Crude oil as fuels and feedstock

A fuel is a substance that can be burnt to produce heat or power

Crude oil formation:

  • Millions of years ago, there were tiny animals and plants in the sea

    • for example, plankton

  • When they died, they fell to the bottom of the sea

  • At the bottom of the sea, they were covered in mud

  • Over time, layers of rock built up on top

  • The rock put the dead organisms in hot, high pressure, low oxygen conditions

  • These conditions, over millions of years, formed crude oil

Crude oil is a mixture of different hydrocarbons.

A hydrocarbon is a compound containing hydrogen and carbon.

The hydrocarbons produces are called alkanes. Each carbon atom has four bonds.

  • Methane - CH4

  • Ethane - C2H6

  • Propane - C3H8

  • Butane - C4H10

  • Pentane - C5H12

Most of the hydrocarbons in crude oil are alkanes. These are hydrocarbons containing only single bonds - this means that they are saturated

Alkanes have the general formula:

CnH2n+2

Properties of short chain alkanes:

  • Low boiling point

  • High volatility (tendency to turn to a gas)

  • Low viscosity (how thick it is)

  • High flammability

Properties of long chain alkanes:

  • High boiling point

  • Low volatility

  • High viscosity

  • Low flammability

The larger the number of carbons, the higher the melting and boiling point.

Fractional distillation can be used to separate a mixture of two miscible liquids (two liquids that mix and don’t separate):

  1. Crude oil is heated in a furnace to vaporise it

  2. Vapours are passed into the fractionating column

  3. The fractional column is cool at the top and hot at the bottom

  4. The vapours then begin to rise up the column and begin to cool

  5. Eventually the vapour will reach a temperature level which matches the boiling point of the hydrocarbon

  6. Fractions are run off at different level of the fractionating column

The different section of the column make:

  • the top one makes petroleum gas (short chain hydrocarbons)

  • Gasoline/petrol

  • Kerosene

  • Diesel oil

  • Residue (containing bitumen - very thick, high boiling ping, long chain hydrocarbons)

In a combustion reaction, fuel is burned and reacts with oxygen to release energy

When we burn fuels we make lots of different products

This depends on the amount of oxygen we have present

If we have plenty of oxygen, we get complete combustion:

Propane + oxygen —> carbon dioxide + water

We always make carbon dioxide and water, as long as we have plenty of air

If we don’t have enough oxygen, we can get incomplete combustion:

Propane + oxygen —> carbon monoxide + water

Some carbon turns into carbon monoxide (CO) which is poisonous

To test for water, you put cobalt chloride paper in the liquid and it should turn from blue to pink

Always balance carbon and hydrogen first, then oxygen.

If the supply of oxygen is very limited, C solid (soot) is made instead.

Lager hydrocarbons can be broken down into smaller ones, this is a process called cracking.

Cracking is the process by which long hydrocarbon chains are broken down into smaller hydrocarbons.

To crack hydrocarbons we can use two methods:

  • Pass the vapour over a hot catalyst

  • Mix with steam and heat to a very high temperature

Cracking is an example of thermal decomposition reaction, when a substance is heated to break it down.

When we crack an alkane, we always make a mixture of alkanes and alkenes

Alkenes general formula:

CnH2n

Alkenes make bromine water go colourless and alkanes do not have an effect.

A positive test for an unsaturated hydrocarbon is that it turns bromine water colourless.