6.2.4 - Fuels from Crude Oil

• what's crude oil?

  complex mixture of hydrocarbons (mostly alkanes)

  also called petroleum

  Made of old organic matter
• what is fractional distillation?

  process of separating crude oil into fractions with different temperatures based on boiling points

  first stage in refining crude oil
• how does fractional distillation separate hydrocarbons?

  hydrocarbons of different lengths have different boiling points so condense at different levels in the fractioning column
• process of fractional distillation

  crude oil is vaporised at 400°C

  vaporised crude oil goes into fractioning column - temperature decreases as you go up the column

  rising vapour condenses at different levels based on their boiling points
• where do small and large hydrocarbons condense in the fractioning column?

  small at the top, large at the bottom
• what happens to hydrocarbons that don't vaporise during fractional distillation?

  they're separated in a vacuum distillation column with lower pressure which lowers their boiling point
• Uses of refinery gases

  main source of butane & propane used as petroleum gas for cooking and heating
• uses of naphtha/gasoline

  fuel for cars

  source of organic compounds for chemical industry
• use of kerosene

  jet fuel
• uses of diesel oil

  fuel for cars, lorries, buses & trains
• use of fuel oil

  heavy fuel oil used in ships' boilers & factories
• uses of lubricating oil

  lubricants, waxes & polishes
• use of bitumen

  road making - asphalt
• trends in properties of fractions

  molecular site, boiling point & viscosity increases down the column

  volatility & flammability decreases down the column
• volatility

  how easily liquid turns into vapour
• viscosity

  how easily fluid flows
• Why are fuels refined?

  to re more sulfur impurities which cause air pollution
• why is petrol blended?

  so engines start reliably and run smoothly
• how is petrol blended?

  by adjusting proportion of volatile hydrocarbons added - higher in winter to prevent cold-starting, lower in summer to prevent much vapour forming
• what's done to ensure smooth combustion?

  fuels with a high octane number are produced by increasing proportions of branched alkenes & arines or blending-in oxygen compounds
• octane number

  measure of the performance of a fuel by Comparison with 2,2,4-trimethylpentane (100) & heptane (0)
• main methods to increase octane number of fuels

  cracking

  reforming

  adding ethand and ethers
• What is cracking?

  breakdown of large hydrocarbon molecules into smaller hydrocarbons by breaking C-C bonds at high temperature & pressure or using a catalyst
• why is cracking done?

  shorter hydrocarbons are in higher demand
• types of cracking

  thermal

  catalytic
• conditions for thermal cracking

  high temperatures - up to 1000°C

  high pressure - up to 70atm
• mechanism for thermal cracking

  C-C bonds are broken heteroly tically forming free radicals

  no intermediates
• What does thermal cracking produce?

  alkenes used to make valuable products like polymers
• conditions for catalytic cracking

  lower pressure & temperature (450°C)

  zeolite catalyst
• zeolyle catalysts

  large lattices of aluminium, silicon & oxygen where these form tunnels and cavities where small molecules can fit

  3D
• why zeolyte catalysts are good catalysts

  they can be developed with active sites to favour shapes & sizes of molecules that are being reacted
• mechanism for catalytic cracking

  zeolyte catalyst removes a hydrogen & 2 electrons bound to a carbon in an alhare forming carbonium ions leading to various products of the reaction

  ionic intermediates
• products of catalytic cracking

  aromatic hydrocarbons and motor fuels
• benefit of catalytic cracking

  cuts costs because reaction can be done at lower temperatures and pressure

  saves time because catalyst speeds up the reaction
• what is reforming?

  processing of straight-chain hydrocarbons into branched chain alkanes and cyclic hydrocarbons for efficient combustion giving fuels a higher octane number
• conditions for reforming

  high temperature - 500°C

  catalyst - platinum or rhodium /
• what does reforming produce?

  hydrogen which is valuable as it can be used in processes elsewhere in refinery
• what is knocking?

  where alkanes explode of their own accord when the fuel/air mixture in the engine is compressed

  straight chain alkanes are most likely to cause knocking
• why are fuels reformed?

  to make knocking less likely to happen so combustion is more efficient