Lube oil refining — Duo-Sol extraction solvent system In the Duo-Sol process for lube oil upgradation (aromatics extraction), which solvent system is used?

Introduction / Context:
Lube base oil refining traditionally uses selective solvent extraction to remove aromatics and improve viscosity index and stability. The Duo-Sol process is a well-known aromatics extraction route.

Given Data / Assumptions:

• Duo-Sol is a liquid–liquid extraction step, not dewaxing.
• Target: remove aromatics from lube distillates.

Concept / Approach:
The Duo-Sol process employs a mixture of phenol and cresylic acids (cresols). This solvent selectively dissolves aromatics, enabling phase separation and producing a raffinate richer in paraffinic/naphthenic components suitable for lube base stocks.

Step-by-Step Solution:
Identify extraction vs. dewaxing: Duo-Sol = extraction, dewaxing often uses MEK/toluene or MEK/MIBK mixtures.Propane and liquid SO2 are associated with other refining operations (deasphalting or refrigeration), not Duo-Sol.Therefore, the correct solvent system is phenol–cresol.

Verification / Alternative check:
Classical refining texts list phenol–cresylic acid as the Duo-Sol solvent pair.

Why Other Options Are Wrong:

• Propane + phenol–cresol: mixing a selective solvent with a deasphalting solvent is not the Duo-Sol scheme.
• MEK + glycol: typical of dewaxing, not aromatics extraction.
• Propane + liquid SO2: unrelated to Duo-Sol lube extraction.

Common Pitfalls:

• Confusing extraction (for aromatics removal) with dewaxing (for pour point control).

Final Answer:
Phenol–cresol (cresylic acid) mixture