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Distillation

Distillation

Distillation is a technique by which two or more substances with different boiling points can be separated from each other. For example, fresh water can be obtained from seawater (water that contains salts) by distillation. When seawater is heated, water turns into a vapor that can be condensed and captured. The salts in the seawater remain behind.

General principles

In contrast to the preceding example, distillation is most commonly used to separate two or more liquids from each other. Imagine a mixture of three liquids, A, B, and C. A has a boiling point of 86°F (30°C); B has a boiling point of 104°F (40°C); and C has a boiling point of 122°F (50°C). Ordinary gasoline is such a mixture, except that it consists of many more than three components.

The three-liquid mixture described above is added to a distillation flask, such as the one in the accompanying figure of the distillation setup. The mixture in the flask is heated by a Bunsen burner or some other apparatus. The temperature of the liquid mixture rises until it reaches the boiling point of any one liquid in the flask. In our example, that liquid is A, which boils at 86°F. Liquid A begins to boil when the temperature in the flask reaches 86°F. It turns into a vapor at that temperature, rises in the distilling flask, and passes out of the flask arm into the condenser.

The condenser consists of a long tube surrounded by a larger tube. The outer tube contains water, which enters near the bottom of the condenser and leaves near the top. The water passing through the outer jacket of the condenser cools the vapor passing through the inner tube. The vapor loses heat and condenses (meaning it changes back to a liquid). It flows out of the condenser and into a receiving containera flask or beaker placed in position to capture the liquid. The liquid (liquid A) is now known as the distillate, or the product of the distillation.

Meanwhile, the temperature in the distilling flask has not changed, as indicated by the thermometer in the mouth of the flask. Heat added to the liquid mixture is used to vaporize liquid A, not to raise the temperature of the mixture. That temperature will begin to rise only when liquid A has completely boiled away. By watching the thermometer, therefore, an observer can know when liquid A has been completely removed from the liquid mixture. At that point, the receiver containing pure liquid A can be removed and replaced by a new receiver.

Once liquid A has boiled away, the temperature in the distilling flask begins to rise again. When it reaches 104°F, liquid B begins to boil away, and the sequence of events observed with liquid A is repeated. Eventually, pure samples of A, B, and C can be collected.

The distillation process described here has been known and used by humans for many centuries. It was used by ancient civilizations to prepare alcoholic beverages such as beer and wine and was perfected by those prechemists of the Middle Ages (4001450) known as alchemists. It has now been refined for use with many kinds of liquids under many different circumstances. For example, some liquids decompose (break apart) at or below

their boiling points. Vacuum distillation is used for such liquids. In vacuum distillation, air is pumped out of the distilling flask. Under reduced pressure in the flask, liquids boil away at temperatures less than their boiling points (below the point at which they would otherwise decompose).

Applications

Distillation is among the most important and widely used industrial operations today. About 95 percent of all separation processes today are carried out in industry with more than 40,000 distillation systems. Those systems generally consist of structures that look very different from the one shown in the distillation setup figure. For example, a petroleum refining plant is usually distinguished by a group of distilling towers that rise more than 100 feet (30 meters) into the air. The principle on which such towers operate, though, is no different from the one described above.

In petroleum refineries, crude oil is heated at the bottom of the refining tower. The hundreds of compounds that make up crude oil each boil off at their own characteristic boiling point. They rise in the refining tower, are cooled, and condense to liquids. Collectors at various heights in the tower are used to draw off those liquids into various fractions known by designations such as gasoline, diesel oil, heating oil, and lubricating oil.

A similar process is used in many other chemical processes. It is common that many by-products are produced along with some desired product in a chemical reaction. The desired product can be separated from the by-products by means of distillation.

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distillation

distillation, process used to separate the substances composing a mixture. It involves a change of state, as of liquid to gas, and subsequent condensation. The process was probably first used in the production of intoxicating beverages. Today, refined methods of distillation are used in many industries, including the alcohol and petroleum industries.

The Basic Distillation Process

A simple distillation apparatus consists essentially of three parts: a flask equipped with a thermometer and with an outlet tube from which the vapor is emitted; a condenser that consists of two tubes of different diameters placed one within the other and so arranged that the smaller (in which the vapor is condensed) is held in a stream of coolant in the larger; and a vessel in which the condensed vapor is collected. The mixture of substances is placed in the flask and heated. Ideally, the substance with the lowest boiling point vaporizes first (see vaporization), the temperature remaining constant until that substance has completely distilled. The vapor is led into the condenser where, on being cooled, it reverts to the liquid (condenses) and runs off into a receiving vessel. The product so obtained is known as the distillate. Those substances having a higher boiling point remain in the flask and constitute the residue.

Since a perfect separation is never effected, the distillate is often redistilled to increase its purity (hence the expression "double distilled" or "triple distilled" ). Many alcoholic beverages are distilled, e.g., brandy, gin, whiskey, and various liqueurs. The apparatus used, called the still, is the same in principle as other distillation apparatus.

The Fractional Distillation Process

When the substance with the lowest boiling point has been removed, the temperature can be raised and the distillation process repeated with the substance having the next lowest boiling point. The process of obtaining portions (or fractions) in this way is one type of fractional distillation. A more efficient method of fractional distillation involves placing a vertical tube called a fractionating column between the flask and the condenser. The column is filled with many objects on which the vapor can repeatedly condense and reevaporate as it moves toward the top, effectively distilling the vapor many times. The less volatile substances in the vapor tend to run back down the column after they condense, concentrating themselves near the bottom. The more volatile ones tend to reevaporate and keep moving upward, concentrating themselves near the top. Because of this the column can be tapped at various levels to draw off different fractions. Fractional distillation is commonly used in refining petroleum, some of the fractions thus obtained being gasoline, benzene, kerosene, fuel oils, lubricating oils, and paraffin.

The Destructive Distillation Process

Another form of distillation involves heating out of free contact with air such substances as wood, coal, and oil shale and collecting separately the portions driven off; this is known as destructive distillation. Wood, for example, when treated in this way yields acetic acid, methyl or wood alcohol, charcoal, and a number of hydrocarbons. Coal yields coal gas, coal tar, ammonia, and coke. Ammonia is also obtained by the destructive distillation of oil shale.

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Distillation

DISTILLATION

Distillation is the process of purifying liquid compounds on the basis of different boiling points or the process of separating liquids from compounds that do not vaporize. Since the actual process causes liquids to precipitate in a wet mist or drops that concentrate and drip, the word derives from the Latin de (from, down, away) + stillare (to drip).

In the simplest form of distillation, saltwater can be purified to yield freshwater by steam distillation, leaving a residue of salt. Distillation is also the process by which alcohol (ethanol, also called ethyl alcohol) as liquors or spirits, are separated from fermenting mashes of grains, fruits, or vegetables. When this process is used to distill alcohol, it is based on the following: Ethyl alcohol (C2H6O) has a lower boiling point than does water (78.5°C versus 100°C), so alcohol vapors rise first into the condenser, where cool water circulates around the outside of the condenser, causing the alcohol vapors to return to liquid form and drop into the collection flask. The purity of the distillate can be increased by repeating the process several times.

About 800 a.d., the process of distillation was evolved by the Arabian alchemist Jabir (or Geber) ibn Hayyah. He may also have named the distillate alcohol, since the word derives from an Arabic root, al-kuhul, which refers to powdered antimony (kohl) used as an eye cosmetic in the Mediterranean region; with time and use it came to mean any finely ground substance, then the "essence," and eventually, the essence of wineits spirit, or alcohol. It came into English from Old Spanish, from the Arabic spoken by the Moors of the Iberian peninsula during their rule there (750-1492 a.d.).

(See also: Beers and Brews ; Distilled Spirits, Types of ; Fermentation )

BIBLIOGRAPHY

LucÍa, S. P. (1963). Alcohol and civilization. New York: McGraw-Hill.

Scott E. Lukas

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distillation

distillation Extraction of a liquid by boiling a solution in which it is contained and cooling the vapour so that it condenses and can be collected. Distillation is used to separate liquids in solution, or liquid solvents from dissolved solids, to yield drinking water from sea water, or to produce alcoholic spirit. Fractional distillation, which uses a vertical column for condensation, is used in oil refining to separate the various fractions of crude oil.

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distiller

dis·till·er / disˈtilər/ • n. a person or company that manufactures liquor: barrels that the master distiller deems to be of superior quality.

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Distiller

Distiller Short for Adobe Acrobat Distiller. See Acrobat.

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