The different process units used in refineries would include the following:

• Desalter unit: Washes out salt from the crude oil before it enters the atmospheric distillation unit.
• Crude oil distillation unit: Distills the incoming crude oil into various fractions for further processing in other units.
• Vacuum distillation unit: Further distills the residue oil from the bottom of the crude oil distillation unit. This is performed below atmospheric pressure.
• Naphtha hydrotreater unit: Uses hydrogen to desulfurize naphtha from atmospheric distillation. The naphtha must by hydrotreated before being sent to a catalytic reforming unit.
• Catalytic reforming unit: Converts the desulfurized naphtha molecules into higher-octane molecules to produce reformate, which is a component of the end-product gasoline or petrol. An important byproduct of a reformer is hydrogen released during the catalyst reaction. The hydrogen is used either in the hydrotreaters or the hydrocracker.
• Alkylation unit: Uses sulfuric acid or hydrofluoric acid to produce high-octane components for gasoline blending.
• Isomerization unit: Converts linear molecules to higher-octane branched molecules for blending into gasoline or feeding to alkylation units.
• Distillate hydrotreater unit: Uses hydrogen to desulfurize some of the other distilled fractions from the crude oil distillation unit (such as diesel oil) after the distillation units.
• Merox (mercaptan oxidizer) or similar units: Desulfurize LPG, kerosene or jet fuel by oxidizing undesired mercaptans to organic disulfides.
• Amine gas treater, Claus unit, and tail gas treatment: Convert hydrogen sulfide from hydrodesulfurization into elemental sulfur.
• Fluid Catalytic Cracking (FCC) unit: Upgrades the heavier, higher boiling point fractions from the crude oil distillation by converting them into lighter and lower boiling point, more valuable products such as high-octane gasoline, light fuel oils and olefin-rich light gases (propylene, butylene…).
• Hydrocracker unit: Uses hydrogen to upgrade heavier fractions from the crude oil distillation and the vacuum distillation units into lighter, more valuable products (naphtha, kerosene, diesel…) in the presence of catalyst beds such as fixed, ebullated and slurry beds.
• Visbreaker unit: Upgrades heavy residual oils from the vacuum distillation unit by thermally cracking them into lighter, more valuable, lower viscosity products.
• Delayed coking and fluid coker units: Convert very heavy residual oils into end-product petroleum coke as well as naphtha and diesel oil byproducts.
• Solvent De-asphalting (SDA) unit: Reduces metal and asphaltene contents of heavy oil cuts before sending them to hydrocracking and hydrotreating units, based on liquid-liquid extraction by using paraffinic solvents.