Drying

Drying is a mass transfer process used to remove water or other solvents from a material. This process occurs through evaporation from solids, semi-solids, or liquids. It is often a vital final production step before products are packaged or sold. For a product to be considered "dried," it must reach a specific physical state. This might mean a continuous sheet like paper or long pieces like wood. It could also result in particles, such as cereal grains, or fine powders like salt and milk powder.

To make this happen, the process usually requires a heat source and an agent to remove vapor. In most cases, a gas stream like air uses convection to apply heat. This air carries away the produced vapor as humidity. Other methods include vacuum drying, where heat comes from conduction, radiation, or microwaves. In vacuum drying, a vacuum system removes the vapor. There is also drum drying, which uses a heated surface to provide energy. In this method, aspirators draw the vapor outside the room. This is often used to manufacture potato flakes.

When a product has high moisture, it goes through distinct drying stages. The first stage is the constant drying rate period. During this time, the moisture content decreases linearly over time. This period mostly involves removing surface moisture from outside individual particles. The speed of drying here is heat-transfer limited. This means the rate depends on how fast heat moves into the material.

Eventually, the process enters the falling rate period. During this stage, the drying rate becomes less steep. The moisture content eventually reaches an equilibrium moisture content. This is the point where the product is in equilibrium with the dehydrating medium. In this period, water moves from the interior to the surface through molecular diffusion. This movement happens because water flows from high moisture zones to low moisture zones. This follows the second law of thermodynamics. This stage is mass-transfer limited because the rate depends on moisture removal from the interior. Many hygroscopic products, like fruits, spend most of their time in this period.

There are several specific methods used to achieve these results. Convective or direct drying uses hot air to increase heat transfer. Heating the air also reduces relative humidity, which increases the driving force for drying. However, if the air is too hot, it can cause case hardening. This is when the surface dries so fast that pores shrink and close. To prevent this in timber, workers might add steam to the heated air. This helps avoid deformation caused by high moisture gradients.

Other advanced methods include freeze drying and supercritical drying. Freeze drying, or lyophilization, involves freezing the solvent before drying. The solvent then undergoes sublimation, which means it passes from a solid to a gas phase below its melting point. This method is excellent for preserving the biological properties of proteins and vitamins. It is used widely in pharmaceuticals and for many foods. Supercritical drying uses superheated steam to dry products. This process can be done in a closed circuit to recover latent heat through recompression.

Applications of drying are found in many industries. In the food industry, drying inhibits microbial development and prevents quality decay. For example, cereals are dried after harvest to ensure microbial stability during storage. Vegetables are often blanched before drying to prevent browning. Milk powder must be dried to very low moisture levels to ensure it flows well and does not cake. In the coatings industry, drying is used to cure solvent-based films.

Non-food industries also rely heavily on these processes. Wood and paper must be dried to prevent the growth of mold. Drying also helps reduce the total volume and weight of materials. In sanitation, drying is used for sewage sludge and fecal materials. This process helps achieve pathogen kill, as many pathogens cannot tolerate high dryness. Drying is also a necessary step if these materials are meant to be incinerated.