Technology Type
- Type
- Crude Vacuum Distillation
- Process
- Crude Oil Processing
- Abbreviation
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Summary
Vacuum Distillation is a part of the refining process that helps to produce petroleum products out of the heavier oils left over from atmospheric distillation. As the name Vacuum Distillation implies, the distillation column is under a vacuum, or significantly less than atmospheric pressure of 760 millimeters of Mercury (mmHg). At low pressures, the boiling point of the ADU bottoms (reduced crude) is low enough that lighter products can vaporize without cracking, or degrading, the oil. Vacuum distillation separates the reduced crude into light, medium, and heavy vacuum gas oil streams. These are slightly heavier than middle distillates (such as jet fuel, kerosene, and diesel). All of the streams are further processed in hydrotreating, hydrocracking, or other downstream residue upgrading units to produce more valuable lighter products. This method is used to enhance the recovery of middle distillate products from the reduced crude.
Process Parameters
The vacuum distillation unit works on the principle that the residuum feed boils and separates under vacuum at a lower temperature without thermal cracking or degradation of the original hydrocarbons present in crude oil. Without a vacuum for gas oil separations much higher temperature would be required that would crack the hydrocarbon materials.
Hot atmospheric residue from the bottom of the atmospheric distillation column is pumped through a series of heat exchangers and preheated by pump-around and hot products of the vacuum distillation unit. The residue can also be taken from storage but its temperature must be above 150°C to avoid line blockage. The preheated feed is then passed through a heater for final heating to 370~420°C. To minimize cracking and coking, steam is also injected into the heater tube and passes at a controlled rate. The feed then enters the vacuum tower and flashes at the lower part of the column with multiple trays under vacuum conditions. A vacuum of 25–40 mm of mercury is maintained at the top of the column and 90–120 mm at the bottom, depending upon the design.
The vacuum distillation column has packings for fractionation and heat exchange zones. This reduces the pressure drop in the column which is necessary for a low vacuum in the lower section of the column. The bottom zone of the vacuum column is equipped with valve trays. The vapors from the flash zone pass through a fractionation zone where the heavy ends are condensed with HVGO reflux. More up in the column, sections are split up by sprays of liquid from the pump-around or the internal reflux streams.The vacuum is applied and maintained by steam ejectors by using medium pressure superheated steam as the motive fluid. The ejectors along with medium pressure steam entrain the hydrocarbon vapors from the tower top, which are condensed in cooling water condensers. Usually, three ejectors are used that send the un-condensed vapors for first condensation stage to a receiver. Un-condensed fraction from first-stage receiver enters the second-stage condenser, and uncondensed vapors from second condensation stage receiver then enters third condensation stage from which after condensation the uncondensed vapors from the third-stage receiver are finally vented out through a flare, a stack, or incinerated in a fired heater.
The condensed vapors and steam from all three condensation stage receivers are collected in a separator drum, where water and hydrocarbons are separated. Water is further sent for treatment in the sour water treatment section and light hydrocarbons are recycled as a light slop.
A portion of the hot vacuum distillate is drawn from the column and returned back after cooling to control the heat load of the column. This stream is called the pump-around which is the only stream compared to circulating refluxes in the crude distillation column.
Products from the Vacuum Distillation Process
Normally, 2 to 5 types of products are received from the vacuum distillation column. But mainly three products - light, intermediate, and heavy vacuum gas oils are obtained from the vacuum column. A few plates below the top plate of the column, light gas oil (LVGO) are drawn. The next oils drawn below the light gas oil tray are intermediate and heavy gas oils as shown in the PFD of the vacuum distillation unit. The products of VDU are further processed to convert them into more desired costly products. The vacuum distillation unit products and their uses are as follows:
Vacuum Products Typical Uses - Light Vacuum Gas Oil (LVGO)
- Intermediate Vacuum Gas Oil (IVGO)
- Heavy Vacuum Gas Oil (HVGO)
Hydrotreating, Hydrocracking feed, Fuel Oil Blending
Hydrocracking, De-asphalting unit, Lube oil production
Hydrocracking, lube oil production, De-asphalting unit, Visbreaking for fuel oil productionThe bottom residue from the vacuum distillation tower is called the short residue (SR) or vacuum residue, which is stripped by the bottom steam followed by cooling through a steam generator. The VDU bottom or vacuum residue is sent to fuel oil blending, to the deasphalting unit or further treated by thermal cracking or catalytic cracking by applying any of the bottom-of-the-barrel upgradation methods.
References
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U.S. Energy Information Administration, 10th Dec 2012, Vacuum distillation is a key part of the petroleum refining process.
- Nasir Hussain, 7th Apr 2021, The Petrosolutions, Vacuum Distillation Unit in Oil Refinery.
- Link
System Info
- Updated by
-  Kokel, Nicolas
- Updated
- 12/8/2024 5:13 PM
- Added
- 11/10/2021 1:00 PM
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