Table of Contents
In the fluidized-bed dryer, hot air is passed through a perforated bottom of the container containing granules to be dried. The granules are suspended in the air stream and rise from the bottom. This condition is called a fluidized state. Hot air surrounds each granule to dry it completely. Therefore, the materials or granules are dried uniformly.
Construction of Fluidized Bed Dryer:
- The fluidized bed dryer consists of a steel shell of a cylindrical or rectangular cross-section. A detachable bowl that has perforated bottom is placed on the bottom of the dryer. It is used to load and unload the materials. The bowl is used to place the materials to be dried. A fan is mounted on the top to circulate hot air. The fresh air inlet, the pre-filter, and the heat exchanger are connected in series to heat the air to the required temperatures. The temperature of the hot air and the exhaust air are monitored. The bag filters are placed on top of the drying container for the recovery of the fines. The airflow is adjusted using recirculation control. The fabric bags are provided to prevent the passage of the fine particles.
Working of Fluidized Bed Dryer:
- The wet granules to be dried are placed in a detachable bowl. The bowl is inserted into the dryer. Fresh air can pass through a pre-filter, which is then heated when passing through a heat exchanger. Hot air flows through the bottom of the bowl. At the same time, the fan starts to rotate. The airspeed increases gradually. When the velocity of the air is greater than the sedimentation rate of the granules, the granules remain partially suspended in the gas stream.
- After a specific time, a pressure point is reached in which the friction drag on the particles is equal to the force of gravity. The granules rise in the container due to the high gas velocity of 1.5 to 7.5 m per minute and then fall back. This condition is said to be a fluidized state. The gas surrounds each granule to dry them completely. The air comes out of the dryer passing through the filters in the bag. The entrained particles remain adhered to the interior of the surface of the bags. Periodically, the bags are shaken to remove entrained particles.
- The residence time for drying is approximately 40 min. The materials are sometimes left in the dryer to reach room temperature. The bowl is removed for unloading. The final product is free-flowing.
Advantages of Fluidized Bed Dryer:
- It takes less time to complete drying as compared to other dryers.
- Drying is achieved at a constant rate.
- Handling time is also short
- It is available in different sizes with the different drying capacity
- The equipment is simple and fewer labor costs are required.
- More thermal efficiency.
- Drying capacity is more than other dryers.
- It facilitates the drying of thermolabile substances the tact time for drying is short.
- It is a batch type or continuous type process
- Many organic powders develop electrostatic charges during drying. To avoid this efficient electrical grounding of the dryer is essential.
- Chances of attrition of some materials resulting in the production of fines.
- It is used for the drying of granules in the production of tablets.
- It is used for the coating of granules.
Make sure you also check our other amazing Article on : Spray Dryer