Why is fractional distillation useful?

Written by cam merritt
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Why is fractional distillation useful?
Refineries are large-scale fractional distillation facilities. (oil refinery 6 image by Aaron Kohr from Fotolia.com)

Fractional distillation makes it possible to extract individual liquids from a mixture of substances. It's what produces gasoline for cars and spirits for cocktails. Fractional distillation can be performed in a lab using Bunsen burners, beakers and glass columns a couple of feet tall, or in giant facilities such as oil refineries with "fractionating towers" that rise hundreds of feet.

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Distillation

Distillation is the process of purifying a liquid by evaporating it out of a mixture, condensing the vapour and then repeating the process. If the mixture you're dealing with is a simple combination of two substances---say, salt and water---then you can just boil one away. This is known as simple distillation. If it's a more complicated mixture---one with more than two liquids, or with liquids that have similar boiling points, then this simple distillation procedure won't be enough. That's when you need fractional distillation. Fractional distillation separates each component---or "fraction"---of a mixture so that it can be collected.

Process

Fractional distillation takes place in an apparatus known as a fractionating column. At the bottom of the column, the mixture is heated until it boils. The vapour rises through the column until it encounters a "tray" or "packing"---a structure, object or material placed in the column to make the vapour condense into liquid. Heat from the rising vapour causes some of the liquid to re-evaporate from the packing; the remaining liquid is drained off at that point in the column. This process continues up the column until the mixture has separated into its component parts; the liquid with the lowest boiling point collects at the top of the column, the liquid with the highest boiling point collects at the bottom and liquids with boiling points somewhere in between collect at different points in the column.

Oil

Probably the most well-known use of fractional distillation occurs at crude-oil refineries. Crude oil is a soup of various hydrocarbons, which are separated in giant fractionating towers. Through fractional distillation, a barrel of crude oil produces gasoline, diesel fuel, kerosene, lightweight alkanes such as butane and propane, and a slew of other substances.

Spirits

The alcohol in beer and wine is the product of fermenting organic material such as grain, but once the alcohol content gets above about 12 per cent, it kills the yeast necessary for fermentation. To produce a stronger alcoholic liquid, you need to distil the alcohol from the mixture. But grain alcohol, or ethanol, boils at 82.8 degrees Celsius, while water boils at 100 degrees C. Simple distillation works only when there is about 23.9 degrees C' difference in boiling points; if the difference is less, the mixture won't separate properly. That's why distilled spirits are produced through fractional distillation.

Other Uses

Other applications of fractional distillation include isolating specific chemicals. Liquid nitrogen and liquid oxygen are also performed through fractional distillation, except that in this case, the mixture---air---is supercooled rather than heated. Each fraction of air turns to liquid at a different temperature.

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