In petroleum refining, the Crude Distillation Unit (CDU) (often referred to as the Atmospheric Distillation Unit) is usually the first processing equipment through which crude oil is fed. Once in the CDU, crude oil is distilled into various products, like naphtha, kerosene, and diesel, that then serve as feedstocks for all other processing units at the refinery.^[1,2]

As oil is being fed into the CDU, the first thing that happens is the crude is heated to a temperature between 215 and 280°F (100 -137°C).^[2] This allows salts, which can be harmful to some equipment, to be removed at the desalter.^[1,2] The now desalted crude continues through the system into the heater where it is further heated to a temperature of over 650°F (350°C).^[1,2] Next, it is fed into the atmospheric column where the vapors and liquids separate.^[2,3] Residues are stripped out at the bottom of the column. The products (naphtha, kerosene, diesel and gas oil) are taken from the side of the column and moved through the refinery for further processing.^[2,3]

References

1. https://en.wikipedia.org/wiki/Petroleum_refining_processes#The_crude_oil_distillation_unit
2. http://www.ou.edu/class/che-design/che5480-07/Petroleum%20Fractionation-Overview.pdf
3. http://www.refiningnz.com/visitors--learning/classroom--learning-resources/learning-centre/how-it-works---the-refining-process/first-step-distillation.aspx

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