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Alkanes: Cracking

Cracking: Catalytic Cracking

Catalytic cracking has been in use since the 1930s. In order to carry out this process, temperatures from 400 to 500 °C and presssures between 200 und 300 bar are required. Since only small amounts of unsaturated hydrocarbons are formed during this process, catalytic cracking is being used for the production of gasoline.

Common to all catalytic cracking processes is the precipitation of coke which deactivates the catalyst and requires regeneration by burning off the deposit. Crystalline sodium aluminosilicates with activating additions, such as Cr2O3, are used most frequently. Na12(AlO2)12(SiO2)12(H2O)27, called zeolite A, is such a catalyst.

Fig.1
Schematic illustration of zeolite A

Fluid catalytic cracking, introduced in 1942, is the process most frequently used. In this moving-bed process, high-boiling petroleum feedstock is introduced into the reactor. Water vapor and catalyst, mostly aluminosilicates e.g. zeolite, are continously added in a counter-current fashion. The stream of water vapor carries the catalyst as well as the lower-boiling hydrocarbon fractions generated during the process upwards. The upper part of the reactor contains a separator which removes lower-molecular weight hydrocarbon fractions and the catalysts from the reactor and re-introduces the higher-boiling fractions into the cracking process. The catalyst is activated by removing the coke deposit with hot, oxygenated air and added again to the process.

Temperatures above 500°C and pressures below 3 bar are being used in modern fluid cracking installations.

Movie of catalytic cracking

Fig.
Catalytic cracking of paraffin oil
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