Cracking

  • Cracking is the breaking of long chain molecules to shorter ones by application of heat or by use of a catalyst.

  • Zeolite catalysts are common.

  • Cracking of heavy fuel oils is done to produce mainly petrol. Cracking is also done to reduce the viscosity of fuel oil (visbreaking), to produce coke (coking), and to produce olefins which are used for petrochemical industry.

  • Cracking processes mainly include pyrolysis, catalytic cracking and hydrocracking.

  • Pyrolysis: Cracking accomplished by heating of higher boiling petroleum fractions like heavy fuel oil at high temperature (above decomposition temperature) and pressure to produce lower boiling lighter fractions. It is also called as thermal cracking.

  • Catalytic Cracking

    • Catalytic cracking involves breaking down and rearranging complex hydrocarbons into lighter molecules in order to increase the quality and quantity of desirable products.

    • Catalytic cracking is the most important and widely used refinery process for converting heavy oils into more valuable gasoline and lighter products. Fluid catalytic cracking (FCC) is the main player for the production of gasoline. The catalyst in this case is a zeolite base for the cracking function.

  • Hydrocracking

    • It is a process that combines catalytic cracking and hydrogenation where the feed is cracked in the presence of hydrogen to produce more desirable products.

    • In this case a dual function catalyst is used. It is composed of a zeolite catalyst for the cracking function and rare earth metals supported on alumina for the hydrogenation function.

    • The main products are kerosene, jet fuel, diesel and fuel oil.

    • It is one of the most versatile petroleum refining schemes adapted to process low value stocks such as atmospheric residues (AR) and vacuum gas oils (VGO). Generally, these feedstocks are not suitable for catalytic cracking because of their high metal, sulfur, nitrogen, and asphaltene contents.

    • Hydrocracking is a hydrogen-consuming reaction that leads to higher gas production and lower liquid yield. It is a highly exothermic reaction.

  • Steam Cracking:

    • It is the main route for producing light olefins, especially ethylene. The feedstocks for steam cracking units range from light paraffinic hydrocarbon gases to various petroleum fractions and residues.

    • Steam cracking reactions are highly endothermic.