Crude Oil, Fractions & Combustion

Hydrocarbons and Crude Oil

• A hydrocarbon contains only hydrogen and carbon atoms.
• Most are alkanes with general formula $C_n{H}_{2n+2}$.
• Crude oil is a finite mixture formed over millions of years from ancient marine biomass.
• It is also a feedstock for the petrochemical industry (plastics, solvents, lubricants).

Fractional Distillation

1. Crude oil is vaporised and fed into a fractionating column hot at the bottom, cool at the top.
2. Fractions condense at their boiling point band and are tapped off.

Fractions (top to bottom)

Trends as chain length increases

• Boiling point increases.
• Viscosity increases.
• Flammability decreases.

Cracking

Long-chain alkanes are broken into shorter alkanes and alkenes using high temperature and a catalyst.

• Driven by supply and demand – too much heavy fraction, not enough petrol.
• Alkenes are used as monomers for addition polymers.

Example: hexane ($C_6{H}_{14}$) can crack to butane ($C_4{H}_{10}$) and ethene ($C_2{H}_4$).

Combustion

Complete combustion (plenty of O₂)

$\text{hydrocarbon}+O_2\to C{O}_2+H_{2}O$

Clean blue flame.

Incomplete combustion (limited O₂)

$\text{hydrocarbon}+O_2\to CO+C+H_{2}O$

Smoky yellow flame – produces soot (carbon).

Pollutants

• CO – colourless, odourless and toxic; binds to haemoglobin and blocks oxygen transport.
• SO₂ (from sulfur impurities) – dissolves in rain to form acid rain.
• NOₓ (from hot engines) – acid rain and respiratory problems.
• Particulates (soot) – respiratory problems and global dimming.