FBD


Principle:

In fluidized bed dryer, hot air (gas) is passed at high pressure through a perforated bottom of the container containing granules to be dried.

The granules are lifted from the bottom and suspended in the steam of air. This condition is called fluidized state.

The hot air (gas) is surrounding every granule to completely dry them. Thus material or granules are uniformly dried.

 

Construction:

Two types of bed dryer are available, vertical fluid bed dryer and horizontal fluid bed dryer.

The construction of vertical fluid bed dryer is shown in the picture.

·      The dryer is made up of stainless steel.

·      A detachable bowl is placed at the bottom of the dryer, which is used for charging and discharging.

·      The bowl has a perforated bottom with a wire mesh support for placing material to be dried.

·      A fan is mounted at the upper part for circulating hot air.

·      Fresh air inlet, pre filter and heat exchanger are connected serially to heat the air to the required temperature.

·      The temperature of hot air and exit air are monitored.

·      Bag filter are placed above the drying bowl for the recovery of fines.



 

Working:

·      The wet granules to be dried are placed in the detachable bowl.

·      The bowl is pushed in to the dryer.

·      Fresh air is allowed to pass through the pre filter, which subsequently get heated by passing through a heat exchanger.

·      The hot air is flows through the bottom of the bowl. Simultaneously fan is allowed to rotate.

·      The air velocity is gradually increased.

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

·      After some time a point of pressure is reached at which frictional drag on the particle is equal to the force of gravity.

·      The granules rise in the container because of high velocity air (gas) and later fall back in a random boiling motion. This condition is said to be fluidized state..

·      The hot air (gas) is surrounding every granule to completely dry them.

·      The air leaves the dryer by passing through the bag filter.

·      The entrained particles remain adhered to the inside surface of the bag, Periodically the bags are shaken to remove the entrained particle.

 

Advantages:

·      It is easy to handle and requires less time to dry particles.

·      High heat transfer coefficient.

·      Low labor cost.

·      Useful for thermolabile substances.

·      Suitable for both small and large scale drying.

·      It shows no hot spots on the final products.

 

Disadvantages:

·      Not suitable for sticky materials.

·      Not the best choice of equipment when organic solvents are to be removed during drying.

·      It has high potential for attrition; and in some cases agglomeration of fine particles.

 

Applications:

·      This process being fast is used in granulation of pharmaceutical powders.

·      It is an efficient method for drying granules.

·      Modified FBDs are used for coating of granules.

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