Fluidized bed dryer (FBD) is well known and widely used equipment in pharmaceutical manufacturing. It is used in the granulation process for drying the material to get desired moisture content in the granules or powders required for perfect compression of tablet formulation. Conventional fluidized bed dryers include batch fluidized bed dryer, semicontinuous fluidized bed dryer, well-mixed continuous fluidized dryer, and plug flow fluidized bed dryer.
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Principle of fluidized bed dryer
The equipment works on a principle of fluidization of the feed materials. In the fluidization process, hot air is introduced at high pressure through a perforated bed of moist solid particulate. If air is allowed to flow through a bed of solid powdered material in the upward direction with a velocity greater than the settling rate of the particles, the solid particles are blown-up and remain suspended in the air stream. At this stage, the solid bed looks like the boiling liquid; therefore this stage is called fluidized. Heat transfer is accomplished by direct contact between the wet solid and hot gases. The use of hot air for fluidizing the bed increases the drying rate of the material. The vaporized liquid is carried away by the drying gases. Sometimes to save energy, the exit gas is partially recycled.
Construction of fluidized bed dryer
A fluidized bed dryer contains a stainless steel chamber having a removable perforated bottom known as the bowl. A typical FBD consists of the air handling unit, product container, exhaust filter, exhaust blower, control panel, air distribution plate, spray nozzle, and solution deliver. The appropriate choice of distributor used during drying process ensures uniform and stable fluidization. The pressure drops across the distributor must be high enough to ensure good and uniform fluidization.
Working of fluidized bed dryer
Material to be dried is placed in the bowl-type vessel. Air is introduced from the top and heated at the required temperature by the heaters. The air is filtered through the filter and then passes through the bed of the material at the bottom. The airflow is generated by the fans fitted at the top of the equipment. The airflow rate and the operating temperature are adjusted by the control panel. As the flow of air increases, the bed expands and particles of powder start to rise up in a turbulent motion. Regular contact with air causes the material to dry. The air leaving the FBD passes through the filter to collect the fine particles of the material. A fluidized bed dryer has a high drying rate and the material is dried in a very short time. Material remains free-flowing and uniform. FBD bags have the finger-like shape to increase the volume of the drying bed that helps to increase the drying rate and decrease the drying time.
(i) FBD has high rates of moisture removal due to excellent air-particle contact which results in high heat and mass transfer rates thus has fast and homogeneous drying.
(ii) High thermal efficiency is usually achieved if part of the thermal energy for drying is supplied by the internal heat exchanger.
(iii) It has low capital and maintenance cost.
(iv) Minimum contact time for drying is best suited for heat-sensitive products.
(v) FBD is highly efficient in material drying to the desired level.
(vi) Handling FBD is easy due to the simple system control panel and thus requires less labour.
(vii) FBD designs come in a wide range of capacities and sizes.
(viii) It shows no hot spots on the final products.
(ix) It is suitable for both continuous and batch material processing.
(i) The high-pressure drop requires more energy to suspend particles.
(ii) Requires increased air handling due to extensive recirculation of exhausts air for high thermal efficiency operation.
(iii) It has poor fluidization and low flexibility in operation, especially if the feed is too wet.
(iv) Not the best choice of equipment when organic solvents are to be removed during drying.
(v) Non-uniform product quality for certain types of materials.
(vi) There may be entertainment of fine particles.
(vii) It has a high potential for attrition and in some cases agglomeration of fine particles.
(viii) The conventional hot air FBD is not a good choice of dryer when handling toxic or flammable solids since there is a danger of fire or explosion if flammability limits are exceeded.
(xi) It has a possibility of product loss.
(x) There are high chances of electrostatic force build-up.
(xi) Using FBD drying of sticky material is quite difficult.
Applications of fluidized bed dryer
(i) Fluidized bed dryers are used in chemical, pharmaceutical, food, dairy, metallurgical, dyes and other process industries for drying of powders, mixing of powders and agglomeration of various materials such as powders, tablets, granules, coals, fertilizers, plastic materials etc.
(ii) This process being fast is used in granulation of pharmaceutical powders.
(iii) Fluidized bed coaters are used widely used for coating of powders, granules, tablets, pellets, beads that are held in suspension by column of air.
(iv) FBD is used for several purposes, such as fluidized bed reactors (types of chemical reactors), solids separation, fluid catalytic cracking, fluidized bed combustion, heat or mass transfer or interface modification, such as applying a coating onto solid items.
(v) The three types (Top spray, Bottom spray, Tangential spray) of FBD are mainly used for aqueous or organic solvent-based polymer film coatings.
(vi) Top-spray fluidized bed coating is used for taste masking, enteric release and barrier films on particles/tablets. Bottom spray coating is used for sustained release and enteric release and Tangential spray coating is used for SR and enteric coating
(vii) FBD is efficiently employed for applications in chemical, pharmaceutical, dyestuff, foodstuff, dairy and various other process industries with the spray dryers and granulation systems for effective drying, mixing, granulation, finishing and cooling of powdered substances.
(viii) It has been preferred over rotary dryers for drying and cooling a wide range of polymer materials which require precise control of residence time and temperature for effective processing.
(ix) It is used for drying moist dibasic calcium phosphate anhydrous (DCPA).
(x) It is suitable for the formulation, development and production of clinical materials.
(xi) The modified versions of FBD are used as precision granulators, top spray granulators, spray drying granulator and granulator coater.
(xii) Various types of modified fluidized bed dryers have been developed and are applied in many industrial processes to overcome some of the problems encountered while using conventional fluidized bed dryer and for a drying process.