Vacuum drying is the mass transfer operation in which the moisture present in a substance, usually a wet solid, is removed by means of creating a vacuum. In chemical processing industries like food processing, pharmacology, agriculture, and textiles, drying is an essential unit operation to remove moisture. 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. 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 0.03–0.06 atm 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.
Vacuum dryer is the equipment with the help of which vacuum drying is carried out. In the pharmaceutical industry vacuum dryer is known by a common name called vacuum oven. Vacuum dryers are sometimes made up of cast iron, but most now 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 .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 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 drying vacuum pump is disconnected and the dried product is collected from the trays.
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 quality of products, such as for fruit preservation, hybrid drying combining osmotic dehydration followed by heat pump drying and microwave-vacuum drying proved effective.