Atmospheric Distillation (CDU) separates the heavy and light crude In Atmospheric Distillation (CDU), it process with a network of heater to heat the crude to about 550 F. Then in the further process the temperature will be raised to about 750 F in furnace. In flash zone, the vapor fraction would go toward the top of the column and the liquid fraction would be send to vacuum distillation (VDU).  The vacuum distillation (VDU) is needed due to the unwanted high temperature. Such high temperature will cause cracking and coking on the surface. In order to lower the temperature, a vacuum chamber is designed to lower the pressure which to lower the temperature.

From the temperature vs Watson Characterization Factor plot that post on class website, it provides us a range of temperature which helps to avoid coking.  The risk of coking is negligible when the operation temperature is low than the band of temperature that showed on plot. However, when it operates under the temperature that is within the area of band, it is possible to cause coking during the process. From the plot, we can also see that higher Watson K factor leads to lower temperature band. Refer to what we learned in lesson 2, Watson K factor is calculate depend on physical properties. Paraffinic has higher Watson K factor than naphthenic and aromatic. It means paraffinic is easier to cause coking than naphthenic and aromatic at high temperature.