<P> The crude oil distillation unit (CDU) is the first processing unit in virtually all petroleum refineries . The CDU distills the incoming crude oil into various fractions of different boiling ranges, each of which are then processed further in the other refinery processing units . The CDU is often referred to as the atmospheric distillation unit because it operates at slightly above atmospheric pressure . </P> <P> Below is a schematic flow diagram of a typical crude oil distillation unit . The incoming crude oil is preheated by exchanging heat with some of the hot, distilled fractions and other streams . It is then desalted to remove inorganic salts (primarily sodium chloride). </P> <P> Following the desalter, the crude oil is further heated by exchanging heat with some of the hot, distilled fractions and other streams . It is then heated in a fuel - fired furnace (fired heater) to a temperature of about 398 ° C and routed into the bottom of the distillation unit . </P> <P> The cooling and condensing of the distillation tower overhead is provided partially by exchanging heat with the incoming crude oil and partially by either an air - cooled or water - cooled condenser . Additional heat is removed from the distillation column by a pumparound system as shown in the diagram below . </P>

When was crude oil first broken into fractions