Filtering & demineralisation Inorganic materials and certain additives are removed [3] from waste oil by filtering and demineralisation. This way cleaner feedstock [3] for burning or re-refining is achieved.
In reaction tank (A) small quantity of sulphuric acid is mixed with waste oil [3]. This mixture is heated to 60°C after which a chemical surface active reagent is added [3] to the reaction tank. After stirring, the mixture is allowed to stand so it can separate to two phases, waste oil and aqueous [3]. The reagent causes the contaminants to accumulate in the aqueous phase, which settles to the bottom of the tank (A) and is drained off [3].
The demineralised oil is then filtered (B) by which suspended fine particles are removed. This way waste oil is run off to storage as clean burner fuel.
This clean burner fuel can be further diluted or “cut” with a lighter petroleum product (called cutter stock) to produce a range of intermediate to light fuel oils depending on the fuel viscosity requirements of the burner [3].
The acid/clay re-refining process The acid/clay treatment is the oldest process with the largest quantity of acid tar as by-product waste streams that represent environmental hazards [4]. This technology is the least environmentally friendly option [4].
The vacuum distillation/clay process Vacuum distillation with clay process undertakes waste oil through distillation under vacuum pressure [4] by which operating temperature is lowered and problem of thermal breakdown is reduced. Clays with high adsorptive capacity are used to remove impurities such as heavy metals and breakdown products arising in the use of oil.
Clays are usually used to provide cleaner feed and to give recovered oil a final polish [4]. However, treating acid tar makes this technology problematic in both an environmental and economic way.
The vacuum distillation/hydrotreating process The modern process is catalytic treatment called hydrotreating of waste oils [4]. The hydrotreating technology removes contaminants by exposing the oil to hydrogen gas. This process is done in the presence of a catalyst at very high pressure and temperature [4]. Chemical reactions that remove trace metals and other contaminants from the lube oil are promoted by the catalyst.
Figure 2: Filtering and demineralisation of waste oil [3]
If the burning fuel isn’t sent to burning it can be used as re-refining stock.
Re-refining of waste oil Historically three technologies were mostly used for re-refining [4]: • The acid/clay re-refining process • The vacuum distillation/clay process • The vacuum distillation/hydrotreating process
Quality of re-refining products Re-refining can produce Group I and II base oils or vacuum gas oil (VGO) that is a suitable feedstock to Fluid Catalytic Cracking (FCC) or hydrocracking (HDC) Refinery Units [5]. Average material balance from 1 litre of used oil is: • Water and Light 7 % vol. • Ends Light Gasoil 5 % vol. • Lube Oil 75 % vol. • Asphaltic Residue 13 % vol.
The more recent hydroprocesses for re-refining are able to make good quality API Group II or Group II+ base oils. Some are even able to make API Group III base oils, depending on the quality of the feedstock.
Application of re-refined oils Although many people are sceptical about the quality of re-refined oils, Mercedes-Benz 228.3, Volkswagen 500.00 and 505.00, and API SN and CJ-4 Continued on page 12
LUBE MAGAZINE NO.166 DECEMBER 2021 11
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