In the refining process, crude oil can be heated to separate its many constituent hydrocarbon molecules. More volatile molecules, such as gasoline, boil at lower temperatures, while less volatile ones boil at higher temperatures. Base oils initial boiling point is around 350 to 370 degrees Celsius (known as the true boiling point, or TBP), so anything in the crude feedstock that boils below this temperature has to be removed first. This is done by distillation at atmospheric pressure, which takes off light end fractions up to and including diesel. These light ends get routed to the fuels pool to produce gasoline, jet fuel, heating oils and diesel.

In order to define the various cuts of base oil at certain viscosity grades, this remaining heavy fraction – referred to as atmospheric residue or long residue – is taken to a vacuum distillation tower where distillate cuts are created.

The vacuum, or at least severely reduced pressure, reduces the TBPs of long residue components so that thermal cracking (heating to a temperature high enough to break molecular bonds) will not occur at this separation stage. Cracking would happen if distillation of long residue without a vacuum were to be attempted. Reduced pressure has a secondary benefit, compared with atmospheric distillation, in that it improves the resolution, or separation efficiency, of the distillates.

The vacuum in the tower is usually created by multiple steam ejectors that exploit the Venturi effect, whereby fluid pressure reduces when it flows through a constricted section of a pipe, similar to a carburettor.

Generally, a vacuum tower will contain a packing material, such as small metal rings, which increase separation efficiency by giving more surface area for vapours to condense on. Metal trays are located at off-take points in the vacuum tower where a given viscosity grade is being run down.

The fractions created in the tower, depending on base oil grade requirements, have a TBP of around 370 to 550 C. In reality, those fractions will have been separated at much lower real temperatures because of the vacuum distillation effect.

TBP is a convenient oil refinery expression of a fractions full range of boiling points in a standard atmospheric pressure format. Base oils can be separated up to about 550 C TBP as the limit of so-called distillate base oils. Often, the totality of the 350 to 550 C range is called vacuum gas oil, or VGO. Around 550 C, TBP tends to be the vacuum towers upper temperature separation limit, because it is difficult to create a vacuum strong enough to reduce the heavy tail-end boiling point sufficiently to prevent thermal cracking at higher real temperatures.

However, crude oil does have a significant residual fraction boiling point above 550 C TBP, and this is where our residual base oils, such as brightstock, derive from. Thus, when the VGO has all been removed we have a fraction sometimes referred to as vacuum residue or short residue.

In the short residue fraction, the initial boing point can be set at around the same level as the upper boiling limit of VGO. However, the final boiling point for short residue cannot be set for reasons already given. But this does not matter – the crude itself can be allowed to define the natural final boiling point of the short residue and indeed the brightstocks made from them.

Returning to the distillate base oil cuts, it is at the distillation stage that the volatility (the evaporation loss, also known as the Noack value) of the base oil is set. If no final redistillation is performed at the end of processing – as is often done when, say, hydrocracked API Group III base oils are manufactured – then this initial distillation will be the only opportunity to set the volatility of the base oil cut. For the lighter grades, for example a Group I solvent neutral 125 or a Group III 4 centistoke (kinematic viscosity at 100 C), controlled cut width to set volatility requires a good vacuum distillation tower, with effective internal packing to get a well-defined boiling range. It is possible to improve Noack volatility by narrowing the cut width – in other words, by cutting off some light ends and a complementary amount of heavy end molecules to keep the viscosity grade constant.

In order to precisely trim the boiling range of a base stock to a specified Noack, the vacuum tower can have side strippers attached to take of a grade as a side stream. These strippers are like mini distillation units containing a few plates that can be used to fine tune the initial and final boiling points of that side stream.

The distillate grades of base oil can be removed from a vacuum tower via trays, with the lightest (low-density) grades from the top of the tower and the heaviest from the bottom. It is normal to have a separation cut – the so-called black oil – between the heaviest distillate grade and the short residue in order to control both the viscosity of the heavy distillate and the initial boing point of the short residue. This black oil influences the eventual characteristics of the brightstock, if that is the intended use of the short residue. Whether the short residue is turned into brightstock or exported from the refinery will depend in part on whether the refinery has a de-asphalter to pre-treat the short residue for brightstock production.

Hopefully, it can be seen where and how production of cuts by vacuum distillation in a base oils lineup can have a very significant impact on final properties.