Fluidized Bed Dryer

Fluidized Bed Dryer

Fluidized Bed Dryer:

Principle: Fluidized bed dryer is operated by the passage of heated air through perforated bed that supports the powder materials. If a gas is allowed to flow upward through the bed of particulate solids at a velocity greater than the settling velocity of the particles and less than the velocity for pneumatic conveying, the solids are blown up and become partially suspended in the gas stream. Instead of gas, air can be assumed for the drying purpose. The resultant mixture of solids and gas behaves like a liquid and the solids are said to be fluidized.

This technique is very efficient and is used in drying granular solids as each particle is surrounded by the drying gas.

The size range of the particles, which can be fluidized, varies greatly from 1m - 2½^// in diameter. It is generally concluded that the particles are distributed in size between 65 mesh to 10m and is best for the smooth fluidization.

  

Mechanism of drying: The materials to be dried are poured into the dryer through the feeder and the materials are setup on the perforated bed. Hot air is generated by the help of a heater and the hot air is sent to a particular direction to the perforated bed with the help of pump.

The harmonizer controls the upward velocity of the air. As the airflow is tremendous and harmonic, the solid particles remain suspended in the air stream. As a result, the upper surface of the particles seems to be wave. In increment of the velocity of the air causes the particles to separate and move freely and the bed is fully fluidized. As a result, each of every particle comes in contact with hot air and dries efficiently.

As the particles move along the hot air, there is always a chance of loosing the fine particles. To prevent this, a filter bag is used and a cover filter is setup behind the outlet so that only moist air goes out.

The dryer capacity ranges from 5 kg – 200 kg and the average drying time is about 20–40min.

Application:

1.      The fluidization technique is efficient for the drying of granular solids because each particle is completely surrounded by the drying gas.

2. Fluidized bed drying has been reported to offer distinct advantages over conventional tray drying for tablet granulation and is widely used for this purpose.

Advantages:

1. Efficient heat and mass transfer gives high drying rate so that drying times are shorter than static bed convection dryer. A batch of tablet granules can be dried in 20–30 min, whereas in compartment bed dryer would require several hours.
2. The fluidized state of the bed gives drying from individual particles and not from the entire bed.
3. The temperature is uniform and can be controlled precisely.
4. The containers are mobile, making the handling simple and reducing labor cost.
5. It produces a free flowing product.
6. Drying time is short, i.e. high output from a small floor space.
7. Capacity ranges from 5–200 kg.

Disadvantages:

1. Due to turbulence of the fluidized state, product loss may occur.
2. Liquid, too wet, sticky and adhesive materials can’t be dried.
3. The vigorous movement of the particles in the hot drying air may lead to the generation of electrostatic change.
4. The cleaning of screen and bag filter is troublesome to avoid contamination.
5. The harmonizer is precise equipment, so the machine is costly.