cracking

1. A general term covering any of a variety of reactions and processes that convert high boiling fractions to more valuable lower boiling fractions.

2. A secondary refining process that uses heat and/or a catalyst to break down large chemical components into smaller, lighter ones which can be used as blending components for fuels.

3. Process carried out in a refinery reactor in which the large molecules in the charge stock are broken up into smaller, lower-boiling, stable hydrocarbon molecules, which leave the vessel overhead as unfinished cracked gasoline, kerosines, and gas oils. At the same time, certain of the unstable or reactive molecules in the charge stock combine to form tar or coke bottoms. The cracking reaction may be carried out with heat and pressure (thermal cracking) or in the presence of a catalyst (catalytic cracking).