Oil, natural gas and coal

Natural gas, crude oil and coal are the main natural sources of hydrocarbons. They are both the chief fuels of the energy sector (power stations, transport, heating) and the raw materials of organic synthesis: plastics, synthetic fibres and rubbers, fertilisers, medicines and dyes all start from oil or gas. Uzbekistan is a major gas producer: the Shurtan, Mubarek, Gazli and Kandym fields, the Ustyurt gas-chemical complex (which produces poly(ethene) and poly(propene)), and the Bukhara and Fergana oil refineries. In this topic it is important to distinguish clearly between physical processing (distillation) and chemical processing (cracking, reforming, pyrolysis).

§1. Natural gas and associated petroleum gas

• Natural gas is mainly methane (80–98 %) with small amounts of ethane, propane and butane, plus N₂, CO₂ and, in some fields, H₂S. It is the cleanest fuel (it produces almost no ash or sulfur oxides) and a raw material for hydrogen, ammonia, methanol, ethyne and carbon black. Natural gas has no smell, so strong-smelling sulfur compounds (thiols) are added for safety.
• Associated petroleum gas comes out with crude oil and is dissolved in it: besides methane it contains much ethane, propane, butane and pentane. It is separated into dry gas (methane), liquefied propane–butane (bottled gas) and natural gasoline.

Methane is also converted by pyrolysis into ethyne (2CH₄ → C₂H₂ + 3H₂) and by chlorination into solvents.

Worked example

What mass of methanol can be made from 1.6 kg of methane via synthesis gas (100 % yield)?

1. n(CH₄) = 1600/16 = 100 mol.
2. CH₄ → CO + 3H₂, CO + 2H₂ → CH₃OH: 1 mol of methane gives 1 mol of methanol (hydrogen is left over).
3. m(CH₃OH) = 100·32 = 3200 g = 3.2 kg.

§2. Crude oil and its distillation

Crude oil is a brown or black oily liquid, less dense than water; it is a mixture of hydrocarbons (alkanes, cycloalkanes, arenes) with small amounts of oxygen, sulfur and nitrogen compounds. Being a mixture, it has no fixed boiling point and boils over a wide temperature range. Extracted crude is first freed from water, salts and dissolved gases and then fractionally distilled in a fractionating column — a physical process: the substances are not changed, only separated by boiling point.

The column is hot at the bottom and cooler at the top, so the lighter fractions rise. The residue is burnt in boilers or redistilled under reduced pressure (vacuum) to give lubricating oils, petroleum jelly, paraffin wax and bitumen (asphalt). Kerosene is aviation fuel; diesel is for lorries and tractors. The more carbon atoms in a fraction, the higher its boiling point and viscosity and the lower its volatility.

§3. Chemical processing of oil

Straight-run petrol is scarce and of low quality, so the heavy fractions are converted chemically.

• Cracking breaks long carbon chains to increase the yield of petrol:

Thermal cracking (470–550 °C) gives products rich in alkenes and an unstable petrol; catalytic cracking (aluminosilicate catalysts, about 450 °C) gives high-quality petrol rich in branched and aromatic hydrocarbons.

• Reforming (aromatisation) turns alkanes and cycloalkanes into arenes: C₆H₁₄ → C₆H₆ + 4H₂, C₇H₁₆ → C₆H₅CH₃ + 4H₂; it raises the octane number of petrol and is a source of arenes and hydrogen.
• Pyrolysis (700–1000 °C) gives the raw materials for polymers: ethene and propene. At Ustyurt, the ethene and propene obtained by pyrolysis of ethane and propane are polymerised.

The octane number measures a petrol’s resistance to premature self-ignition (knocking) in the engine: 92-octane petrol burns like a mixture of 92 % isooctane and 8 % n-heptane.

Worked example

How can you check whether cracking products contain an alkene?

1. Add bromine water or KMnO₄ solution to a sample.
2. If it is decolourised, an unsaturated hydrocarbon (alkene) is present.
3. Straight-run petrol does not decolourise these reagents — this is how they are told apart.

§4. Coal and its processing

Coal is a solid fuel consisting mainly of carbon, with compounds of hydrogen, oxygen, nitrogen and sulfur. Coking means heating coal without air to about 1000 °C; the products are:

• coke — almost pure carbon, used to reduce iron ore in blast furnaces;
• coke-oven gas (H₂, CH₄, CO) — a fuel;
• coal tar — a source of benzene, methylbenzene, phenol and naphthalene;
• ammoniacal liquor — used to make ammonium sulfate (a fertiliser).

Burning coal produces more ash, soot and sulfur oxides than burning gas. Coal gasification (heating with steam: C + H₂O → CO + H₂) also yields synthesis gas, from which liquid fuels and methanol are made. So all three natural sources — gas, oil and coal — ultimately feed organic synthesis through the same intermediates (synthesis gas, alkenes, arenes).

§5. Environment and heat of combustion

Burning hydrocarbon fuels affects the environment: CO₂ (greenhouse effect), CO (highly toxic, formed by incomplete combustion), SO₂ and nitrogen oxides NOₓ (acid rain), soot and dust. Remedies include catalytic converters in cars, removing sulfur from fuels, switching to gas and to renewable energy. Oil spills are especially dangerous for water: a thin film forms on the surface, stops gas exchange and kills living things.

Fuels are compared by their heat of combustion (heat released when 1 kg or 1 m³ burns) and by their emissions. Natural gas has one of the highest heats of combustion per unit mass and the lowest harmful emissions, so it is environmentally preferable to coal and fuel oil. Oil and gas reserves are limited and non-renewable, so saving them, processing them more deeply and using solar and wind power are important tasks.

Worked example

How much heat is released when 1 m³ of natural gas (95 % CH₄, STP) burns? (CH₄ + 2O₂ → CO₂ + 2H₂O + 890 kJ)

1. V(CH₄) = 1000·0.95 = 950 L → n = 950/22.4 ≈ 42.4 mol.
2. Q = 42.4·890 ≈ 37 700 kJ.
3. Answer: about 37.7 MJ.

Check yourself

1. Is fractional distillation of oil a physical or a chemical process?

2. Cracking C₁₀H₂₂ gave one alkane and one alkene with equal numbers of carbon atoms. Write the products.

3. Which resists knocking better: n-octane or isooctane?

4. What is synthesis gas made of?

5. What does reforming heptane give?

6. Which gases cause acid rain?

7. What is the main component of natural gas?

8. How many moles of O₂ are needed to burn 2 mol of octane completely?

Common mistakes

• Calling fractional distillation a chemical process.
• Thinking cracking gives only alkanes: alkenes form too.
• Confusing octane number with the number of carbon atoms in a molecule.
• Assuming associated gas and natural gas have the same composition.
• Mixing up reforming and cracking: reforming gives arenes, cracking shortens chains.