Evaporation

Evaporation is a process that is commonly used to concentrate an aqueous solution of nonvolatile solutes and a volatile solvent. In evaporation, a portion of the solvent is vaporized or boiled away, leaving a thick liquid or solid precipitate as the final product. The vapor is condensed to recover the solvent or it can simply be discarded. A typical example is the evaporation of brine to produce salt.

Evaporation may also be used as a method to produce a liquid or gaseous product obtained from the condensed vapor. For instance, in desalinization processes, sea water is vaporized and condensed in a water-cooled heat exchanger and forms the fresh water product.

In general, evaporation processes can be expressed as:

The separating agent is heat, which is usually supplied by a low-pressure steam to provide the latent heat of vaporization. When the liquid, say, a water solution, is heated, boiling occurs at the heated surface and the liquid circulates. Because the dissolved solids (solutes) are less volatile than water (solvent), water will first escape gradually from the solution. As sufficient water is boiled off, the resulting liquid becomes saturated, and then the dissolved solids crystallize. To reduce energy consumption, via utilizing the latent heat of the generated vapor over and over again, heat introduced into one evaporator can be used in other evaporators involved in a multistage, or formally called multi-effect process.

A variety of approaches are employed to vaporize liquids or solutions. Liquids can flow as a thin-film layer on the walls of a heated tube, can be diffused on the heated surface, or can be spread in fine droplets into a hot, dry gas. Wet cloth or paper generally can be dried by evaporation of the moisture into a gas stream. For some heat-sensitive liquids, such as pharmaceutical products and foods, evaporation must be carried out under reduced pressure, in which the boiling point occurs at lower temperature. Alternatives to this approach are to increase heat-transfer area or to inject steam directly into the solution to heat it rapidly. Very often, fouling layers can build up next to heat-transfer surfaces and reduce the heat-transfer coefficient across the evaporator. In certain situations, material tends to foam during vaporization. Liquid can boil over into the vapor, resulting in the failure to separate components or concentrating solutions. Therefore, good evaporator designs and the understanding of liquid characteristics are very crucial in evaporation efficiency.

Evaporation is often employed to produce a concentrated liquid for industrial purposes. A slurry of crystals in a saturated liquid can be obtained by means of evaporation. In the sugar industry, water is boiled off to produce sucrose prior to crystallization. Solutions such as caustic soda and organic colloids all can be concentrated by evaporation. Because of the their substantial thermal sensitivity, vacuum evaporation is normally applied for concentration of fruit juices. Concentrated juices are easily transported and during storage, are more resistant to environmental degradation than fresh juices.