Crude Oil and Separation Into Useful Products
In 1859, Edwin Drake drilled the first oil well in Pennsylvania and struck crude at 21 metres deep, within 10 years, fractional distillation towers were separating it into kerosene, lubricating oil, and paraffin wax.
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Q1 · If crude oil is a single black liquid pumped from the ground, how do you think a refinery could possibly turn it into very different products like petrol, diesel, jet fuel, and candle wax?
Q2 · Why do you think different hydrocarbon fractions separated from crude oil boil at different temperatures?
● Know
- What crude oil is and how it formed
- The principle of fractional distillation
- The main fractions and their uses (gas, petrol, kerosene, diesel, lubricants, bitumen)
● Understand
- Why shorter-chain hydrocarbons collect at the top of the distillation column
- How fractional distillation exploits differences in boiling point
- Why crude oil must be separated before it is useful
● Can do
- Describe the process of fractional distillation
- Explain which fractions collect at which levels and why
- Rank fractions by boiling point and chain length
When crude oil is pumped out of the ground, it is a viscous black liquid with a strong smell, it looks nothing like the clear petrol you pump at a service station, yet both contain the same family of molecules. Crude oil is a mixture of hydrocarbons, organic compounds containing only carbon and hydrogen atoms. It formed over 300 million years from the remains of marine microorganisms (phytoplankton and zooplankton) that died, sank to the ocean floor, and were compressed under heat and pressure deep in the Earth's crust. This process takes millions of years, making crude oil a non-renewable resource. Because it formed from ancient living organisms, it is also classified as a fossil fuel.
Crude oil is not a pure substance, it contains hundreds of different hydrocarbon molecules, ranging from methane (CH₄, 1 carbon) to asphalt-like molecules with 50+ carbons. This mixture means crude oil cannot be used directly; it must be separated by refining. The physical process that achieves this separation is fractional distillation, which exploits the fact that different-sized hydrocarbon molecules have different boiling points, directly related to chain length and the strength of intermolecular forces between molecules.
Bass Strait crude oil (produced offshore Victoria by Esso and BHP) is a light, low-sulfur crude, it contains a higher proportion of shorter-chain hydrocarbons (petrol and kerosene fractions) than heavy Middle Eastern crudes. This makes it more valuable because refiners get more high-demand products per barrel.
Australia produces about 300,000 barrels of crude oil per day domestically (mostly Bass Strait and NW Shelf), but imports roughly 90% of its total petroleum needs. Viva Energy's Geelong refinery in Victoria and Ampol's Lytton refinery in Queensland are the last two operating oil refineries in Australia.
Fractional distillation separates crude oil by exploiting differences in boiling point between different hydrocarbon fractions. The crude oil is first heated to about 400 °C in a furnace, converting most of it to vapour. The vapour enters the bottom of a tall fractionating column, which is hot at the bottom and progressively cooler towards the top. As vapours rise through the column, they cool. Each fraction condenses at the temperature corresponding to its boiling point and is collected at that height. Short-chain molecules (fewer carbons) have weaker intermolecular forces, lower boiling points, and condense high up; long-chain molecules condense lower down.
The column is not perfectly clean, each fraction is a range of hydrocarbon chain lengths, not a single compound. This is acceptable because the fractions are defined by their intended use: petrol-range hydrocarbons (5–10 carbons), kerosene-range (10–16 carbons), diesel-range (14–20 carbons). The consistency required for each application is achieved by further refining and blending after the column. The key principle is: boiling point increases with chain length, because longer chains have more surface area for intermolecular (van der Waals) forces, more energy is needed to separate them.
In a fractionating column at 200 °C and 0.5 m height: kerosene fraction (C₁₀–C₁₆, boiling point 150–250 °C) condenses and flows out. At 300 °C and 0.1 m height: diesel (C₁₄–C₂₀, boiling point 200–300 °C) condenses. At the very bottom, bitumen (C₄₀+, mp above 300 °C) flows out as a liquid residue.
Viva Energy's Geelong refinery processes about 130,000 barrels of crude oil per day through its fractionating columns, producing petrol, jet fuel, diesel, and bitumen for road paving used across Victoria and NSW. The bitumen fraction, the heaviest, highest-boiling fraction, paves approximately 20,000 km of Australian roads per year.
Each fraction from the fractionating column has a characteristic carbon chain length range, boiling point range, and set of applications. LPG (liquefied petroleum gas, 1–4 carbons): boiling point below 0 °C, stored under pressure as liquid, used for cooking gas and camping stoves. Petrol (5–10 carbons): boiling point 40–200 °C, volatile liquid, used in car engines. Kerosene (10–16 carbons): 150–250 °C, used for jet fuel (aviation turbine fuel, ATF) and heating. Diesel (14–20 carbons): 200–300 °C, higher viscosity, used in trucks, trains, and ships.
The heavier fractions, heavy fuel oil (20–70 carbons), power large ships and industrial boilers. Bitumen (70+ carbons) is the solid-like residue used in road surfacing. Carbon chain length controls viscosity, longer chains make thicker liquids, as well as flash point (minimum temperature for ignition). Petrol ignites below 0 °C (extremely flammable); diesel ignites above 55 °C (less flammable, safer for trucks). These differences are not arbitrary, they arise directly from the molecular chain lengths of each fraction.
Jet A-1 aviation fuel (kerosene fraction, C₁₀–C₁₆) has a flash point of 38 °C and freezes at −47 °C. At Qantas's Sydney airport fuel farm, jet A-1 is stored in tanks holding over 100 million litres, pumped to aircraft through underground lines. The narrow boiling point range ensures consistent fuel performance across all aircraft types.
Sydney Airport consumes about 2.5 billion litres of jet A-1 per year, Australia's largest single site of petroleum product consumption. The fuel arrives by pipeline from Ampol's Lytton refinery in Queensland, flowing 750 km in the same pipeline that supplies diesel for NSW trucks and LPG for Queensland households.
Each fraction from the fractionating column has a characteristic carbon chain range and boiling point range. has 1–4 carbons and is stored under pressure for cooking and camping stoves. Petrol has 5–10 carbons and is used in car . has 10–16 carbons and is used as jet fuel. The heaviest residue, , has 70+ carbons and is used to surface roads.
At the start of this lesson, you heard that a single black liquid pumped from the ground can be turned into petrol, diesel, jet fuel, and candle wax, and that one industrial process, fractional distillation, makes this possible by sorting hydrocarbon molecules according to their boiling points.
Now that you've worked through the lesson, can you explain what crude oil actually is at the molecular level, and why different fractions separate at different points in the column based on their boiling points and chain lengths?
Q1. Explain why crude oil must be separated by fractional distillation before it can be used as petrol or diesel.
Q2. A student thinks petrol and bitumen are collected at the same level of the distillation column. Explain why this is incorrect, referring to chain length and boiling point.
Q3. Describe the process of fractional distillation from start to finish. Explain the role of temperature gradient in the column and why different fractions condense at different heights.