How Distillation Separates Mixtures

In simple distillation, a liquid mixture is heated in a flask. The component with the lower boiling point tends to enter the gas phase more readily. Vapour rises from the flask and passes into a condenser.

The condenser has a cool outer jacket surrounding an inner tube. As the vapour loses thermal energy, it changes back into a liquid. This liquid, called the distillate, collects in a separate container. Less volatile substances remain behind in the original flask.

Distillation does not create a new substance. It uses physical changes of state to separate existing components. The particles in the vapour are the same kind that were present in the liquid. They have simply been persuaded to change location through the traditional scientific method of heating something and watching it leave.

Simple distillation works well when separating a solvent from a dissolved solid. Salt water can be heated so that water vaporises and later condenses, while most of the salt remains in the flask. The collected water is separated from the salt, although additional treatment may be needed before it is safe to drink.

Separating two liquids is harder if their boiling points are close. Both may enter the vapour, so one round of evaporation and condensation gives only a partial separation. Fractional distillation improves the result by adding a fractionating column above the heated flask.

Inside the column, vapour repeatedly condenses and evaporates. The more volatile component becomes increasingly concentrated in the rising vapour, while the less volatile component tends to return to the flask. Industrial columns use many surfaces or trays to repeat this process efficiently. The column creates many small separation stages inside one piece of equipment.

Distillation is used in laboratories, petroleum refining, water treatment and the production of chemicals. It can require large amounts of energy because liquids must be heated and cooled. Some substances also decompose before boiling, so lower pressure or another separation method may be needed.

A careful setup should include:

• •A flask that is not sealed shut.
• •Controlled heating rather than an open flame when unsafe.
• •A thermometer positioned to measure vapour temperature.
• •Cooling water flowing through the condenser.
• •A separate container for the distillate.

Distillation has a surprising limit. Some mixtures of liquids reach a point where the vapour has exactly the same composition as the liquid. Boiling such a mixture no longer enriches the vapour in either component, so further distillation cannot improve the separation. This special mixture is called an azeotrope. The best known example is ethanol and water. At normal atmospheric pressure, fractional distillation can raise the ethanol content to roughly ninety-five percent by mass, but not beyond, however tall the column. The last few percent of water stays with the ethanol.

Chemists get around this in several ways. One method adds a third substance that changes how the components behave, creating a new mixture that can be separated. Another removes the remaining water with materials that absorb it, such as molecular sieves, which have pores sized to trap small water molecules but not the larger ethanol ones. Because the composition of an azeotrope depends on pressure, distilling at two different pressures can also help. Azeotropes explain why pure ethanol for laboratories is prepared differently from ordinary spirits, and why distillation is not always the final step.

Crude oil is a mixture of thousands of different hydrocarbons, molecules made of hydrogen and carbon in chains and rings of many sizes. Refineries separate it with fractional distillation on a giant scale. The crude is heated in a furnace until much of it turns to vapour, then fed into the bottom of a tall column. The column is hottest at the bottom and coolest at the top. Vapour rises through a series of trays, and each substance condenses at the height where the temperature falls below its boiling point.

Small molecules with low boiling points, such as the gases used for bottled fuel, leave from the top. Further down, liquids are drawn off as fractions used for petrol, jet fuel and kerosene, then diesel, and finally heavy oils. Each fraction is still a mixture, but of molecules with a similar range of boiling points. The thick residue at the bottom would start to break down if heated much further, so it is often sent to a second column at reduced pressure, where lower boiling points let more useful material be separated without cracking it.

Many of the plant oils that give lavender, peppermint or eucalyptus their smell boil at temperatures well above that of water, and heating them that hot could damage the delicate molecules. Steam distillation gets round this with a neat trick. Steam is passed through the plant material, carrying the oil molecules away with it, and the mixed vapour is then cooled in a condenser. The liquid that collects separates into two layers, because the oil does not dissolve in water. Most of these oils float on top and can be drawn off, while the watery layer below keeps a faint scent.

The reason it works is that two liquids which do not mix each push vapour into the space above them independently. Their vapour pressures add together, and the mixture boils when the total equals the pressure of the surrounding air. So oil and water boil together below 100 degrees Celsius, the boiling point of water alone, even though the oil by itself would need far more heat. That gives a gentle separation at a modest temperature. The same principle is used in industry to purify compounds that would decompose if they were heated close to their normal boiling points.

Distillation does not always mean heating a liquid on a bench. The oxygen used in hospitals and the nitrogen used to pack crisps usually come from air separated by fractional distillation at very low temperatures. First the air is filtered and stripped of water vapour and carbon dioxide, which would otherwise freeze and block the pipes. It is then compressed and cooled, partly by letting it expand, until most of it turns into a pale blue liquid at temperatures close to minus 200 degrees Celsius. The German engineer Carl von Linde developed large-scale equipment for liquefying air and separating it around the end of the nineteenth century.

The liquid air is fed into a tall, insulated column, and the familiar logic of boiling points takes over. Nitrogen boils at about minus 196 degrees Celsius, so it is the more volatile component and rises as gas to the top. Oxygen boils at about minus 183 degrees, so it gathers as liquid near the bottom. Argon, with a boiling point between the two, collects in the middle of the column and can be drawn off separately. The oxygen goes to medicine and steelmaking, while argon is used in welding, where its lack of reactivity shields hot metal from the air.

Distillation separates substances by turning a more volatile liquid into vapour and then condensing it elsewhere. Simple distillation suits large boiling point differences or a solvent mixed with a solid, while fractional distillation handles closer boiling points. Follow vaporisation, travel and condensation, and the apparatus becomes a logical path rather than a glass maze.