Naphthenic base oils sit mainly in API Group V because they have inherently low viscosity indices – almost always less than 80 and often between zero and 40. The VI limitation means that naphthenics are not generally suited to the broad range of lubricant applications. Rather they find most use as process oils and in specialty lubricants such as refrigerator oils that require their exceptionally low pour points and wax-free nature.
Naphthenic base stocks are produced from naphthenic crudes, which often tend to be high in TAN (total acid number) due to the presence of naphthenic acids. Although high TAN crudes are often traded at a discount, naphthenic crudes that are good for base oil production can be more difficult to source than paraffinics nowadays. Like paraffinic lube crudes, naphthenic crudes can range from relatively heavy, high-sulfur varieties such as the Venezuelan naphthenic crudes, to lower-sulfur North Sea types.
Naphthenic high-TAN crude, used for base oil production, is distilled in high-vacuum units with acid resistant metallurgy such as stainless steel. Thus paraffinic base oil plants cant routinely switch to naphthenic production without proper conversion.
Otherwise, the processing steps are largely the same as for paraffinic stocks, with the important exception that base stocks from good naphthenic crudes do not need a step for dewaxing. This is because the crude is largely devoid of the waxes or normal paraffinictype molecules that boost the VI of paraffinic base stocks but also result in much higher pour points.
Naphthenic stocks can be processed either by the classic solvent extraction approach used for Group I base stocks, or the severe hydro-processing approach largely used in the production of Group II or III stocks. Thus naphthenic stocks can have compositions that resemble Group I or Group II/ III in terms of sulfur and saturates (but not viscosity index), depending on how they have been processed.
Given the lack of dewaxing con-straints, naphthenics can extend to the full range of viscosity grades – from just heavier than gas oils, up to bright stocks. This, together with their wide variety of chemical constitutions, gives them great flexibility in application, covering everything from less demanding lube and process oil applications, to technical white oil or even medicinal white oil applications.
One of the selling points made for naphthenics is their solvency for process oil applications. In terms of aniline number, their solvency is very little different, grade-for-grade from many Group I paraffinics. The aromatics contents of naphthenic base stocks can be as high as in Group I stocks, and in this case it may be the aromatic molecules rather than the naphthenics that determine the solvency of the base stock. It is just that naphthenics can provide solvency without the downside of having limited low-temperature performance due to residual waxes. This is important in, say, electrical oil applications, where many original equipment manufacturers prefer to avoid use of additives such as pour-point depressants, which would be needed if using paraffinic stocks. Transformers in remote locations could be exceptionally cold and cannot afford to have their dielectric fluid solidify.
Since naphthenics, if suitably hydrotreated, can have saturates (i.e paraffinic plus naphthenic) content just as high as Group II or Group III base stocks, we can create a base stock that essentially has just one component – the naphthenic component – since the paraffinic content is essentially zero.
Among saturated hydrocarbon molecules, the naphthenic component has a very high traction or coefficient of friction.* This makes naphthenics highly suitable for controlling frictional performance in some gear box and other industrial lubes. Most of the modern synthetic traction fluids on the market today are based on just the same type of saturated naphthenic ring structures, albeit with a narrower range of molecular detail, as highly saturated naphthenic base stocks.
Whilst naphthenic molecules are inherently low in VI because of their condensed, ring-like nature, naphthenic stocks at the higher end of the grade structures can have surprisingly high VI. In fact there are commercial naphthenic bright stocks that have sufficiently low aromatics content (the main killer of VI) that their VI is greater than 80. This, combined with low sulfur content from hydro-processing, means they are formally Group II bright stocks, nailing the myth that it is not possible to make a Group II bright stock! The main constraint on making Group II bright stock is the ability to catalytically dewax properly, rather than the ability to process to the correct saturates and sulfur level.
Large integrated oil companies exited naphthenic base stock production, but Chinas CNOOC joined the market a few years ago. Production volumes decreased in the 1990s and early 2000s but have since stabilized and slightly increased.