Vacuum Dryer - How it Works?

Vacuum drying is a viable technology used successfully for many years in the pharmaceutical, food, plastics, and textile industries. It is an indirect-heat dryer and best used for drying heat-sensitive materials at comparatively low temperatures. These materials are dried by applying a vacuum to evaporate the water or solvent. The use of vacuum lowers the environmental pressure and hence lowers the boiling point of solvents this process is known as vacuum drying. The combination of heat and vacuum together is a very effective source of drying at a relatively lower temperature. The moisture content in the material dried via vacuum dryers is comparatively lower than those of normal dryers without vacuum.

Principle of Vacuum Dryer

Vacuum drying is the mass transfer operation in which the moisture present in wet solid is removed using creating a vacuum. The principle involved is creating a vacuum to decrease the pressure below the vapor pressure of the water. With the help of vacuum pumps, the pressure is reduced around the substance to be dried. This decreases the boiling point of water inside that product and thereby increases the rate of evaporation significantly. The result is a significantly increased drying rate of the product. The pressure maintained in vacuum drying is generally 0.0296 –0.059 atmospheres and the boiling point of water is 25 - 30 °C. The vacuum drying process is a batch operation performed at reduced pressures and lower relative humidity compared to ambient pressure, enabling faster drying.

Construction of Vacuum Dryer

In the pharmaceutical industry, a vacuum dryer is also known as a vacuum oven. Vacuum dryers are made up of stainless steel or cast iron so that they can bear the high vacuum pressure without any kind of deformation. 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. Heat is usually supplied by passing steam or hot water through hollow shelves.

Fig.1: Vacuum Dryer

Working of Vacuum Dryer

The materials to be dried are kept on the trays inside the vacuum dryer and pressure is reduced using a vacuum pump. The dryer door is tightly shut and steam is passed through the space between trays and jacket so that the heat transfer occurs by conduction. Water vapors from the feed are sent into the condenser. After drying vacuum pump is disconnected and the dried product is collected from the trays. The reduced pressure lowers the heat needed for rapid drying. Drying temperatures can be carefully controlled and, for the major part of the drying cycle, the material remains at the boiling point of the wetting agent. Drying times are long, usually about 12 to 48 h. The heat is transferred to the material as it contacts the dryer’s heated surface, drying the material by conduction.

Advantages of Vacuum Dryer

1. Vacuum dryers offer low-temperature drying of thermolabile materials and are suitable for solvent recovery from solid products containing solvents.
2. Materials can be dried in final containers or enclosures by removing a large amount of moisture compared to normal dryers.
3. The average drying temperature is much lower than normal standard dryers and the quality of dried material is much better than that of the normal dryers.
4. Drying action becomes faster as heat is easily transferred throughout the body of the dryers, due to its large surface area.
5. A major advantage is its energy conservation. It requires less energy for drying, cutting down on the economic and environmental costs associated with drying a product for storage, sale, or other purposes.
6. Vacuum-drying processes also tend to work faster than other drying methods, cutting down on processing times.
7. Another advantage of drying materials using this dryer is that it is a less damaging drying process. Vacuum drying tends to retain the integrity of the original item without damaging it with heat.
8. It is predominately a batch unit operation however it can be integrated as a continuous process.
9. This equipment reduces risks to workers from vented fumes and particles that can usually make people sick or that force people to wear protective garments while operating other dryers.

Disadvantages of Vacuum Dryer

1. In general vacuum drying process is a batch type of drying process with low efficiency.
2. Vacuum dryers are expensive in terms of their cost and there is a requirement of skilled labor to operate them.
3. The cost of maintenance is comparatively high.
4. Its upper-temperature limit (typically about 600 °F) is lower than that of a direct-heat dryer. Thus the rate at which material temperature can be raised in a vacuum dryer is also limited. The vacuum pump is primarily responsible for the vacuum level inside the dryer.

Applications of Vacuum Dryer

The following examples describe areas in which vacuum drying is employed.

1. Type of industries: Vacuum drying equipment has many applications across most industrial sectors such as chemical, pharmaceutical, food, and plastics.
2. Type of operation: It is typically used for batch operation as it removes water or removes and recovers solvents from a moist material.
3. Changing physical state: It can be used to change a material's molecular and physical chemistry (called a phase change) in specialized operations such as chemical reactions and polymer solid stating.
4. Separation: It is typically used for separating volatile liquid by vaporization from a powder, cake, slurry, or other moist material.
5. Indirect-heat drying: The heat is transferred to the material as it contacts the dryer's heated surface, drying the material by conduction.
6. Safety: With a vacuum dryer, ventilation does not require and personnel working near the dryer are safer. It is also possible to recover the precipitated moisture collected during the drying for further use.
7. Drying pharmaceuticals: It is used in the preparation of mouth-dissolving tablets and granules of nimesulide, camphor, crospovidone, and lactose by wet granulation. Camphor sublimes from the granules by exposure to vacuum. The porous granules are compressed to get superior tablets.
8. Drying proteins: Vacuum-drying overcomes long processing times (typically 3–5 d), and the inherent steps in the freeze-drying lead to instabilities in the protein structure. Due to the complex structural properties, proteins tend to denature and undergo irreversible aggregation during various processing steps of freeze-drying. Vacuum drying is used in the formulation of proteins as dry powders using mannitol as the bulking agent.
9. Drying bacteria: Normally, probiotic bacterial strains, and starter cultures are dry-frozen to preserve them until use. It consumes a very high amount of energy and some bacterial strains do not survive temperatures below 0 °C. Thus low-temperature vacuum drying can be used to dry probiotics at moderate temperatures above 0°C without causing too much damage to the cell structure.
10. Vaccines and other injectables: Freeze drying involves the use of a vacuum to produce pharmaceutical products. Freeze-drying is used to increase the shelf life of products, such as vaccines and other injectables. By removing water from the material and sealing the material in a vial under a vacuum, the material can be easily stored, shipped, and later reconstituted to its original form for injection.

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