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why do we need to crack hydrocarbons

CRACKING ALKANES This page describes what cracking is, and the differences between catalytic cracking and thermal cracking used in the petrochemical industry. Cracking What is cracking? Cracking is the name given to breaking up large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst. The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (petroleum). These fractions are obtained from the distillation process as liquids, but are re-vaporised before cracking.

There isn't any single unique reaction happening in the cracker. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds. One possible reaction involving the hydrocarbon C
Or, showing more clearly what happens to the various atoms and bonds: This is only one way in which this particular molecule might break up. The ethene and propene are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol (gasoline). Catalytic cracking Modern cracking uses zeolites as the catalyst. These are complex aluminosilicates, and are large lattices of aluminium, silicon and oxygen atoms carrying a negative charge.

They are, of course, associated with positive ions such as sodium ions. You may have come across a zeolite if you know about ion exchange resins used in water softeners. The alkane is brought into contact with the catalyst at a temperature of about 500 C and moderately low pressures. The zeolites used in catalytic cracking are chosen to give high percentages of hydrocarbons with between 5 and 10 carbon atoms - particularly useful for petrol (gasoline). It also produces high proportions of branched alkanes and aromatic hydrocarbons like benzene.

For UK A level (and equivalent) purposes, you aren't expected to know how the catalyst works, but you may be expected to know that it involves an ionic intermediate. If you were a refinery, you'd throughput hundreds of tonnes/day from one reactor. To keep process volumes manageable, high gas pressures and fast throughputs. If you thermally cracked long chain alkanes, you would need red heat and get coking. If you did it over acid catalyst, typically zeolite whose pores define product by fit, you'd get coking. The way to do it is to reform under high pressure and modestly high temperature over noble metal catalyst-doped zeolite plus added hydrogen. Coking is then deeply suppressed - "Platformate. " Ethylene is better made otherwise; alpha-olefins overall are assembled catalytically.

You do not want straight-chain products. You want a maximally branched ~$ce{C8}$-cut for gasoline, with maximum octane number. Linear ~$ce{C10}$-cut and up is cheap diesel and kerosene, with maximum cetane number. Government intervention tortures your access to liquid fuels. The most recently built refinery is ancient. Enviro-whinerism has carefully lowered the energy content/gallon. "Sustainability" means you pay for it big time but don't have a right to receive it. How is it bottom of the barrel diesel costs so much? It's the law! written by idiots and crooks.

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why do we need to crack hydrocarbons