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.
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.
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.
• Pharmaceuticals and Biochemical
• Food and Dairy Products
• 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.
Fluid Bed Types:
Fluid bed processors may be configured for either continuous or batch operation.
There are two types of basic fluid bed designs according to the solids flow pattern in the dryer.
• The continuous back-mix flow design for feeds that require a degree of drying before fluidization is established.
• The plug flow design for feeds that is directly fluidizable on entering the fluid bed.
Transport of the solids through the fluid bed may be achieved either by the fluidization alone or a combination of fluidization and vibration.
The flow of gas relative to the solids is characterized either as cross flow in a single tier fluid bed or as cross/counter-current in a multi-tier fluid bed.
Back-mix flow fluid beds
These are applied for feeds that are non-fluidizable 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 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. Heating surfaces immersed in the fluidized layer improve the thermal efficiency and performance of this system. Back-mix fluid beds of both rectangular and circular designs are available.
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. Thereby the residence time distribution of the solids becomes narrow. Plug flow fluid beds of either rectangular or circular designs are especially used for removal of bound volatiles or for heating and cooling. 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.
Plug flow may be achieved in different ways depending upon the shape and size of the bed.
• In rectangular beds, baffles are often arranged to create an alternating flow of solids from side to side.
• In circular beds, baffles are spiral.
• In relatively small circular beds with high powder layers, baffles are radial.
Vibrating fluid beds
This design, marketed under the name VIBRO-FLUIDIZER, 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.
Vibro-Fluidizers incorporate pressure shock resistance and sanitary features if clean operation is required.
This is a rectangular fluid bed dryer incorporating back-mix and plug 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.
Advantages of the CONTACT FLUIDIZER- compared to fluid beds without heating surfaces, two-stage flash/fluid bed dryers, or rotary dryers - include its compact design, high thermal efficiency, and low gas throughput.
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 counter-current 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.
Fluid Bed Drying - Industrial Applications
1. Food Stuffs and Dairy Products
• Baby foods
• Dairy products
• Foodstuff additives
• Health food extracts
• Soup mixers
2. Chemicals: In this wide category of industries, the whole spectrum of fluid bed dryers and process systems are applied. The process may involve drying, cooling, agglomeration, or granulation. The fluid beds may also be used for finishing of powders produced by spray drying. For each application, the process concept and design details are selected to meet the specific requirements based on industrial experience and testing of the product.
• Dyestuffs, pigments
• Inorganic salts
• Organic chemicals
3. Pharmaceuticals and Biochemical: These industries use both continuous and batch processing. The execution of dryers is characterized by high hygienic standards, pressure shock protection, self-emptying capability, and automatic cleaning systems. Batch systems are used for mixing, drying, granulation, and also for coating of powders, pellets, tablets, and capsules.
A wide range of plant systems and sizes are available.
• Fermentation products
4. Polymers: Today, fluid beds are preferred to rotary dryers for drying and cooling a wide range of polymer materials. The heat sensitivity of these materials requires precise control of residence time and temperature. Polymers having either water and organic volatile or monomer content are processed. For the latter, gastight, closed cycle arrangements with solvent recovery are used. Process safety systems are carefully designed for personnel, plant, and environmental protection. Fluid bed dryers with heating panels are often used due to high production rates and demand for lowest energy consumption.
• Polyvinyl chlorides