Fluidized Bed Dryer and Bag Filters


Fluidized bed dryer

Drying is very much important for wet granules for compression into the tablet and for modifying the characteristics of viscous and sticky materials. Drying is commonly the last step in unit process before compression into tablet form and packaging.

Principle: Fluidized bed dryer is better known by name because hot air (gas) or ambient air is allowed to flow upward at high pressure through a perforated bottom of vessel containing bed of particulate solids or wet granules. The velocity of hot air is more than the settling velocity of the granules/particles by which they remain suspended in stream of hot air. This condition is called fluidized state. The hot air surrounds every granule/particle and completely dries them.

Construction: Two types of FBD are available, vertical and horizontal which are made up of stainless steel or plastic. Vertical type is used for batch drying and horizontal is used for continuous drying. A detachable vessel is placed at the bottom of dryer for loading and unloading of material. This vessel has perforated mesh through which hot air with high pressure is passed. Bag filters are placed above the vessel for collection of dried material.

Working: The material to be dried is placed in detachable vessel at bottom of the dryer. The air is introduced from below through the prefilter, which is heated by means of heaters installed therein. Simultaneously fan is also allowed to work. As the velocity of air is increased, the bed begins to expand and further increase causes turbulent motion of particles called fluidization. The granules/particles remain suspended in air stream. Later on, a state of pressure is reached at which frictional drag on the granules/particles is equal to the force of gravity. The particles rise in stream of air due to high velocity of gas, this condition is called fluidised state.

Bag filters: These are placed above the drying vessel for the recovery of particles and dried material.  Various features of theses filter bags are:

  1. These are made of different types of material or fabrics- cotton, nylon, polyester, satin, polypropylene.
  2. These can bear high pressure of hot air or high flow rates
  3. Durable construction
  4. Good resistance to wear and abrasion
  5. These are available in different pore sizes, thickness, fabrics, permeability.
  6. Anti-static fabrics are also available.
  7. These are available in different sizes which can easily fit in equipment.

Advantages of fluidized bed dryer:

  1. Thermal efficiency is 2-8 times than tray drier
  2. It is available in different sizes having drying capacity in range of 5- 250 kg/hour.
  3. Free flowing granules/particles are made.
  4. It can also be used for mixing the material with high efficiency.
  5. It can be used for both batch and continuous drying.
  6. Loading and unloading of material is very easy due to detachable vessel
  7. Higher drying temperatures than tray and truck dryer.


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