2.2 Fractional distillation and theory

How do you separate two liquids that have similar boiling points and are miscible? Miscible means they completely mix (eg alcohol and water) and do not form two layers (eg oil and water).

How can you separate a complex mixture of liquids by a method of distillation? Simple distillation isn't good enough to do an efficient job of separating liquids with boiling points that may be relatively close together. The liquids must all dissolve in each other i.e. they are all fully miscible.

Fractional distillation involves 2 main stages and both are physical state changes. It can only work with liquids with different boiling points but the boiling points can be quite close together. However, this method only works if all the liquids in the mixture are miscible (e.g. alcohol & water, hydrocarbons in crude oil etc.) and do NOT separate out into layers like oil & water (immiscible).

(1) For a laboratory demonstration of fractional distillation, the liquid or solution mixture is poured into the round-bottomed flask, heated and boiled to vaporise the most volatile component in the mixture (liquid ==> gas).

Anti-bumping granules give a smoother boiling action by providing a surface for small bubbles of vapour to form on.

(2) The vapour passes up through a fractionating column, where the separation takes place (theory at the end).

The fractionating column is packed with glass beads or short rods/tubes to give a large surface area for both evaporation and condensation to take place on.

This column is not used in the simple distillation described above, but is essential to the procedure of fractional distillation. When the temperature at the top of the column reaches the boiling point of the lowest boiling component, that component will then distil over into the condenser.

The higher boiling liquids (with greater intermolecular bonding forces between the molecules), are the most easily condensed liquids, condense and separate out, running back into the flask mixture, whereas the lower boiling liquid's vapour passes through into the Liebig condenser.

This is because the temperature falls as you rise up the special fractionating column.

In fractional distillation condensers you must think of the boiling point as equal to the condensation point as it decreases up the condenser column.

(3) The (e.g. ethanol) vapour is cooled by cold water in the condenser to condense (gas ==> liquid) it back to a liquid (the distillate) which is collected in the flask.

The 1st liquid, with the lowest boiling point, evaporates-boils off first, and is called the 1st fraction.

Then, each liquid distils, with increasing boiling point, over the top of the column, when it reaches its particular boiling point to give the 2nd, 3rd fraction etc.

The boiling point of each fraction can be read off from the thermometer.

In the diagram, think of the yellow liquid as water and the blue liquid as ethanol ('alcohol') - just to help visually to understand what is happening, I do know they are both colourless! 

The mixture might that of ethanol ('alcohol') and water from the fermentation of sugar. Imagine the green liquid is a mixture of a blue liquid (boiling point 80^oC) and a yellow liquid (boiling point 100^oC), so we have a coloured diagram simulation of the fractional distillation colourless alcohol and water mixture!

As the vapour from the boiling mixture enters the fractionating column it begins to cool and condense.

The highest boiling or least volatile liquid tends to condense more i.e. the yellow liquid (water).

The lower boiling more volatile blue liquid gets further up the column.

Gradually up the column the blue and yellow separate from each other so that yellow condenses back into the flask and pure blue distils over to be collected.

This ensures that only reasonably pure ethanol distils over.

The continuous evaporation and condensation over a large surface area is crucial to the success of fractional distillation and that large surface area is provided by using lots of little pieces of glass tubing or glass beads

The same arguments apply to the fractional distillation of complex multi-component mixtures like crude oil. The diagram above illustrates the school laboratory demonstration of a distilling simulated crude oil. As the vapour ascends the fractionating column the highest boiling liquids condense out and the lowest boiling liquid's vapour exits the top of the fractionating column and enters the condenser and runs into the collection tube. In this way you can progressively distil over higher and higher boiling fractions, which are themselves narrow boiling ranges of hydrocarbons.

Fractional distillation is used to separate oxygen and nitrogen from liquefied air, though the distillation takes place at very low temperatures, below -160^oC !

Fractional distillation is used on a large scale to separate the components of crude oil, because the different hydrocarbons have different boiling and condensation points (see oil). Very tall fractionating columns are used to ensure the separation of an extremely complex mixture of hydrocarbons.

However, in the school/college laboratory, to increase the separation efficiency of the glass fractionating column, it is usually packed with glass beads, short glass tubes or glass rings etc. which greatly increase the surface area for evaporation and condensation in the fractional distillation process. 

In the fractional distillation of crude oil, the system works slightly differently, the different fractions are condensed out at different points in a huge fractionating column. At the top are the very low boiling fuel gases like butane and at the bottom are the high boiling big molecules of waxes and tar.

More details on the Fractional distillation of crude oil & uses of fractions

AND in these separation procedures there always loss of product so at a higher level (GCSE/IGCSE/A Level) you need to know about

 % reaction yield & reasons for loss of product

atom economy  *  % purity of a product

I'm afraid there is a bit more to it than just a distillation apparatus !!!