Principle of Fluidized Bed Dryer (FBD):

Fluidized Bed Dryer works on a principle of fluidization of the feed materials. In fluidization process, hot air is introduced at high pressure through a perforated bed of moist solid particulate. The wet solids are lifted from the bottom and suspended in a stream of air (fluidized state). The hot gas Surrounding every granule to completely dry them. Thus, materials or granules are uniformly dried. 

Types of Fluidized Bed Dryer (FBD):

Two types of bed dryers are available

1. Vertical fluid bed dryer

2. Horizontal fluid bed dryer

1. Vertical 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. 

Working of VERTICAL Fluidized Bed Dryer:

Heated air is introduced from upper part of the apparatus through the wet materials in a drying chamber.

A mesh support is fitted at the bottom to hold wet materials.

Vertical bed dryer may be batch type (shown as above figure) or continuous type dryer.

Vertical fluid bed dryer unit can be used as granulator. The Dry ingredients/powder is placed in chamber & Fluidized while the granulating liquid is sprayed into the bed causing particles to agglomerate into granules. After that granules are dried heating air.

• The wet granules to be dried are placed in a detachable bowl. The bowl is inserted in the drier.
• Fresh air can pass trough a prefilter, which is then heated when passing trough a heat exchanger.
• Hot air flows through the bottom of the bowl.
• At the same time, fan start to rotate.
• The air speed increases gradually.
• When the velocity of air is greater than the sedimentation rate of the granules, the granules remain suspended in the gas stream. After specific time, a pressure point is reached in which the friction drag on a particle is equal to the force of gravity. The granules rise in the container due to high gas velocity of 1.5 to 7.5 meter per minute and then fall back. This state is known as fluidized state.
• The gas surround to each granule do 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 surface of bags.
• Periodically bags are shaken to remove entrained particles

2. Horizontal fluid bed dryer

Working of Horizontal fluid bed dryer

• Horizontal vibrating conveyer FBD is a continuous type FBD which is suitable for drying of Pharmaceutical products.
• Wet materials is placed on perforated conveying surface. Heated air is passed from below conveying perforated surface through the bed of wet solids.
• Vibration is used to maintained uniform density & thickness of fluidized bed in drying zone.
• Dryer can be divided into several different zones with control of airflow & TEMPERATURE so that drying can take place at the maximum desirable rate in each stage without sacrificing efficiency or damaging heat-sensitive materials.

Advantages of Fluidized Bed Dryer (FBD):

• Short Drying Time (20-40 minutes)- It takes less time to complete drying as compared to other dryer
• Excellent mixing action of dryer
• No hot spot are produced- facilitates the drying of thermolabile substances since the contact time of drying is short.
• Higher temperature can be employed if required
• available at different sizes with different drying capacity.
• Available as batch type or continuous type process.
• More thermal efficiency

Disadvantages of Fluidized Bed Dryer:

• Many organic powders develop electrostatic charge during drying. To avoid this efficient electrical grounding of the dryer is essential.
• Chances of attrition of some materials resulting in production of fines.

Pharmaceutical applications of Fluidized Bed Dryer:

• It is used for drying of granules in the production of tablets.
• It is used for coating of granules.
• It can be used for three operations such as mixing, granulation and drying.

Variants:

Plug flow dryer: It is a rectangular fluid bed dryer having different compartments for fluidisation. The material is made to move from inlet through different compartments to outlet. Different drying condition can be maintained in the compartments. Often the last compartment is fluidised with cold gas to cool the solids before discharge

Also Read…

Classification of Dryers

Freeze Dryer/Lyophilization