In every refinery there are two distillation processes, the first process goes through the Crude Distillation Unit (CDU) followed by the Vacuum Distillation Unit (VDU). The CDU uses flash characteristics to separate the crude into a vapor and liquid stream. The liquid stream is then introduced to steam to further separate it from the low boiling distillates and the heavier components. The reheated 730-850⁰F heavier components are then further distilled in the VDU to prevent cracking which causes coke to form on the metal distillation columns. Normally, distillation happens at 25 to 40 mmHg but to improve the vaporization the pressure is lowered to 10mmHg or less in the VDU. In the VDU, steam and high tube velocities are used to minimize thermal cracking and subsequently the formation of coke. The VDU can be characterized in three different operations; dry, wet, and damp. Dry operation does not use steam and has the highest furnace outlet temperatures, Wet operation use steam and has the lowest furnace outlet temperature, and damp operation is the combination of the two. [1]
Since the overall goal of the VDU is to prevent coke formation during crude distillation, the Watson Characterization Factor is used to estimate these temperature limits. The temperature limits are usually set to where the temperature of the crude is lower than the lowest temperature that initializes the decomposition zone. Another use of the Watson Characteristic Factor in the VDU process is that it characterizes the thermal reactivity of the crude. Crude with a high Kw factor is highly paraffinic which stresses a lower distillation temperature. Whereas, crude with a lower Kw factor is less likely to thermally crack and produce coke and can withstand a higher distillation temperature.
Sources:
1. Gary, J. H., & Handwerk, G. E. (2007). Petroleum refining: technology and economics. New York: M. Dekker.