Crude Oil, Fractional Distillation and Cracking
Crude oil contains a mixture of hydrocarbons. Fractional distillation separates fractions by boiling range, and cracking converts some surplus larger hydrocarbons into smaller fuel molecules and alkenes.
What a learner can do afterwards
- Explains how the column's temperature gradient separates fractions by boiling range and relates those ranges to typical hydrocarbon chain lengths
- Links chain length to viscosity, how easily a fraction evaporates, and how easily it burns
- Describes the conditions for cracking and writes an equation showing a long chain becoming an alkane plus an alkene
- Explains why refineries use cracking by comparing production with demand
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Crude oil is a fossil fuel formed from ancient organic remains, including tiny marine organisms. Sediment buried the remains. Heat and pressure over millions of years helped turn the material into oil. Crude oil contains many hydrocarbons. A hydrocarbon contains only carbon and hydrogen. Its molecules may contain chains, branches or rings.
You have met vaporisation and condensation in distillation. Heating crude oil vaporises much of it. A fractionating column is hot below and cooler above. Higher-boiling hydrocarbons condense lower down. Lower-boiling ones stay as vapour until cooler levels; some gases leave the top. The heaviest material, including bitumen, remains as residue near the base. Each fraction contains hydrocarbons with a similar boiling range.
For similar hydrocarbons, longer chains generally have higher boiling points, greater viscosity and lower volatility. Viscosity means resistance to flow. Volatility means how readily a substance vaporises. Shorter-chain fractions usually flow and ignite more easily. These are trends, not exact tests: molecular structure and temperature also affect properties.
Catalytic cracking heats vaporised larger hydrocarbons and passes them over a suitable catalyst. Smaller molecules form, including alkanes and alkenes. Steam cracking instead uses high temperature and steam. Equations conserve each kind of atom. For example, C12H26 -> C10H22 + C2H4 has 12 carbon atoms and 26 hydrogen atoms on each side.
Distillation separates existing molecules; cracking changes them by breaking bonds. Refineries can have more heavy fractions than customers demand and greater demand for smaller fuel molecules. Cracking converts some surplus larger molecules into smaller products. It also makes alkenes for polymers. Heavy fractions still have uses, including fuels and bitumen for roads.
A temperature gradient separates boiling ranges. Cracking changes surplus larger hydrocarbons into smaller fuels and useful alkenes.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Linked videos: Distillation and Simple and fractional distillations, Khan Academy.
More to read
What fossil fuel means
A fossil fuel has an ancient biological origin. Crude oil did not form because rocks melted or rivers dried up. Buried organic remains, including tiny marine organisms, changed over millions of years under heat and pressure.
Oil contains many different molecules. Separating it at a refinery does not reverse its geological formation or make new crude oil. Fractional distillation sorts molecules already present. Cracking later changes some larger molecules into smaller ones.
Keep these processes apart: formation happens over geological time; refinery separation and chemical processing happen after extraction.
Use a trend with care
Carbon count helps explain hydrocarbon properties, but it does not describe the whole molecule. Hydrocarbons can contain branches or rings as well as unbranched chains.
Compare similar hydrocarbons under the same conditions. Longer chains generally have higher boiling points and greater viscosity. Shorter chains generally vaporise more readily. Temperature can also change how readily a liquid flows.
A fraction contains a range of molecules, so its properties are not an exact test of one chain length. Its boiling range and typical behaviour help explain its uses.
Follow the molecules
Ask whether molecules stay intact or change. In fractional distillation, hydrocarbons vaporise and condense without being split. The temperature gradient lets fractions collect at different levels, while heavy residue stays near the base.
In cracking, bonds break and smaller molecules form. Count atoms to check a proposed equation: each carbon atom and hydrogen atom must appear in the products.
A refinery can use both processes. Distillation sorts the mixture. Cracking converts some surplus larger molecules into smaller fuels and alkenes. This does not mean heavy fractions have no uses.
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