All hydrocarbon base oils contain a certain amount of aromatic molecules (or in the case of polyalphaolefins, residual olefins) that have no beneficial function. In particular, the poly-aromatics in API Group I, II and III base stocks are detrimental to both viscosity index and in the long-term to oxidative stability.

There are two principal approaches to dealing with poly-aromatics, both of which are classified as base oil finishing processes. The first is hydrofinishing, a conversion that is by far the most widely applied, when appropriate, because of its inherent simplicity.

The second is a chromatographic-type separation for the physical removal of poly-aromatics, often referred to as clay finishing. Here, the base stock is trickled over a fresh clay bed where the most polar molecules – the poly-aromatics – are adsorbed into the clay leaving the less polar molecules, namely saturates such as paraffinics and naphthenics and even the less polar mono- and di-aromatics, in the bulk base stock.

As well as clay, some base stock manufacturers use activated carbon beds as the solid adsorbent for finishing, which can make disposal issues easier, such by simply incinerating it.

In fact, the second approach is used in a slightly modified form as both a preparative chromatography technique in base oil characterization (specifically, for test IP 368 Determination of hydrocarbon types in lubricating oil base stocks – preparative high performance liquid chromatography method) or as the basis for the definition of the API groups (specifically, ASTM D-2007 Standard test method for characteristic groups in rubber extender and processing oils and other petroleum-derived oils by the clay-gel absorption chromatographic method).

The adsorption/chromatographic technique of base oil finishing has one advantage, in that the species you most want to remove, i.e., the poly-aromatics, come out most easily. This is simply because poly-aromatics are the most polar base oil molecules and are adsorbed into the clay or carbon bed most strongly. The opposite is the case with hydrofinishing.

Returning to the hydrofinishing approach, this is a completely necessary part of Group II and Group III base stock production, irrespective of the feedstock. The hydrofinishing step stabilizes the part-finished stocks against daylight instability issues. Any residual poly-aromatics can absorb visible and near-ultra-violet radiation if exposed to daylight. This initiates oxidation reactions and forms a cloudy haze in the base stocks, derived from insoluble oxidation products.

However, the hydrofinishing step is not necessarily required in solvent-extracted Group I production. Some manufacturers choose to apply it here, but mainly for improvement in the color appearance of Group I stocks rather than for stability reasons. Group I stocks can be quite yellow-to-brown in color, which some customers find unattractive. However, properly made Group I stocks, even with these colors, will still have good performance levels. Color correction of Group I using the hydrofinishing approach actually requires significant process skills and the capability to do properly.

Applying the hydrofinishing step to Group I base stocks is tricky if you do not want to curtail lubricant performance levels. This is because Group I stocks contain a fairly high level of residual sulfur – usually greater than 300 parts per million, by the APIs definition. Around half of this sulfur is beneficial and acts as a natural secondary antioxidant. The effect of a hydrofinishing step in Group I production can be, ironically, to remove the beneficial sulfur first, leaving the non-beneficial/non-antioxidant sulfur.

Even if Group I is quite severely hydrofinished, and therefore removing essentially all sulfur, there is still the issue of the poly-aromatics, which are the last to respond to the hydrofinishing step, the mono- and di-aromatics converting much more easily. The poly-aromatics are referred to as being refractory in this sense.

Since Group I normally contains greater than or equal to 10 percent by weight aromatic molecules, the problem of hydrofinishing Group I becomes obvious. Thus, where Group I is required in technical white oil applications, such as food-grade lubricants, the easier process route of using clay finishing beckons. Even this approach, though, leaves the producer with used and contaminated clay to dispose of or recycle.

The benefits of properly applied and severe finishing processes to paraffinic and naphthenic base stocks are considerable. They can upgrade Group II, Group III and naphthenic base oils all the way to medicinal white oil performance levels, with the associated significantly higher manufacturing margins. Such medicinal white oil stocks are essentially aromatics free, but are especially poly-aromatics free. They will pass tests such as the sulfuric acid carbonization test, which is a color test for trace poly-aromatics in base oils.

Returning to PAOs, these base oils are normally hydrogenated as a finishing step – hydrofinished, by any other name – not because they contain any aromatics but because they may contain residual olefins after the oligomerization process. Again, this is a necessary step to produce very stable base stocks. Clay finishing is not really an option here. In fact, such hydrofinishing steps are often applied also to viscosity modifiers too, since they are essentially polyolefins and may contain some residual unsaturated molecules after polymerization, which can initiate oxidation processes in finished lubricants.

So it can be seen how important finishing is, in whichever form, to hydrocarbon base stocks and even some additives used in lubricant formulations.