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Option C2 (SL/HL): Part 2 Cracking

Option C2 (SL/HL): Part 2 Cracking. The petroleum fractions with 1 to 12 C atoms are in greater demand than other fractions.

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Option C2 (SL/HL): Part 2 Cracking

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  1. Option C2 (SL/HL): Part 2 Cracking • The petroleum fractions with 1 to 12 C atoms are in greater demand than other fractions. • Cracking is the process of breaking down large, less useful, hydrocarbon fractions into shorter, but more useful alkanes and alkenes, which are used as fuels and petrochemical feedstock. • This can be achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst.

  2. CRACKING Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products THERMALproceeds via a free radical mechanism CATALYTICproceeds via a carbocation (carbonium ion) mechanism THERMAL HIGH PRESSURE ... 7000 kPa HIGH TEMPERATURE ... 400°C to 900°C FREE RADICAL MECHANISM HOMOLYTIC FISSION PRODUCES MOSTLY ALKENES ... e.g. ETHENE for making polymers and ethanol PRODUCES HYDROGEN ... used in the Haber Process and in margarine manufacture Bonds can be broken anywhere in the molecule by C-C bond fission or C-H bond fission

  3. 1. Thermal cracking / Pyrolysis Thermal cracking – using heat at high temperature + dodecane 2-methylpropene 2,2,4-trimethylpentane • Feedstock: reactant molecules • Kerosene: 10-16 C chain liquids (190-250 oC b.p. range) • Reaction conditions: Moderate temp. range: 500°C to 750°C, high pressure (70 atm), no catalyst, longer contact time • Uses of products: lighter fractions as fuel

  4. Thermal cracking in the lab

  5. CRACKING CATALYTIC SLIGHT PRESSURE HIGH TEMPERATURE ... 450°C ZEOLITE CATALYST CARBOCATION (IONIC) MECHANISM HETEROLYTIC FISSION PRODUCES BRANCHED AND CYCLIC ALKANES, AROMATIC HYDROCARBONS USED FOR MOTOR FUELS ZEOLITES are crystalline aluminosilicates; clay like substances

  6. 2. Catalytic cracking Catalysts used are: Molecular formula SiO2 • Silica (silicon dioxide) • Alumina (aluminium oxide) • Zeolite (silicate compounds) Al2O3 http://www.cybered.net/commerce.asp?CatId=311&ProdId=X5005108

  7. Catalytic cracking – catalyst used No one equation can represent the complex reactions that take place in a practical catalytic furnace. For example: C10H22 C8H18 C2H4 + decane octane ethene • Feedstock: reactant molecules • Gas oil: 14-20 C chain liquids (250-350 oC b.p. range) • Uses of products: High grade gasoline (branched chain alkanes increase fuel octane rating)

  8. Catalytic Cracking of Decane Bond often breaks here H H H H H H H H H H H H C C C C C C C C C C H H H H H H H H H H decane Catalyst at high temperature:500°C Moderately low pressure Catalyst: _______ Short contact time C10H22 Heat energy used to break carbon to carbon bonds H H H H H H H H H H H C C C C C C C C H + C C H H H H H H H H H H Octane Ethene A saturated hydrocarbon An unsaturated hydrocarbon

  9. Catalytic Cracking of Decane – showing bonds breaking Bond often breaks here H H H H H H H H H H H H C C C C C C C C C C H H H H H H H H H H This hydrogen atom can move here to form a new C H bond to produce octane decane C10H22 Two electrons join to form a carbon to carbon double bond Heat energy used to break carbon to carbon bonds C H bond breaks H H H H H H H H H H H H C C C C C C C C H + + C C H H H H H H H H H H H Octane Single electron available to form a new bond Ethene A saturated hydrocarbon An unsaturated hydrocarbon

  10. Catalytic cracking • Cracking the long, not very useful, hydrocarbons may produce: • A small alkene, ethene, which can be made into plastics (e.g. polyethene) • A medium-sized alkane, C8H18, Octane, which is petrol (i.e. the fuel for cars) • At 500 oC, Decomposition of hydrocarbons produces carbon, which deposits on the catalyst and deactivates it. • From time to time, air is passed through the reactor / furnace so that carbon is burnt off in air to regenerate the catalyst. • The combustion of carbon produces energy which is used for further cracking. • The regenerated catalyst is circulated and mixed with new residue molecules, and then on to the reactor again

  11. 3. Steam cracking Steam cracking – The hydrocarbons are heated, vaporised, and mixed with steam at higher temperatures + hexane propane propene + butane ethene hydrogen • Feedstock: reactant molecules • Naphtha: 5-10 C chain liquids (40-190 oC b.p. range) • Reaction conditions: High temp. range: 800°C to 900°C, Moderately low pressures, no catalyst, very short contact time • Uses of products: Lower molar mass alkanes are used in polymer manufacture, hydrogen used in hydrocracking

  12. Key Websites 1. Catalytic cracking: http://bbc.net.uk/schools/gcsebitesize/chemistry/usefulproductsoil/oil_and_oilproductsrev7.shtml http://www.chemguide.co.uk/physical/catalysis/petrochem.html http://en.wikipedia.org/wiki/Cracking_(chemistry) http://www.uyseg.org/catalysis/petrol/petrol2.htm http://science.howstuffworks.com/oil-refining5.htm

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