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Vacuum drying can be a useful tool for solid products that are heat-sensitive.
Drying is an essential unit operation in a variety of chemical process industries (CPI) sectors. Food, pharmaceutical, chemical, plastic, timber, paper and other industries use drying equipment to eliminate moisture during product processing. Most dryers are classified as direct dryers, where hot air (at near atmospheric pressure) is used to supply the heat to evaporate water or other solvents from the product. Another important dryer category, vacuum dryers, involves the use of a reduced-pressure atmosphere to surround the product.
Drying is among the most energy-intensive unit operations, due to the high latent heat of vaporization of water and the inherent inefficiency of using hot air as the (most common) drying medium. Depending on the specific product attributes required, different industry sectors require different types of drying technology. Drying high-value products that are likely to be heat-sensitive, such as food, pharmaceuticals and biological products, demands special attention. When dried by convection at higher temperatures, these heat-sensitive products degrade, change color and appearance and have lower vitamin or nutrient content. Vacuum dryers offer an alternate path. This article discusses the operation and selection of vacuum dryers, and provides examples of applications in which vacuum drying is used.
Drying principles
Drying involves two distinct drying periods, known as the constant drying period and the falling drying period (Figure 1). Drying occurs when liquid is vaporized by supplying heat to the wet feedstock. The liquid removed by the drying process could be either free moisture (unbound) or bound within the structure of the solid. The unbound moisture, normally present as a liquid film on the surface of a solid particle, is easily evaporated, while the bound moisture could be found within the solid material, trapped in the microstructure of the solid. In this case, the moisture must travel to the surface to be evaporated. When a solid product is subjected to drying, removal of unbound and bound moisture depends on the rates at which these two processes proceed. Removal of unbound moisture depends on external conditions of air or gas temperature, flow, humidity, area of exposed surface and pressure. The movement of bound moisture depends on the nature of the product being dried and the...