Cracking

Explained

Cracking, and the mismatch it exists to fix

Fractional distillation separates crude oil into fractions, but it cannot change what is in them. Whatever proportions the crude oil arrived with are the proportions you are left with.

That is the problem. Crude oil contains a great deal of the long chain fractions, and the world wants very little of them. It wants petrol and other short chain fuels, and it wants them in far larger quantities than distillation alone can supply.

Cracking converts the surplus into the shortage. It breaks long chain alkanes into shorter, more useful molecules, and it is the reason a barrel of crude oil can be turned into something close to what the market actually needs.

What comes out

Cracking a long alkane produces a shorter alkane and at least one alkene.

An alkene has to be produced, and the reason is arithmetic. The original alkane did not carry enough hydrogen to make two saturated molecules out of one, so the fragments cannot all be alkanes. One of them forms a carbon to carbon double bond instead, which is what makes it an alkene.

So both products are useful, for different reasons. The shorter alkane is a better fuel: it is more volatile, ignites more easily and burns more cleanly. The alkene is a raw material rather than a fuel, because that double bond makes it reactive enough to be built into polymers and converted into alcohols.

Writing an equation

Carbon and hydrogen atoms must balance across the arrow, exactly as in any other equation, and that is all there is to it.

Take decane, C₁₀H₂₂. Cracking it might give octane, C₈H₁₈, plus ethene, C₂H₄. Count the carbons: 8 plus 2 makes 10. Count the hydrogens: 18 plus 4 makes 22. The equation balances, so it is a valid answer.

There is usually more than one correct answer, because the molecule can break at different points. C₁₀H₂₂ could equally give C₇H₁₆ plus C₃H₆, or C₆H₁₄ plus C₄H₈. If your atoms balance and one product is an alkene, you have a working answer, so check by counting rather than by trying to remember a particular split.

The alkene formula follows the general formula C_nH_2n, and the alkane follows C_nH_2n plus 2. Knowing both lets you write the second product once you have chosen the first.

The conditions

Cracking needs heat, and it is done in one of two ways.

Catalytic cracking uses a temperature of about 600 to 700 degrees Celsius over a catalyst of silica or alumina. The catalyst lowers the temperature needed, which reduces the energy cost.

Steam cracking mixes the vaporised hydrocarbon with steam at a higher temperature and no catalyst. It favours alkene production, so it is used when the alkenes are what you are after rather than the fuel.

Either way the hydrocarbon must be vaporised first, because the reaction happens in the gas phase. That is why the process is described as passing the vapour over the catalyst rather than heating the liquid.

Where the alkenes go

Ethene is polymerised into poly(ethene), and propene into poly(propene), by joining thousands of molecules end to end across their double bonds.

Ethene also reacts with steam over a catalyst to make ethanol, which is the industrial route to alcohol as opposed to the fermentation route.

So cracking sits at the start of two chains of topics. Follow the alkane and you are in fuels and combustion. Follow the alkene and you are in polymers and alcohols.