How does crude oil distillation work?This is accomplished in oil refineries by a distillation process: the crude oil is first heated in a tubular furnace at atmospheric pressure to about 370°C, causing it to evaporate. The crude oil vapour is transferred to a distillation column (the ‘rectification column'), in which the vapour rises and cools. Various hydrocarbons then condense at different levels according to their boiling points, and may be drawn off separately.
Temperatures of 400°C are not exceeded in atmospheric distillation processes, as otherwise the crude oil components would degrade (‘crack'). However, at these temperatures complete separation cannot be achieved – a mixture of high boiling point hydrocarbons remains at the base of the column, the so-called atmospheric residue.
This residual liquid is subjected to a second distillation under vacuum. At reduced pressure (rough vacuum 10 to 50 mbar) the boiling point of the mixture is lower, allowing further separation at only slightly higher temperatures. The process allows the following valuable products to be separated:
Vacuum gas oil, wax distillates, and vacuum residue as an intermediate product for further processing.