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Working Principle and Applications of FBD (Fluid Bed Dryer)

Fluid Bed Dryers have a single stage operation useful to convert solutions, suspensions, and slurries into dry powder. This is a high-speed process in controlled temperature which ensures its efficiency. This process has many different applications in several industries. It is useful in the chemical industry, pharmaceutical industry, food industry, dyestuff and several other processes.

Fluidization: It’s the process of making fine solid particles behave like particles of a fluid, which freely move about, slide past each other, and even collide with other molecules. Fluidization is done by suspending solid particles in a stream of fast-moving air
Bed: Refers to the layer of suspended solid molecules
Dryer Equipment: It’s the equipment where wet substances are dried

In a fluid bed dryer, the inlet air stream is blown past solid powder molecules at rest. The particles are taken up in the dry & hot air stream and dehydrated through evaporation.

Working Principle
  • The main principle behind the working of the fluid bed dryers is, Air is allowed to flow through a bed of solid powdered material in an upward direction. This made so that the velocity is higher than the setting rate of all particles. 
  • When this happens all the solid particles will get blown up and be suspended in an air stream. The solid bed is thus converted into a boiling liquid which is known as the fluidized state. It is then possible to make use of hot air to help with the process of fluidizing the bed. This increases the drying rate of the material.
  • This machine consists of a stainless steel chamber with a removable perforated bottom; also known as a bowl. All the material that needs to be dried has to be placed in this bowl. The way it works, the air is introduced from the bottom of a bowl and heated to a set temperature by the heaters. 
  • Once the air is filtered with the help of a filter it is passed through the bed of the material. The constant airflow is as a result of the fans fitted at the top of the equipment. All parameters of the equipment like the rate of flow, operating temperature are adjustable with the help of a control panel.

This machine is known for its high drying rate since the material is dried in quick time. The materials are allowed to remain in a free-flowing state at all times. The bags used here have a finger-like structure which helps increase the surface area of the drying bed. This type of design helps increase the drying rate and decrease the drying time during the fluid bed drying process.

This machine can have some serious issues if it is not handled in the correct manner. Here are some of the common problems:
  1. Development of an electrostatic charge in the particles.
  2. Burning of material in the fluid-bed-dryer equipment.
  3. Outlet filter blockage can potentially increase the pressure in the vessel resulting in an accident.

A typical fluidized bed dryer consists of the following components
  1. Processing Unit
  2. Air gate
  3. Bowl of FBD
  4. Product Trolley
  5. Filters
  6. Fan Motor
  7. Product temperature sensor
  8. SFM(Solid flow meter)
  9. Safety Window
  10. HMI

Processing Unit
  • Environmental air is processed by the processing unit, in processing unit air is filtered through different filters. First of all air pass-through coarse filters or the mesh filters also called primary filters. After mesh filters air pass-through secondary filters or bag filters. 
  • After bag filters, this air passes through HEPA filters because this air comes in direct contact with the product. This processed air is heated by a heating system which brings the temperature of air according to the specifications.

Air Gate
  • Air from processing unit comes through the supply duct and enter into a bowl of FBD or fluid bed dryer through Air gate. Air gate is controlled by HMI to open and close it. When it is opened air enters into the bowl of fluid bed dryer and when it is closed air entry is also stopped.

Bowl of FBD
  • Bowl is the hollow lower portion of fluid bed dryer over which product trolley is fitted and air from air gate enters into fluid bed dryer bowl and then go to drying chamber through the perforated bottom of the product trolley.

Product Trolley
  • It is the part of FBD which is movable and contains product which is required to be dried. After wet sieving of granules, granules are added into the product trolley and product trolley is sealed over bowl of FBD in the drying chamber. 
  • The lower side of product trolley is fitted with a perforated bottom. In some FBD moving or rotating rods are attached which rotate after some time intervals and prevent the build-up of local mass formation.

  • Filters are a very important part of FBD. These filters prevent product loss during drying. FBD has two types of filters, Stainless steel Filters and Bag Filters. SS filters are made of stainless steel and have fine perforations which prevent proud loss and allow air to pass. Bag filters are made of canvas cloth. 
  • Bag filters are also called candle filters or finger bag filters. The fluid bed dryer which has stainless steel filters is equipped with blowback system which removes the powder attached with filters and prevents blockage of filters. 
  • The Fluid bed dryers having bag filter is attached with a shaking system which prevents blockage of the filter. Filters of Fluid bed dryer should be inspected before use to detect any damage or leakage.

Fan Motor
  • Fan motor controls the air pressure and carrying capacity. When fan motor speed is increased air velocity is increased and vice versa.

Product Temperature sensor
  • A sensor is attached with FBD which is inserted into product trolley and monitors the temperature of dried product.

Solid Flow Meter (SFM)
  • Advance type of fluid bed dryer is attached with SFM solid flow meter which monitors the flow of solid. The solid flow meter is attached within the exhaust air duct. If the filter bag or SS filters installed in FBD are damaged or ruptured product loss occurs. 
  • In this situation, the Solid flow meter plays an important role to prevent product loss from the fluid bed dryer. when filters are damaged powder comes out through exhaust duct where SFM sense the powder flow and shut down the dryer.

Safety window
  • Safety window is installed in a fluid bed dryer to prevent blast of fluid bed dryer in pharmaceutical industries. When the pressure inside the drying chamber increases due to blockage of Filters or any other technical fault the safety window is opened and fumes or pressure built inside is released.

  • HMI is a human-machine interface. It is a touch screen where the operator gives instructions to FBD to change the parameters. All the parameters like inlet air temperature, outlet air temperature etc. is controlled by HMI of fluid bed dryer.

Critical Parameters of FBD
During operation of fluid bed dryer following parameters should be observed critically,
  1. Inlet Air temperature
  2. Outlet air temperature
  3. Product temperature
  4. Fan motor speed
  5. LOD of product

Parameters to be controlled in fluidized bed dryers (system)
In order to improve operation, efficiency, and reproducibility of a fluidized bed dryer, some parameters need to be controlled. These parameters are categorized into:
  1. Apparatus parameters: Those controlled by the equipment.
  2. Process parameters: Those controlled by the process.
  3. Product parameters: Those controlled by the product.

1. Apparatus/Equipment Parameter
  • Position of the air distribution plate: This parameter influences the pattern.
  • Shape of the instrument: The annular based design gives a better product.
  • Nozzle height: Nozzle height plays a vital role when fluidized bed dryers are used as a coating machine. The atomized coating solution should not get dried before reaching the tablet surface.

2. Process/Operating Parameter
  • Temperature: Increased temperature leads to increased moisture diffusivity and hence increased drying rate and decreased drying time. The nature of the material plays an important role in choosing the operating temperature.
  • Humidity: Faster drying is achieved when the moisture content of the inlet air is maintained at its minimum.
  • Airflow rate/gas velocity: Increasing gas velocity increases drying rate but should be maintained at an optimized rate (not to fast or too slow). Gas velocity has no effect on particles with high internal resistance to moisture transfer.

3. Product Parameters
  • Moisture content of the feed material.
  • Feed rate/batch size.
  • Product moisture content.
  • Particle size, shape, and diameter.

Advantages of Fluidized-bed Dryers
  1. High rates of moisture removal due to excellent gas-particle constant which results in high heat and mass transfer rates.
  2. High thermal efficiency is usually achieved if part of the thermal energy for drying is supplied by the internal heat exchanger
  3. Lower capital and maintenance cost
  4. Reduced contact time for drying.
  5. Ease of control.

Disadvantages of Fluidized Dryers
  1. High pressure drops results as a result of the need to suspend the entire bed in gas which equally leads to high energy consumption.
  2. Requires increased gas handling due to extensive recirculation of exhaust gas for high thermal efficiency operation.
  3. Poor fluidization and low flexibility especially if the feed is too wet.
  4. Not the best choice of equipment when organic solvents need to be removed during drying.
  5. Non-uniform product quality for certain types of fluidized bed dryers.
  6. Entertainment of fine particles.
  7. High potential for attrition; and in some cases agglomeration of fine particles.
  8. The conventional hot air fluidized bed dryers are not a good choice of dryer when handling toxic or flammable solids since there is danger of fire or explosion of flammability limits are exceeded.

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