Fluidized Bed Dryer and Vacuum Dryer

 Fluidized Bed Dryer

When a solid material present in a holding vessel is placed under suitable conditions for the solid/fluid mixture to act as a fluid, a fluidized bed is formed. This is possible by exposing the material to pressurized fluid.

Principle

The principle of fluidization fluidized bed drying involves introducing hot air or gas (at high pressure) within a container through its perforated bottom. The container is filled with granules to be dried. The gas introduced lifts up the granules from the bottom and sues— them in the air stream, thus, resulting in a fluidized state. This dryer is useful for uniform drying methods of the material as the hot gas vacuum drying process surrounds all the granules completely.

Construction

Vertical fluid bed dryer and horizontal fluid bed dryer are the two types of bed dryers. A vertical fluidized bed dryer is made up of stainless steel or plastic.

At the bottom of the dryer, a detachable bowl used for charging and discharging is present. This bowl has a perforated bottom with a wire mesh which supports the materials to be dry heat sensitive The upper part of the dryer is equipped with a fan for circulating hot air. The air is heated up to the desired temperature with the help of fresh air inlet, pre-filter, and heat exchanger connected in a series. Temperature of the air at the inlet and outlet are monitored. Filter bags are placed above the drying bowl to recover the fines.

Working

The wet granules to be dried are put in the detachable bowl, which is then placed into the dryer. The fresh air introduced is allowed to pass through a pre-filter, and then through a heat exchanger after which it gets heated. The hot air flows through the bottom of the bowl and the fans are allowed to rotate. Velocity of the air is increased gradually and when it exceeds the settling velocity of granules, the granules remain partially suspended in the gas stream. At a point of pressure, the frictional drag on moisture content the particles equals the gravity force. At this point, the granules rise in the container because of high velocity gas (1.5-7.5m/min) and then fall back in a random boiling motion. This condition is the fluidized state.

The hot air completely dries the material by surrounding each and every granule. The air passes through the filter bags and exits the dryer. The particles trapped in the filter bags remain adhered to their inside surface which can be removed by shaking the bags at frequent intervals. Intense mixing of granules and hot inlet air temperature give uniform conditions of temperature, composition, and particle size distribution. Drying is achieved at a constant rate and a short falling rate period. The residence time for drying is 40 minutes. The material reaches the ambient temperature when left for some time in the dryer. Thereafter, the bowl is pulled out for discharging the free-flowing end product.

Uses

Fluidized bed dryer has the following uses:

1) It is used for drying time granules to be punched into tablets.

2) It is used for mixing, granulation, and drying.

3) It is used for coating granules.

4) It is used for mixing ingredients.

Merits

Fluidized bed dryer has the following merits:

1) It is less time-consuming as it takes 20-40 minutes for drying in comparison to tray dryer which takes 24 hours.

2) Its handling time of material is 15 times faster than the tray dryer.

3) Its handling of material is simple and demands reduced labour as the drying containers are mobile.

4) It is available in different sizes with 5-200kg/hour drying capacity.

5) Its thermal efficiency is 2-6 times than tray dryer.

6) Its mixing efficiency is high.

7) Its efficient mixing and drying capacities remove the hot spots in the dryer.

8) Its temperatures for drying can be higher than that of the tray dryer and truck

9) Its time of contact for drying is short, thus, suitable for thermolabile Substances.

10) It is used either as a batch or continuous type.

11) It occupies a small floor space.

12) Migration of soluble material (as in case of static beds) does not occur due to the free movement of individual particles.

Demerits

Fluidized bed dryer has the following demerits:

l) Drying of organic powders results in electrostatic charges which can be avoided by electrical earthing of the dryer.

2) The turbulence occurring in the fluidized state of granules results in attrition of some materials, thus, producing fines. Although, this can be avoided by using an appropriate binding agent.

3) Fine particles get entrapped, therefore, should be collected using filter bags.

 

 

Vacuum Dryer

In pharmaceutical industries, a vacuum oven is the most common vacuum dryer consisting of a jacketed vessel made up of substances which can endure steam pressure in a jacket and vacuum in the oven.

Principle

The principle of vacuum or its application is used by a vacuum dryer to dry the ingredients. When by generating vacuum, the pressure is dropped. water boils at a lower temperature leading to rapid evaporation.

Construction

Vacuum dryer consists of a heavy-jacketed vessel made up of cast iron and can endure steam pressure in a jacket and high vacuum in the oven.

 

Around 20 hollow shelves which are part of the jacket divide the 1.5m3 bounded space within the jacket into different parts. For efficient heat conduction, these shelves provide approximately 45-50m2 of surface area. The material to be dried is kept on the metal trays placed over the shelves. The oven should be sealed airtight by locking the door. The condenser is positioned between the oven and the vacuum pump.

Working

The trays containing the material to be dried are placed on the shelves. By using a vacuum pump, pressure is reduced up to 30-6 kilopascals and the door is tightly closed. In the hollow space of jackets and shelves, steam or hot air is introduced, and heat transmission occurs via conduction. The boiling point is reduced at this vacuum and water gets evaporated at 25-30C. Condensation occurs when the water vapor enters the condenser. The vacuum line is detached when the process of drying is over, and the dried product is recovered from the trays.

Uses

Vacuum dryer has the following uses:

1) It is used for drying chamber thermolabile materials.

2) It is used for drying dusty and hygroscopic materials.

3) It is used for dying drugs containing toxic solvents.

4) It is used for drying feed containing valuable solvents, which can be recovered by condensation.

5) It is used for drying drugs required as porous end products and food industries.

6) It is used for drying friable dry extracts.

Merits

Vacuum dryer has the following merits:

1) As trays are present within the dryer, material handling becomes easy.

2) Material changing is also easy.

3) Shelves which are hollow and can be heated electrically are used.

4) It provides larger surface area so that rapid and efficient heat transfer occurs throughout the dryer body.

5) The process of drying occurs at a required temperature as hot water can be supplied all over the dryer.

Demerits

Vacuum dryer has the following demerits:

1) It is a batch type process.

2) Its efficiency is low.

3) It is expensive.

4) It demands high labor.

5) Its maintenance cost is high.

 

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