Vacuum Dryer.

 

Principle:

• Vacuum drying is generally used for the drying of substances which are hygroscopic and heat sensitive, and is based on the principle of creating a vacuum to decrease the chamber pressure below the vapor pressure of the water, causing it to boil. 

• Hence, water evaporates faster, and the rate of drying increases.

Construction:

• The oven is divided into hollow trays which increases the surface area for heat conduction.

• The oven door is locked airtight and is connected to a vacuum pump to reduce the pressure.

• The materials to be dried are kept on the trays inside the vacuum dryer and pressure is reduced by means of vacuum pump.

• The enclosed space ( approximately 1.5 meter cube ) is divided in to a number of portions by means of 20 hollow shelves, which are part of the jacket. 

• These shelves provide a larger surface area ( about 45 to 50 meter square ) for conduction of heat. 

• Over the shelves, metal trays are placed for keeping the material. 

• The oven door can be locked tightly to give an airtight seal. 

• The oven is connected to a vacuum pump by placing a condenser in between.

Working:

• The trays that are present in the dryer are used to dry the material that is placed in the shelves and the pressure is reduced to 30 to 60 Kps by vacuum pump.

• The door closes firmly and steam passes through the jacket space and the shelves.

• So the heat transfer is carried out by the conduction mechanism.

• When evaporated under vacuum, the water is evaporated from the material at 25 - 30℃.

• The vapor goes to the condenser.

• After drying, the vacuum line is disconnected.

• Then the materials are collected from the tray.

Uses:

1. Vacuum dryer can be used for drying of following:

1. Heat sensitive materials, which undergo decomposition.

2. Dusty and hygroscopic material.

3. Drugs containing toxic solvents. These can be separated in to closed containers.

4. Feed containing valuable solvents. These are recovered by condensation.

5. Drugs which are required as porous end products.

6. Friable dry extracts.

Advantages:

1. Material handling is easy.

2. Hollow shelves which are electrically heated can be used.

3. It provides a large surface area. So the heat can be easily transferred through the body of the dryer and the last drying action takes place.

4. Hot water can be supplied through the dryer, which help in drying process at the desired temperature.

Disadvantages:

1. Batch type process.

2. Low efficiency.

3. More expensive.

4. Labour cost is too high.

5. High maintenance.

6. Danger of overheating due to the vacuum.

Commonly Asked Questions.

1. With a well labeled diagram describe construction and working of Vacuum Dryer.

2. Write a short note on Vacuum Dryer.

Silverson Homogenizer.

 

Principle:

• Silverson mixer emulsifiers produce intense shearing force and turbulence by the use of high-speed rotors. 

• This turbulence causes the liquids to pass through fine space formed by closely placed perforated metal sheets. 

• Circulation of material takes place through the head by the suction produced in the inlet at the bottom of the head. 

• Circulation of the material ensures the rapid breakdown of the dispersed liquid into smaller globules.

Construction:

•   It consists of long supporting columns connected to a motor that gives support to the head. 

• The central portion contains a shaft, one end of which is connected to the motor and the other end is connected to the head. 

• The head carries turbine blades. 

• The blades are surrounded by a mesh, which is further enclosed by a cover having openings.

Working:

• The emulsifier head is placed in the vessel containing immiscible liquids (or coarse emulsion) in such a way that it should get completely dipped in the liquid. 

• When the motor is started, the central rotating shaft rotates the head, which in turn rotates turbine blades at a very high speed. 

• This creates a pressure difference. As a result, liquids are sucked into the head from the center of the base and subjected to intense mixing action. 

• Centrifugal forces expel the contents of the head with great force through the mesh and onto the cover. 

• As a result, a fine emulsion emerges through the openings of the outer cover. 

• The intake and expulsion of the mixture set up a pattern of circulation to ensure the rapid breakdown of the bigger globules into smaller globules.

Uses:

1. Silverson mixer is used for the preparation of emulsions and creams of fine particle size.

Advantages:

1. Silverson mixer is available in different sizes to handle liquids ranging from a few milliliters to several thousand liters.

2. It can be used for batch operations. It is also used for continuous operation by incorporating into a pipeline, through which the immiscible liquids flow.

Disadvantages:

1. Occasionally, there is a chance of clogging of the pores of the mesh.

2. Foaming.

Commonly Asked Questions:

1. Define Mixing. With help of a diagram describe the principle, construction and working of the Silverson Homogenizer.

2. Write a short note on Silverson Homogenizer.

Fluidized Bed Dryer (FBD).

 

Principle of FBD:

• The equipment works on a principle of fluidization of the feed materials.

•  In the 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)

Construction of FBD:

• The dryer is made up of stainless steel or plastic.

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

• The bowel has a perforated bottom with a wire mesh support for placing materials to be dried.

• A fan is mounted in the upper part for circulating hot air.

• Fresh air inlet, prefilter 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 filters are placed above the drying bowl for the recovery of fines.

Working:

• The wet granules to be dried are placed in a detachable bowl. The bowl is inserted in the drier.

• Fresh air can pass through a prefilter, which is then heated when passing through a heat exchanger.

• Hot air flows through the bottom of the bowl.

• At the same time, fans 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 a 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 a fluidized state.

• The gas surrounding each granule does 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.

Uses:

1. It is used for drying granules in the production of tablets.

2. It is used for the coating of granules.

3. It can be used for three operations such as mixing, granulation and drying.

Advantages:

1. It takes less time to complete drying as compared to other dryers.

2. Drying is achieved at a constant rate.

3. Handling time is also short.

4. It is available at different sizes with different drying capacities.

5. The equipment is simple and requires less labor.

6. More thermal efficiency.

7. Drying capacity is more than other dryers.

8. It facilitates the drying of thermolabile substances since the contact time of drying is short.

9. It is a batch-type or continuous-type process.

Disadvantages:

1. Many organic powders develop electrostatic charge during drying. To avoid this, efficient electrical earthing of the dryer is essential.

2. Chances of attrition of some materials resulting in production of fines.

Commonly Asked Questions.

1. With a well labeled diagram describe construction and working of FBD.

2. Write a short note on the FBD.

Filtration: Meta Filter.

 

Meta Filter:

Principle:

• Mechanism is surface filtration.

• In this, metal rings contain semicircular projections, which are arranged as a nest to form channels on the edges.

• This channel offers resistance (strainer) to the flow of solids (coarse particles).

• The clear liquid is collected into the receiver from the top. 

Construction:

• Metafilter consists of a series of metal rings.

• These are threaded so that a channel is formed on the edges.

• It contains a grooved drainage column on which a series of metal rings are packed.

• These rings are usually made up of stainless steel and have dimensions of about 15.0 mm internal diameter and 22.0 mm external diameter.

• Each metal ring has a number of semicircular projections (0.8 mm in thickness) on one side of the surface.

• The projections are arranged as a nest to form channels on the edges.

• These rings are tightened on the drainage column with a nut.

• Meta Filters are also known as edge filters. 

Working:

• Filters are placed in a vessel

• Slurry is pumped under pressure or occasionally by applying reduced pressure to the outlet side

• Slurry passes through the channels formed on the edges between the rings

• The clear liquid rises up and collected from the outlet into receiver

• For separation of fine particles, a bed of suitable materials such as kieselguhr is first built up.

• The pack of rings serves essentially as a base on which the true filter medium is supported. 

Advantages of filter leaf:

1. Can be used under high pressures, without any danger of bursting the filter medium.

2. Running cost is low, as separate filter medium is not used. 

3. Can be constructed from a material that can provide excellent resistance to corrosion and avoid contamination of sensitive products.

4. It is an extremely versatile filter because fine as well as large both types of particles can be separated.

5. Removal of cake can be carried out by simply back- flushing with water.

6. Change over from one batch to another or one product to another is easy.

7. Sterile products can be handled. 

Disadvantages of meta filter:

1. It is not a continuous process.

2. It is used where the proportion of solid in the liquid to be filtered is about  5% or less.

Uses:

• Meta Filters are usually used for-

1. Clarification of syrups.

2. Filtration of injection solutions.

3. Clarification of insulin liquors.

4. Filtration of viscous liquids can be achieved by applying pressure.

Filter Leaf

 

Filter Leaf:

Principle:

• It is an apparatus consisting of a longitudinal drainage screen covered with a filter cloth.

• The mechanism is surface filtration and acts as a sieve or strainer.

• Vacuum or pressure can be applied to increase the rate of filtration

Construction:

• The leaf filter consists of a frame enclosing a drainage screen or grooved plate.

• The frame may be any shape circular, square or rectangular.

• The whole unit is covered with filter cloth.

• The outlet for the filtrate connects to the inside of the frame through suction. 

Working:

• The filter leaf is immersed in the slurry.

• Vacuum system is connected to the outlet.

• The slurry passes through the filter cloth.

• Finally filtrate enters the drainage canal and goes through the outlet into the receiver.

• Air is passed to flow in reverse direction which facilitates removal of cake 

Advantages of filter leaf:

1. Simplest form of filter used for batch process.

2. A number of units can be connected in parallel to increase the surface area of filtration.

3. Pressure difference can be obtained either with vacuum or using pressure up to the order of 800 kilopascals.

4. Labor costs for operating the filter leaf are fairly moderate.

5. The efficiency of washing is high.

6. The slurry can be filtered from any vessel.

7. The cake can be washed simply by immersing the filter in a vessel of Water. 

Disadvantages of filter leaf:

1. It is not a continuous process.

2. It is used where the proportion of solid in the liquid to be filtered is about  5% or less.