Lubrication science
From crude oil to base oil: refining routes in plain language
Quick answer
Why Group I, II and III oils differ: the process steps that remove wax, sulphur and aromatics.
Start with crude oil
Crude oil is a mix of hydrocarbon molecules of many sizes. Atmospheric distillation separates fuels. The heavy residue is distilled again under vacuum to give lubricating oil cuts of different thickness.
Two ways to clean the cut
| Route | Main steps | Result |
|---|---|---|
| Solvent route (Group I) | Solvent extraction removes aromatics, then solvent dewaxing removes wax so the oil flows when cold, then hydrofinishing polishes it. | Useful, lower cost, less stable oil with more sulphur and aromatics. |
| Hydroprocessing route (Groups II and III) | Hydrotreating or hydrocracking with hydrogen and a catalyst removes sulphur, nitrogen and aromatics and rearranges molecules. Catalytic dewaxing and hydrofinishing follow. | Cleaner, more stable oil. Severe hydrocracking gives the higher viscosity index of Group III. |
| Synthesis (Group IV) | Small molecules (olefins from ethylene) are joined in a controlled way into uniform polyalphaolefin chains. | Uniform molecules, very high viscosity index, excellent low-temperature flow. |
| Gas-to-liquid wax route | Wax made from natural gas is hydroisomerised into base oil. | Very pure Group III-type base oil. |
What the process changes
- Saturates rise, so oxidation stability rises.
- Sulphur and aromatics fall, which cuts sludge, deposits and corrosive by-products.
- Wax removal sets the pour point.
- Molecular uniformity sets the viscosity index and the volatility.
Aromatics and sulphur: not only bad
Aromatics dissolve additives and seal rubber well, which is why some older oils and some greases still use more solvent-refined stock. Very pure oils can need extra care with seal compatibility and additive solubility.
Takeaway. Process decides purity. More severe processing gives a more stable, more uniform base oil at a higher cost.
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