Fluidized (fluid) bed dryers are used extensively in the pharmaceutical industries to reduce moisture content of pharmaceutical powder and granules. They have also found use in the drying of suspension, slurries, solutions, dilute paste or sludges. A typical fluidized bed dryer consists of the following components.
- Air preparatory unit.
- Product container.
- Exhaust filter.
- Exhaust blower.
- Control panel.
- Air distribution plate.
- Spray nozzle.
- Solution deliver.
In fluidized bed dryer, hot air is passed at high pressure through a perforated bottom of the container containing the wet solids. The wet solids are lifted from the bottom and suspended in a stream of air (fluidized state). The hot air then surrounds every granules. Heat transfer is accomplished by direct contact between the wet solid and hot gases. The vapourized liquid is carried away by the drying gasses. Sometimes to save energy, the exit gas is partially recycled.
The choice of distributor used during and drying process apart from ensuring uniform and stable fluidization also prevents
- Poor fluidization quality of solids in certain regions in the fluidized bed dryer.
- Plugging of distributor –perforated holes.
- Solids from dropping into wind box or gas plenum located beneath the fluidized bed.
The pressure drops across the distributor, must be high enough to ensure good and uniform fluidization.
Parameters to be controlled in fluidized bed dryers (system)
In order to improve operation, efficiency and reproduability of a fluidized bed dryer, some parameters need to be controlled. These parameters are categorized into:
- Apparatus parameters: Those controlled by equipment.
- Process parameters: These controlled by process.
- Product parameters: Those controlled by product.
- Position of the air distribution plate: This parameter influences the pattern.
- Shape of the instrument: The annular based design gives better product.
- Nozzle height: Nozzle height plays a vital role when fluidized bed dryer is used as a coating machine. The atomized coating solution should not get dried before reaching the tablet surface.
- Temperature: Increased temperature leads to increased moisture diffusivity and hence increased drying rate and decreased drying time. The nature of the material plays an important role in the choosing operating temperature.
- Humidity: Faster drying is achieved when the moisture content of the inlet air is maintained at its minimum.
- Air flow rate/gas velocity: Increasing gas velocity increases drying rate but should be maintained at optimized rate (not to fast or too slow). Gas velocity has no effect on particles with high internal resistance to moisture transfer.
- Moisture content of the feed material.
- Feed rate/batch size.
- Product moisture content.
- Particle size, shape and diameter.
There are various types of fluidized bed dryer and an individual’s familiarity with the specific characteristics of these various types enables one to make a logical and cost effective selection of fluidized bed dryer for a drying operation. In many instances, several different types may provide similar performance at the same cost. It should be noted also that not all modified fluidized bed dryers are necessarily better than the conventional dryers in terms of product quality, or energy efficiency, or drug performance.
Conventional fluidized bed dryers include;
- Batch fluidized bed dryers.
- Semi-continuous fluidized bed dryers.
- Well-mixed, continuous fluidized dryers.
- Plug flow fluidized bed dryer.
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. They include but not limited to;
- Hybrid fluidized bed dryers
- Pulsating fluidized bed dryers
- Fluidized bed dryer with immersed heat exchange.
- Mechanically assisted fluidized bed dryer.
- Vibrated fluidized bed dryer.
- Agitated fluidized bed dryer/swirl fluidizers.
- Fluidized bed dryers of inert particles.
- Spouted bed dryer.
- Recirculating fluidized bed dryer.
- Jetting fluidized bed dryer.
- Super heated steam fluidized bed dryer.
- Fluidized bed freeze dryer.
- Heat pump fluidized dryers.
Pharmaceutical uses of fluidized bed dryers
- Used to make effervescent granulations
Advantages of Fluidized bed Dryer
- High rates of moisture removal due to excellent gas-particle constant which results to high heat and mass transfer rates.
- High thermal efficiency is usually achieved if part of the thermal energy for drying is supplied by internal heat exchanger
- Lower capital and maintenance cost
- Reduced contact time for drying.
- Ease of control.
Disadvantages of Fluidized Dryer
- High pressure drops results as a result of the need to suspend the entire bed in gas which equally leads to high energy consumption.
- Requires increased gas handling due to extensive recirculation of exhausts gas for high thermal efficiency operation.
- Poor fluidization and low flexibility especially if the feed is too wet.
- Not the best choice of equipment when organic solvents need to be removed during drying.
- Non uniform product quality for certain types of fluidized bed dryer.
- Entertainment of fine particles.
- High potential of attrition; and in some cases agglomeration of fine particles.
- The conventional hot air fluidized bed dryer is not a good choice of dryer when handling toxic or flammable solids since there is danger of fire or explosion of flammability limits are exceeded.
- Arun S. Mujumdar (2015). Handbook of Industrial Drying. CRC Press, Taylor and Francis Group, New York.