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Evaporation is a standard thermally driven unit operation. Learn three methods to improve evaporator efficiency.
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Evaporation is a unit operation that separates a liquid from solids by means of heat transfer via vaporization or boiling. The purpose of evaporation is to concentrate a solution of a nonvolatile solute (i.e., solids) and a solvent (i.e., liquid), which is typically water. Evaporating a portion of the solvent concentrates the solute into a moreviscous liquid product. Evaporation is regularly used in the food processing, chemical, kraft paper, and pharmaceutical industries to produce liquid concentrates.
Evaporation differs from dehydration and drying in that the product of evaporation is a concentrated liquid, not a solid. Evaporation can be used as the initial step in producing a dried product if the liquid concentrate then undergoes a drying process such as spray drying. The combination of evaporation and spray drying is often used to make powdered products, such as powdered milk. This combination of processes is economically attractive because high-efficiency evaporation is significantly less costly than drying and other methods of removing water (I). Evaporation also produces a higher concentration of solids than other methods of concentration (Figure 1) (2).
Evaporation differs from distillation in that the concentrated solution, rather than the condensed evaporate, is typically the valuable product. A common exception is the evaporation of solutions with a high mineral content, where the vapor is condensed as the product and the concentrated brine is discarded. This process is commonly referred to as water distillation, although the process is more akin to a thermally driven liquid-solids separation operation.
Evaporation may be carried out as a batch or continuous process. This article focuses on evaporation as a continuous process, in which the feed and product streams are continuous and their concentrations remain constant.
The basics
All evaporators are comprised of two sections: a heating section (called a steam chest) and a vapor/liquid separation section. These sections can be located within a single vessel (body), or the heating section may be external to the vessel that houses the vapor/liquid separation section.
Evaporators may be composed of one or more effects, where an effect is defined as one or more bodies operating at the same boiling temperature. In a...