cat cracker

[miss sixty]

why are small molecules sometimes put into a catalytic cracker?

[pedant]

Presumably you are talking the oil industry. Most commercial catalytic crackers use catalysts that are fairly indiscriminate what they will crack. So a fairly short molecule will get cracked into smaller molecules. There are 2 main reasons why this is done. Firstly certain crude oils are richer in some cuts (LPG, naphtha, kerosene, lube, wax, asphalt, resin etc) than others, the excess may be put through the cracker. Secondly a particular cut of crude may produce a base stock that is not as commercially attractive as chucking it through a cracker to make a greater volume of a lighter cut. After organic leads were removed from gasoline there was an increase in the amount of shorter chain molecules that were redirected though catalytic crackers to make high octane value components that could used to boost the RON of gasoline rather than having to rely on the comparatively expensive and rather unpleasant alternative MTBE.