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If we have a long-chain hydrocarbon, such as decane, and we split it through thermal cracking (say in an industrial plant), we use high temperatures, and high pressures. Cracking produces smaller molecules - alkanes, and alkenes. It might look like this:

$$\ce{CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3}$$ Which is cracked into products like this: $$\ce{CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3-> 2CH2=CH2 + C3H6 + C8H18}$$

Why do we use high pressure in this case? Le chateliers principle states that the equilibrium will shift to resist the change - in this case it would move the position of equilibrium to the left, not the right. In cracking we want maximum amount of products (ie. the right) - therefore surely a low pressure would shift the equilibrium that way.

The reaction is not dependent on molecules colliding - merely enough thermal energy has to be provided to enough molecules to cause them to split - homolytic fission - and become free radicals which react to form smaller molecules. Therefore a pressure requirement to increase the collisions doesn't seem necessary.

Why then do we use high pressure when cracking hydrocarbons?

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2 Answers 2

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First of all, cracking is not an equilibrium process, therefore La Chatelier-Brown principle doesn't apply! (irrelevant) Also, the mechanism is much more complicated than this:

  • In gas phase, radical reactions generally have extremely complex reaction networks. E.g. burning of methane with oxygen contains thousands if not more elementary reactions. I am pretty sure that gas phase cracking is not a pure first order reaction.

  • In industry, cracking is generally done on heterogeneous catalyst, therefore not a gas phase reaction. Heterogeneous catalysis is pretty much controlled by the absorbed amount of molecules on the catalyst surface: high pressure, faster reactions.

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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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I think its worthwhile defining what coking is. –  Ari Ben Canaan Apr 15 '14 at 16:09
I suspect there may, in fact, be a good answer to this question hidden in your first paragraph above, and perhaps the second one too, if you just unpacked it enough so that even people who are not industrial chemists could understand it. As for the last paragraph, whether valid or not, it doesn't seem to have any relevance to the question (or to the topic of this site) whatsoever. –  Ilmari Karonen Apr 15 '14 at 17:34
Chair parade. Given the answer, look up its sources. Pyrolyzing organics absent air produces coke (graphite) as by-product. This wastes carbon in light streams and ruins catalysts. Low value still bottoms are terminally pyrolyzed to recover lighter ends plus coke. Part (most?) of the National Petroleum Reserve is refinery waste purchased as Saudi light crude then disappeared in Louisiana solution-mined salt domes, re third paragraph. –  Uncle Al Apr 15 '14 at 18:24

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