The Crude Oil Refinery

Chain length decides everything

Crude oil is a mixture of hydrocarbons, mostly alkanes, formed from the remains of ancient biomass over a very long time, and it is finite: we are using it far faster than it forms. That is the starting point. What makes this topic manageable is that everything else in it comes back to a single fact. These molecules differ mainly in how long their carbon chains are, and chain length decides how they behave. A refinery separates them by boiling point, and boiling point depends on chain length. The properties you have to know as trends are chain length again. Cracking exists entirely because the chain lengths crude oil happens to contain are not the chain lengths people want to buy. Even how easily a fuel catches light turns out to be chain length showing up in another form. So this is not four topics to memorise. It is one idea, applied four times, and if you can say what chain length does you can reconstruct most of the rest.

Words for the refinery

Six terms your specification uses precisely. The first three describe what crude oil is; the last three describe what is done to it.

The first four alkanes

Alkanes have the general formula CnH2n+2, where n is the number of carbon atoms. Put n equal to 1 and you get _____, which is methane. Put n equal to 2 and you get _____, which is ethane. Propane has three carbons and so is _____, and butane has four and so is _____. Notice that each step up the series adds one carbon and _____ hydrogens, which is what the formula is telling you.

What the formula gives you

An alkane has five carbon atoms. Using the general formula CnH2n+2, how many hydrogen atoms does it have?

• Twelve, because two times five is ten, plus two
• Ten, because there are twice as many hydrogens as carbons
• Eleven
• Five, one for each carbon

Longer chains, heavier behaviour

Three properties change as the carbon chain gets longer, and your specification asks for the trends rather than for numbers. Two rise and one falls, which is the thing to get straight.

Which two follow from longer chains

Select the TWO statements that are true of hydrocarbons with longer carbon chains.

• They have higher boiling points, so they condense lower down a fractionating column
• They are more viscous, so they flow less easily
• They are more flammable, so they ignite more easily
• They have lower boiling points, so they leave from the top of the column

Why the column has a temperature

Fractional distillation is easy to describe and easy to describe wrongly, so it is worth being exact about the mechanism. The crude oil is heated until most of it becomes vapour, and that vapour enters a tall column which is hot at the bottom and steadily cooler towards the top. As the vapour rises it cools. A hydrocarbon condenses back to liquid when it reaches a level cool enough for its own boiling point, and is drawn off there. So the column is not sorting molecules by size directly. It is sorting them by the temperature at which they turn back into a liquid, and that temperature depends on chain length. Long chains have high boiling points, so they condense early and leave near the bottom. Short chains have low boiling points, so they have to travel much further up before they are cool enough, and the shortest of all leave the top still as gases. Notice too that what comes off is a fraction rather than a pure substance: a group of hydrocarbons with similar boiling points, separated together. That is why the products are called fractions, and why they still have to be processed afterwards.

Where each fraction leaves

Two ways a fuel can burn

When a hydrocarbon burns, the carbon and the hydrogen in it are both oxidised, and what you get depends on whether there is enough oxygen. With a plentiful supply, combustion is complete: the carbon becomes carbon dioxide and the hydrogen becomes water. Write it as a sentence and it is straightforward, because everything in the fuel has been fully oxidised. With a poor supply of air, combustion is incomplete, and the carbon does not make it all the way to carbon dioxide. Some becomes carbon monoxide, and some may not be oxidised at all, appearing as particles of carbon, which is soot. The hydrogen still becomes water. This matters for two reasons your specification cares about. Carbon monoxide is a toxic gas and is difficult to detect without equipment, because it has no colour and no smell. Soot deposits as particles, which is why an appliance burning with a poor air supply may leave black marks, and why a yellow smoky flame is a sign of incomplete combustion where a clean blue one is not. Notice what settles which happens: not the fuel, but the air supply.

Which two come from a poor air supply

A hydrocarbon burns in a limited supply of air. Select the TWO products that this can produce which complete combustion does not.

• Carbon monoxide
• Carbon, as soot particles
• Water
• Carbon dioxide

Why a refinery cracks anything

Put this reasoning into the order that explains why cracking exists at all.

• Crude oil contains a fixed mixture of chain lengths, decided by how it formed
• Fractional distillation separates that mixture but cannot change it
• Demand is highest for short-chain hydrocarbons, because they make the most useful fuels
• Crude oil yields more long-chain fractions than the market wants
• Long chains are therefore broken into shorter ones by cracking
• This produces short-chain alkanes and also alkenes, which are useful as feedstock

The claim about chain length

Four statements about hydrocarbons. Select the ONE that has a trend the wrong way round.

• Longer-chain hydrocarbons have higher boiling points than shorter ones.
• Longer-chain hydrocarbons are more viscous than shorter ones.
• Longer-chain hydrocarbons are more flammable than shorter ones.
• Shorter-chain hydrocarbons condense higher up a fractionating column.

A trend explained properly

Assemble a sentence that explains a fraction’s position by its property rather than just stating it.

Running the column

You are tracing what happens to a batch of crude oil through a refinery. Take the decisions in order.

• The crude oil is heated and enters the column as vapour. What decides where each hydrocarbon leaves?
• One fraction is drawn off near the top. What can you say about it?
• The refinery finds it has more long-chain fractions than it can sell. What does it do, and why?
• A burner fed with one of the fuels produces a yellow smoky flame. What does that indicate?

Explain how a refinery works

Explain what crude oil is and what a refinery does with it, from the raw mixture through to the products, using the terms from this topic.

• Say what crude oil is, what it is mostly made of, and why it is described as finite
• Give the general formula for alkanes and use it to work out one molecular formula
• Explain how fractional distillation separates the mixture, naming what the column does
• Explain why a hydrocarbon leaves the column where it does
• Give the three property trends with chain length, being careful about which way each runs
• Explain the difference between complete and incomplete combustion, naming the products of each
• Explain why cracking exists, using supply and demand rather than just naming the process
• Finish by explaining how chain length connects every part of your answer