*Drying process.*

Drying has a number of synonyms that are very close. Dehydration is the process where a material is excluded of its water or water is lost as a component. In fooddrying operations, the term is often used to describe processes which strive to expel moisture but retain other volatile components in the original material, and that are responsible for valuable aromatic and flavouring properties. A more thorough removal of water is indicated by desiccation. It's being used to indicate almost complete dehydration of these materials for preservation when drying foodstuffs. To describe the thorough removal of moisture from gases, the term is also popularly used. While heat can be used to drive moisture away from a wet substance, by the action of pressure gradients, moisture can be severed with its host material. This process is known as dewatering, and when the moisture-solid bond is not strong, it is generally used as a precursor to the drying of very wet materials. Mechanical means, such as pressing or centrifuging, will be used for dewatering.

Although air is usually considered to be the drying medium, the use of other media does have advantages. If a combustible powder is developed by the solid being dried or the moisture itself is a flammable solvent, then it is advisable to use an inertized or inevitably inert gas. Steam drying has the added advantages of lower use of energy and higher rates of heat transfer. Drying in steam is faster than drying at the same temperature in perfectly dry air above the so-called inversion-point temperature. Moisture is uniformly released during the superheated-steam drying of wood under vacuum. This process is commonly used to produce high-quality seasoned panel timber with minimal degradation caused by drying stresses [2].

For steam drying, a confined vacuum or high-pressure vessel is not necessary. The steam will remain in the dryer by allowing air in the drying chamber to be displaced by water vapour as the vessel warms up and moisture evolution begins, and no complex sealing arrangements are needed for solids to be consumed and released. At 100°C, steam has only 55% of the air density at the same temperature and therefore will remain trapped inside the chamber.

Airless drying is known as the patented method, and the arrangements for batch operation are shown in **Figure 2**. If the vented steam can be used for other purposes, such as hot water production, the airless drying system is capable of showing considerable thermal savings over conventional air drying.

Drying occurs when the wet material contains more moisture than the equilibrium position for its environment. Liquid moisture diffuses to the surface of a wet body where it evaporates, diffusing the vapour into the surrounding air through the

#### **Figure 2.**

*Airless drying system with heat recovery.*

boundary layer. This was the initial concept of convective drying. This perspective is inadequate, except when drying uniform substances with dissolved moisture [3].

Moisture movement mechanisms are generally more complex. Most materials, such as particles and fibers, are composed of sub-entities that may be loose or held in some kind of matrix. The quantity of moisture retained and the extent of bonding to the solids govern the number and nature of the voids between these entities and the pores within them. The material is said to be capillary-porous if the openings make up a capillary network. A capillary-porous material may be non-hygroscopic: that is, its full vapour pressure is exerted by the moisture held within the body. In some coarse, nonporous mineral aggregates, this is a restrictive case. Between the particles, moisture is simply trapped [4].
