Before the crude can enter the vacuum distillation unit (VDU), it must first be introduced into the Crude Distillation Unit (CDU) flash zone in order to be separated into vapor and liquid streams. Separation of light and heavy crudes is achieved through Atmospheric Distillation (CDU). Vapor fractions rise to the top of the column while the liquid fraction can be sent to VDU after it is introduced to stripping by steam, in order to recover the components dissolved in the heavier liquid that has settled at the bottom of the CDU. This vacuum chamber is vital since there must be a pressure drop which ultimately results in a decrease in temperature. High temperatures are unwanted since this would bring about cracking and inevitably coking on the column’s metal surfaces, interfering with fractionation.

To control the risk of coking, the ideal vacuum distillation temperature in the vacuum distillation column can be found using the Watson Characterization Factor (Kw). This will calculate the upper bound temperature limit for vacuum distillation to avoid coking. Below this range of temperatures, coking risk would be considered negligible while the upper bound represents uncertainty of the probability of coking. To be sure, the distillation temperature would be set lower than this temperature band. One should also take into account the hydrocarbon composition of the crude as paraffinic crudes are more susceptible to coking.