Distillation

Summary

Distillation is a process for purifying liquid mixtures by collecting vapors from a boiling substance and condensing them back into the original liquid. Various forms of this technique, practiced since antiquity, continue to be used extensively in the petroleum, petrochemical, coal tar, chemical, and pharmaceutical industries to separate mixtures of mostly organic compounds as well as to isolate individual components in chemically pure form. Distillation has also been employed to acquire chemically pure water, including potable water through the desalination of seawater.

Definition and Basic Principles

Matter commonly exists in one of three physical statessolid, liquid, or gas. Any phase of matter can be changed reversibly into another at a temperature and pressure characteristic of that particular sample. When a liquid is heated to a temperature called the boiling point, it begins to boil and is transformed into a gas. Unlike the melting point of a solid, the boiling point of a liquid is proportional to the applied pressure, increasing at high pressures and decreasing at low pressures.

When a mixture of several miscible liquids is heated, the component with the lowest boiling point is converted to the gaseous phase preferentially over those with higher boiling points, which enriches the vapor with the more volatile component. The distillation operation removes this vapor and condenses it back to the liquid phase in a different receiving flask. Thus, liquids with unequal boiling points can be separated by collecting the condensed vapors sequentially as fractions. Distillation also removes nonvolatile components, which remain behind as a residue.

Background and History

Applications of fundamental concepts such as evaporation, sublimation, and condensation were mentioned by Aristotle and others in antiquity. However, many historians consider distillation to be a discovery of Alexandrian alchemists (300 BCE to 200 CE), who added a lid (called the head) to the still and prepared oil of turpentine by distilling pine resin. The Arabians improved the apparatus by cooling the head (which came to be known as the alembic) with water, which allowed the isolation of a number of essential oils by distilling plant material and, by 800 CE, permitted the Islamic scholar Jbir ibn Hayyn to obtain acetic acid from vinegar.

Alembic stills and retorts were widely employed by alchemists of medieval Europe. The first fractional distillation, also called differential distillation, was developed by Taddeo Alderotti in the thirteenth century. The first comprehensive manual of distillation techniques was Liber de arte distillandi de simplicibus, written by Hieronymus Brunschwig and published in 1500 in Strasbourg, France. The first account of the destructive distillation of coal was published in 1726. Large-scale continuous stills with fractionating towers similar to modern industrial stills were devised for the distillation of alcoholic beverages in the first half of the nineteenth century and later adapted to coal and oil refining. Laboratory distillation similarly advanced with the introduction of the Liebig condenser around 1850. The modern theory of distillation was developed by Ernest Sorel and reduced to engineering terms in his Distillation et Rectification Industrielles (1899).

How It Works

Simple Distillation. A difference in boiling point of at least 25 degrees Celsius is generally required for successful separations with simple distillation. The glass apparatus for laboratory-scale distillations consists of a round-bottomed boiling flask, a condenser, and a receiving flask. Vapors from the boiling liquid are returned to the liquid state by the cooling action of the condenser and are collected as distillate in the receiving flask. An air condenser may be sufficient for high-boiling liquids, but often, a jacketed condenser—in which a cooling liquid such as cold water is circulated—is required. The design of many styles of condensers, such as the Liebig condenser and the West condenser, enhances the cooling effect of the circulating liquid. An adapter called a still head connects the condenser to the boiling flask at a 45-degree angle and is topped with a fitting in which a thermometer is inserted to measure the temperature of the vapor (the boiling point). A second take-off adapter is often used to attach the receiving flask to the condenser at a 45-degree angle so that it is vertical and parallel to the boiling flask. One should never heat a closed system, so the take-off adapter contains a sidearm for connection to either a drying tube or a vacuum source for distillations under reduced pressure. The apparatus was formerly assembled by connecting individual pieces with cork or rubber stoppers, but the ground-glass joints of modern glassware make these stoppers unnecessary.

Fractional Distillation. When the boiling points of miscible liquids are within about 25 degrees Celsius, simple distillation does not yield separate fractions. Instead, the process produces a distillate whose composition contains varying amounts of the components, being initially enriched in the lower-boiling and more volatile one. The still assembly is modified to improve efficiency by placing a distilling column between the still head and the boiling flask. This promotes multiple cycles of condensation and revaporization. Each of these steps is an equilibration of the liquid and gaseous phases and is, therefore, equivalent to a simple distillation. Thus, the distillate from a single fractional distillation has the composition of one obtained from numerous successive simple distillations. Still heads that allow higher reflux ratios and distilling columns with greater surface areas permit more contact between vapor and liquid, which increases the number of equilibrations. Thus, a Vigreux column having a series of protruding fingers is more efficient than a smooth column. Even more efficient are columns packed with glass beads, single- or multiple-turn glass or wire helices, ceramic pieces, copper mesh, or stainless-steel wool. The limit of efficiency is approached by a spinning-band column that contains a very rapidly rotating spiral of metal or Teflon over its entire length.

Applications and Products

Batch and Continuous Distillation. Distilling very large quantities of liquids as a single batch is impractical, so industrial-scale distillations are often conducted by continuously introducing the material to be distilled. Continuous distillation is practiced in petrochemical and coal-tar processing and can also be used for the low-temperature separation and purification of liquefied gases such as hydrogen, oxygen, nitrogen, and helium.

Vacuum Distillation. Heating liquids to temperatures above about 150 degrees Celsius is generally avoided to conserve energy, minimize the difficulty of insulating the still head and the distilling column, and prevent the thermal decomposition of heat-sensitive organic compounds. A vacuum distillation takes advantage of the fact that a liquid boils when its vapor pressure equals the external pressure, which causes the boiling point to be lowered when the pressure decreases. For example, the boiling point of water is 100 degrees Celsius at a pressure of 101.3 kilopascals (kPa), but this drops to 79 degrees Celsius at 45.5 kPa and rises to 120 degrees Celsius at 198.5 kPa. Vacuum distillation can be applied to solids that melt when heated in the boiling flask. However, the condenser may require higher temperatures to prevent the distillate from crystallizing. The term vacuum distillation is actually a misnomer, for these distillations are conducted at a reduced pressure rather than under an absolute vacuum. A pressure of about 2.7 kPa is obtainable with ordinary water aspirators, and 0.13 kPa (1 millimeter of mercury, or mmHg) can be achieved with a laboratory vacuum pump.

Molecular Distillation. When the pressure of residual air in the still is lower than about 0.0013 kPa (0.01 mmHg), the vapor can easily travel from the boiling liquid to the condenser, and distillate is collected at the lowest possible temperature. Distillation under high-vacuum conditions permits the purification of thermally unstable compounds of high molecular weight, such as glyceride fats and natural oils and waxes, that would otherwise decompose at temperatures encountered in an ordinary vacuum distillation. Molecular stills often have a simple design that minimizes refluxing and accelerates condensation. For example, the high-vacuum short path still consists of two plates, one heated and one cooled, separated by a very short distance. Industrially, the distillate can be condensed on a rapidly rotating cone and removed quickly by centrifugal force.

Steam Distillation. Another method to lower boiling temperature is steam distillation. When a homogeneous mixture of two miscible liquids is distilled, the vapor pressure of each liquid is lowered according to Raoult's law of thermodynamicsnamed for French physicist François-Marie Raoultand Dalton's law of partial pressurenamed for English chemist and physicist John Dalton. However, when a heterogeneous mixture of two immiscible (incapable of mixing) liquids is distilled, the mixture's boiling point is lower than that of its most volatile component because the vapor pressure of each liquid is now independent of the other liquid. Steam distillation occurs when one of these components is water and the other an immiscible organic compound. The steam may be introduced into the boiling flask from an external source or generated internally by mixing water with the material to be distilled. Steam distillation is especially useful in isolating the volatile oils of plants, like when extracting lemongrass and eucalyptus oil to make fragrances and personal hygiene products.

Azeotropic Extractive Distillation. Specific nonideal solutions of two or more liquids form an azeotrope, a constant-boiling mixture whose composition does not change during distillation. Water (boiling point of 100.0 degrees Celsius) and ethanol (boiling point of 78.3 degrees Celsius) form a binary azeotrope (boiling point of 78.2 degrees Celsius) consisting of 4 percent water and 96 percent ethanol. No amount of further distillation will remove the remaining water. However, adding benzene (boiling point 80.2 degrees Celsius) to this distillate forms a tertiary benzene-water-ethanol azeotrope (boiling point of 64.9 degrees Celsius) that leaves pure ethanol behind when the water is removed. This is an example of azeotropic drying—a special case of azeotropic extractive distillation.

Microscale Distillation. Microscale organic chemistry, with a history that spans more than a century, is not a new concept to research scientists. However, the traditional 5- to 100-gram macroscale of student laboratories was reduced by one hundred to one thousand times by introducing microscale glassware in the 1980s to reduce the risk of fire and explosion, limit exposure to toxic substances, and minimize hazardous waste. Microscale glassware comes in various configurations, such as Mayo-Pike or Williamson styles. Distillation procedures are especially troublesome on a microscale because the ratio of wetted-glass surface area to the volume of distillate increases as the sample size is reduced, causing a significant loss of product. Specialized microscale glassware such as the Hickman still head has been designed to overcome this difficulty.

Analytical Distillation. The composition of liquid mixtures can be quantitatively determined by weighing the individual fractions collected during a carefully conducted fractional distillation. However, this technique has been largely replaced by instrumental methods such as chromatography, particularly gas chromatography and high-performance liquid chromatography.

Membrane Distillation. Membrane distillation, a process involving the use of a hydrophobic membrane for separation, can produce distilled-quality water from seawater and has a variety of industrial applications. Most membrane distillation processes, like reverse osmosis, function by static pressure or concentration difference in the substances, like medical dialysis. As with many forms of distillation, a drawback is energy inefficiency. The most efficient solar energy model for desalinating water in terms of operating cost and daily productivity is the photovoltaic reverse osmosis.

Careers and Course Work

The art of distillation is most commonly practiced by chemists who majored in chemistry or chemical engineering, the former distilling samples on a laboratory scale and the latter conducting distillations on larger pilot plants and considerably larger industrial scales. The difference between chemistry and chemical engineering majors is their advanced and elective coursework. Chemistry majors concentrate on molecular structure to better understand matter's chemical and physical properties. In contrast, chemical engineering majors focus more on the properties of bulk matter involved in the large-scale, economical, and safe manufacture of useful products. Advanced graduate study can be pursued in both disciplines at the master's and doctoral levels, the latter degree being prevalent among professors of chemistry and chemical engineering at colleges and universities. Massachusetts Institute of Technology offers courses in distillation and separation processes. Aspirants can work as distillation operators, chemists, and extraction technicians.

Both chemistry and chemical engineering majors need a strong foundation in physics and mathematics. They typically study a year of general chemistry, followed by a year of organic chemistry and another year of physical chemistry. The ability to predict the relative boiling points of chemical substances based on the strength of intermolecular forces—a skill honed in general and organic chemistry—is crucial. Distillation is an essential technique used in student laboratories to isolate and purify products of synthesis, often in small quantities, using microscale glassware. Theoretical and quantitative aspects of phase transitions are covered in physical chemistry lectures and labs. Participating in research with established groups is a valuable way for students to gain practical experience in laboratory procedures, methodologies, and protocols. Understanding proper procedures for handling toxic materials and disposing of hazardous waste is essential for all practicing chemists.

Social Context and Future Prospects

More than any other technology, the process and apparatus of distillation gave birth to the modern chemical industry because of the numerous chemical products derived first from coal tar and later from petroleum. Though distillation is a mature science, its importance in chemical process industries and continued technological advancements should not be overlooked. Beginning in the 1990s, distillation technology experienced three decades of advancements, including electrolyte modeling, reactive distillation and enhanced distillation, high-capacity trays, high-capacity packings, and enhanced heat-transfer surfaces.

The continued role of distillation in modern technology will depend on several factors, including energy conservation and the sustainability of raw materials. Increased demand and diminishing supplies of raw materials, together with the accumulation of increasing amounts of hazardous waste, have made recycling economically feasible on an industrial scale, and distillation has a role to play in many of these processes. Likewise, the increasing cost of crude oil due to diminishing supplies of this finite resource encourages the use of alternate sources of oil, such as coal (nearly 75 percent of total fossil fuel reserves), and distillation would be expected to play the same central role as it does in refining petroleum. However, distillation is also an energy-intensive technology in the requirements of heating liquids to boiling and cooling the resulting vapors so they condense back to liquid products. Thus, one would expect that the chemical industry of the future would seek alternate energy sources, such as solar power, as well as ways to conserve energy through improvements in distillation efficiency.

According to the US Centers for Disease Control and Prevention (CDC), the distillation process is effective at removing many pathogens and chemical contaminants. However, though distillation is used in desalination, its byproducts have become an environmental concern in the early decades of the twenty-first century, as the process uses various chemicals such as chlorine, hydrochloric acid, and hydrogen peroxide to increase efficiency.

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