Refining is a process in which crude oil or other fats and oils are distilled without altering the molecular structure, and refineries process crude oil through thermal cracking and produce liquid fuels of different densities, characteristics and functions. It generally refers to petroleum refining, that is, the cracking of crude oil, etc., into fuels such as kerosene, gasoline, diesel, heavy oil, etc., which are compatible with the use of internal combustion engines, and to the production of chemical feedstocks such as olefins and aromatics, etc., and the reprocessing of refining residues, depending on their type, actual needs, and local norms.