Absorb Chemistry for GCSE by Lawrie Ryan
     
Fractional Distillation of Crude Oil
Introduction

In the unit Introduction to Crude Oil, we saw how we can find sources of crude oil. Once the crude oil has been extracted from the ground, it is transported to an oil refinery by pipeline or by giant oil tankers. In this unit we will see what happens to the oil in the first process at the refinery.
Distilling crude oil in the lab
As you probably know,
crude oil
Crude oil is a liquid formed from tiny sea creatures and plants that died millions of years ago. It contains a mixture of hydrocarbons. It is the raw material for many fuels and plastics.
crude oil
is a mixture of a large number of compounds (mainly hydrocarbons). The thick, gooey liquid is of little use to anyone in this form.

Figure 1.  Crude oil.


At an oil refinery, the various hydrocarbons are separated into mixtures of compounds with similar boiling points. This is done by a process called fractional distillation. We say that the crude oil is separated into fractions. In order to understand this process, we can look at what happens when we distil crude oil in the lab, as shown in Fig.2 below:

Figure 2.  Distilling crude oil in the lab.
What happens to the colour of the fractions as the boiling range of the
fraction
The fraction is the mixture of hydrocarbons collected between a certain range of temperatures in the fractional distillation of crude oil.
fraction
increases?
 
They remain the same colour.
They get lighter in colour.
They get darker in colour.
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As we heat the crude oil, the lighter hydrocarbons (those with the smaller molecules) are the first to boil off. They are collected as the first (light) fraction, which is a mixture of hydrocarbons with similarly low boiling points. Then larger molecules evaporate from the oil as heating continues. They make up the heavier fractions.

This happens because, in general, small molecules have weaker forces between their molecules than large molecules, so it takes more heat energy to make the molecules move apart sufficiently to evaporate and form a gas.

Figure 3.  Hydrocarbons with small molecules have lower boiling points than large hydrocarbons.


Some of the molecules in crude oil have branched chains of carbon atoms. Look at the three molecules below and their boiling points:

Figure 4.  Alkanes in crude oil.
Which of the molecules (A, B, or C) has the lowest boiling point?
 
A
B
C
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For molecules of a similar mass, a branched-chain
molecule
A molecule is a group of two or more atoms bonded together.
molecule
has a lower boiling point than a straight-chain molecule. With branches sticking out, the molecules cannot pack as closely together, so they are easier to separate from neighbouring molecules.

Notice that the molecules shown above all have the formula C5H12, but they each have a different structure.

Molecules with the same formula but different arrangements of atoms within their molecules are called isomers.

What is the straight-chain molecule of C5H12 called?
 
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Properties of the different fractions
You have already seen how the larger molecules that make up the higher boiling point fractions of crude oil have compounds that are darker in colour. Look at some other properties of the different fractions shown in Figs.5 and 6 below:

Figure 5.  Pouring fractions from crude oil.
Figure 6.  Burning fractions from crude oil.
Match the properties shown above to the type of fraction:
  Light in colour, runny, and easy to ignite
  Dark, viscous (thick liquid), hard to ignite, burns with a smoky flame
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Fractional distillation in an oil refinery
In an oil refinery, the oil is separated into its fractions in giant fractionating columns, as shown in Fig.7:

Figure 7.  A fractionating column in an oil refinery.


Look at the fractionating column below and the uses we make of the different fractions:

Figure 8.  Uses of crude oil.
Match the fraction to its use:
  Petroleum gases
  Gasoline
  Kerosene
  Diesel
  Lubricating oil
  Heavy fuel oil
  Bitumen
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In the fractionation process, the oil is heated and enters the column as a vapour. The column is hottest at the bottom so only the hydrocarbons with the highest boiling points condense back to liquid there. They are piped from the bottom of the column. The other hydrocarbons rise up the column.

As we go up the column, the temperature falls and successively lighter fractions are collected as they too condense. The lightest fraction (which is made up of gases) comes from the top of the column.

Which fraction is collected in the following locations:
  At the top of the fractionating column, where the temperature is lowest
  At the bottom of the fractionating column, where the temperature is highest
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Summary


Crude oil can be separated into its fractions (mixtures of hydrocarbons with similar boiling points) by fractional distillation. This is sometimes called fractionation and takes place in huge fractionating columns in oil refineries.

Here are some properties of the different fractions:


Fraction Size of molecules Colour Viscosity (thickness) Flammability
Light fraction, with low boiling point range small colourless thin, runny liquid ignites easily (burns with clean flame)
Medium fraction, with mid-range boiling point medium yellow thicker liquid more difficult to ignite (some smoke in flame)
Heavier fraction, with higher boilng points large dark orange viscous (thick) liquid difficult to ignite (burns with smoky flame)



So, as the fractions get heavier (and the chains in their molecules get longer), they get darker in colour, more viscous, and less flammable. They also burn with smokier flames.

Exercises
1. Which fraction from crude oil contains the largest
hydrocarbon
A hydrocarbon is a compound containing only hydrogen and carbon.
hydrocarbon
molecules?
 
Petroleum gases
Gasoline
Fuel oil
Bitumen
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2. In an experiment to distil crude oil, a teacher collected three fractions. Match the colour below to the type of fraction:
  Colourless liquid
  Dark orange liquid
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3. What happens to the boiling points of hydrocarbons as their molecules get larger?
 
The boiling points decrease.
The boiling points increase.
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4. The hydrocarbon with the formula C4 H10 has two
isomers
Isomers are olecules that have the same chemical formula but different arrangements of atoms. For example:
isomers
– a straight-chain molecule and a molecule with a branched chain. What is the name of the straight-chain isomer?
 
Propane
Butane
Pentane
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5. Which of the two isomers will have the higher boiling point?
 
Straight-chain isomer
Branched-chain isomer
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6. Match the fraction from crude oil to its properties:
  Lower boiling point range
  Higher boiling point
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7. Which fraction from crude oil is used to surface roads and make roofs waterproof?
 
Petroleum gases
Gasoline
Heavy fuel oil
Bitumen
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8. Which fraction from crude oil is used as a fuel for aeroplanes?
 
Petroleum gases
Gasoline
Kerosene
Heavy fuel oil
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