In a fluidised bed scrubber the typical single bed of a packed tower structure is replaced by two or more shallow beds, and the high surface area angular packings are replaced by hollow ellipsoids which are fluidised by the gas stream.
This is relatively fast moving when compared to the velocity of the gas flow through a packed tower. The residence time of the gas in the tower is thus rather less than in a packed tower but this tends to be compensated by the higher mass transfer induced by the gas turbulence in the fluidised packings.
Fluidised bed scrubbers are not normally used for odour control because of the short residence time of the gas flow within the tower.
A prime advantage of the fluidised bed is that the mobility of the packings minimises the aggregation of particulates and insoluble depositions. Very small particulates, 8µm and below, will however tend to pass through a fluidised bed.
With the higher gas velocity the tower diameter can be narrower and more compact unit designed. Additional costs are, however, incurred with more expensive packing, the complexity of the structure, and the capital outlay and running costs of larger fans.
The efficient function of the fluidised bed depends on the velocity of the gas stream being maintained between specified minimum and maximum levels and this would typically be a narrower range than a packed tower.