Vacuum distillation is the distillation of crude oil at a pressure lower than atmospheric pressure because reducing the pressure lowers the boiling point of the crude oil. Furthermore, the vacuum is also used to separate the heavier portion of the crude oil into fractions because the extremely high temperatures that are necessary to vaporize the topped crude at atmospheric pressure would cause thermal cracking to occur. Other disadvantages that would be avoided include the loss due to dry gas, discoloration of the product, and equipment damage as a result of coke formation. Also, addition of steam to the process increases the furnace tube velocity and reduces coke formation in the furnace as well. It also decreases the total hydrocarbon partial pressure in the vacuum tower.

Selecting the vacuum distillation temperature is crucial for the process to control the coking in the system. The Watson Characterization Factor (Kw) is a value derived from the physical properties of the crude oil used to classify it. The Kw for the crude oil may be used to approximate the upper temperature limit for vacuum distillation that would cause coking. The empirical correlation between Kw and the temperatures above is known as the decomposition zone. Thus, a band of temperatures maybe determined whereby there maybe a possibility of coking occurring. For a temperature below such a band, there is negligible coking. In addition, vacuum distillation temperatures should be selected with particular consideration regarding the composition of the crude oil. Crude oils with high Kw are highly paraffinic and are heated using lower temperatures to avoid thermal cracking. However, higher temperatures maybe used for crude oil with lower Kw factors, such as naphthenes and aromatics, because they are more stable.

References:

Gary, James H., Glenn E. Handwerk, and Mark J. Kaiser. “4 Crude Distillation.” Petroleum Refining: Technology and Economics. Boca Raton: CRC, 2007. Print.

Websites:

http://en.wikipedia.org/wiki/Vacuum_distillation

http://en.citizendium.org/wiki/Vacuum_distillation

https://www.e-education.psu.edu/fsc432/content/selecting-right-temperature