Vacuum distillation is needed when the temperatures required for atmospheric distillation of the heavier crude oils are so high that thermal coking would be possible. Thermal coking in a distillation column has the negative effects of loss of product, plugging of the furnace pipes with coke, and other equipment damage. In order to prevent this problem, the mixture is distilled under a vacuum since the boiling point temperature decreases with the decrease of the pressure in the process. The absolute pressure in the column is set from 25 to 40 mmHg. In order to increase vaporization, steam is added to lower the pressure to less than 10 mmHg. Steam addition also benefits the process by increasing the tube velocity in the furnace, which minimizes the formation of coke and also decreases the partial pressure of the mixture in the column. In order to minimize the pressure difference between the top and bottom of the column, packing is added to increase the contact between the liquid and vapor so that more fractionation occurs.

The Watson Characterization factor (Kw) can be used to determine whether vacuum distillation is needed and at what temperatures it should be implemented at in order to control thermal coking in the process. Kw is calculated using the physical properties of the crude oil. This factor could determine the temperatures for which above them a significant amount of coking production would occur. The Watson Characterization factor also can determine if the mixture is paraffinic, naphthenic, or aromatic, which also gives insight into what temperatures will need to be used. This allows the operators to know what temperatures they need the run the column at to prevent harmful thermal coking.

References

1.)    Gary, James H., Handwerk, Glenn E., Kaiser, Mark J. Petroleum Refining Technology and Economics. Taylor and Francis Group. Florida. 2007. Print.