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Petroleum Processing

Selecting the Right Temperature

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Selecting the Right Temperature

Selecting the right temperature in the vacuum distillation column is critical to control the risk of coking in the column. The Watson Characterization Factor (Kw) may be used to estimate the upper temperature limit for vacuum distillation to avoid coking. Figure 4.6 shows an empirical correlation between Kw and the temperatures above which significant thermal decomposition could take place. This region is labeled as the decomposition zone in Figure 4.6. Because of the complex variability in crude oil  composition as it relates to coking propensity, one could draw a band of temperatures below which coking risk is negligible, and the area within the band represents uncertainty in terms of the probability of coking. To be on the safe side, the temperature in the column should be lower than the lower temperature line of the band. Figure 4.6 also shows that crudes with high Kw (paraffinic) should be heated to lower temperatures in the column than crudes with lower Kw (less paraffinic). Hydrocarbon composition is closely related to thermal reactivity, since paraffins could be more readily cracked than naphthenes, whereas aromatic compounds are the most stable hydrocarbons. To sum up, vacuum distillation temperatures should be selected with particular care for paraffinic crudes because of the relative ease of cracking of paraffins that leads to the formation of coke on surfaces.

Graph with temp on the y-axis and the Watson k factor on the x-axis. Shows range of viable temperatures. Described above
Figure 4.6. Use of KW factor to select the operating temperatures in VDU.
Credit: Dr. Semih Eser © Penn State is licensed under CC BY-NC-SA 4.0

Severe cases of coking can plug the flow paths in the distillation column and require shutting down the unit. Shutting down a distillation unit would be catastrophic, as it would require shutting down the whole refinery that must run around the clock except for the scheduled maintenance period.

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Question 1: Why would shutting down a refinery be a catastrophic event?

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ANSWER: Because of the very large crude throughput in refineries reaching, for example, as high as 560,640 bbl/d (89,135 m3/d) for the Baytown Refinery in Texas. It would be impossible to have large enough additional storage capacity in a refinery to hold the atmospheric distillation residue, if the VDU is shut down.