스크랩 distillation (증류)

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 chemical process

process involving the conversion of a liquid into vapour that is subsequently condensed back to liquid form. It is exemplified at its simplest when steam from a kettle becomes deposited as drops of distilled water on a cold surface. Distillation is used to separate liquids from nonvolatile solids, as in the separation of alcoholic liquors from fermented materials, or in the separation of two or more liquids having different boiling points, as in the separation of gasoline, kerosene, and lubricating oil from crude oil. Other industrial applications include the processing of such chemical products as formaldehyde and phenol and the desalination of seawater. The distillation process appears to have been utilized by the earliest experimentalists. Aristotle (384?322 bc) mentioned that pure water is made by the evaporation of seawater. Pliny the Elder (ad 23?79) described a primitive method of condensation in which the oil obtained by heating rosin is collected on wool placed in the upper part of an apparatus known as a still.

Most methods of distillation used by industry and in laboratory research are variations of simple distillation. This basic operation requires the use of a still or retort in which a liquid is heated, a condenser to cool the vapour, and a receiver to collect the distillate. In heating a mixture of substances, the most volatile or the lowest boiling distills first, and the others subsequently or not at all. This simple apparatus is entirely satisfactory for the purification of a liquid containing nonvolatile material and is reasonably adequate for separating liquids of widely divergent boiling points. For laboratory use, the apparatus is commonly made of glass and connected with corks, rubber bungs, or ground-glass joints. For industrial applications, larger equipment of metal or ceramic is employed.

A method called fractional distillation, or differential distillation, has been developed for certain applications, such as petroleum refining, because simple distillation is not efficient for separating liquids whose boiling points lie close to one another. In this operation the vapours from a distillation are repeatedly condensed and revaporized in an insulated vertical column. Especially important in this connection are the still heads, fractionating columns, and condensers that permit the return of some of the condensed vapour toward the still. The objective is to achieve the closest possible contact between rising vapour and descending liquid so as to allow only the most volatile material to proceed in the form of vapour to the receiver while returning the less volatile material as liquid toward the still. The purification of the more volatile component by contact between such countercurrent streams of vapour and liquid is referred to as rectification, or enrichment.

Multiple-effect distillation, often called multistage-flash evaporation, is another elaboration of simple distillation. This operation, used primarily by large commercial desalting plants, does not require heating to convert a liquid into vapour. The liquid is simply passed from a container under high atmospheric pressure to one under lower pressure. The reduced pressure causes the liquid to vaporize rapidly; the resulting vapour is then condensed into distillate.

A variation of the reduced-pressure process uses a vacuum pump to produce a very high vacuum. This method, called vacuum distillation, is sometimes employed when dealing with substances that normally boil at inconveniently high temperatures or that decompose when boiling under atmospheric pressure. Steam distillation is an alternative method of achieving distillation at temperatures lower than the normal boiling point. It is applicable when the material to be distilled is immiscible (incapable of mixing) and chemically nonreactive with water. Examples of such materials include fatty acids and soybean oils. The usual procedure is to pass steam into the liquid in the still to supply heat and cause evaporation of the liquid.

Related Articles

Aspects of the topic distillation are discussed in the following places at Britannica.

Assorted References

• chemical separation (in analysis (physics and chemistry): Distillation;

  During distillation a mixture of either liquid or liquid and solid components is placed in a glass vessel, called a pot (or boiling flask), and heated. The more volatile components?i.e., those with the lower boiling points?are converted to a gaseous state and...

  in separation and purification (chemistry): Distillation)

  Distillation (as discussed in analysis: Interference removal: Distillation), is a method of separation based on differences in the boiling points of substances. It has been known for centuries. The essential operation in distillation is the boiling of a liquid; after being converted to a vapour, the substance is then condensed to a liquid that is collected separately rather than allowed to...

• desalination (in desalination (chemical process);

  Distillation remains the most widely used desalination process. Either a multiple-effect or a flash evaporator may be used. The first consists of a series of evaporators in which salt water is heated and vaporized in long, vertical tubes. The hot vapour is used to heat salt water entering the next...

  in ocean (Earth feature): Desalination)

  At present desalination is accomplished primarily by distillation and membrane processes. Distillation processes involve some form of evaporation and subsequent condensation. Many of the largest commercial desalination plants use multiple-effect distillation (i.e., multistage-flash distillation). Roughly half of the world’s desalting facilities employ distillation processes and account...

• diesel fuel production (in diesel engine: Fuel for diesels)

  Petroleum products normally used as fuel for diesel engines are distillates composed of heavy hydrocarbons, with at least 12 to 16 carbon atoms per molecule. These heavier distillates are taken from crude oil after the more volatile portions used in gasoline are removed. The boiling points of these heavier distillates range from 177 to 343...

• dye manufacturing (in dye: Synthetic dyes)

  ...a fraction was utilized as wood preservative, road binder, and a source of the solvent naphtha. Fortunately, it attracted the attention of chemists as a source of new organic compounds, isolable by distillation. A leading researcher in this area, German chemist August Wilhelm von Hofmann, had been attracted to England in 1845 to direct the Royal College of Chemistry. In the following 20 years,...

• ethyl alcohol (in ethyl alcohol (chemical compound))

  Ethyl alcohol produced either by fermentation or by synthesis is obtained as a dilute aqueous solution and must be concentrated by fractional distillation. Direct distillation can yield at best the constant-boiling-point mixture containing 95.6 percent by weight of ethyl alcohol. Dehydration of the constant-boiling-point mixture yields anhydrous, or absolute, alcohol. Ethyl alcohol intended for...

• gasoline production (in gasoline (chemistry))

  Gasoline was at first produced by distillation, simply separating the volatile, more valuable fractions of crude petroleum. Later processes, designed to raise the yield of gasoline from crude oil, split large molecules into smaller ones by processes known as cracking. Thermal cracking, employing heat and high pressures, was...

• isoprenoids (in isoprenoid (chemical compound): Isolation and identification of isoprenoids)

  ...by factors such as the chemical and physical properties of the compound and its abundance and distribution in nature. Volatile and plentiful substances such as turpentine are obtainable by distillation of oleoresins; rosin acids and fatty acids occur together in tall oil, a by-product obtained in the manufacture of paper pulp from pine...

• isotope separation (in isotope (chemistry): Distillation)

  The same factors that lead to the enrichment of alcohol in the vapour above a solution of water and alcohol permit the enrichment of isotopes. At temperatures below 220 °C (428 °F), for example, light water (¹₁H₂O) vaporizes to a slightly greater extent than heavy water...

• kerosine production (in kerosene (chemical compound))

  Kerosene accounts for 10 to 25 percent of the total volume of crude petroleum. It is separated physically from the other portions by fractional distillation. It can also be produced chemically by cracking, or decomposing, the less volatile portions of mineral oils at atmospheric pressure...

• liqueur production (in liqueur)

  ...are produced by the infusion method, in which fruit is steeped in the spirit, which absorbs aroma, flavour, and colour. Plant liqueurs, naturally colourless, are produced by either percolation or distillation. Percolation is accomplished in an apparatus much like a coffee percolator. Leaves or herbs are placed in the top section, and the base spirit in the bottom section is pumped up over the...

• rare-earth metals processing (in rare-earth element: Standard processes for specific groups)

  For the Group IV metals, only a distillation process is used. The pure oxide of the rare earth is dissolved in acid and reprecipitated using ultrapure chemicals to remove traces of calcium and magnesium often introduced from the water and chemicals used in the ion-exchange process. The precipitate is again converted to the oxide and placed in the still. Pure metallic lanthanum, cerium, or misch...

• retort use (in retort (chemistry and industry))

  vessel used for distillation of substances that are placed inside and subjected to heat. The simple form of retort, used in some laboratories, is a glass or metal bulb having a long, curved spout through which the distillate may pass to enter a receiving vessel. The design dates back to the cucurbit (flask) used by medieval alchemists.

• sample preparation (in sample preparation (chemistry): Isolation and preconcentration)

  In organic work, thermal evolution techniques include fractional distillation and gas chromatography. In the former, careful temperature control allows the collection of constant-boiling species from simple mixtures.

• water treatment and desalination (in environmental works (civil engineering): Thermal processes)

  Distillation, a thermal process that includes heating, evaporation, and condensation, is the oldest and most widely used of desalination technologies. Modern methods for the distillation of large quantities of salt water rely on the fact that the boiling temperature of water is lowered as air pressure drops, significantly reducing the amount of energy needed to vaporize the water. Systems that...

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