Conversion process units are used to convert one hydrocarbon stream into another by changing molecule size and structure. The objective is to shift the yield of the refinery away from less valuable products (eg residual fuel oil, LPG) and towards more valuable ones (eg gasoline, diesel).
They do this by breaking large molecules into smaller ones (cracking), joining small molecules into larger ones, or changing the shape of molecules without changing overall size.
A number of different types of conversion units can be found in refineries. The choice the refiner has made of which ones to have often in their refinery usually depends on the nature of the crude feedstock available and the highest demand (and value) products in their local market:
FCC - Cracks VGO to form a mix of lighter products, especially gasoline
RCC - Cracks atmospheric resid to form a mix of lighter products, especially gasoline
Coker - Cracks vacuum resid to form a mix of lighter products, VGO and petcoke
Hydrocracker - Cracks VGO while injecting hydrogen to form a mix of lighter products, favoring saturated hydrocarbon products, such as diesel
Resid hydrocracker - Cracks vacuum resid while injecting hydrogen to form a mix of lighter products, also favoring diesel
Reformer - Changes the moleculer structure of heavy naphtha through creating more aromatics, naphthenic rings and isomers to raise octane for blending into gasoline
Alkylation - Combines a light olefin (butylene, propylene) with isobutane to form a high-quality gasoline blend stock
Dimer or poly - Combines two light olefins to form a high quality gasoline blend stock
C4 isomerization - Changes to molecular structure of butane to make iso-butane
C5/C6 isomerization - Changes the molecular structure of light naphtha to increase the isomers and raise its octane for gasoline blending
visbreaker - Cracks vacuum resid to reduce its viscosity for fuel oil blending
Thermal cracker - Cracks VGO to form a mix of lighter products and reduce the viscosity of the VGO