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 container—a 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 (400–1450) 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.

Distillation

General principles

Applications

Distillation is the process of separating liquids by boiling them, and then condensing the resulting vapor. Islamic alchemist, astronomer, and physicist Abu Musa Jabir ibn Hayyan (c. 721–c. 815) is generally considered today as the inventor of the modern distillation process. Distillation is one of the most important processes for separating the components of a solution. The solution is heated to form a vapor of the more volatile components in the system, and the vapor is then cooled, condensed, and collected as drops of liquid. By repeating vaporization and condensation, individual components

in the solution can be recovered in a pure state. Whiskey, essences, and many pure products from the oil refinery industry are processed via distillation.

General principles

Distillation has been used widely to separate volatile components from nonvolatile compounds. The underlying mechanism of distillation is the differences in volatility between individual components. With sufficient heat applied, a gas phase is formed from the liquid solution. The liquid product is subsequently condensed from the gas phase by removal of the heat. Therefore, heat is used as the separating agent during distillation. Feed material to the distillation apparatus can be liquid and/or vapor, and the final product may consist of liquid and vapor. A typical apparatus for simple distillation used in chemistry laboratories is one in which the still pot can be heated with a water, steam, or oil bath. When liquids tend to decompose or react with oxygen during the course of distillation, the working pressure can be reduced to lower the boiling points of the substances and hence the temperature of the distillation process.

In general, distillation can be carried out either with or without reflux involved. For the case of single-stage differential distillation, the liquid mixture is heated to form a vapor that is in equilibrium with the residual liquid. The vapor is then condensed and removed from the system without any liquid allowed to return to the still pot. This vapor is richer in the more volatile component than the liquid removed as the bottom product at the end of the process. However, when products of much higher purity are desired, part of the condensate has to be brought into contact with the vapor on its way to the condenser and recycled to the still pot. This procedure can be repeated for many times to increase the degree of separation in the original mixture. Such a process is normally called rectification.

Applications

Distillation has long been used as the separation process in the chemical and petroleum industries because of its reliability, simplicity, and low-capital cost. It is employed to separate benzene from toluene, methanol or ethanol from water, acetone from acetic acid, and many multi-component mixtures. Fractionation of crude oil and the production of deuterium also rely on distillation.

Today, with over 40, 000 distillation towers in operation, distillation makes about 95% of all current industrial separation processes; however, distillation systems also have relatively high energy consumption. Significant effort, therefore, has been made to reduce the energy consumption and to improve the efficiency in distillation systems. This includes incorporating new analytical sensors and reliable hardware into the system to achieve advanced process control, using heat rejected from a

KEY TERMS

Activity coefficient— The ratio of the partial pressure of a component in the gas phase to the product of its mole fraction in the liquid phase and its vapor pressure as a pure liquid which is an important factor encountered in many vapor-liquid separation processes.

Bubble point— For a saturated liquid, because any rise in temperature will form bubbles of vapor, the liquid is said at its bubble point.

Dew point— The point at which air or a gas begins to condense to a liquid.

Differential distillation— During distillation, only a very small portion of the liquid is flashed each time and the vapor formed on boiling the liquid is removed at once from the system.

Distillate— The product withdrawn continuously at the top of the distillation column.

Reflux— Part of the condensate from the condenser is returned to the top tray of the distillation column as reflux to provide liquid flow above the feed point for increasing separation efficiency.

condenser of one column to reboil other columns, and coupling other advanced process such as adsorption and crystallization with distillation to form energy-saving hybrid operation systems.

Pang-Jen Kung

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 wine—its 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

Distillation

Distillation is one of the most important processes for separating the components of a solution . The solution is heated to form a vapor of the more volatile components in the system, and the vapor is then cooled, condensed, and collected as drops of liquid. By repeating vaporization and condensation, individual components in the solution can be recovered in a pure state. Whiskey, essences, and many pure products from the oil refinery industry are processed via distillation.

General principles

Distillation has been used widely to separate volatile components from nonvolatile compounds. The underlying mechanism of distillation is the differences in volatility between individual components. With sufficient heat applied, a gas phase is formed from the liquid solution. The liquid product is subsequently condensed from the gas phase by removal of the heat. Therefore, heat is used as the separating agent during distillation. Feed material to the distillation apparatus can be liquid and/or vapor, and the final product may consist of liquid and vapor. A typical apparatus for simple distillation used in chemistry laboratories is one in which the still pot can be heated with a water , steam, or oil bath. When liquids tend to decompose or react with oxygen during the course of distillation, the working pressure can be reduced to lower the boiling points of the substances and hence the temperature of the distillation process.

In general, distillation can be carried out either with or without reflux involved. For the case of single-stage differential distillation, the liquid mixture is heated to form a vapor that is in equilibrium with the residual liquid. The vapor is then condensed and removed from the system without any liquid allowed to return to the still pot. This vapor is richer in the more volatile component than the liquid removed as the bottom product at the end of the process. However, when products of much higher purity are desired, part of the condensate has to be brought into contact with the vapor on its way to the condenser and recycled to the still pot. This procedure can be repeated for many times to increase the degree of separation in the original mixture. Such a process is normally called "rectification."

Applications

Distillation has long been used as the separation process in the chemical and petroleum industries because of its reliability, simplicity, and low-capital cost. It is employed to separate benzene from toluene, methanol or ethanol from water, acetone from acetic acid , and many multicomponent mixtures. Fractionation of crude oil and the production of deuterium also rely on distillation.

Today, with 40,000 distillation towers in operation, distillation makes about 95% of all current industrial separation processes; however, distillation systems also have relatively high energy consumption. Significant effort, therefore, has been made to reduce the energy consumption and to improve the efficiency in distillation systems. This includes incorporating new analytical sensors and reliable hardware into the system to achieve advanced process control, using heat rejected from a condenser of one column to reboil other columns, and coupling other advanced process such as adsorption and crystallization with distillation to form energy-saving hybrid operation systems.

Pang-Jen Kung

KEY TERMS

Activity coefficient

—The ratio of the partial pressure of a component in the gas phase to the product of its mole fraction in the liquid phase and its vapor pressure as a pure liquid which is an important factor encountered in many vapor-liquid separation processes.

Bubble point

—For a saturated liquid, because any rise in temperature will form bubbles of vapor, the liquid is said at its bubble point.

Dew point

—The point at which air or a gas begins to condense to a liquid.

Differential distillation

—During distillation, only a very small portion of the liquid is flashed each time and the vapor formed on boiling the liquid is removed at once from the system.

Distillate

—The product withdrawn continuously at the top of the distillation column.

Reflux

—Part of the condensate from the condenser is returned to the top tray of the distillation column as reflux to provide liquid flow above the feed point for increasing separation efficiency.

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.