Oil refining begins with the distillation or fractionation of crude oil into separate hydrocarbon groups. The resultant products are directly related with the characteristics of the processed crude oil. Most of these products of distillation are then sorted into more useful products by changing their physical properties and structures of molecules under the action of cracking, reforming and other conversion processes. These products are sequentially subjected to various processes of purification and separation, such as extraction, Hydrotreating, and the removal of active sulfur to form the final products. While the most basic operations for oil processing are usually limited to atmospheric and vacuum distillation, large refineries carry out the fractionation, conversion, treatment and blending with lubricant, the production of heavy fuel oils and asphalt; they can also produce processing of oil products.
The first refinery, which opened in 1861, produced kerosene by simple atmospheric distillation. By-products were tar and naphtha (crude oil). It was soon discovered that high quality lubricants can be produced by distillation in vacuum