Fluid Bed Driers

Fluid bed processing involves drying, cooling, agglomeration, granulation, and coating of particulate materials. 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, the fluidization gas supplies heat, but the gas flow need not be the only source. Fluid bed drying offers important advantages over other methods for drying of particulate materials. Particle fluidization gives easy material transport, high rates of drying at high thermal efficiency while preventing overheating of individual particles. Fluid bed drying is suitable for powders, granules,agglomerates and pellets with 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.

Fluid bed drying is an efficient method of drying milk for production of instant milk powder. The fluid bed is connected to the bottom of the drying chamber and consists of a casing with a perforated bottom. The casing is spring mounted and can be vibrated by a motor. The powder from the drying chamber is admitted to the first section of the fluid-bed dryer where it is humidified by steam. The vibrator conveys the powder through the drying sections, where air at a gradually decreasing temperature is admitted through the powder bed. Agglomeration takes place in the first stage of drying when the particles adhere to each other. Subsequently water is evaporated from the agglomerates during their passage through the drying sections.In fluid bed for 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 involves the spraying of a liquid on to the fluidized layer under strictly controlled conditions.

There are two basic types of fluid bed designs according to the solids flow pattern in the dryer. 

• The back-mix flow design for feeds that require a degree of drying before fluidization is established.
• The plug flow design for feeds, those are directly fluidizable on entering the fluid bed

i) Back-mix flow fluid beds: These are applied for feeds that are nonfluidizable in their original state, but become fluidizable after a short time in the dryer, e.g. after removal of surface volatiles from the particles. The condition of the fluidizing material is kept well below this fluidization point. Proper fluidization is obtained by distributing the feed over the bed surface and by designing the fluid bed to allow total solids mixing (back-mix flow) within its confines. The product temperature and moisture are uniform throughout the fluidized layer.

ii) Plug flow fluid beds: These are applied for feeds that are directly fluidizable. Plug flow of solids is obtained by designing the fluid bed with baffles to limit solids mixing in the horizontal direction. The volatile content and temperature vary uniformly as solids pass through the bed, and the plug flow enables the solids to come close to equilibrium with the incoming gas.In plug-flow fluidized bed dryers, the bed usually has a length-to-width ratio in the range of 5:1 to 30:1; the solids flow continuously as a plug through the channel from the inlet to the exit. This ensures approximately equal residence time for all particles, regardless of their size. The main operational problems occur at the feed end where wet feedstock must be fluidized directly rather than mixed with drier material as in a well-mixed unit.

iii) Vibrating fluid bed dryer: Vibrating fluid bed dryer is basically of the plug flow type. It is especially applied for drying and cooling products that fluidize poorly due to a broad particle size distribution, highly irregular particle shape, or require relatively low fluidization velocities to prevent attrition. The vibro-fluidizer operates with a shallow powder layer of less than 200 mm. This gives a much lower product residence time per unit bed area than non-vibrating beds, which can have powder layers up to 1500 mm.

iv) Contact fluidizers: This is a rectangular fluid bed dryer incorporating back-mix and mix flow sections. A rotary distributor disperses the wet feed evenly over the back-mix section equipped with contact heating surfaces immersed in the fluidized layer. The heating surfaces provide a significant portion of the required energy, and therefore, it is possible to reduce both the temperature and the flow of gas through the system. This is particularly important for heat sensitive products. Subsequent plug flow sections are used for post drying and cooling, if required.

v) Batch fluidized bed dryer: Batch fluidized bed dryers are used for low throughput (normally <50 kg/h and for 100 kg/h) and multi-product applications. Drying air is heated directly or indirectly usually to a fixed temperature. The drying air flow rate is also usually fixed. However, it is possible to start drying at a higher inlet gas temperature and flow rate and lower it since the product moisture content falls below the critical value.Mechanical agitators or vibration may be needed if the material is difficult to fluidize.

vi) Multi-tier fluid beds: These fluid beds consist of two or more stacked fluid beds. The upper tier (back-mix or plug flow) is for pre drying and the lower tier (plug flow) for post drying. The drying gas travels countercurrent to the solids. The gas leaving the lower tier contains sensible heat, which is transferred to the upper tier. Furthermore, each fluid bed may be provided with immersed heating surfaces. These designs result in a low gas throughput and high thermal efficiency, which are of great importance in closed cycle drying systems. Various possibilities of combining fluid beds are presented in Table 

Multi stage fluidized bed dryers