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Alkanes: Cracking

Cracking: Hydrocracking

For technical reasons, hydrocracking was only made possible in the late Fifties of the last century. In this process, catalytic cracking is carried out in an atmosphere of hydrogen (300 - 500 m3 hydrogen per ton of hydrocarbon) with the advantage that impurities, such as sulfur and nitrogen, can be removed from the product easily as hydrogen sulfide and ammonia, respectively. Hydrocracking does not generate any alkenes because hydrogen reacts with the generated radicals forming alkanes with low boiling points instead. Reaction conditions in the hydrocracking process are very drastic with pressures of 80 - 200 bar and temperatures in the range of 270 - 450°C.

The single-stage process employs bi-functional systems consisting of metal sulfides (cobalt, molybdenum and nickel) and aluminum oxide as catalysts. The disadvantages of this process are moderate yields (ca. 60%) and very small fractions. Additionally, the activity of the catalysts is lowered by the ammonia formed in the process. To circumvent these problems, a multi-stage process has been developed in which the undesired by-products, ammonia and hydrogen sulfide, are removed in a special reactor before entering the actual cracking process.

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