Hydrocarbon Products, Use and Energy Transition

Imagine a refinery producing 1,000 barrels of crude every hour: the distillation column automatically fills tanks of petrol, diesel, and bitumen, but the bitumen tank fills up much faster than anyone can sell it, while the petrol tank empties before trucks can carry it away, creating a permanent mismatch between what crude oil contains and what customers actually need. When crude oil is fractionally distilled, the proportions of each fraction produced don't match what consumers want to buy. A typical crude produces about 20% petrol-range hydrocarbons but consumer demand for petrol is far higher, about 40% of all refined products. Meanwhile, the crude produces 30–40% heavy fuel oil, but demand for heavy fuel oil is declining as shipping moves to lower-sulfur fuels. Cracking bridges this gap by converting the excess heavy fractions into the lighter, higher-value fractions that are in shortage.

Without cracking, refineries would be forced to sell heavy fractions at a large discount or pay for disposal, while simultaneously not meeting petrol demand. Cracking turns a potential liability (surplus heavy oil) into a valuable product (petrol, ethene, propene). This is why every major refinery in the world uses cracking, it is not optional chemistry but an economic necessity that determines the profitability of the entire oil-refining industry. Australia's Lytton and Geelong refineries use catalytic cracking as a core process.

Two methods are used industrially to crack hydrocarbons. Thermal cracking uses extreme heat (400–900 °C) and pressure to break C–C single bonds. At these temperatures, C–C bonds have enough thermal energy to break homolytically, the chain fragments randomly. Thermal cracking produces a mixture of alkanes and alkenes; it is used primarily to produce ethene and propene for the petrochemical industry. The process is energy-intensive because high temperature must be maintained.

Catalytic cracking uses a zeolite catalyst (an aluminium silicate mineral with a porous, cage-like structure) at lower temperatures (450–550 °C). The catalyst provides a surface where C–C bonds can break with much lower activation energy, the reaction proceeds at lower temperature, saving energy. Catalytic cracking also produces a higher proportion of branched alkanes (which have better octane ratings for petrol) and alkenes. Both methods always produce alkenes among the products, this is a key diagnostic feature of cracked products, distinguishing them from straight distillation fractions which are all alkanes.

The products of cracking include petrol-range alkanes (5–10 carbons) and valuable small alkenes. Ethene (C₂H₄) from cracking is the world's most important petrochemical building block: it is polymerised to polyethylene (plastic bags, pipes, packaging film), or reacted with water to make ethanol. Propene (C₃H₆) is polymerised to polypropylene (car bumpers, food containers, carpet fibres). The production of alkenes by cracking is the direct link from crude oil refining to the plastics industry: without cracking, there would be no cheap source of alkene monomers for polymer production.

The economic chain is: crude oil → fractional distillation → heavy fractions → cracking → ethene + propene → polymerisation → polyethylene + polypropylene → every plastic product you use. Australia imports large quantities of polymer granules from overseas petrochemical plants (particularly in Singapore and South Korea), which in turn source their ethene and propene from cracking units. Understanding cracking is understanding how a barrel of crude oil ultimately becomes a plastic bottle, a food container, and a carpet fibre simultaneously.