Vacuum distillation is a necessary process because the outlet temperatures of the atmospheric pressure distillation are so high that thermal would begin the break down the crude oil. In order to continue distilling the heaviest crude fractions of the atmospheric distillation process, the pressure must be reduced to 25 to 40mmHg. Cracking, the breaking of the chemical bonds between carbon atoms would cause coking on the metal surfaces in the column, which interferes with distillation.

Watson Characterization is used to determine an upper temperature limit for the vacuum distillation process, which avoids coking. If severe coking occurs it can plug the flow of the distillation column, which would lead to the shutting down of the entire refinery. By comparing the Watson characterization factor (Kw) to temperatures, which relate to coking propensity, a safe temperature can be determined. Bands of temperatures show where coke formation is negligible as well as uncertain. Generally the temperature that is chosen is lower than the lower temperature line of the band.