing a flow of medium from the chute. This flow may be interrupted at any time
by turning off the fluidising air.
2) On a daily basis at least, the bed should be cleaned of floating residues by means of a wire
mesh hand scoop. This procedure removes carbon char which impairs fluidisation and acts
as an absorbent.
3) The air line filter supplied with the fluidised bath is self draining. However, it should be kept in
good condition by inspection at two weekly intervals and by cleaning the bowls and washing
or replacing the filter elements as necessary. With exceptionally dirty or wet air supplies this
frequency may have to be increased. Free water or water vapour in the air supply is a
notorious source for the production of hydrochloric acid in the bed when PVC is being
processed.
In addition, oil vapours in the air supply which reach the porous plate are carbonised within
the pores of the plate, quickly causing blockage and consequent poor fluidisation, local
overheating, premature heater failure and possible distortion of the inner container.
4) The fume extraction system (if used) should be regularly maintained. The cyclone trap
collection bin should be emptied at the end of each days use; the collected fluidising medium
being returned to the fluidised bath. It is important to ensure that the air tight seal between
the cyclone and collector bin is remade.
For maintenance of other fume treatment equipment see the associated maintenance
manuals.
SPECIAL MAINTENANCE PROCEDURES TO BE ADOPTED WHEN BURNING OFF PVC
OR OTHER HALOGENATED POLYMERS
Burning off PVC (polyvinyl chloride) in a fluidised bath offers one of the most severe
conditions of operation. Hydrogen chloride (HCL), liberated on the breakdown of PVC, is
absorbed by the bed medium, creating an acidic environment within the bed, especially when
the bed also absorbs water from the atmosphere or fluidising air when cold. HCL is
extremely corrosive, especially when aerated and wet. In addition, in water it produces water
ions which,even in neutral or alkaline solutions, promote corrosion and rusting in steel;
witness for example, the corrosive nature of sea water and calcium chloride road de-icer.
Consequently, fluidised beds used for burning off PVC require strict supervision to minimise
corrosion of the bed its self and of the parts cleaned in it, especially if these are of uncoated
steel. The purpose of most of the recommended maintenance procedure is aimed at keeping
I F B - 1 0 1 O P E R A T O R ' S
M A N U A L
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