Drying is just one of a myriad of industrial heat and mass transfer operations. I am frequently asked questions such as “What is the difference between drying and evaporating?”, or “drying and curing,” or “What is roasting and what is calcining?” These processes are related but distinct enough to be characterized.
This file will detail some of the more common industrial moisture removal and thermal processes and will distinguish some of the peculiarities relating to them.
DryingAlthough drying is classically defined as a process that removes a liquid (usually water) from a solid in equipment termed a dryer, there remains a wide application of various technologies to moisture removal. In my mind, drying is differentiated from dewatering in that in drying, the moisture leaves the product as a vapor or gas. This is achieved by imparting energy to the moisture molecule or changing the environment so that the molecule has sufficient latent energy to leave the product.
Moisture reduction also can be achieved by mechanical methods such as squeezing, blowing at high velocity, wiping or shaking. Examples of these in industrial terms are filtration (membrane, plate-and-frame or vacuum), air knives, desiccant drying and dewatering screens, respectively. This is the technology of dewatering.
The drying process is intended to remove moisture from a feed substance and thereby condition the feed into a “dry” final product. This does not imply that the product will have no moisture content -- only that it will be “dry” by the definition of the specification. Some dry materials such as certain agricultural products may possess 12 to 17 percent moisture (wet basis) and be considered dry.
Typically, in drying, one does not physically or chemically change the composition of the feed, other than by removing the moisture.
A classic example of a drying process would be the removal of water from milk to manufacture powdered milk, or the removal of a solvent from processed sand.
DehydrationDehydration is commonly associated with the drying of foods and foodstuffs. In some instances, it may be construed to infer the removal of water with an associated chemical change. Personally, thermal dehydration and drying are synonymous.
EvaporatingWhile drying does evaporate moisture, the process of evaporation, as commonly referred to in industry, is a process in which a relatively large volume of liquid is removed from a solid or a liquid to obtain a concentrated product. This product is not necessarily dry; it may be a liquid, a slurry or the condensed form of the so-formed vapor.
As a process, evaporation is fundamentally involved with changing the state of a liquid to a vapor. One does not change the chemical characteristics of any of the components.
A common example of evaporation is the production of salt concentrate from a brine solution. Evaporation is used extensively in the food and kindred industries.
CuringCuring is the process of applying heat to chemically change or stabilize the structure of a material without necessarily removing any of the material constituents.
Examples of curing are found in the painting industry, and in the application of resins and some thermoset plastics.
CrystallizingThis is the formation of a solid by supersaturating the solute, allowing it to form a “rigid” crystalline or particulate structure. To achieve this, solvents (organic or other, such as water) need to be driven off, and this may be achieved by drying or evaporating the solution. Crystallizing will change the chemical structure of the compound(s) in the feed material.
Prilling is a form of crystallizing using a spray dryer. Prills are most widely recognized as fertilizer agglomerates.
Examples of crystallization include the production of sugar and salt.
Next month, I'll continue this series with a look at distilling, dehumidifying and calcining.