Fluid bed processing involves drying, cooling, agglomeration, granulation, and coating of particulate materials. It is ideal for a wide range of both heat sensitive and non-heat sensitive products. Uniform processing conditions are achieved by passing a gas (usually air) through a product layer under controlled velocity conditions to create a fluidized state.
In fluid bed drying, heat is supplied by the fluidization gas, but the gas flow need not be the only source. Heat may be effectively introduced by heating surfaces (panels or tubes) immersed in the fluidized layer.
In fluid bed cooling, cold gas (usually ambient or conditioned air) is used. Conditioning of the gas may be required to achieve sufficient product cooling in an economically sized plant and to prevent pick up of volatiles (usually moisture). Heat may also be removed by cooling surfaces immersed in the fluidized layer.
Agglomeration and granulation may be performed in a number of ways depending upon the feed to be processed and the product properties to be achieved.
Fluid bed coating of powders, granules, or tablets involves the spraying of a liquid on the fluidized powder under strictly controlled conditions.
The Niro Group offers a range of systems for fluid bed drying. Continuous and batch dryers, coolers, agglomeration, coating, pelletizing and granulation systems are designed to operate in open cycle (involving water evaporation) or closed cycle (involving mostly organic solvent evaporation). For products posing a risk of dust explosion during processing, pressure shock resistant designs, self-inertized and closed cycle systems are available.
Fluid bed drying offers important advantages over other methods of drying particulate materials.
Particle fluidization gives easy material transport, high rates of heat exchange at high thermal efficiency while preventing overheating of individual particles.
The properties of a given product are determined from drying rate data, i.e. how volatile content changes with time in a batch fluid bed operating under controlled conditions. Other important properties are fluidization gas velocity, fluidization point (i.e. the volatile content below which fluidization without mechanical agitation or vibration is possible), equilibrium volatile content, and heat transfer coefficient for immersed heating surfaces.
These and other data are applied in a computational model of fluid bed processing, thus enabling dimensioning of industrial drying systems.
Fluid bed drying is suited for powders, granules, agglomerates, and pellets with an average particle size normally between 50 and 5,000 microns. Very fine, light powders or highly elongated particles may require vibration for successful fluid bed drying.