Alkanes and Fuels
5. Crude Oil and Fractional Distillation
Learning outcomes
- I can describe crude oil as a mixture of hydrocarbons.
- I can explain how fractional distillation separates crude oil into useful fractions.
- I can identify the major fractions produced during distillation.
- I can relate hydrocarbon chain length to boiling point.
- I can explain the importance of petroleum products in modern society.
What Is Crude Oil?
Crude oil is a naturally occurring liquid mixture containing a very large number of different compounds.
Most of these compounds are hydrocarbons.
A hydrocarbon contains only:
- carbon
- hydrogen
Many of the hydrocarbons in crude oil are alkanes, although crude oil also contains other types of hydrocarbons and small amounts of compounds containing elements such as sulfur, nitrogen, and oxygen.
Crude oil is therefore a mixture, not a single pure substance.
A Mixture of Hydrocarbons
The hydrocarbons in crude oil have different:
- molecular sizes
- carbon-chain lengths
- structures
- boiling points
- viscosities
- volatilities
Some contain only a few carbon atoms.
Others contain dozens of carbon atoms or more.
These differences in physical properties allow crude oil to be separated into useful groups.
How Crude Oil Forms
Crude oil formed from the remains of ancient organisms that were buried beneath sediments.
Over millions of years, conditions involving:
- burial
- pressure
- heat
- geological processes
helped transform organic material into petroleum and natural gas.
Because crude oil takes geological timescales to form, it is considered a non-renewable resource on human timescales.
Why Crude Oil Must Be Separated
Crude oil itself is not usually the most useful form of petroleum.
Instead, industry separates it into groups of hydrocarbons with similar properties.
These groups are called:
fractions
Different fractions are useful for different purposes.
Examples include:
- fuel gases
- gasoline
- petrochemical feedstocks
- kerosene
- diesel
- fuel oil
- lubricants
- waxes
- bitumen
What Is a Fraction?
A fraction is a mixture containing hydrocarbons with a similar range of:
- boiling points
- molecular sizes
- carbon-chain lengths
A fraction is therefore not normally one pure compound.
For example, a gasoline fraction contains several different hydrocarbons with boiling points within a useful range.
Fractional Distillation
Crude oil is separated using:
fractional distillation
Fractional distillation separates substances because they have different boiling points.
The process takes place in a large:
fractionating column
The column is:
hot at the bottom
and:
cooler toward the top
This temperature gradient is essential to the separation.
Step 1: Heating the Crude Oil
Crude oil is first heated strongly.
Much of the crude oil:
vaporizes
The hot mixture of gases and vapours enters the fractionating column near the bottom.
Very large hydrocarbons with extremely high boiling points may remain as liquids or residues.
Step 2: Vapours Enter the Column
Inside the fractionating column, the temperature decreases with height.
At the bottom:
temperature is high
Near the top:
temperature is much lower
Hydrocarbon vapours rise through this temperature gradient.
Step 3: Vapours Cool
As hydrocarbon molecules move upward, the surrounding temperature decreases.
Eventually, each hydrocarbon reaches a region where the temperature is low enough for it to:
condense
Condensation is the physical change:
gas → liquid
Different hydrocarbons condense at different heights because they have different boiling points.
Step 4: Fractions Are Collected
Hydrocarbons with similar boiling points condense in similar regions of the column.
They are collected together as:
fractions
Therefore:
high-boiling hydrocarbons condense lower in the column
while:
low-boiling hydrocarbons condense higher in the column
Very small hydrocarbons may remain gases even near the top and leave the column as refinery gases.
Why Fractional Distillation Works
Fractional distillation works because the hydrocarbons in crude oil have different:
boiling points
The boiling point depends partly on molecular size and intermolecular forces.
As hydrocarbon molecules become larger:
- they contain more electrons
- their electron clouds become more polarizable
- London dispersion forces become stronger
- more energy is required to separate the molecules
Therefore:
longer hydrocarbon chain → generally higher boiling point
Chain Length and Boiling Point
Consider these alkanes:
Methane:
CH₄
Butane:
C₄H₁₀
Octane:
C₈H₁₈
Hexadecane:
C₁₆H₃₄
As carbon-chain length increases, boiling point generally increases.
This relationship is one of the key ideas behind fractional distillation.
Intermolecular Forces
Hydrocarbon molecules are generally nonpolar.
The main attractions between them are:
London dispersion forces
Small hydrocarbons have relatively weak dispersion forces.
Large hydrocarbons have stronger dispersion forces.
Therefore:
small molecules → lower boiling points
large molecules → higher boiling points
What Happens Inside the Column?
Imagine three hydrocarbon molecules.
Hydrocarbon A
Short chain
Low boiling point
Travels high in the column before condensing.
Hydrocarbon B
Medium chain
Intermediate boiling point
Condenses around the middle.
Hydrocarbon C
Long chain
High boiling point
Condenses low in the column.
This allows thousands of compounds to be separated into manageable groups.
Major Fractions of Crude Oil
The exact names and boiling ranges used for fractions can vary between refineries and textbooks, but a simplified school-level sequence from the top to the bottom of a fractionating column is:
Refinery gases
↓
Gasoline / petrol
↓
Naphtha
↓
Kerosene
↓
Diesel / gas oil
↓
Fuel oil
↓
Lubricating oils and waxes
↓
Bitumen / residue
As we move downward:
carbon-chain length increases
boiling point increases
viscosity increases
volatility decreases
Refinery Gases
Near the top of the column are the smallest hydrocarbons.
These are often called:
refinery gases
They contain very short hydrocarbon molecules.
Properties include:
- very low boiling points
- high volatility
- low viscosity
- gases at ordinary conditions
Uses include:
- heating
- cooking
- LPG
Propane and butane are important examples.
Gasoline or Petrol Fraction
The gasoline fraction contains relatively short-chain hydrocarbons.
Properties include:
- low boiling points compared with heavier fractions
- relatively high volatility
- low viscosity
- easy ignition
Major use:
fuel for spark-ignition vehicle engines
Its ability to vaporize relatively easily is important for its use as a fuel.
Naphtha
Naphtha is an important petroleum fraction used largely as a chemical feedstock.
It can be used to produce chemicals that eventually become:
- plastics
- solvents
- synthetic materials
- other useful organic compounds
This illustrates an important point:
Crude oil is not used only to produce fuels.
It is also an important source of raw materials for the chemical industry.
Kerosene
The kerosene fraction contains larger hydrocarbons than gasoline.
It has:
- higher boiling points
- lower volatility
- greater viscosity
One major use is:
aviation turbine fuel
Kerosene-type fuels are also used in some heating and lighting applications.
Diesel or Gas Oil
The diesel fraction contains still larger hydrocarbon molecules.
Compared with gasoline, diesel generally has:
- higher boiling points
- lower volatility
- greater viscosity
Uses include:
- diesel engines
- heavy vehicles
- machinery
- some heating systems
Fuel Oil
Fuel oil contains relatively large hydrocarbon molecules.
It has:
- high boiling points
- relatively high viscosity
- low volatility
It has historically been used in applications such as:
- large ships
- industrial heating
- some power-generation systems
Lubricating Oils
Even larger hydrocarbons can be found in lubricating fractions.
They tend to have:
- high boiling points
- high viscosity
- low volatility
These properties make them useful for reducing friction between moving surfaces.
A lubricant needs to remain between surfaces rather than evaporating quickly.
Waxes
Some long-chain hydrocarbons are solid or waxy at room temperature.
Paraffin wax is a familiar petroleum-derived material.
Uses can include:
- candles
- coatings
- polishes
- waterproofing
- packaging applications
Longer chains have stronger intermolecular attractions, contributing to their higher melting and boiling temperatures.
Bitumen
At the bottom of the fractionating system is a very heavy residue containing very large hydrocarbon molecules and other heavy components.
One important product is:
bitumen
Bitumen is:
- very viscous
- very low in volatility
- dark
- thick
- useful for road surfacing and roofing materials
Comparing the Top and Bottom of the Column
Near the top:
- shorter hydrocarbon chains
- smaller molecules
- weaker intermolecular forces
- lower boiling points
- greater volatility
- lower viscosity
- easier vaporization
Near the bottom:
- longer hydrocarbon chains
- larger molecules
- stronger intermolecular forces
- higher boiling points
- lower volatility
- greater viscosity
This pattern is central to understanding petroleum fractions.
A Useful Trend
Moving from the top toward the bottom of a fractionating column:
Chain length ↑
Molecular size ↑
Boiling point ↑
Viscosity ↑
Volatility ↓
Flammability and ease of ignition also generally decrease as the fractions become heavier, although actual combustion behavior depends on the specific fuel and conditions.
Why Long-Chain Hydrocarbons Have Higher Boiling Points
Long-chain hydrocarbons contain more electrons.
Their larger electron clouds are more polarizable.
Therefore, they experience stronger:
London dispersion forces
More energy is needed to overcome these intermolecular attractions.
Therefore:
boiling point increases
This gives the full explanation:
Longer chain
↓
Larger molecule
↓
More electrons and greater polarizability
↓
Stronger London dispersion forces
↓
More energy required to separate molecules
↓
Higher boiling point
Fractional Distillation Is a Physical Process
Fractional distillation does not normally convert one hydrocarbon into another.
It separates hydrocarbons according to differences in physical properties.
No new substance needs to be produced during the separation itself.
Therefore:
fractional distillation is a physical separation process
rather than a chemical reaction.
The hydrocarbons repeatedly vaporize and condense as they move through different temperature regions of the column.
Distillation vs Fractional Distillation
Simple distillation can be useful for separating a liquid from dissolved substances or liquids whose boiling points are widely separated.
Fractional distillation is particularly useful when separating mixtures containing many liquids with different but sometimes relatively close boiling ranges.
Crude oil contains many hydrocarbons.
Therefore, a fractionating column is used to achieve much more effective separation.
Fractions Are Boiling Ranges
A petroleum fraction does not normally have one exact boiling point.
Instead, it has a:
boiling range
This is because a fraction contains several different hydrocarbon molecules.
For example, a fraction might contain hydrocarbons that boil across a particular range of temperatures.
Therefore:
fraction ≠ single pure hydrocarbon
Why Some Fractions Are More Useful Than Others
Demand for petroleum products is not equal.
Modern societies often require large quantities of:
- transportation fuels
- petrochemical feedstocks
- aviation fuels
- diesel
However, crude oil does not necessarily contain these fractions in exactly the proportions society wants.
This creates an important industrial problem.
Cracking
Large hydrocarbon molecules can be converted into smaller, more useful molecules through a process called:
cracking
Cracking can produce:
- shorter alkanes
- alkenes
Shorter hydrocarbons can be useful as fuels.
Alkenes are especially valuable as raw materials for making polymers and other chemicals.
Cracking is a chemical process, unlike fractional distillation.
Fractional Distillation vs Cracking
Fractional distillation:
separates existing molecules
Cracking:
changes molecules into different molecules
Fractional distillation is primarily:
physical
Cracking is:
chemical
This distinction is important.
Petroleum as a Fuel Source
A major use of petroleum is the production of fuels.
Petroleum-derived fuels are used in:
- cars
- trucks
- aircraft
- ships
- machinery
- heating
- some electricity generation
These fuels have high energy densities and have historically been convenient to store and transport.
Petroleum as a Chemical Feedstock
Petroleum is also extremely important as a raw material for the chemical industry.
Petrochemical feedstocks can be used to manufacture:
- plastics
- synthetic fibres
- synthetic rubber
- paints
- detergents
- solvents
- adhesives
- coatings
- packaging
- many industrial chemicals
This means that petroleum affects everyday life far beyond transportation fuels.
Petroleum and Plastics
Many plastics begin with small organic molecules produced by processing petroleum fractions.
For example, cracking can produce alkenes such as ethene.
Ethene can then be used to make:
poly(ethene)
This creates a connection between:
crude oil → fractions → cracking → alkenes → polymers
Petroleum chemistry therefore provides important raw materials for the plastics industry.
Petroleum in Transportation
Different transportation systems require fuels with different physical properties.
Gasoline is suitable for many spark-ignition engines.
Diesel is suitable for compression-ignition engines.
Kerosene-type fuels are used in turbine-powered aircraft.
These fuels differ in properties such as:
- volatility
- boiling range
- viscosity
- ignition behaviour
Their molecular composition therefore affects their practical use.
Petroleum and Roads
Not every useful petroleum product is burned.
Bitumen is a major example.
Its high viscosity and waterproof properties make it useful in:
- roads
- roofing
- waterproof surfaces
This shows how physical properties determine applications.
Environmental Considerations
Petroleum products provide important materials and energy, but their production and use also have environmental consequences.
These include:
- carbon dioxide emissions from combustion
- air pollution from combustion
- accidental oil spills
- habitat disruption associated with extraction and infrastructure
- plastic waste
- greenhouse-gas emissions across production and use
- dependence on a finite resource
Understanding petroleum therefore involves considering both its usefulness and its environmental impacts.
Petroleum Is Non-Renewable
Crude oil takes millions of years to form naturally.
Humans consume petroleum far faster than geological processes replace it.
Therefore, crude oil is classified as:
non-renewable
This does not mean that petroleum will suddenly disappear at a particular moment.
It means that it is a finite geological resource that is not replenished on a human timescale.
Worked Example 1: Identifying a Mixture
A student says:
"Crude oil is one giant hydrocarbon molecule."
Is this correct?
No.
Crude oil is a:
mixture of many different hydrocarbons and other compounds.
Worked Example 2: Predicting Boiling Point
Which would generally have the higher boiling point?
C₅H₁₂
or:
C₁₂H₂₆
Answer:
C₁₂H₂₆
It is larger and contains more electrons.
Therefore, it experiences stronger London dispersion forces.
More energy is required to separate its molecules.
Worked Example 3: Position in the Column
Which would generally condense higher in a fractionating column?
A short-chain hydrocarbon or a long-chain hydrocarbon?
Short-chain hydrocarbon
It has a lower boiling point and must reach a cooler region before it condenses.
Worked Example 4: Heavy Fraction
A petroleum fraction is:
- very viscous
- not very volatile
- composed mainly of large hydrocarbon molecules
Where would you expect it to be collected?
Near the bottom of the fractionating system.
Its large molecules have high boiling points.
Worked Example 5: Light Fraction
A fraction contains small hydrocarbons with low boiling points.
Predict two properties.
It will generally have:
high volatility
and:
low viscosity
It would be collected relatively high in the column.
Worked Example 6: Gasoline vs Fuel Oil
Which fraction generally contains shorter hydrocarbons?
Gasoline
Which generally has the higher boiling range?
Fuel oil
Which is generally more viscous?
Fuel oil
Which is generally more volatile?
Gasoline
Worked Example 7: Fractional Distillation
Why do different hydrocarbons condense at different heights?
Because they have:
different boiling points
The column has a:
temperature gradient
so different hydrocarbons condense where the temperature falls below the range at which they remain vaporized.
Worked Example 8: Physical or Chemical?
Is fractional distillation a chemical reaction?
No.
It separates substances according to physical properties.
The molecules are not converted into different substances simply by the distillation process.
Worked Example 9: Petroleum Product
Which fraction would be most closely associated with road surfacing?
Bitumen
Bitumen consists of very heavy material remaining near the bottom of petroleum processing.
Worked Example 10: Explaining a Trend
A student writes:
"Long hydrocarbons boil at higher temperatures because they have stronger covalent bonds."
This is not the correct explanation.
The important difference during boiling is:
intermolecular forces
Longer hydrocarbons have larger, more polarizable electron clouds.
Therefore, they experience stronger London dispersion forces.
More energy is required to separate the molecules.
Therefore:
their boiling points are generally higher.
Reading a Fractionating Column Diagram
When interpreting a fractional distillation diagram, remember:
Top of column
Cooler
Shorter chains
Lower boiling points
More volatile
Less viscous
↓
Middle
Intermediate properties
↓
Bottom of column
Hotter
Longer chains
Higher boiling points
Less volatile
More viscous
A Reliable Fractional Distillation Explanation
If asked to explain fractional distillation, a strong answer could follow this sequence:
1. Crude oil is heated.
2. Most hydrocarbons vaporize.
3. The vapours enter a fractionating column.
4. The column is hot at the bottom and cooler toward the top.
5. Hydrocarbon vapours rise and cool.
6. Different hydrocarbons have different boiling points.
7. They condense at different heights.
8. Hydrocarbons with similar boiling ranges are collected together as fractions.
This explains both the process and the scientific reason it works.
Common Mistakes
Mistake 1: Saying crude oil is a pure substance
Crude oil is a:
mixture
Mistake 2: Saying crude oil contains only alkanes
Crude oil contains many hydrocarbons, including but not limited to alkanes, as well as smaller amounts of other compounds.
Mistake 3: Saying fractional distillation creates hydrocarbons
Fractional distillation:
separates hydrocarbons already present in the mixture.
Mistake 4: Saying the column is hottest at the top
The fractionating column is:
hotter at the bottom
and:
cooler toward the top
Mistake 5: Saying long-chain hydrocarbons have lower boiling points
The general trend is:
longer chain → higher boiling point
Mistake 6: Saying fractions are pure substances
Each fraction normally contains:
a mixture of hydrocarbons with similar boiling ranges.
Mistake 7: Confusing fractional distillation and cracking
Fractional distillation:
separates molecules
Cracking:
breaks large molecules into smaller molecules
Mistake 8: Saying boiling breaks C–C bonds
Boiling primarily overcomes:
intermolecular forces
It does not normally break the covalent bonds within hydrocarbon molecules.
Did You Know?
A modern oil refinery does much more than simply separate crude oil.
After fractional distillation, fractions may undergo additional processes to change their composition or improve their usefulness.
These processes can include:
- cracking
- reforming
- removal of sulfur compounds
- blending
- purification
The refinery therefore combines physical separation with chemical processing to produce materials with particular properties.
Key Terms
- Crude oil: Naturally occurring mixture containing many hydrocarbons and smaller amounts of other compounds.
- Hydrocarbon: Compound containing only carbon and hydrogen.
- Petroleum: Naturally occurring hydrocarbon mixture and the products derived from it.
- Fraction: Mixture of hydrocarbons with a similar range of boiling points.
- Fractional distillation: Separation of a liquid mixture using differences in boiling points.
- Fractionating column: Industrial column in which hydrocarbon vapours separate according to boiling range.
- Boiling point: Temperature at which a liquid boils at a specified pressure.
- Vaporization: Physical change from liquid to gas.
- Condensation: Physical change from gas to liquid.
- Volatility: Tendency of a substance to vaporize.
- Viscosity: Resistance of a fluid to flowing.
- London dispersion force: Intermolecular attraction important between hydrocarbon molecules.
- Refinery gas: Light hydrocarbon fraction containing very small molecules.
- Naphtha: Petroleum fraction commonly used as a petrochemical feedstock.
- Kerosene: Petroleum fraction used particularly in aviation turbine fuels.
- Diesel: Petroleum-derived fuel used in compression-ignition engines.
- Bitumen: Heavy petroleum material used in applications such as road surfacing and roofing.
- Feedstock: Raw material used to manufacture other chemicals.
- Cracking: Chemical process that converts larger hydrocarbons into smaller molecules.
- Non-renewable resource: Resource that is not replenished on a human timescale.
Key Relationships
Moving up the fractionating column:
Temperature ↓
Average chain length ↓
Boiling point ↓
Viscosity ↓
Volatility ↑
Moving down the column:
Temperature ↑
Average chain length ↑
Boiling point ↑
Viscosity ↑
Volatility ↓
Key Takeaways
- Crude oil is a mixture, not a pure substance.
- Most compounds in crude oil are hydrocarbons.
- Hydrocarbons contain only carbon and hydrogen.
- Crude oil contains molecules with many different carbon-chain lengths and boiling points.
- Crude oil is separated into useful fractions using fractional distillation.
- The fractionating column is hot at the bottom and cooler toward the top.
- Crude oil is heated so that much of it vaporizes before entering the column.
- Hydrocarbon vapours rise through the column and cool.
- Different hydrocarbons condense at different heights because they have different boiling points.
- Fractions contain mixtures of hydrocarbons with similar boiling ranges.
- Short-chain hydrocarbons generally have lower boiling points, lower viscosity, and greater volatility.
- Long-chain hydrocarbons generally have higher boiling points, greater viscosity, and lower volatility.
- The increase in boiling point with chain length is explained by stronger London dispersion forces between larger molecules.
- Important petroleum fractions include refinery gases, gasoline, naphtha, kerosene, diesel, fuel oil, lubricating materials, waxes, and heavy residues such as bitumen.
- Fractional distillation is a physical separation process.
- Cracking is different because it is a chemical process that converts large hydrocarbons into smaller molecules.
- Petroleum is important not only as a source of fuels but also as a source of chemical feedstocks used to manufacture plastics and many other materials.
- Petroleum products have played a major role in transportation, industry, construction, manufacturing, and modern chemical production.
- Crude oil is a non-renewable resource, and its extraction, processing, use, and disposal can have significant environmental impacts.