It is too simplistic to attribute the decline to what some call impurities. Those impurities (mainly aromatics, sulfur and nitrogen species) are just variants on the molecular soup, can be controlled and, in the case of sulfur, can actually provide some free secondary antioxidancy. Also, aromatics are an essential component when strong solvency is required, such as in dissolving additive packages.

On the other hand, trace nitrogen-containing molecules in Group I are a significant performance limiter, providing no benefits and promoting both oxidation and lacquer formation in engines. In contrast, nitrogen containing molecules are essentially absent in Group II oils.

There has been a kind of pincer movement conspiring against Group I, one part performance limitations, the other part manufacturing technology advances. The main performance limitation in crankcase lubes was identified during the development of the API CG-4 heavy-duty diesel additive technologies in the mid-1990s. This was the first heavy-duty category to have a really severe soot dispersancy engine test, the Mack T-8. It soon became evident that this test responded mainly to the aromatics content of the base stock. The higher the aromatics, the worse the performance.

Soot dispersancy was not really an issue for Group II or Group III base oils because they normally only trace levels of aromatics, about 1 percent weight. For Group I base stocks, aromatics can vary from about 20 to 30 percent weight, depending on crude source, grade and processing severity.

The technical problem is that small aromatic molecules compete with the larger ashless dispersant for the polar soot surface and, hence, prevent the dispersant from properly stabilizing soot particle by aggregation. As later heavy-duty diesel engine categories were developed and the associated soot dispersancy engine tests became even more severe (e.g. Mack T-11), Group I base stocks were progressively marginalized and had to be used in mixtures with Group II to get the aromatics balance right. Eventually, they were completely excluded from the latest diesel engine lubricant formulations. With heavy-duty diesel lubricants being such relatively large consumers of base stocks, this was a significant market to lose.

On top of the aromatics issue came the evolution of chemical limitations on all crankcase lubricants, especially sulfur limits. At a time when sulfur was being constrained overall, developers of crankcase oil additive packages had first call on sulfur levels for their antiwear and antioxidant components, leaving little for the base oil.

At that time, Group I was by far the most significant paraffinic base stock in terms of production volume. Group II production was only one-half as large as Group I and was almost exclusively located in United States. Even today, there is no meaningful Group II production in Europe. All recent Group II capacity additions have been in the rapidly developing economies of the Far East and Asia.

For new capacity additions, it was inevitable that Group I technology would not be the plant technology of choice, not only because of the com-position and performance limitations noted above, but also because Group I production cannot easily be scaled up in the same way as Group II and III plants. Where we see large Group I plants (> 500,000 tons per year), it is usually as parallel production trains rather than as more cost-effective large single trains.

Far more manual intervention is required for Group I production, especially in the dewaxing units. And feedstock choice is also an issue for Group I production because all of the final base oil molecules must be in the crude from the beginning; no chemical interconversion is possible as with Group II and III. Thus, so-called Group I lube crudes limit crude procurement flexibility in refineries and, hence, add to feedstock costs.

So why are Group I stocks still with us? After all, only recently has Group II approached Group I in terms of global high-viscosity index paraffinic base oil production – forecast to dominate by 2016. (See Lubes n Greases, January 2012.) Perhaps the simplest explanation for the continued presence of Group I base stocks is because theyre there.

If we were starting with the proverbial clean sheet of paper with all current technology options at our disposal, there would not be a good business case for a Group I plant. However, where they do exist and the capital investment was written down years ago, there is still a good financial case for maintaining plants to supply the markets they can still serve – process oils, additives manufacturing, less demanding crankcase lubricants and, of course, wax manufacturing. Markets even include diesel engine oils for areas where significant volumes of API CF performance levels are still used and where aromatics dont seem to be much of a worry.

And dont forget bright stocks. Along with naphthenics, Group I oils have a niche here that Group II cannot routinely serve. Not all Group I plants will close; some may just evolve into bright stock trains.