Lube refining: Which treatment produces appreciable improvement in the viscosity index (VI) of lubricating oil by selectively removing low-VI components?

Introduction / Context:
Lubricating oil base stocks require good viscosity-temperature behavior. Refining steps aim to remove components that degrade VI, oxidation stability, or low-temperature flow.

Given Data / Assumptions:

• Objective: appreciable increase in VI.
• Processes considered: acid, solvent extraction, alkali, and clay treatment.
• Feed: lube distillates prior to dewaxing/finishing.

Concept / Approach:
Solvent extraction (e.g., with furfural or N-methyl-2-pyrrolidone) removes aromatics and polar compounds that depress VI. This yields raffinate with higher VI, often followed by dewaxing and hydrotreating/ hydrofinishing.

Step-by-Step Solution:

Verification / Alternative check:
Process data consistently show VI uplift after solvent extraction compared to untreated distillates.

Why Other Options Are Wrong:

• Acid treatment: Historical sludge/asphaltene removal and color improvement; limited VI uplift.
• Alkali treatment: Neutralizes acidic impurities; does not appreciably raise VI.
• Clay treatment: Adsorptive finishing mainly for color/trace impurities; small VI effect.

Common Pitfalls:
Assuming dewaxing alone defines VI; dewaxing targets pour point, while extraction targets solvency profile affecting VI.

Final Answer:
Solvent extraction