Vacuum drying is the

mass transfer Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction or component) to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtrati ...

operation in which the moisture present in a substance, usually a wet solid, is removed by means of creating a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

. In chemical processing industries like

food processing Food processing is the transformation of agricultural products into food, or of one form of food into other forms. Food processing includes many forms of processing foods, from grinding grain to make raw flour to home cooking to complex indu ...

, pharmacology, agriculture, and textiles, drying is an essential unit operation to remove moisture. Vacuum drying is generally used for the drying of substances that are

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substa ...

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. With the help of vacuum pumps, the pressure is reduced around the substance to be dried. This decreases the

boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding en ...

of water inside that product and thereby increases the rate of

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...

significantly. The result is a significantly increased drying rate of the product. The vacuum drying process is a batch operation performed at reduced pressures and lower

relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity de ...

compared to ambient pressure, enabling faster drying.

Vacuum dryer

Vacuum dryer is the equipment with the help of which vacuum drying is carried out. Vacuum dryers are sometimes made up of cast iron, but most are made of stainless steel, 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 Conduction is the process by which heat is transferred from the hotter end to the colder end of an object. The ability of the object to conduct heat is known as its ''thermal conductivity'', and is denoted . Heat spontaneously flows along a tem ...

. The oven door is locked air tight and is connected to 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 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 is sent into the condenser and after the drying vacuum pump is disconnected and the dried product is collected from the trays.

Microwave vacuum drying

Because conventional drying approaches (e.g., convective drying) may cause great nutritional and textural changes (like a darker color due to

Maillard reaction The Maillard reaction ( ; ) is a chemical reaction between amino acids and reducing sugars that gives browned food its distinctive flavor. Seared steaks, fried dumplings, cookies and other kinds of biscuits, breads, toasted marshmallows, and m ...

), microwave vacuum drying is an alternative for pharmaceuticals and food drying, a method known since 1989. The microwaves speed up the drying process and lower temperature in vacuum system, reducing overall drying cycle time and temperature-induced effects of the food product. Microwave vacuum drying may be used for production of dried pharmaceuticals and food. Industrialized euipment, however, may require pre-treating samples before processing in the industrial vacuum system; pre-drying is used by conventional methods to reduce bulk water content.

Applications

Vacuum dryer can be used to dry heat sensitive hygroscopic and toxic materials. If the feed for drying is a solution, it can be dried using vacuum dryer as the solvent can be recovered by condensation. To improve the quality of products, such as for fruit preservation, hybrid drying combining osmotic dehydration followed by

heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...

drying and

microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...

-vacuum drying proved effective.

References

{{Reflist Drying