Although crude oil contains a vast diversity of hydrocarbons, a comparatively small number of these make up the bulk of demand for oil. Cracking solves this mismatch: most of the demand is for light, short-carbon-chain molecules, so a cracker breaks down long-chain hydrocarbons into lighter, more commercially-valuable chemicals. This is usually done in massive industrial plants, but as [Markus Bindhammer] showed, it’s possible even in a tabletop apparatus.
There are several methods of cracking, but [Markus] used catalytic fluid cracking: a feedstock high in alkanes (hydrocarbons containing fully saturated carbon-carbon bonds) is heated in the presence of a catalyst, whereupon its long alkane chains split to form alkenes (hydrocarbons with a carbon-carbon double bond) with the loss of a hydrogen molecule. In [Markus]’s setup, a heating mantle heated a boiling flask containing paraffin oil and an amorphous silica-alumina catalyst. Vapors from this flask passed through a condenser tube and a bottle of bromine water, then escaped through a flashback arrestor. Bromine reacts far more readily with alkenes than with alkanes, so the disappearance of its characteristic yellow color would visually indicate the production of alkenes.
To avoid unwanted oxidation, [Markus] purged the cracker with argon before using it. While running the cracker, a flammable mixture of light hydrocarbons and hydrogen escaped from the flask of bromine water. The yellow color of bromine disappeared, and two phases formed: one aqueous, and a lighter phase of hydrocarbons and brominated hydrocarbons. The hot side of the reactor did not survive well; the catalyst turned black with coke, and the heating mantel’s cover fused to the boiling flask. However, the reaction undoubtedly succeeded: while a pool of normal paraffin oil wouldn’t ignite, the cracked oil lit easily.
To go the other way, from small molecules to larger hydrocarbon chains, [Markus] has also used the Fischer-Tropsch process.
Source: Blog – Hackaday