evaporation

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Evaporation

Evaporation is the movement of a substance from its liquid to its vapor phase. (Movement directly from solid to vapor phase is called sublimation.) It is commonly used to concentrate solutions of nonvolatile solutes and volatile solvents. 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.

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 industrial evaporation processes, 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

KEY TERMS

Fouling A term to describe buildup of a semisolid layer next to the heat-transfer surface of the tubes in heat exchangers to reduce the overall heat transfer coefficient.

Heat-transfer coefficient The ratio of the heat transferred per unit time to the product of the heat transfer surface area and the temperature difference between the two steams next to the heat-transfer surface.

Latent heat of vaporization When a change of phase, either from liquid to vapor or vice versa, occurs, latent heat of vaporization has to be absorbed during vaporization or evolved during condensation at constant temperature, which is resulted from the difference in the values of internal energy between liquid and vapor phases.

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 juicesthough they also do not always taste as good.

See also Heat transfer; States of matter.

Pang-Jen Kung

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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.

See also Heat transfer; States of matter.

Pang-Jen Kung

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Fouling

—A term to describe buildup of a semisolid layer next to the heat-transfer surface of the tubes in heat exchangers to reduce the overall heat transfer coefficient.

Heat-transfer coefficient

—The ratio of the heat transferred per unit time to the product of the heat transfer surface area and the temperature difference between the two steams next to the heat-transfer surface.

Latent heat of vaporization

—When a change of phase, either from liquid to vapor or vice versa, occurs, latent heat of vaporization has to be absorbed during vaporization or evolved during condensation at constant temperature, which is resulted from the difference in the values of internal energy between liquid and vapor phases.

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Evaporation

Evaporation is a geologic process that concentrates the ion solute residues in the ocean basins. At a fundamental level, evaporation is the transition of the molecule of a liquid from the liquid state to the gaseous state by diffusion from the surface of the liquid.

Driven by solar energy , the only significant loss of water from the ocean basin occurs via evaporation. As the ocean surface and atmospheric interface is small compared to the total volume of the ocean, estimates of the time a particular molecule remains in the liquid phase range in the order of thousands to tens of thousands of years before once again entering the atmosphere as part of the hydrologic cycle .

Because solutes (e.g., dissolved salts) from weathering and erosion are not as volatile (i.e., as easy to move into the gas or vapor phase as the water molecules, evaporation plays a significant role in the formation of many geologic features (e.g., Great Salt Lake, Dead Sea, etc.).

Evaporation is usually also responsible for the majority of the loss of water from precipitation and results in a high cycling of water molecules during the hydrologic cycle.

Evaporation may be driven by solar energy or be a directed process 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 sea water 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.

Although evaporation can be driven by the random motion of molecules near the liquid-gas interface, the addition of heat to a system speeds the evaporative process.

See also Caliche; Condensation; Drainage calculations and engineering; Leaching; Oceans and seas; Phase state changes; Runoff

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Evaporation

Evaporation is the name given to the process in which a liquid is converted to the gaseous state. Everyone is familiar with the process of evaporation. Suppose that you spill a teaspoon of water on the kitchen table. If you come back a few hours later, the water will have disappeared. It has changed from liquid water into water vapor, or evaporated.

Molecular explanation

Evaporation occurs because all molecules of all substances are constantly in motion. Consider the molecules that make up a teaspoon of water, for example. Those molecules are constantly in motion, flying back and forth within the water, sometimes colliding with each other. When collisions occur, some molecules gain energy from other molecules.

Those changes make little difference for molecules deep within the water. But for molecules at the surface of the water, the situation is different. Molecules at the surface that pick up energy from other molecules begin to travel faster. Eventually, they may be able to travel fast enough to escape from the surface of the water or to evaporate from the water.

This process continues as long as water molecules remain. Molecules that were once inside the water eventually work their way to the surface. When they pick up enough energy by colliding with other water molecules, they too escape. Eventually, no water molecules remain. The liquid has completely evaporated.

The remaining liquid

This description explains an interesting fact about an evaporating liquid: its temperature decreases as evaporation occurs. Remember that surface molecules escape from the liquid as they pick up energy from other molecules. The molecules left behind, therefore, have less energy than they had before the collisions. Since they have less energy, they also have a lower temperature.

The human body uses this principle to remain cool. On a warm day, we perspire (sweat). Sweat evaporates from the skin, taking body heat with it. As a result, the body is cooled.

Commercial applications

Evaporation is an important commercial process by which liquids are removed from solids. In many instances, a product is formed as the result of a chemical reaction that takes place in water. One way to obtain the final product is to simply allow the water to evaporate leaving the solid product behind.

[See also Matter, states of ]

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evaporation Process by which a liquid or solid becomes a vapour. The reverse process is condensation. Solids and liquids cool when they evaporate because they give up energy (latent heat) to the escaping molecules. Evaporation of sweat on the skin helps control the human body's temperature.

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evaporation See EVAPOTRANSPIRATION.

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evaporation See evapotranspiration.

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