Refining of crude oil and its products

Refining separates crude petroleum, a mixture of many hydrocarbons, into useful products and then upgrades them to meet quality requirements.

Steps of refining

1. Desalting and heating: water and salts are removed, and the crude is heated to about 350–400 °C in a furnace.
2. Fractional distillation: the hot vapour-liquid mixture enters an atmospheric column. Vapours rise through bubble-cap trays; lighter components with low boiling points condense higher up, and heavier components condense lower down.
3. Vacuum distillation of the bottom residue separates heavy gas oils and lubricating oil without cracking them.
4. Conversion and treatment: cracking, reforming, sweetening and blending.

   Gas (C1-C4)     <-- top, below 30 C
  ---------------
   Gasoline        <-- 40-200 C
  ---------------
   Kerosene        <-- 150-250 C
  ---------------
   Diesel/gas oil  <-- 250-350 C
  ---------------
   Fuel oil        <-- 350-450 C
  ---------------
  Crude --> [furnace] --> enters here
   Residue/bitumen <-- bottom

Main fractions

Cracking

Cracking breaks the large molecules of heavy oil into smaller, more valuable ones such as gasoline. Thermal cracking uses 450–750 °C and high pressure; catalytic cracking uses a zeolite catalyst at about 500 °C and gives gasoline of higher octane number.

Reforming

Reforming uses a platinum catalyst to rearrange the molecules of low-octane naphtha into branched and aromatic compounds, raising the octane number of gasoline.