Fluidised Bed Dryer

Principle

Hot air is passed at high pressure through a perforated bottom of container containing granules to be dried

The granules are lifted from the bottom & suspended in the stream of air. This condition is called Fluidized state

The hot gas surrounding every granule to completely dry them

Thus, materials or granules are uniformly dried.

Construction

Two types of bed dryers are available:

1. Vertical fluid bed dryer

2. Horizontal fluid bed dryer

• The dryer is made up of stainless steel or plastic. A detachable bowl is placed at the bottom of the dryer, which is used for charging & discharging

• The bowl has perforated bottom with a wire mesh support for placing materials to be dried. A fan is mounted in the upper part of circulating hot air

• Fresh air inlet, prefilter & heat exchanger are connected serially to heat the air to the required temperature. The temp. of hot air & exit air are monitored

• Bag filters are placed above the drying bowl for the recovery of fines

• The wet granules to be dried are placed in the detachable bowl & it is pushed into dryer

• Fresh air is allowed to pass through a prefilter, which subsequently gets heated by passing through a heat exchanger. The hot air flows through the bottom of the bowl. The air velocity is gradually increased

• When the velocity of the air is greater than settling velocity of granules, the granules remain partially suspended in the gas stream

• The granules rise in the container because of high velocity gas & later fall back in a random boiling motion. This condition said to be Fluidised State.

• The gas surrounds every granule to completely dry them. The air leaves the dryer by passing through bag filters

• The entrained particles remain adhered to the inside of the bags. Periodically the bags are shaken to remove the entrained particles

Advantages

1. It requires less time to complete drying i.e. 20 to 40 min. compared to 24hr of tray dryer

2. Different sizes with different drying capacity from 5 to 200 kg/hr

3. Handling simple & low labour cost

4. Thermal efficiency is 2 to 6 times than tray dryer

5. Mixing efficiency is also high as compared to other dryers

6. Hot spots are not observed in the dryer

7. It facilitates the drying of thermolabile substances, as contact time is short

8. It can be used for batch type or continuous type

9. It has high output from a small floor space

10. The free movement of individual particles eliminates the risk of soluble material migrating as may occur in static bed

Disadvantages

1. Many organic powders develop electrostatic charge during drying. To avoid this, efficient earthing of dryer is essential

2. The turbulence of the fluidized state of granules may cause attrition of some materials resulting in the production of fines. but using a suitable binding agent this problem can be solved

3. Fine particles may become entrained & must be collected by bag filters

Uses

1. It is popularly used for drying of granules in the production of tablets

2. It can be used in three operations like mixing, granulation & drying

3. It is modified for coating of granules