Making Crude Oil Useful—Fractional Distillation and Cracking
What is Crude Oil?
Simply put - useless. Crude Oil as dug from the ground is completely useless. And yet this 'black gold' gives us petrol, LPG, paraffin, bitumen, kerosene, plastic and a whole host of other compounds vital to modern (western?) life.
Crude Oil is one of three types of fossil fuel, the other two being Gas and Coal, and is arguably the most useful. The applications of this particular fossil fuel are far beyond that of mere electricity generation. As such the world turns on the price of oil, and countries have grown fabulously wealthy, and even gone to war, over this thick black gloop.
Crude Oil, the Mixture.
Crude Oil is a liquid fossil fuel that is very viscous and black in appearance (it also stinks to high heaven). It is a mixture of lots of different hydrocarbons, some of these hydrocarbon chains are very long, others are very short. Depending on the length of the hydrocarbon we have different uses for each one.
The longer the hydrocarbon:
- The higher the boiling point
- The higher the viscosity
- The darker the colour
- The lower the flammability
Due to the different boiling points, crude oil can be separated into fractions (parts) by heating it in a process called fractional distillation.
up to 25°C
Fractional Distillation - How Does it Work?
Each fraction collected by fractional distillation consists of a mixture of hydrocarbons whose boiling points fall within a particular range. But how does this work? The whole process hinges around boiling points, intermolecular forces and intramolecular forces.
- Long chain hydrocarbons have lots of intermolecular forces (think of lots of necklaces getting tangled in a jewellery box) making them difficult to separate. This gives them a high boiling point.
- Due to the high number of intermolecular forces, the forces are more difficult to break in large molecules. As such long chain hydrocarbons are thick, viscous liquids, or waxy solids
- Short chain hydrocarbons have very few intermolecular forces (think lots of earrings in a jewellery box)
- Small molecules have very small forces of attraction between them and are easy to break by heating. As such, these short chain hydrocarbons are volatile liquids or gases with low boiling points.
Industrial Fractionating Column
Fractional Distillation: Step-by-Step
- Crude oil is vapourised and fed into the bottom of the fractionating column.
- As the vapour rises up the column, the temperature falls.
- Fractions with different boiling points condense at different levels of the column and can be collected.
- The fractions with high boiling points (long chain hydrocarbons) condense and are collected at the bottom of the column
- Fractions with low boiling points (short chain hydrocarbons) rise to the top of the column where they condense and are collected.
Fractional Distillation in 90 seconds
Supply and Demand
Crude Oil is useless until we separate this mixture using fractional distillation. The resulting fractions have different uses depending on their properties, and some fractions are more useful than others. In general, shorter chain hydrocarbons are more useful than longer chains. The majority of the use we get out of crude oil is as fuel. As shorter chain molecules are more flammable (and burn with a cleaner flame) these are in higher demand.
As a result, the smaller fractions are in high demand. In fact, we cannot meet this demand through the products of fractional distillation alone. Fortunately, we have much more of the larger fractions than are needed.
To solve this supply-and-demand problem, we use a process called catalytic cracking to break the long chain hydrocarbons into shorter, more useful, hydrocarbons.
Cracking converts large alkane molecules into smaller, more useful, alkane and alkene molecules. The alkenes can then undergo polymerisation to make polymers (such as plastics) while the shorter alkanes are typically used for fuel.
As you can see in the video opposite, cracking needs a catalyst and a high temperature. If you struggle to remember that, just think of CHristmas crackers (C for catalyst, H for heat).
Cracking by the RSC
Where Next? Fractional Distillation and Cracking
- BBC - GCSE Bitesize: Fractional distillation
A secondary school revision resource for OCR GCSE Science about carbon chemistry and making crude oil useful
- Cracking Alkanes - thermal and catalytic
A brief description of the difference between thermal and catalytic cracking of alkanes
An eclectic set of resources all about alkanes
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