Making Crude Oil Useful—Fractional Distillation and Cracking

Author:
Rhys Baker
Updated date:
Jun 15, 2016

Crude Oil

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.

The Fractions

Little Peter Parrot Digs Holes For Blondes - a mnemonic to remember the fractions in order of their boiling points. Each Fraction is actually a mixture in itself, hence boiling ranges rather than distinct boiling points
Fraction	Boiling Range
LPG	up to 25°C
Petroleum	40-100°C
Paraffin	150-250°C
Diesel	220-350°C
Heating Oil	>350°C
Fuel Oil	>400°C
Bitumen	>400°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

The vapourised mixture enters the fractionating column at around 450°C. As the vapour travels up the column, it cools. As each fraction has a unique boiling point, each fraction condenses (and is collected at) a set point up the column
BBC.co.uk

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

Knowledge Check

For each question, choose the best answer. The answer key is below.

What property of hydrocarbons lets fractional distillation work?
- Viscosity
- Boiling Point
- Flammability
- Charge
Where does the fraction with the lowest boiling point leave the column??
- Top
- Bottom
As hydrocarbon chains increase in size...
- Intermolecular forces decrease
- Intermolecular forces increase
Bitumen is used to
- Fuel cars
- Heat homes
- Make roads
- Fuel Powerstations

Answer Key

Boiling Point
Top
Intermolecular forces increase
Make roads

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?

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
Alkanes
An eclectic set of resources all about alkanes

Comments

apooo on February 08, 2020:

it was too confusing I just wanted to find out how alkenes are produced through fractional distillation I'm ver sad now

Victoria [student] on September 10, 2019:

helpful and very cool

Alex (student) on January 17, 2019:

Very useful and interesting

Abrar on April 06, 2018:

i have to do a presentation about this and it helped a lot :) THANKS!!!

Joseph on June 14, 2017:

Hey there

youre mom on January 19, 2017:

this sux

mr kyawmoemyint on April 11, 2015:

I am myanmar. N ow I working myanmar No(1) refinery Thanlyin town . So l learned refinery that l got knowledge very very thank you.

shahzad minhaj on February 09, 2013:

i like it

sceince on January 18, 2013:

thanks

omo daddy on July 10, 2012:

I felt refreshed after reading that. Very informative, thanks.

Venugopaal from India on June 14, 2012:

Nice hub with lots of information. Thanks

Rhys Baker (author) from Peterborough, UK on May 31, 2012:

Woops! Thanks for that Marie! All changed

Marie on May 31, 2012:

Why does the quiz say bitumen is used to fuel cars?

Rhys Baker (author) from Peterborough, UK on February 22, 2012:

Thank-you, claudiafox. I am glad this was interesting to people other than my GCSE students. I am pleased you enjoyed it - thanks for the vote up

claudiafox from Sydney, Australia on January 22, 2012:

Well, hello TFscientist. This is good. Very good. Excellent. I learned new things. Do like your use of bullets and also the quiz boxes. Very pretty, too. I vote you up.