6. CRACKING a problem of Supply and Demand in the Oil Industry!

6a. Why do we need cracking?

There isn't enough petrol in the original crude oil and crude oil doesn't have alkenes in it for plastics but cracking reactions can help supply them! AND no left over waste oil!

Alkanes can be 'cracked' in a thermal decomposition reaction to make more smaller more useful molecules because, quite simply, we need a lot more petrol and diesel than is in crude oil AND the cracking process make alkene molecules (NOT in oil) from which we make many industrial chemicals and in particular polymer - plastic products.

Cracking converts longer alkane hydrocarbon molecules into smaller alkane & alkene molecules

• The figures in the table are quite variable depending on the source of oil and what the chemical industry markets want.

• When crude oil has been distilled into useful fractions it is found that the quantities produced do not match the ratio required for commercial needs e.g. we have an insatiable appetite for petrol and diesel in our cars and there are too many left-overs of the larger molecules which do not make good fuels or have other uses

  • e.g. fuel oil, lubricating oil, naphtha, wax and bitumen in crude oil exceed demand.

  • The data table above gives you an idea of the imbalance between what we get from crude oil and what industry and domestic consumers actually want.

    • The data in the table was gleaned from several internet sources and cab a bit variable, so I've quoted typical ranges, so don't expect the figures to add up to a 100%.

    • The table shows that the hydrocarbon demands don't match the composition of crude oil particularly the demands for petrol and diesel.

    • You can see there is quite a deficiency of gas (half required), petrol (< third required), but a surplus of naphtha, heavy oils, waxes etc., so these heavier fractions are cracked to make more of the smaller molecules we need.

    • Kerosene can be cracked to make more petrol and heavy oil and waxes can be cracked to make more petrol and diesel, AND all cracking makes the incredibly useful hydrocarbon molecules called alkenes.

  • So, the excess of these bigger hydrocarbon molecules can be converted to smaller ones by cracking them apart!

• Also, alkenes are NOT found in crude oil and they are one of the most valuable types of organic molecule in the chemical industry e.g. to make polymers (plastics like poly(ethene)) or ethanol (an alcohol).

  • The chemistry of ALKENES – unsaturated hydrocarbons – reaction with bromine & hydrogen

• The two deficiencies are remedied by the process of cracking which converts useless big long 'sticky' molecules into useful smaller ones!

  • eg cracking naphtha or diesel oil fractions to convert these 'less demanded' longer molecules to give more shorter useful molecules like petrol/diesel fuel molecules AND the extremely useful alkenes.

• Therefore cracking is an important economic process in the petrochemical industry to make the best and most varied use of the resource we call crude oil.

  • Crude oil is a non-renewable resource, so the more efficiently we use it the longer it lasts, irrespective of the issue of burning fossil fuels creating global warming and climate change!

6b. How do we crack the fractions from oil distillation?

• CRACKING is done by heating some of the less used fractions of bigger molecules to a high temperature vapour and passing over a suitable hot catalyst at high pressure.

  • Sometimes the fraction is heated with steam to a very high temperature without a catalyst.

  • Using different conditions i.e. by varying with/without steam, temperature, pressure or catalyst you can control the composition of the mixture and make a variety of different hydrocarbon products.

  • The cracking reaction is an example of a thermal decomposition, that is, breaking a molecules down with heat, by heating them to high temperature.

  • The high temperature and catalyst are needed to facilitate the cracking reaction because you have to break the very strong carbon-carbon covalent bonds in the alkane molecule.

  • This is an endothermic thermal decomposition reaction.

  • 

  • You can safely demonstrate cracking in the laboratory by heating paraffin grease over an aluminium oxide (or porcelain chips) catalyst at 400–700^oC, and collecting the smaller gaseous hydrocarbon molecules over water – readily shown to be flammable.

    • This experiment needs to be done as a teacher demonstration – most carefully!

    • The mineral wool (or glass wool) is soaked in paraffin oil or any other heavy oil, even paraffin grease will do.

    • This and the mineral catalyst are set up as shown in a pyrex boiling tube.

    • The catalyst is strongly heated and the heat will also vapourise some of the oil/grease whose vapour will then pass over the catalyst to 'cracked' i.e. broken down by the combined effect of the heat and catalyst.

    • Some of the hydrocarbon liquid molecules collected in the bottle AND some of the gases in the inverted test tube from the cracking process should decolourise bromine water – the simple test for alkenes.

      • See diagram below, you just add orange bromine water to a sample of the collected hydrocarbon gases or liquids).

    • Test for alkenes

    • See The chemistry of ALKENES – unsaturated hydrocarbons – reaction with bromine

    • The experimental setup above allows the safe collection of the very short molecules e.g. methane, ethane, ethene, propene and propane gases and liquid alkanes and alkenes of at least five carbon atoms in the chain.

    • The bottle is a safety measure, if the gases inside the pyrex tube cool down, they contract, but any water sucked back from the gas collection system is trapped in this bottle. This means no cold water gets into the hot pyrex tube to crack it and a cause a nasty accident! We just want to crack the hydrocarbon molecules!

  • The cracking reaction is an example of thermal decomposition – a reaction that breaks down molecules into smaller ones using heat and it takes place on the very hot surface of the catalyst.

    • The main products from cracking alkanes from oil are ...

      • smaller alkanes e.g. for petrol or diesel,

      • alkenes e.g. for polymers–plastics, alcohols and many other important organic compounds.

• The equations below illustrate the process, small molecules are used to show the overall molecular change clearly BUT in practice the 'starter' molecules are likely to be more like the larger alkane hydrocarbon molecules shown in equations (3) and (4).

  • The cracking involves breaking single carbon–carbon bonds to form the alkanes (saturated hydrocarbons) and alkenes (unsaturated hydrocarbons) products. Examples of word equations and balanced symbol equations for cracking reactions are given below.

    • There are lots of possibilities! eg four examples below show in each case the formation of a shorter alkane and an alkene from a cracking reaction ...

Cracking reaction (1) butane ethane + ethene

C₄H₁₀ C₂H₆ + C₂H₄

this is probably not used commercially, but illustrates the principle of cracking with small simple molecules to give a smaller alkane and an alkene eg ethene to make the plastic poly(ethene)

Cracking reaction (2) butane methane + propene

C₄H₁₀ CH₄ + C₃H₆

this is probably not used commercially, but illustrates the principle of cracking with small simple molecules to give a smaller alkane methane and an alkene propene to make the plastic poly(propene)

Cracking reaction (3) octane hexane + ethene

C₈H₁₈ C₆H₁₄ + C₂H₄

this cracking reaction is used commercially to make a volatile petrol fuel molecule hexane plus ethene for polymerisation to make poly(ethene)

Cracking reaction (4) dodecane hexane + propene

C₁₂H₂₆ C₆H₁₄ + 2C₃H₆

2

this reaction is used commercially to crack a naphtha/kerosene molecule into a petrol molecule plus two alkene propene molecules from which you make the plastic poly(propene)

Cracking reactions (5) Other catalytic cracking reactions at high temperature produce hydrogen as well e.g.

ethane ethene + hydrogen

C₂H₆ C₂H₄ + H₂

or

propane propene + hydrogen

C₃H₈ C₃H₆ + H₂