Vacuum distillation is necessary to separate the heavier components of crude. This is due to the temperature limit imposed in order to avoid cracking of hydrocarbons, an desirable precursor to accumulation of carbonaceous solids (coking) in the distillation column. The Vacuum Distillation Unit receives the residue of the Atmospheric Distillation Unit and operates between 10 to 30 mmHg. The presence of a vacuum essentially lowers the vapor pressure of the atmospheric residue, allowing fractional distillation of heavy distillates while still preventing cracking. The Watson Characterization Factor provides an upper temperature limit for vacuum distillation, but actual temperatures used are typically lower in order to avoid uncertainties and risks of coking. Higher values of the Watson Characterization Factor require lower temperatures and lower values of the characterization factor allow for higher temperatures. This is due to the nature of the hydrocarbons associated with the Watson Characterization Factor – values less that 10 indicate a highly aromatic composition, high stability, and a lower tendency to crack; values between 10 to 11 indicate a napthenic crude with moderate stability and moderate tendency to crack; and values ranging from 11 to 12.9 indicate a more paraffinic composition, which is relatively easier to crack and has the lowest stability among the three.