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Fluid bed drying is done in fluid beds. These fluid beds are designed to dry powder particles where the residual moisture content is higher than what is wanted in the final powder. Hot drying air is distributed through specially designed perforated plates on which the powder particles/agglomerates are resting.
The velocity of the drying air makes the powder ‘fluidize’, i.e. the particles get airborne; however, to the extent that only the finest and smallest particles leave the fluid bed with the air, while the bigger particles/agglomerates are transported with the upward and forward directed flow of air into the subsequent section, due to the perforation technique of the plate.
Fluid bed drying is suited for powders, granules, agglomerates, and pellets with an average particle size between 50 microns and 5 mm.
We distinguish between two different types of fluid beds:
Back mix fluid bed drying is used for powders that are not fluidizable due to high moisture content. The moist powder enters the back mix fluid bed where powder with lower moisture content is already fluidizing. The product temperature and moisture content remains uniform throughout the fluidized layer. By controlling the powder layer in the back mix fluid bed, residence time can be controlled. By controlling the amount and temperature of the drying air, the moisture content in the fluidized layer can be controlled. Heat panels, where hot water or steam is circulated, can be submerged into the fluidized powder layer for an additional heat transfer. This will reduce the amount of drying air.
Plug flow fluid bed drying is used for powders that are directly fluidizable. The design of this type of fluid bed is important for achieving a precise, narrow and controlled residence time distribution. The plug flow fluid bed can be stationary or vibrating as in the VIBRO-FLUIDIZER™. The vibration facilitates the fluidization and forwards transport of the powder on the perforated plate. The powder passes through the plug flow fluid bed while coming close to equilibrium with the incoming drying air. By using cold and/or dehumidified air in a subsequent section, the powder can be cooled. The unit operates with a powder layer of approx. 200 mm. This gives a much more lenient powder treatment, since powder residence times are shorter than in non-vibrating plug flow fluid beds that operate at much deeper powder layers.
What does GEA Niro offer?
GEA Niro offers the two types of basic fluid bed models, both individually designed for the solids flow pattern in the dryer, but also as stand alone units or combined to form specialty dryers.
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