Lube oil upgrading — which operation is not a key quality-improvement step? In the refining of lubricating base oils, which of the following is not considered an important process for upgrading base oil quality?

Introduction / Context:
Base oil quality depends on saturates level, aromatic content, sulfur/nitrogen, and wax content. Refiners apply multiple finishing steps to improve viscosity index, stability, color, and cleanliness. Not every step directly upgrades quality; some are auxiliary or applied in other product lines.

Given Data / Assumptions:

• Solvent refining removes aromatics to raise viscosity index and stability.
• Clay treatment (adsorptive finishing) improves color and stability.
• Hydrotreating/hydrofinishing saturates olefins/aromatics and removes sulfur/nitrogen.
• “Deoiling” here refers to wax–oil separation tied to wax production, not direct lube quality upgrading.

Concept / Approach:
Key quality upgrades for lube base stocks are solvent extraction, hydrotreating, and finishing adsorption. Deoiling is more closely associated with wax processing; lube dewaxing (not “deoiling”) is the step that lowers pour point and improves low-temperature flow.

Step-by-Step Solution:
List core upgrading steps: solvent refining, hydrotreatment, clay finishing → all quality upgrades.Contrast with “deoiling”: process used in wax recovery rather than base oil upgrading per se.Hence, “Deoiling” is not an important base oil quality-improvement step.

Verification / Alternative check:
Refinery schemes show dewaxing (MEK–toluene, catalytic) for lubes; “deoiling” relates to slack wax processing rather than base stock improvement.

Why Other Options Are Wrong:

• Solvent refining: core aromatics removal step.
• Clay treatment: finishing for stability/color.
• Hydrotreatment: essential to remove heteroatoms and improve stability.

Common Pitfalls:

• Confusing “deoiling” of slack wax with “dewaxing” of base stock.

Final Answer:
Deoiling