It is well known that a significant number of API Group I base oil plants have closed, mainly triggered by manufacturing economics such as lack of scale, lube crude supply constraints and sulphur or aromatics levels that do not suit current blender requirements. One consequence is that a gap began to appear in the supply of base oils in the kinematic viscosity range between about 12 cSt and 32 cSt (measured at 100 degrees C). The gap has manifested itself as a bright stock premium.
How has this situation arisen and what can be done to mitigate it? Group II base oils, which are the principal replacement for Group Is, tend to be limited to the so-called distillate grades – around 4 to 12 cSt at 100 degrees C. Group I bright stock, a residual grade, can be easily made using the same processing units as Group I distillate base oils. But the units found in a typical Group II plant – particularly the dewaxing units -are less suited to making acceptable bright stocks.
Group II base oils are made by hydroprocessing – in its simplest form a saturation/cracking stage that converts aromatics to naphthenics or paraffinics followed by a dewaxing stage that converts normal paraffins (waxes) to iso-paraffins (flowable base oil molecules). Dewaxing is facilitated by catalysts, most of whichrely on molecularly porous zeolites, into which base oil molecules can diffuse and hence become isomerised to iso-paraffins. However, there is a limit to the size of wax molecules that current zeolites can accommodate, such that some large waxy molecules would get excluded or partially excluded, and not get converted. This would affect the base oils appearance and performance.
Even if the molecular framework of zeolites could be made larger to accommodate large waxy molecules, they would ultimately become mechanically unstable, plus their performance/ selectivity characteristics would change. If non-waxy feedstocks were used, such as naphthenic crudes, then this problem could be circumvented, but plants would be limited in the viscosity index they achieved for residual grades. These are some reasons why its not easy to make Group II bright stock, even though it has been done on a limited basis. As a result, a shift from Group I to Group II capacity has reduced bright stock supply and caused a price premium.
But the heavy grade premium is not limited to Group I bright stocks. If we look at the prices of Group II base oils across the grade structure, we see a premium for heavier grades that makes the Group I bright stock premium seem small. This is largely driven by the lower yields of heavier Group II grades. There are a limited number of heavier distillate molecules, and they can be more difficult to hydrotreat. If the hydrotreating severity is increased too much, they will crack down to lighter base oil grades or even gas oil.
Where is the need for heavy base oils that Group II plants are not producing? Lets assume that this means greater than 12 cSt in terms of kinematic viscosity at 100 degrees (KV100) or greater than ISO 100 for KV40, the more common viscosity measurement for base oils used in industrial lubricants.
Base oils heavier than 12 cSt KV100 can be used in SAE 40 or higher monograde engine oils, especially where the additive package provides little thickening power. Products such as SAE 50 low-speed marine cylinder oils do contain additives that contribute significantly to KV100, but they still require base oils of around 15 cSt KV100.
When bright stock cant be used to thicken, what alternatives might be used? Viscosity modifiers should not be used to thicken monogrades; most original equipment manufacturers just dont like this approach. Polybutylenes or polyisobutylenes are an option, but even these can be used only selectively. Most PIBs are manufactured with a reactive olefinic end-group in order that they can be further functionalised for additive manufacture. This means that lubricants requiring long-term oxidative stability require either fully saturated PIBs, which will have a price premium, or that standard PIBs should be limited to lubricants that dont require long term oxidative stability, such as two-stroke engine oils.
Another alternative is heavy polyalphaolefins such as the PAO 40 and PAO 100 (KV100) which are base stock polymers not limited by crude oil carbon numbers. These have long-term oxidative stability, since they are fully saturated, but also have a price premium which must be considered in the overall lubricant blending cost.
One final option is to migrate some heavy applications to lower viscosity lubricants. We are beginning to see some SAE 40 marine cylinder oils formulated with Group II, for example. SAE 15W-40 multigrades that can be blended with standard Group II grades are largely replacing SAE 20W-50 multigrades, which normally required a bright stock component. So lightening the finished lubricant grade slate could give the dual benefits of fuel economy whilst mitigating the requirements for heavy neutrals or bright stocks.