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adapted to manufacture higher performance base oils, but it was not until the 1990’s that these began to move to centre stage.
With so many types of base oil available, the American Petroleum Institute (API) developed a scheme to classify products in a simpler way using a narrow range of selected chemical and physical parameters. Currently this is known as the API Base Oil Classification scheme, where five Groups cover both conventional mineral oil and synthetic base oils giving the lubricating oil formulator a wide range of possibilities to deliver the required performance.
At present, the majority of oils produced by solvent extraction are of the Group I type and conversely those produced by higher pressure hydrogen/catalytic treatments are of the Group II (and above) type, which are over 40% of the world supply and growing at the expense of Group I.
Modern base oil re-refining – sustainability comes of age
A typical feedstock arising from the main sources of used automotive and industrial oils will typically contain a base oil content of above 80%, hence re-refining to recover that component is both commercially and environmentally attractive. That’s a simple view, but compared with the conventional refining processes described earlier, if some of the steps can be avoided and the yields are potentially high, then focusing on the recovering the ‘heart’ of the feedstock should deliver both energy efficiency and carbon savings. Today the main driving force and raison d’être for re-refining is sustainability, while it also leads to other economic and supply benefits.
The composition of a ULO will vary as a function of a country’s industrialisation and may typically be described as containing ontaminants... • Water – that may be free or bound to additives • Remaining performance additives – some unused and some part spent residues
• Some light ends, mainly from associated use of fuels in the diesel range and light distillates e.g., gasoline
• Carbon as soot associated with internal combustion engine pre-use, that may be bound to additives
• Hydrocarbon oxidation and nitration substances, varnishes, and acids
• Wear metals 12 LUBE MAGAZINE NO.177 OCTOBER 2023
• General dirt, inorganics, and other adventitious matter, and;
• Occasionally fatty oils, more rarely anti-freeze, and solvents.
While applying some of the principles of conventional refining to recover the desired base oil from the contaminants, the re-refining process needs some detailed modifications including mass balance considerations.
Process dependent, many re-refiners only accept feedstocks meeting a certain specification placing controls on contaminants, for example, these may include water content, solids, chlorine, silicones, biolubes, or Persistent Organic Pollutants (POPs). Feedstocks that cannot be accepted are directed towards energy recovery treatments, while POPs are sent for high temperature incineration.
Re-refiners have a number of treatment tools at their disposal including distillation, heat, rarer acid treatment, use of complexing chemicals, solvent extraction, and catalytic hydrogenation processes. When choosing a tailored process, before all else it needs to be decided what type of output base oil is sought, for example, selection by API Group type. It is possible to produce a Group I or II oil by solvent extraction, in the latter case providing the used oil feedstock is essentially of Group II type. Similarly, by using the correct catalyst technology and appropriate hydrogen pressures and temperatures, a predominately Group I or II feedstock can result in a Group II or III output.
In thin film distillation processes, it is common to pre-treat feedstock by carrying out a separate front-end pre-distillation at temperatures around 130°C to remove water and light solvents.
A distillation stage on dried oil operating at +300°C under high vacuum is a common feature to many processes, while some rely on a particularly transient high flash temperature to destroy both used and unused additives. These processes partition base oil and fuel fractions leaving a heavy bottoms residue. Suffice to say that additive separation is often achieved by stressing chemicals to temperatures well above their design operating range. For example, nearly all common zinc-based anti-wear additives are destroyed at bulk oil temperatures above 220°C, however, the
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