10.1: Gas distributor
The distributor is a device designed to ensure that the fluidizing gas
is always evenly distributed across the cross-section of the bed.
It is a critical part of the design of a fluidized bed system.
Good design is based on achieving a pressure drop which is
a sufficient fraction of the bed pressure drop.
Readers are referred to Geldart (1986) for guidelines on distributor design.
Many operating problems can be traced back to poor distributor design.
Some distributor designs in common use are shown in Figure 19.
Figure 19: Some distributor designs in common use:
(a) drilled plate, (b) cap design, (c) continuous
horizontal slots, (d) stand pipe design,
(e) sparge tubes with holes pointing downwards.
10.2: Loss of fluidizing gas
Loss of fluidizing gas will lead to collapse of the fluidized bed.
If the process involves the emission of heat then this heat will not
be dissipated as well from the packed bed as it was from the fluidized bed.
The consequences of this should be considered at the design stage.
10.3: Erosion
All parts of the fluidized bed unit are subject to erosion by the solid particles.
Heat transfer tubes within the bed or freeboard are particularly at risk
and erosion here may lead to tube failure. Erosion of the distributor
may lead to poor fluidization and areas of the bed becoming deaerated.
10.4: Loss of fines
Loss of fine solids from the bed reduces the quality of fluidization
and reduces the area of contact between the solids and the gas in the process.
In a catalytic process this means lower conversion.
10.5: Cyclones
Cyclone separators are often used in fluidized beds for separating entrained
solids from the gas stream (see Rhodes, 1998, Chapter 7)
Video 12 [16.0 min to download at 28.8kbaud]:
A gas cyclone in operation separating solids
from gas at the exit of a circulating fluidized bed riser.
Cyclones installed within the fluidized bed vessel would be fitted with
a dip-leg and seal in order to prevent gas entering the solids exit.
Fluidized systems may have two or more stages of cyclone in series in order
to improve separation efficiency. Cyclones are also the subject of erosion
and must be designed to cope with this.
10.6: Solids feeders
Various devices are available for feeding solids into the fluidized bed.
The choice of device depends largely on the nature of the solids feed.
Screw conveyors, spray feeders and pneumatic conveying are in common use.