Continued from page 9
Re-refining of waste oil Through history three technologies were mostly used for re-refining [5]:
Figure 2: Dewatering of waste oil [4]
Filtering & demineralisation Inorganic materials and certain additives are removed from waste oil by filtering and demineralisation (see Figure 3 below). This way cleaner feedstock for burning or re-refining is achieved. [4]
In the reaction tank (A) small quantity of sulphuric acid is mixed with the waste oil. This mixture is heated to 60°C, after which a chemical surface-active reagent is added to reaction tank. After stirring, the mixture is allowed to stand so it can separate into two phases: waste oil and aqueous. In the aqueous phase, the reagent causes the contaminants to accumulate, deposits settle at the tank base (A) and are then drained off. [4]
The demineralised oil is then filtered (B) removing particles, thus 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. [4]
The acid/clay treatment is the oldest process and produces the largest quantity of acid tar as by-product waste streams that create environmental hazards. [5] This technology isn’t environmentally friendly and therefore is the least best option to choose. [5]
The vacuum distillation with clay process undertakes waste oil through distillation under vacuum pressure [5] by which operating temperature is lowered and the 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. [5] However, the problem with treating acid tar makes this technology environmentally and economically problematic.
The preferred modern process is catalytic treatment called hydrotreating of waste oils. [5] The hydrotreating technology removes contaminants by exposing the oil to hydrogen gas. This process is performed in the presence of a catalyst at very high pressure and temperature. [5] Chemical reactions that remove trace metals and other contaminants from the lube oil are promoted by the catalyst.
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 FCC (fluid catalytic cracking) or HDC (hydrocracking) Refinery Units. [6] The 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.
Figure 3: Filtering and demineralisation of waste oil [4]
If the burning fuel isn’t sent to burning it can be used as re-refining stock.
10 LUBE MAGAZINE NO.176 AUGUST 2023
Hydro re-refining processes are now producing good quality API Group II or Group I+ base oils. Some are even able to make API Group III base oils, depending on the quality of the feedstock.
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