The crude oil separates into fractions according to weight and boiling point. The heaviest fractions with the highest boiling points settle at the very bottom. Dark colour Petrol comes off near the top of the column. Fuel oil comes off near the bottom of the column. The fractions are now ready for piping to the next areas within the refinery.
Some fractions require very little additional processing.
However, most molecules require much more processing to become high-value products. This is where a refinery makes money, because the low-value fractions that aren't in great demand can be converted to petrol and other useful chemicals. A cracking unit consists of one or more tall, thick-walled, reactors and a network of furnaces, heat exchangers and other vessels. Some refineries also have cokers, which use heat and moderate pressure to turn the really heavy fractions into lighter products and a hard, coal like substance that is used as an industrial fuel. Cracking and coking are not the only forms of conversion.