An industrial process plant where crude oil is converted into useful products such as naphtha, diesel fuel, kerosene, and LPG. Also known as a petroleum refinery, the process involves the separation of the crude oil into fractions in the process of fractional distillation. By boiling the crude oil, the light or more volatile components with the lowest boiling point rise towards the top of the column, whereas the heavy fractions with the highest boiling points remain at the bottom. The heavy bottom fractions are then thermally cracked to form more useful light products. All the fractions are then processed further in other parts of the oil refinery, which may typically feature vacuum distillation used to distill the bottoms; hydrotreating, which is used to remove sulphur from naphtha; catalytic reforming; fluid catalytic cracking; hydrocracking; visbreaking; isomerization; steam reforming; alkylation; hydrodesulphurization; and the Claus process used to convert hydrogen sulphide into sulphur, solvent dewaxing, and water treatment.