# Fractional Distillation

In this worksheet, students will learn about fractional distillation, the process used to separate crude oil into it's different products.

### QUESTION 1 of 10

When we get crude oil out of the ground, it isn't very useful. It's a sticky mess, a bit like treacle- but not as nice to eat.

Crude oil is a mixture of hydrocarbons (substances made of carbon and hydrogen only). Each different hydrocarbon is much more useful- and much more valuable- if we can separate them out. In order to separate crude oil into its components it needs to be heated through a process called fractional distillation. Here is a diagram that shows how crude oil is separated:

Crude oil is pumped into the bottom of the fractionating column, and heated to about 350 °C. This is hot enough for most of the liquid oil to turn to gas. As the gas rises up the fractionating column, it cools. When it gets cool enough, some of the gas condenses back into liquid. The liquid is collected where it forms, and pumped out. Each part of the fractionating column collects a different fraction; part of the oil with a certain boiling temperature.

Each of these fractions (refinery gas, petrol, naptha and so on) is still a mixture, but each fraction only contains a few, very similar, types of hydrocarbon. The molecules in each fraction are similar enough to be useful, even when mixed together.

What are the different fractions like?

Here is a table of some of the properties of the different fractions. You don't need to memorise the numbers in detail (mostly they are on arbitrary scales, and people don't always agree about where to put the boundaries between the different fractions), but the patterns are important.

Fraction Number of carbon atoms Boiling point (°C) Viscosity
(1= low, 10 = high)
Flammability
(1 = hard to light, 10 = burns easily)
Energy content
(1 = low, 10 = high)
Refinery gas 1 to 4 below 25 0 (gas) 10 5
Petrol 5 to 10 80 to 100 1 8 6
Naphtha 8 to 12 110 to 150 2 7 7
Kerosene 10 to 16 180 to 250 4 5 8
Diesel 16 to 20 260 to 350 6 4 9
Residue more than 20 more than 350 10 2 10

How does chain length affect the properties of the hydrocarbons?

Longer molecules have higher boiling points
The component hydrocarbon molecules are long chains held together by intermolecular forces (inter means between). The forces are broken while boiling.

Lighter molecules with shorter chains need less energy for the forces to be broken, so they are easily separated. They have low boiling points.

Heavy molecules, like the ones in bitumen, have longer chains and more energy is needed to separate them. They have high boiling points.

Shorter molecules flow more easily, longer molecules are more viscous.

The smaller chains are also able to flow over each other better, meaning that they will be less viscous (viscous means sticky, like treacle). The longer the chains, the harder it is for them to flow over each other making them more viscous.

There's a lot to think about when you are learning about fractional distillation. Make sure you know the names of the fractions, and what they are used for. The patterns will help you; small molecules leave the refinery at the top of the column, and large ones at the bottom. Those patterns link to what you know about intermolecular forces, and the structures of alkanes.

Fill in the gaps in this paragraph about crude oil. Use these words:

distillation

fractional

hydrocarbons

lengths

mixture

separate

Look at this table of properties of different fractions of crude oil.

Fraction Number of carbon atoms Boiling point (°C) Viscosity
(1= low, 10 = high)
Flammability
(1 = hard to light, 10 = burns easily)
Energy content
(1 = low, 10 = high)
Refinery gas 1 to 4 below 25 0 (gas) 10 5
Petrol 5 to 10 80 to 100 1 8 6
Naphtha 8 to 12 110 to 150 2 7 7
Kerosene 10 to 16 180 to 250 4 5 8
Diesel 16 to 20 260 to 350 6 4 9
Residue more than 20 more than 350 10 2 10

Which fraction should we use for fuel for domestic cooking? Pick a fraction and a reason.

Why do longer chain hydrocarbons have higher boiling points? Pick two correct answers.

There are stronger intermolecular forces

There are stronger intramolecular forces

More energy is needed to separate the molecules from each other

Less energy is needed to separate molecules from each other

In fractional distillation, we inject crude oil at the bottom of the column. Mark this description of what happens next; tick each correct sentence.

As the hydrocarbons rise up the column, they become cooler.

This causes them to evaporate when they get cool enough.

As each fraction turns to a liquid, it is collected and piped away.

Each fraction is a pure substance.

At the top of the column, we have small molecules with a low boiling point.

Sort these fractions of crude oil into order, from the ones collected at the top to the ones collected at the bottom.

## Column B

1 (top)
diesel oil
2
naptha
3
refinery gas
4
residue
5
petrol
6 (bottom)
kerosene

Which of these phrases describes kerosene? Tick all the correct answers.

pure substance

hydrocarbon

gas at room temperature

mixture

used as a fuel

Look at this table of properties of different fractions of crude oil.

Fraction Number of carbon atoms Boiling point (°C) Viscosity
(1= low, 10 = high)
Flammability
(1 = hard to light, 10 = burns easily)
Energy content
(1 = low, 10 = high)
Refinery gas 1 to 4 below 25 0 (gas) 10 5
Petrol 5 to 10 80 to 100 1 8 6
Naphtha 8 to 12 110 to 150 2 7 7
Kerosene 10 to 16 180 to 250 4 5 8
Diesel 16 to 20 260 to 350 6 4 9
Residue more than 20 more than 350 10 2 10

Which fraction should we use for powering a ship? Pick a fraction and a reason.

pure substance

hydrocarbon

gas at room temperature

mixture

used as a fuel

From your knowledge of the boiling temperature of petrol and diesel, which one is more likely to work badly in cold, wintry conditions?

Match these temperatures with the names of the resulting fractions.

## Column B

25 °C
petrol
40 °C
residue
110 °C
kerosene
180 °C
diesel oil
260 °C
refinery gas
340 °C
naphtha

Match these fractions of crude oil with their uses.

## Column B

refinery gas
fuel for diesel engines
petrol
jet airplane fuel
naphtha
fuel for home use
kerosene
fuel for ships, lubricating oil
diesel oil
chemical production
residue
fuel for cars
• Question 1

Fill in the gaps in this paragraph about crude oil. Use these words:

distillation

fractional

hydrocarbons

lengths

mixture

separate

EDDIE SAYS
Crude oil is a mixture, with lots of different types of hydrocarbon molecule in it. That's because it forms naturally, but it makes it less useful than we would like. Fractional distillation is the answer to that problem.
• Question 2

Look at this table of properties of different fractions of crude oil.

Fraction Number of carbon atoms Boiling point (°C) Viscosity
(1= low, 10 = high)
Flammability
(1 = hard to light, 10 = burns easily)
Energy content
(1 = low, 10 = high)
Refinery gas 1 to 4 below 25 0 (gas) 10 5
Petrol 5 to 10 80 to 100 1 8 6
Naphtha 8 to 12 110 to 150 2 7 7
Kerosene 10 to 16 180 to 250 4 5 8
Diesel 16 to 20 260 to 350 6 4 9
Residue more than 20 more than 350 10 2 10

Which fraction should we use for fuel for domestic cooking? Pick a fraction and a reason.

EDDIE SAYS
For domestic use, convenience is the important thing; we need something which flows and burns easily. It doesn't matter so much that hydrocarbons in refinery gas don't release as much energy as others do.
• Question 3

Why do longer chain hydrocarbons have higher boiling points? Pick two correct answers.

There are stronger intermolecular forces
More energy is needed to separate the molecules from each other
EDDIE SAYS
You can think about breaking the molecules apart in terms of either forces or energy- stronger forces need more energy to overcome. The important thing is that its intermolecular forces that count.
• Question 4

In fractional distillation, we inject crude oil at the bottom of the column. Mark this description of what happens next; tick each correct sentence.

As the hydrocarbons rise up the column, they become cooler.
As each fraction turns to a liquid, it is collected and piped away.
At the top of the column, we have small molecules with a low boiling point.
EDDIE SAYS
As the hydrocarbons cool, they turn from a gas to a liquid, which is condensation. Each fraction is a mixture of a few similar hydrocarbons; not pure, but similar enough to be useful together.
• Question 5

Sort these fractions of crude oil into order, from the ones collected at the top to the ones collected at the bottom.

## Column B

1 (top)
refinery gas
2
petrol
3
naptha
4
kerosene
5
diesel oil
6 (bottom)
residue
EDDIE SAYS
Because this question is about names, there aren\'t really any short cuts to learning them. Keep testing yourself on this order, until you get it right every time. It will happen!
• Question 6

Which of these phrases describes kerosene? Tick all the correct answers.

hydrocarbon
mixture
used as a fuel
EDDIE SAYS
Kerosene is a mixture of hydrocarbons (between about 10 and 16 carbon atoms per molecule), which is one of the fractions of crude oil. Its boiling point is about 200 °C, so it can't be a gas at room temperature (it's a liquid; it freezes at about -40 °C, depending on what is added to it). It is used as fuel in airplanes, because it has a good balance of convenience and energy content.
• Question 7

Look at this table of properties of different fractions of crude oil.

Fraction Number of carbon atoms Boiling point (°C) Viscosity
(1= low, 10 = high)
Flammability
(1 = hard to light, 10 = burns easily)
Energy content
(1 = low, 10 = high)
Refinery gas 1 to 4 below 25 0 (gas) 10 5
Petrol 5 to 10 80 to 100 1 8 6
Naphtha 8 to 12 110 to 150 2 7 7
Kerosene 10 to 16 180 to 250 4 5 8
Diesel 16 to 20 260 to 350 6 4 9
Residue more than 20 more than 350 10 2 10

Which fraction should we use for powering a ship? Pick a fraction and a reason.

EDDIE SAYS
In industrial uses, mess and convenience are less important than energy content. In many cases, the fuel used for ships would be "fuel oil" taken from the residue at the bottom of the fractionating column.
• Question 8

From your knowledge of the boiling temperature of petrol and diesel, which one is more likely to work badly in cold, wintry conditions?

diesel
EDDIE SAYS
The point here is that the freezing point of covalent compounds also depends on the strength of the intermolecular forces. So the higher the boiling point, the higher the freezing point as well. Diesel has a higher boiling point than petrol, and it stops flowing as a proper liquid at about -15 °C; enough to be a problem on cold nights.
• Question 9

Match these temperatures with the names of the resulting fractions.

## Column B

25 °C
refinery gas
40 °C
petrol
110 °C
naphtha
180 °C
kerosene
260 °C
diesel oil
340 °C
residue
EDDIE SAYS
The exact temperatures aren't critical here; different sources quote slightly different values. The range and order are the important points to learn.
• Question 10

Match these fractions of crude oil with their uses.

## Column B

refinery gas
fuel for home use
petrol
fuel for cars
naphtha
chemical production
kerosene
jet airplane fuel
diesel oil
fuel for diesel engines
residue
fuel for ships, lubricating oil
EDDIE SAYS
Notice the pattern; homes and cars use the light (small molecule) fractions- because they are easier to use. The heavier (large molecule) fractions are harder to use, and get used in industry and large ships. The very large molecules don't work as fuel, but get used for lubrication, or sticking things together (like tar).
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