Petroleum: How It Forms, What It Contains and Its Impacts

If petroleum began as ancient life, why isn’t crude oil full of recognisable plant or animal material? And how can one dark liquid be turned into bottled gas, petrol, jet fuel, lubricants, and road bitumen?

The answer involves two very different stages of chemistry. Geological processes transform a small fraction of buried organic matter into a complex mixture of hydrocarbons. Refineries then separate that mixture and chemically change selected components to produce useful materials.

Understanding the difference between these stages is essential. Petroleum is not a pure substance, and refining does not involve just one process.

01The evidence for petroleum’s biological origin

Imagine finding a liquid underground containing thousands of different carbon compounds. Some resemble molecules associated with living organisms. Others have structures that have changed through millions of years of heating.

How could you establish where that liquid came from?

Before reading on, make a prediction. If petroleum formed from ancient biological material, what might survive after millions of years underground?

You probably wouldn’t expect intact leaves, bones, or cells. You might, however, expect recognisable molecular fragments left behind when biological molecules break down.

These fragments became important evidence in the investigation of petroleum’s origins.

From shale oil to laboratory experiments

During the nineteenth century, organic-rich shale was heated in special retorts to release oil. The oil could be collected and used for lighting.

This demonstrated something important: heating organic-rich sedimentary material could produce an oil-like liquid.

In 1912, the chemist Friedrich Bergius and other researchers were investigating related processes, while Friedrich Engler’s earlier work had demonstrated that heating animal-derived material could produce petroleum-like substances through thermal decomposition.

However, there was a problem with these early investigations.

Heating organic material in a laboratory did not necessarily reproduce the conditions experienced by sediments buried deep underground. Natural petroleum formation involves water, pressure, temperature changes, and extremely long periods of time.

Scientists needed experiments that better represented those conditions.

Worked into the laboratory: hydrous pyrolysis

Modern hydrous pyrolysis experiments investigate whether organic-rich sedimentary rocks can produce petroleum under controlled conditions.

The basic experimental arrangement is straightforward.

A sample of organic-rich shale and water is placed inside a sealed, pressure-rated reaction vessel. The vessel is heated at a controlled temperature, with oxygen excluded to prevent combustion.

After heating, the vessel is cooled. The resulting gases, liquid hydrocarbons, and solid residue can then be collected and chemically analysed.

What should the experiment produce if buried organic matter really can become petroleum?

It should produce a mixture containing hydrocarbons with physical and chemical characteristics similar to naturally occurring crude oil.

That is what hydrous pyrolysis experiments have demonstrated. Heating suitable organic-rich source rocks in the presence of water can generate products resembling natural petroleum.

The experiment supports the proposed geological mechanism, but it isn’t a perfect reproduction of nature. A laboratory cannot simply recreate millions of years of burial, changing temperature, and interactions between different rock layers.

The improved experimental design nevertheless addresses an important limitation of early shale-retorting experiments: it introduces water and allows temperature and pressure to be controlled more realistically.

The molecular clue: biomarkers

Another important development came in 1934, when Alfred Treibs identified chlorophyll-related compounds in oil shale.

Chlorophyll is a pigment used by photosynthetic organisms. Its molecular structure contains a complex nitrogen-containing ring system.

When biological molecules break down, some of their structural features can survive in altered forms. These surviving compounds are called biomarkers, or molecular fossils.

Petroleum contains several types of biomarkers associated with ancient organisms, including compounds related to chlorophyll and other biological molecules.

The important observation is not that petroleum contains intact biological molecules. It is that certain compounds retain structural features that connect them to molecules originally produced by living organisms.

Scientists can also examine how these molecules have changed during burial and heating. Their molecular arrangements provide information about the thermal maturity of the petroleum.

Together, laboratory experiments, biomarkers, and geological evidence strongly support the biological origin of most conventional commercial petroleum.

There is an important qualification. Some methane and other small hydrocarbons can form through non-biological reactions in particular geological environments. This is called an abiogenic origin. However, the existence of abiogenic hydrocarbons does not mean that most commercial petroleum deposits formed this way.

For a deeper scientific discussion, see Sephton and Hazen’s review of petroleum origins.

02How petroleum forms underground

Now that we have evidence for its origin, we can follow the process that produces an oil deposit.

1. Organic matter accumulates

Millions of years ago, aquatic environments contained enormous populations of microscopic organisms, including algae, plankton, and other microorganisms.

When these organisms died, their remains settled onto the bottom of oceans, lakes, and other sedimentary environments.

Some organic material also came from land plants.

Normally, dead organisms are decomposed by microorganisms. Much of their carbon returns to the environment.

However, when organic material becomes buried in oxygen-poor sediment, decomposition is incomplete. A small proportion of the original organic matter survives.

Fine mud and other sediments gradually cover this material.

2. Burial produces kerogen

As more sediment accumulates, the layers below become compacted.

Microorganisms and chemical reactions gradually transform the preserved organic matter. The result is a complex, solid organic material called kerogen.

Kerogen is not a single compound. It is a mixture of large, complex organic structures trapped within sedimentary rock.

The rock containing sufficient organic material to generate petroleum is called a source rock.

3. Heat transforms kerogen into petroleum

As burial continues, the temperature of the sedimentary rock increases.

Over geological time, heat causes chemical changes in kerogen. Its large organic structures gradually break down, producing smaller hydrocarbon molecules.

This process is part of thermal maturation.

A useful approximate temperature range for oil generation is around 60-120°C, although the actual range depends on the type of kerogen and how long it experiences elevated temperatures.

This is sometimes called the oil window.

There is an important detail here: more heat does not always mean more liquid oil.

At sufficiently high temperatures, existing oil molecules can undergo further thermal decomposition, producing smaller hydrocarbons, particularly methane.

A source rock can therefore become overmature and produce predominantly gas rather than recoverable liquid oil.

Pressure also increases during burial, but temperature and time are particularly important in driving the chemical transformation of kerogen.

4. Petroleum migrates through rock

Once generated, petroleum can move out of the source rock and into surrounding rocks.

Why does it move?

Oil is generally less dense than the water already occupying the pore spaces in sedimentary rocks. Buoyancy therefore encourages it to migrate upwards.

A rock’s porosity describes how much empty space it contains. Its permeability describes how easily fluids can move through those connected spaces.

Sandstone and some limestone formations can make good reservoir rocks because they contain interconnected pores.

Think of a reservoir rock as a sponge containing water. Petroleum can gradually move into its connected spaces, displacing some of the water.

The analogy has a limit: an oil reservoir is not an enormous underground cavern. Petroleum generally occupies microscopic spaces between rock grains.

5. A geological trap allows petroleum to accumulate

If petroleum continues moving upwards without obstruction, it may eventually reach the surface.

However, it can become trapped beneath an impermeable layer of rock called a cap rock or seal.

The porous rock containing the accumulated petroleum is the reservoir rock.

Together, the source rock, migration pathway, reservoir rock, and cap rock form the essential parts of a petroleum accumulation.

The different fluids separate according to density:

• Natural gas accumulates above the oil.
• Crude oil occupies the region below the gas.
• Water generally occupies the lowest part of the reservoir.

This arrangement explains why drilling into a petroleum reservoir can encounter different fluids at different depths.

It also explains why finding petroleum in a source rock does not guarantee a commercially useful oil deposit. The petroleum must migrate, accumulate, and remain trapped.

03What is crude oil actually made of?

Crude oil is a complex mixture containing hundreds or thousands of different chemical compounds.

Most are hydrocarbons, which are compounds containing only carbon and hydrogen.

However, crude oil also contains compounds containing sulfur, nitrogen, and oxygen, as well as small amounts of metals and other impurities.

The exact composition varies between oil deposits. Two crude oil samples can have noticeably different colours, viscosities, densities, sulfur contents, and boiling-point distributions.

The main hydrocarbon families

Three important hydrocarbon groups are found in petroleum.

Let’s examine why these structures matter.

Alkanes

Alkanes contain only single covalent bonds between carbon atoms.

Their general molecular formula is:

\[
\mathrm{C}_{n}\mathrm{H}_{2n+2}
\]

For example, octane has the molecular formula \(\mathrm{C_8H_{18}}\).

Alkanes can form straight chains or branched structures. Both arrangements occur in petroleum.

Cycloalkanes

Cycloalkanes contain carbon atoms arranged in rings, with single bonds between the carbon atoms.

For a molecule containing one ring, their general formula is:

\[
\mathrm{C}_{n}\mathrm{H}_{2n}
\]

Cyclohexane, \(\mathrm{C_6H_{12}}\), is one example.

Aromatic hydrocarbons

Aromatic hydrocarbons contain one or more aromatic rings, such as the six-carbon ring found in benzene.

Some aromatic compounds are important industrial feedstocks. Others present significant health and environmental concerns.

For example, benzene is a known human carcinogen. Some larger aromatic compounds, including certain polycyclic aromatic hydrocarbons (PAHs), are also associated with serious health risks.

What about sulfur and other elements?

Not every compound in crude oil is a hydrocarbon.

Some molecules contain sulfur, nitrogen, or oxygen atoms within their structures.

Crude oil may also contain trace quantities of metals such as nickel and vanadium, alongside water, dissolved salts, and sediment.

Sulfur content is particularly important when evaluating crude oil quality.

Crude oils with relatively low sulfur content are commonly called sweet crude oils. Those with higher sulfur content are called sour crude oils.

Sulfur-containing compounds can create processing difficulties and contribute to sulfur dioxide emissions when petroleum products are burned.

Why does molecular structure matter?

Different hydrocarbon molecules have different physical properties.

Consider two straight-chain alkanes. One has six carbon atoms, while the other has sixteen.

The longer molecule generally experiences stronger London dispersion forces because it has more electrons and a larger electron cloud.

More energy is therefore needed to separate its molecules during boiling.

As a general trend:

• Longer hydrocarbon chains have higher boiling points.
• Longer chains are generally less volatile.
• Longer chains tend to have greater viscosity.
• Branching generally lowers the boiling point compared with a straight-chain isomer of the same molecular formula.

These differences explain how crude oil can be separated into useful groups.

They also help explain why lighter petroleum components evaporate more readily during an oil spill.

04From crude oil to useful products

Petroleum is valuable because its different components can be separated, converted, and processed into products with specific properties.

It is important to distinguish three different operations.

1. Physical separation: fractional distillation

Fractional distillation separates components according to their boiling-point ranges.

The molecules are not chemically changed during this process.

Instead, different groups of compounds are collected at different stages of the separation process.

A fraction is not necessarily a pure substance. It usually contains many compounds with similar boiling-point ranges.

The detailed column apparatus and the distinction between the separation process and chemical conversion are covered in the separate tutorgum resource Fractional Distillation vs Cracking.

2. Chemical conversion: cracking

Sometimes a crude oil fraction contains large hydrocarbon molecules that are less useful for producing the products consumers need.

Cracking breaks larger hydrocarbon molecules into smaller ones.

It involves breaking and rearranging covalent bonds, so it is a chemical reaction rather than physical separation.

For example, an illustrative cracking reaction is:

\[
\ce{C10H22 -> C8H18 + C2H4}
\]

A decane molecule produces an octane molecule and an ethene molecule.

Notice that ethene is an alkene. Cracking can therefore produce both smaller alkanes and alkenes.

These smaller molecules are useful in fuels and the petrochemical industry.

3. Chemical treatment and blending

Some petroleum fractions require further treatment before they can be used.

For example, hydrodesulfurisation uses hydrogen to convert sulfur-containing compounds into hydrogen sulfide, \(\ce{H2S}\).

The hydrogen sulfide can then be processed to recover sulfur or undergo further treatment.

This reduces the sulfur content of the finished fuel.

Refineries also blend different components and add suitable additives to meet product specifications.

These three operations have different purposes:

Important petroleum fractions and their uses

Different fractions have different physical properties, making them suitable for different applications.

One particularly important use of petroleum is as a chemical feedstock.

Naphtha can be processed to produce small molecules such as ethene and propene. These molecules are monomers used to manufacture polymers.

For example, propene undergoes addition polymerisation to form polypropylene.

See Polypropylene: Addition Polymerisation, Structure and Properties for a closer look at this process.

Petroleum therefore supplies more than fuels. It also provides starting materials for plastics, synthetic materials, and many industrial chemicals.

05Environmental impacts throughout the petroleum life cycle

It is tempting to evaluate petroleum only by looking at the exhaust from a car.

However, petroleum has environmental impacts before it reaches a vehicle and after its products have been used.

A proper evaluation considers extraction, transport, refining, combustion, and disposal.

1. Extraction and transportation

Petroleum extraction requires drilling and infrastructure.

These activities can disturb natural habitats, affect marine and terrestrial ecosystems, and require substantial energy and water.

There are also risks associated with transporting crude oil through pipelines and tankers.

Accidental releases can contaminate soil, groundwater, rivers, and oceans.

Oil spills are particularly damaging because petroleum is a mixture of compounds with different environmental behaviours.

Some lighter hydrocarbons evaporate relatively quickly. Some compounds dissolve into water, while heavier components can remain on shorelines and sediments.

Extraction can also release methane through leaks, venting, and flaring.

Methane is a powerful greenhouse gas. Capturing associated gas, repairing leaks, and reducing unnecessary flaring can reduce these emissions.

The International Energy Agency’s Global Methane Tracker 2026 discusses the scale of methane emissions and available methods for reducing them.

2. Refining

Refineries require substantial energy to heat crude oil and operate their chemical processing equipment.

Burning fuels to provide this energy releases carbon dioxide.

Refining can also produce air pollutants, including:

• Sulfur dioxide (\(\ce{SO2}\)).
• Nitrogen oxides (\(\ce{NO_x}\)).
• Carbon monoxide (\(\ce{CO}\)).
• Particulate matter.
• Volatile organic compounds (VOCs).

These pollutants can contribute to respiratory health problems, poor air quality, and environmental damage.

In Australia, petroleum refining is recognised as an industrial source of several important air pollutants. The State of the Environment 2021 report discusses these industrial emissions.

However, refining can also reduce environmental impacts associated with the final product.

For example, removing sulfur from fuels reduces the amount of sulfur dioxide produced during combustion.

This creates a trade-off: sulfur removal improves the fuel, but the treatment process requires resources and produces sulfur-containing waste streams that must be managed.

3. Combustion: what happens when petroleum fuels burn?

Most petroleum fuels are hydrocarbons.

When a hydrocarbon undergoes complete combustion in sufficient oxygen, its carbon atoms form carbon dioxide, while its hydrogen atoms form water.

For example, the complete combustion of octane can be represented by:

\[
\ce{2C8H18 + 25O2 -> 16CO2 + 18H2O}
\]

The production of carbon dioxide is unavoidable when fossil carbon undergoes complete combustion.

Carbon dioxide is a greenhouse gas that contributes to global warming.

However, complete combustion is not always achieved.

When oxygen is insufficient or combustion conditions are poor, incomplete combustion can produce carbon monoxide and solid carbon particles (soot).

High-temperature combustion can also produce nitrogen oxides, while sulfur compounds in fuel can produce sulfur dioxide.

These pollutants have different environmental effects:

4. Oil spills and environmental weathering

An oil spill does not remain chemically identical to the original crude oil.

After petroleum enters the environment, its composition changes through processes collectively known as weathering.

Several processes can occur simultaneously.

Evaporation

Smaller, more volatile hydrocarbons evaporate into the atmosphere more readily.

This is why lighter crude oils can lose substantial amounts of their original mass following a spill.

Dissolution

Some petroleum compounds dissolve into water.

Aromatic hydrocarbons are generally more water-soluble than comparable aliphatic hydrocarbons. Some dissolved compounds can therefore affect aquatic organisms even when they are no longer visible as a surface slick.

Photooxidation

Sunlight can promote chemical reactions that oxidise certain petroleum compounds, changing their molecular structures.

Biodegradation

Microorganisms can break down some hydrocarbons through biological oxidation.

However, the process is not instantaneous. Its rate depends on factors such as temperature, oxygen availability, nutrients, and the molecular structure of the hydrocarbons.

The important distinction is that a reduction in visible surface oil does not necessarily mean the pollution has disappeared.

Some components have moved into the atmosphere or water, while others remain in the environment.

The ITRC technical guidance on petroleum hydrocarbons explains how the composition of petroleum mixtures changes during environmental weathering.

5. Disposal of petroleum-based products

Petroleum is also used to manufacture plastics, lubricants, coatings, and many other materials.

These products have environmental impacts that depend on their chemical structure and how they are disposed of.

For example, plastic products can remain in the environment for long periods. Physical breakdown can produce smaller plastic fragments, including microplastics.

Burning petroleum-based plastics releases carbon dioxide and may produce other pollutants depending on the material and combustion conditions.

Recycling and reusing suitable materials can reduce demand for virgin petroleum feedstocks. However, collection, sorting, contamination, and the different properties of polymers can limit recycling.

It is also important to distinguish petroleum from the polymers manufactured using petroleum feedstocks.

A plastic polymer is not simply crude oil in solid form. Its molecules have been chemically transformed into long chains with different properties.

For a closer explanation, see Monomers vs Polymers: Health and Environmental Impacts.

Evaluating the overall trade-off

Petroleum has several practical advantages.

It has high energy density, is relatively easy to transport and store, and supplies important feedstocks for manufacturing.

Existing transport, refining, and distribution infrastructure also makes petroleum products widely available.

However, these advantages need to be considered alongside the environmental costs of extraction, processing, combustion, and disposal.

Reducing sulfur content can improve air quality, but it does not eliminate carbon dioxide emissions from burning the fuel.

Capturing methane reduces upstream greenhouse gas emissions, but it does not prevent the carbon in petrol or diesel from becoming carbon dioxide when burned.

Using petroleum as a chemical feedstock rather than immediately burning it can delay the release of its carbon. However, the eventual disposal of products, particularly through incineration, can still return that carbon to the atmosphere.

There is no single environmental impact that describes every petroleum product. A sensible evaluation considers the particular crude oil, the refining processes, the final application, and the product’s entire life cycle.

06Questions and solutions

Question 1

A refinery receives crude oil and separates it into a petrol fraction and a heavy residue.

A student claims that fractional distillation chemically converts crude oil into two new substances.

Explain why this claim is incorrect. Identify one physical property that helps explain why the two fractions have different uses.

Solution 1

Fractional distillation physically separates existing compounds rather than chemically creating new substances.

Crude oil contains many hydrocarbons with different boiling points. During distillation, these compounds are separated into fractions according to their boiling-point ranges.

Their molecular structures remain unchanged during the separation.

The petrol fraction contains relatively volatile hydrocarbons, making it suitable for use as a vehicle fuel. The heavy residue contains larger, less volatile compounds and can be used to produce materials such as bitumen.

Both fractions remain mixtures rather than pure substances.

The key distinction is that physical separation changes the composition of a sample, while chemical conversion changes the molecular structures of its components.

Question 2

Diesel fuel can be approximated as hexadecane, \(\ce{C16H34}\), for a simplified combustion calculation.

A truck completely burns 2.00 kg of hexadecane.

Calculate the mass of carbon dioxide produced.

Explain why the mass of carbon dioxide is greater than the mass of the original fuel.

Solution 2

The combustion produces approximately 6.22 kg of carbon dioxide. Its mass exceeds the original fuel mass because oxygen from the atmosphere is incorporated into the combustion products.

Step 1: Write the balanced chemical equation.

\[
\ce{2C16H34 + 49O2 -> 32CO2 + 34H2O}
\]

One mole of hexadecane produces 16 moles of carbon dioxide.

Step 2: Calculate the molar mass of hexadecane.

\[
M(\ce{C16H34})=16(12.01)+34(1.008)
\]

\[
=226.4\text{ g mol}^{-1}
\]

Step 3: Calculate the moles of hexadecane.

Convert the fuel mass into grams:

\[
2.00\text{ kg}=2000\text{ g}
\]

\[
n=\frac{m}{M}
\]

\[
n(\ce{C16H34})=\frac{2000}{226.4}
\]

\[
=8.83\text{ mol}
\]

Step 4: Calculate the moles of carbon dioxide.

\[
n(\ce{CO2})=8.83\times16
\]

\[
=141.3\text{ mol}
\]

Step 5: Calculate the mass of carbon dioxide.

\[
m=nM
\]

\[
m(\ce{CO2})=141.3\times44.0
\]

\[
=6217\text{ g}
\]

Therefore:

\[
\boxed{m(\ce{CO2})=6.22\text{ kg}}
\]

The carbon atoms originally present in the fuel combine with oxygen from the atmosphere. The additional oxygen accounts for the greater mass of the carbon dioxide.

In reality, diesel is a mixture rather than pure hexadecane, and actual combustion may also produce carbon monoxide, particulates, and nitrogen oxides.

Question 3

Two equal masses of crude oil are accidentally released into separate sections of seawater.

One sample is a light crude oil, while the other is a heavy crude oil.

Both spills experience the same temperature, sunlight, and wind conditions.

After several hours, the light crude has lost considerably more mass, while the heavy crude leaves a thicker residue.

(a) Explain why the light crude loses more mass over this period.

(b) Predict how increasing the water temperature would affect the evaporation of the lighter hydrocarbons.

(c) Explain why the reduction in visible oil does not necessarily indicate that the environmental damage has ended.

Solution 3

The light crude loses more mass primarily because it contains a greater proportion of relatively volatile, low-boiling-point hydrocarbons.

(a) Light crude generally contains a larger proportion of smaller hydrocarbons that evaporate readily.

These molecules have higher volatility than the larger, heavier components found in greater proportions in many heavy crude oils.

As a result, more of the light crude can enter the atmosphere through evaporation.

Heavy crude contains a larger proportion of less volatile components, so more of it remains as a thick liquid residue.

(b) Increasing the water temperature would generally increase the evaporation rate of the lighter hydrocarbons.

At higher temperatures, molecules have greater average kinetic energy. A greater proportion can overcome the intermolecular attractions holding them in the liquid.

Consequently, more volatile components enter the atmosphere over a given period.

(c) A reduction in visible surface oil does not mean that all petroleum components have been removed from the environment.

Some compounds evaporate into the atmosphere, while others dissolve into seawater.

Heavier components can remain attached to sediments, shorelines, and marine organisms.

Some hydrocarbons undergo biodegradation, but this process can be slow and incomplete.

The tempting conclusion is that less visible oil means less pollution. The problem is that weathering redistributes and transforms the mixture rather than instantly eliminating every harmful component.

Question 4

Two organic-rich source rocks originated from the same type of sedimentary material.

Their burial histories were different.

• Source rock A reached a maximum temperature of approximately 90°C and remained there long enough to reach oil-generating maturity.
• Source rock B remained near 160°C for a sufficiently long period to become overmature.

Assume both rocks had comparable initial organic composition.

(a) Which source rock is more likely to retain a greater proportion of liquid petroleum?

(b) Explain why the hotter source rock does not necessarily produce more recoverable liquid oil.

(c) Identify another geological condition required for a substantial petroleum accumulation to develop.

Solution 4

Source rock A is more likely to retain a greater proportion of liquid petroleum because it has reached oil-generating maturity without progressing as far into thermal overmaturity.

(a) Source rock A is more likely to contain a greater proportion of liquid hydrocarbons.

Its temperature history has allowed kerogen to generate petroleum within the oil window.

Source rock B has experienced substantially greater heating and has become overmature.

(b) Higher temperatures initially accelerate the chemical reactions that transform kerogen into petroleum.

However, continued heating can also break down the larger hydrocarbon molecules that have already formed.

This secondary thermal cracking produces smaller hydrocarbons, particularly methane.

Therefore, although source rock B may have generated substantial quantities of hydrocarbons, it is more likely to contain less recoverable liquid oil than source rock A.

The tempting prediction is that more heat always produces more oil. This ignores the fact that petroleum itself can undergo further chemical reactions after it forms.

Thermal maturation changes the distribution of products, not simply the total amount of organic matter converted.

(c) A suitable geological trap is also required.

Petroleum must migrate from its source rock into a porous and permeable reservoir rock. An impermeable cap rock must then prevent further upward migration.

Without a suitable trap, petroleum may continue towards the surface rather than accumulating in a recoverable deposit.

This demonstrates that petroleum generation and petroleum accumulation are separate geological processes.

Question 5

A refinery processes 1.00 tonne of crude-derived feed containing 1.00% sulfur by mass.

Fractional distillation produces two streams:

• 600 kg of distillate containing 0.40% sulfur by mass.
• 400 kg of heavy residue.

Assume no sulfur is lost or chemically changed during fractional distillation.

The distillate then undergoes hydrodesulfurisation. Its sulfur concentration falls to 0.050% by mass.

Assume the mass of the treated distillate remains 600 kg and that sulfur removed during treatment enters a separate hydrogen sulfide-containing recovery stream.

Finally, all 600 kg of treated fuel is burned. Assume all sulfur remaining in the fuel is converted into sulfur dioxide.

(a) Calculate the initial mass of sulfur in the feed.

(b) Calculate the mass of sulfur in the distillate and the mass remaining in the heavy residue. Determine the sulfur percentage of the residue.

(c) Calculate the mass of sulfur removed during hydrodesulfurisation and the mass remaining in the final fuel.

(d) Calculate the mass of sulfur dioxide produced when the final fuel is completely burned.

(e) A refinery manager claims:

“Fractional distillation removed 60% of the sulfur from the original feed. The final fuel also has 95% less sulfur than the original feed, so the entire refining operation has reduced sulfur pollution by 95%.”

Evaluate this statement. Explain why tracking the mass of sulfur through every output stream gives a more accurate environmental assessment than comparing percentages alone.

Solution 5

The refinery does not destroy sulfur during fractional distillation. It redistributes sulfur-containing compounds between the distillate and residue. Chemical treatment subsequently transfers some sulfur into a separate recovery stream.

The final fuel produces approximately 0.60 kg of sulfur dioxide when burned.

(a) Calculate the initial sulfur mass.

The original feed has a mass of 1.00 tonne.

Convert this into kilograms:

\[
1.00\text{ tonne}=1000\text{ kg}
\]

The sulfur concentration is 1.00%, or 0.0100 as a decimal.

\[
m(\mathrm{S})=1000\times0.0100
\]

\[
\boxed{m(\mathrm{S})=10.0\text{ kg}}
\]

The original feed contains 10.0 kg of sulfur.

(b) Calculate the sulfur in each distillation stream.

The distillate has a mass of 600 kg and contains 0.40% sulfur.

\[
m(\mathrm{S})_{\text{distillate}}
=600\times0.0040
\]

\[
\boxed{m(\mathrm{S})_{\text{distillate}}=2.40\text{ kg}}
\]

Since no sulfur is lost during distillation, the remaining sulfur must be in the heavy residue.

\[
m(\mathrm{S})_{\text{residue}}=10.0-2.40
\]

\[
\boxed{m(\mathrm{S})_{\text{residue}}=7.60\text{ kg}}
\]

The residue has a total mass of 400 kg.

Its sulfur percentage is:

\[
\%\mathrm{S}=\frac{7.60}{400}\times100
\]

\[
\boxed{\%\mathrm{S}=1.90\%}
\]

The residue is therefore considerably more sulfur-rich than the original feed.

This makes sense because many heavier sulfur-containing compounds preferentially remain in the heavier fraction.

(c) Calculate the sulfur removed during hydrodesulfurisation.

The final fuel has a mass of 600 kg and a sulfur concentration of 0.050%.

\[
m(\mathrm{S})_{\text{final}}
=600\times0.00050
\]

\[
\boxed{m(\mathrm{S})_{\text{final}}=0.300\text{ kg}}
\]

The distillate initially contained 2.40 kg of sulfur.

Therefore, the mass removed is:

\[
m(\mathrm{S})_{\text{removed}}
=2.40-0.300
\]

\[
\boxed{m(\mathrm{S})_{\text{removed}}=2.10\text{ kg}}
\]

The sulfur mass balance is:

\[
10.0=7.60+2.10+0.300
\]

All 10.0 kg of sulfur is accounted for.

The sulfur removed from the distillate has entered the separate hydrogen sulfide-containing stream. It has not ceased to exist.

(d) Calculate the sulfur dioxide produced.

The balanced equation is:

\[
\ce{S + O2 -> SO2}
\]

One mole of sulfur produces one mole of sulfur dioxide.

The molar masses are:

\[
M(\mathrm{S})=32.1\text{ g mol}^{-1}
\]

\[
M(\ce{SO2})=64.1\text{ g mol}^{-1}
\]

Using the mass ratio:

\[
m(\ce{SO2})=
0.300\times\frac{64.1}{32.1}
\]

\[
\boxed{m(\ce{SO2})=0.599\text{ kg}}
\]

Approximately 0.60 kg of sulfur dioxide is produced.

(e) Evaluate the refinery manager’s statement.

The statement is misleading because it confuses changes in concentration with changes in the total mass of sulfur.

The original feed contained 10.0 kg of sulfur, while the distillate contains 2.40 kg.

Therefore, the proportion of the original sulfur entering the distillate is:

\[
\frac{2.40}{10.0}\times100=24\%
\]

The remaining 76% is in the heavy residue.

The distillate’s sulfur concentration is 60% lower than the original feed concentration, but this does not mean that 60% of the original sulfur has been removed from the entire refinery system.

There is a second misleading comparison.

The final fuel contains 0.050% sulfur, compared with the original feed’s 1.00%.

This is a 95% reduction in sulfur concentration relative to the original feed. However, it does not mean the entire refining operation has reduced sulfur pollution by 95%.

The residue still contains 7.60 kg of sulfur, and the hydrogen sulfide-containing recovery stream contains another 2.10 kg.

The environmental outcome depends on how these streams are managed.

If the residue is burned without appropriate controls, its sulfur can contribute to further sulfur dioxide emissions. If the hydrogen sulfide is safely processed, sulfur can be recovered rather than released into the atmosphere.

The important lesson is that environmental assessments must follow the mass of a substance through every relevant input and output stream.

A lower concentration in one product does not automatically mean that the total amount of an unwanted element has disappeared.

This is why mass balance is such a useful tool when evaluating industrial chemical processes.

Understanding petroleum requires keeping three ideas connected: how geological processes produce it, how molecular structure determines its properties, and how physical separation and chemical conversion create useful products.

The next important distinction is between separating molecules according to their boiling points and changing their molecular structures through chemical reactions. That is the central idea behind Fractional Distillation vs Cracking.