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Vacuum distillation is the distillation of liquids performed at a pressure lower than atmospheric pressure to take advantage of the fact that reducing the pressure lowers the boiling point of liquids.

Petroleum crude oil is a complex mixture of hundreds of different hydrocarbon compounds generally having from 3 to 60 carbon atoms per molecule, although there may be small amounts of hydrocarbons outside that range.[1][2][3]

The refining of petroleum crude oil begins with distilling the incoming crude oil in a so-called atmospheric crude oil distillation unit (CDU) operating at pressures slightly above atmospheric pressure.[1][2][4] In distilling the crude oil, it is important not to subject the crude oil to temperatures above 370 to 380 °C because the high molecular weight components in the crude oil will undergo thermal cracking and form petroleum coke at temperatures above that.

The formation of coke is very undesirable and would result in plugging the tubes in the furnace that heats the feed stream to the crude oil distillation column. Plugging would also occur in the piping from the furnace to the distillation column as well as in the column itself.

The constraint imposed by limiting the column inlet crude oil to a temperature of less than than 370 to 380 °C yields a residual oil from the bottom of the atmospheric distillation column consisting entirely of hydrocarbons that boil above 370 to 380 °C.

To further distill the residual oil from the crude oil distillation unit, the distillation is performed at absolute pressures as low as 10 to 40 mmHg (also referred to as torr) so as to limit the operating temperature to less than 370 to 380 °C.

Figure 1 is a photograph of a large vacuum distillation column in a petroleum refinery and Figure 2 is a schematic process flow diagram of a petroleum refinery vacuum distillation column that depicts the internals of the column.
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The 10 to 40 mmHg absolute pressure in a vacuum distillation column increases the volume of vapor formed per volume of liquid distilled. The result is that such columns have very large diameters.[5] Vacuum distillation columns, such those in Images 1 and 2, may have diameters of 15 meters or more, heights ranging up to about 50 meters, and feed rates ranging up to about 25,400 cubic meters per day (160,000 barrels per day).

The vacuum distillation column internals must provide good vapor-liquid contacting while, at the same time, maintaining a very low pressure increase from the column top to the column bottom. Therefore, refinery vacuum columns often use distillation trays only where withdrawing products from the side of the column (referred to as side draws). The remainder of the column uses packing material for the vapor-liquid contacting because such packing has a lower pressure drop than distillation trays. This packing material can be either structured sheet metal or randomly dumped packing such as Raschig rings.

The absolute pressure of 10 to 40 mmHg in a refinery vacuum distillation column is most often achieved by a vacuum system using multiple stages of steam jet ejectors.[6][7]

Many industries, other than the petroleum refining industry, use vacuum distillation on a much smaller scale.

References


  1. J.H. Gary and G.E. Handwerk (1984), Petroleum Refining Technology and Economics, 2nd Edition, Marcel Dekker, Inc. ISBN 0-8247-7150-8.
  2. W.L. Leffler (1985), Petroleum refining for the nontechnical person, 2nd Edition, PennWell Books. ISBN 0-87814-280-0.
  3. James G. Speight (2006), The Chemistry and Technology of Petroleum, Fourth Edition, CRC Press. 0-8493-9067-2.
  4. Henry Z. Kister (1992), Distillation Design, 1st Edition, McGraw-Hill. ISBN 0-07-034909-6.
  5. Karl Kolmetz, Andrew W. Sloley et al (2004), "Designing Distillation Columns for Vacuum Service", 11th India Oil and Gas Symposium and International Exhibition, Mumbai, India (also published in Hydrocarbon Processing, May 2005).
  6. Robert B. Power (1993), Steam Jet Ejectors For The Process Industries, First Edition, McGraw-Hill. ISBN 0-07-050618-3.
  7. Editor:Richard Darton (1997), Distillation and Absorption, Volume 2, First Edition, page 594, Institution of Chemical Engineers, United Kingdom. ISBN 0-85295-393-3.