The United States was the first region to embrace Group II manufacturing in a big way when it replaced or upgraded small-scale, low-efficiency Group I capacity in the mid-1990s. Today, Europe has significantly increased its Group II production, after largely relying on U.S. and Far East imports.

Group II is associated with hydroprocessing, which is largely the enabler behind its phenomenal growth. Hydroprocessing allows better use of a wide range of crudes by saturating, cracking and isomerizing feedstock components that otherwise would have been by-products in Group I manufacture.

In theory, Group II can be made by solvent extraction of the better lube crudes, but the yields would not be economical. This is especially true today because distillate aromatic extracts, the main by-products of solvent extraction, have severely limited markets, and they make poor cracker feeds. So, it is far better to hydro-convert them to base stock molecules.

Some manufacturers do run hybrid process line-ups for Group II, in which initial aromatics extraction followed by less severe hydroprocessing can be used. This has the advantage of removing some of the polyaromatics up-front that would have to be dealt with by hydrofinishing at the end of the process. This approach can be a cost-effective way to convert a Group I plant to Group II manufacture. A similar hybrid approach was occasionally used in Group I manufacture, again mainly for benefits on feedstock yield.

Many of the benefits of high saturates Group II were discovered in use rather than being predicted, such as their much superior dispersant additive response in heavy-duty diesel applications. This was a significant technical boost, coincidental with their increasing availabilities around the mid to late 1990s. It allowed original equipment manufacturers to push soot management in heavy-duty diesel engines to previously undreamt of levels. Now, soot management, though still important, is not the determinant of diesel crankcase lube performance that it once was, due to progress in diesel emissions control systems.

There is an assumption that because all major Group II base stocks contain around 99 percent saturates, that is the end of the story in terms of quality. As always, though, there is more subtlety to it. Within that 99 percent saturates, there is considerable scope for compositional variation such as the ratio of isoparaffinics to naphthenics. And even within that, naphthenics can be subdivided into one-ring through to fused multiring naphthenics, all with quite different antioxidant response.

Also, even within the approximately 1 percent residual aromatics, appropriate hydrofinishing is essential to knock out most of the polycyclic aromatics to ensure reasonable uninhibited stability of the stocks in the supply chain, especially with respect to light exposure. So, there is significant scope for quality differentiation within Group II offerings.

The chemical conversion nature of the principal Group II hydrocracking process has the benefit of allowing a much broader spectrum of lube crudes for processing, with all the refinery economic benefits that entails. One frequently made point – though not altogether correct now – is that Group II slates do not have brightstocks associated with them. That is true in the sense that few have attempted to make and market Group II brightstocks, but it certainly is technically possible.

Where Group II is made at a dedicated lube hydrocracker site, the refinery may lack a de-asphalter, required to produce the de-asphalted oil vacuum bottoms feedstock (DAO). But DAO feedstock can still be imported. The main issue detracting from Group II brightstock production is the grade limitations on the production of a haze-free product from the current catalytic dewaxing technologies with simple line-ups. But even there, progress is being made, both on the catalyst front and in smarter line-ups.

While a Group II brightstock might not be the 32-centiStoke (Vk100C) material normally associated with Group I production, a product in the mid-20 cSt is perfectly feasible and also acceptable for the majority of heavy grade finished lubricants. A 32-cSt material would normally be cut back anyway to produce the desired base oil viscosity in all but the heaviest lube grades.

The lighter Group II brightstock grade limit is primarily a consequence of the main chain cracking of intermediates by hydroprocessing during the upgrading processes. This of course does not occur in Group I production.

Group II paraffinic brightstock is available in the Far East, and a Group II naphthenic-derived brightstock is available in China. The latter skirts the issue of catalytic dewaxing and haze limitations because true naphthenics do not require dewaxing. Using a naphthenic feedstock limits achievable viscosity index, but acceptable VIs can be produced, certainly above 80 to meet the Group II specification.