Product treating: the Unisol process (using caustic soda with methyl alcohol) is employed for the complete removal of which impurity from gasoline?

Introduction:
Gasoline “sweetening” aims to reduce odour and corrosivity by removing sulphur species, especially mercaptans. Several processes exist, but Unisol is a specific extraction-based method using caustic with alcohol to enhance solubility and mass transfer of sulphur compounds.

Given Data / Assumptions:

• Process: Unisol (caustic soda + methyl alcohol).
• Feed: gasoline-range hydrocarbons.
• Target: specific sulphur species removal.

Concept / Approach:
Mercaptans (R–SH) can be extracted into an alkaline methanolic phase, forming mercaptides that preferentially partition out of the hydrocarbon phase. This differs from oxidation sweetening (e.g., Merox), where mercaptans are oxidized to disulphides. Waxes and asphalt are high–molecular weight non-volatile species irrelevant to gasoline; diolefins affect gum formation but are not the Unisol target.

Step-by-Step Solution:
Identify impurity: mercaptans cause odour/corrosion.Apply Unisol: extract with NaOH/methanol to remove R–SH.Result: “sweet” gasoline with lower mercaptan sulphur.

Verification / Alternative check:
Refinery treating schemes often show Unisol/Merox options depending on product slate and sulphur specifications. Lab tests (doctor test) confirm sweetening efficacy.

Why Other Options Are Wrong:

• Waxes/asphalt: not present in the gasoline range; removed in other refinery steps.
• Diolefins: managed by hydrotreating/polymerization control, not Unisol extraction.

Common Pitfalls:
Confusing extraction sweetening with catalytic oxidation sweetening; selection depends on sulphur spec and economics.

Final Answer:
Mercaptans (sulphur compounds)