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

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!

Alkenes can be 'cracked' in a thermal decomposition reaction to make smaller more useful molecules

• 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 two many left-overs of the larger molecules which do not make good fuels or have other uses. Fuel oil, naphtha and bitumen in crude oil exceed demand.

• 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) 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 longer molecules into shorter petrol fuel molecules, paraffin fuel, alkenes etc.

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

• CRACKING is done by heating some of the less used fractions to a high temperature vapour and passing over a suitable hot catalyst at high pressure. Sometimes the fraction is heated with steam to a high temperature. Using different conditions ie 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.

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  • You can demonstrate cracking in the laboratory by heating paraffin grease over an aluminium oxide 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!

    • Any hydrocarbon liquids collected in the bottle should decolourise bromine water - the test for alkenes.

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

  • 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) and alkenes (e.g. for polymers-plastics).

• 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 those 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.

    • There are lots of possibilities! eg four examples below ...

(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)

(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)

(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)

(4) C₁₂H₂₆ C₆H₁₄ + 2C₃H₆

dodecane ==> hexane + propene

2

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