Written By: Souad LOUSDAD
The next step in the process of crude oil separation is the vacuum distillation of the atmospheric residue that was drawn off from the bottom of the atmospheric column.
Let’s start with some simple questions:
Why is it called vacuum fractioning? What is the main advantage of it? And what is the difference between the atmospheric and the vaccum fractionation?
First of all; vacuum is a space entirely devoid of matter.
In practice, vacuum is referred to when the air pressure in an area goes below atmospheric pressure. It can be created by removing air from a space using a vaccum pump or by reducing the pressure using a fast flow of fluid.
In our case, it is clearly obvious that is called a vaccum distillation because the pressure is below the atmospheric which allows for the heavier materials to distill at lower temperatures than those that would be required at atmospheric pressure.
read also An introduction to Crude Oil Refining
At atmospheric condition of pressure, the flash temperature (in the flash zone) of normal atmospheric residue feed to achieve any meaningful degree of vaporization would be extremely high (say in excess of 900◦F).
At this kind of temperatures, the heavy residue will begin to break up or crack and this forms coke in the extreme and olefinic products which may not be desirable to the whole process of refining. Effective vaporization and fractionation can be achieved however at reduced pressures.
Under this condition a reasonable flash temperature (say 343–399 ◦C) can be easily obtained, and that is the main advantage of the crude oil vacuum distillation.
The main difference between the two types is the lower pressure as compared to normal distillation. Another difference that involves from the lower pressure is the lower temperatures of the boiling points, compared to the atmospheric distillation .
Also, larger diameter columns and simpler internals are used to maintain comparable vapor velocities at reduced operating pressure.
Actually, several designs of vacuum columns are implemented, each with conditions and specifications:
- “Dry” vacuum columns:
They are used without any injection of steam, that’s why are called “dry”.
They need to run at a very low pressure from 10 to 15 mmHg at the top and require the use of a booster ejector from which a high – velocity jet of steam is discharged to produce a continuous flow vacuum for the process equipment.
- “Wet “vacuum columns:
These are carried out with injection of steam in the furnace feed and stripping steam in the bottom of the tower.
The pressure is a bit higher than the previous mentioned columns; it goes from 40 to 60 mmHg at the top.
- “Semi Wet”:
The only steal injected is at the bottom of the column, using essentially a booster ejector.
The choosing of the column depends on whether steam is used to lower the partial hydrocarbon pressure.
The most common type for the crude vacuum distillation is the first one, the Dry vacuum process.
But, how does it go?
Well, the process follows very much the same pattern as the atmospheric distillation.
Normally, the atmospheric residue from the crude distillation unit is routed hot directly to the fired furnace of the vacuum unit heated to approximately 400 °C.
If the atmospheric residue is sent to storage, than the temperature should not be below 150°C in order to control the viscosity necessary for a proper flow.
The absolute operating pressure in a vacuum tower can be reduced to 20 mmHg or less, note that 1 atmosphere of pressure equals 760 mmHg.
Generally, vacuum distillation units handling reduced crude operate at 3-5 mmHg at the top of the column and 25-30 mmHg in the flash zone.
The vacuum is maintained on the tower overhead using steam ejectors and vacuum pumps.
Shortly, how do these ejectors work?
“Ejectors recompress the gases through a nozzle where vapors from the column are sucked by a stream of medium or low pressure steam. The vapor phase at the ejector exit is partially condensed in exchange with cooling water. The liquid phase is then sent to the overhead drum.
The vapor phase goes from the condenser to another ejector-condenser stage.
One pump can replace two or three stages of ejectors in dry or wet type vacuum distillation.
They do not use steam and can significantly reduce hydrocarbon-rich aqueous condensates in a system using ejectors.
But systems with ejectors are much more flexible and rapid to put into operation.”
Now back to our process, neither the vacuum residue that leaves the bottom of the tower in this process nor the side-streams are steam stripped.
The vacuum condition is produced by steam ejectors taking suction from the top of the tower and these ejectors remove inert and other vapor that may exist and pull a vacuum of about 5 mmHg absolute.
Thereafter the distillate vapors are condensed in the tower with the reflux steams moving down the tower in the same way as the side steams in the Atmospheric unit.
The products drawn off at the appropriate sections are cooled either by heat exchangers with colder steams in the atmospheric unit, or in some particular cases as heating mediums to light end reboilers, then pumped to storage for further processes.
Two typical cuts are separated in the vacuum distillation column depending on their boiling point ranges: Light vacuum gas oil or LVGO and Heavy vacuum gas oil or HVGO.
These two products pursue essentially are further converted within further processes such as Fluid Catalytic Cracking and others, all referred to as Conversion Processes.
Heavy hydrocarbons that cannot be boiled even under reduced pressure remain at the bottom of the column and are pumped out as vacuum residue, which can only be used for producing coke in a Coker unit or essentially to produce bitumen.
To sum up the “separation processes”:
Both processes, atmospheric and vacuum distillation aim principally to separate the crude oil into fine cuts that pursue further processes, considering the quality of the crude and the wanted products and following a specific order and, all to avoid any complications that might occur in the refinery.
We end up having different valuable products like LPG, Kerosene, Naphtha, Gas oils, and others…
And despite the names, some of these give you all the luxury you are experiencing nowadays.
Resources:
– Petroleum refining (separation processes).
– Student’s Guide to Refining.
– Refining Crude Oil.
-Design of Distillation Column Control Systems.