Vacuum Distillation Units process the fuel’s heavy distillates after settling on the bottom of the Atmospheric Distillation Unit and not vaporizing like the lighter fractions. Atmospheric Distillation is used for separation of light and heavy crudes. The heavy distillates require a vacuum because of how temperature and pressure affect molecules. Creating a vacuum chamber will greatly lower the pressure and uses Ideal Gas Law, one can use the correlation with a pressure drop then the required temperature will drop as well. This is desired because high temperatures will cause vibrations and allow for thermal cracking or destruction of bonds within a given molecule carbon chain.  Cracking the molecules would lead to coking on the metal surface of the distillation column and interfere with fractionation in distillation.

The Watson Characterization Factor gives refineries an idea of the range of temperatures for vacuum distillation to help avoid coking. A graph is comprised of temperature vs. Watson K factor and a band is used for plotting this range on the graph. In most cases, the refineries will operate below the lower end temperature to avoid as much accumulation of carbonaceous material as possible.  Higher K factors are at lower temperatures (paraffins), which makes sense because paraffins are easily cracked. In comparison to aromatics, such as benzene, which are stabilized molecules and require higher temperatures to dissociate the molecules bonds.