Sweetening of petroleum fractions – role of solutizer In caustic-based mercaptan sweetening, which additive is used as a solutizer to improve phase contact and reaction rate with the hydrocarbon?

Introduction / Context:
Refinery “sweetening” processes convert odorous, corrosive mercaptans in light petroleum fractions into less objectionable disulphides. In caustic systems, mass transfer between the aqueous caustic phase and the hydrocarbon phase can limit reaction rates. A solutizer is added to improve mutual solubility and contact.

Given Data / Assumptions:

• Process: caustic-based sweetening (e.g., Merox type), not the classical Doctor test reagent alone.
• Additives considered: phenolic solutizers vs non-solutizer species.
• Objective: identify the solutizer used with caustic.

Concept / Approach:
Phenolic compounds (e.g., phenol, cresols/cresylic acid) are classic solutizers that increase the distribution of caustic into the hydrocarbon phase and stabilise catalytic activity, thereby accelerating mercaptan oxidation to disulphides. Sodium plumbite is associated with the Doctor test chemistry, not the solutizer role. Alcohols like methanol may aid phase behaviour in some systems but the exam-standard answer for “solutizer with caustic” is phenolic (phenol/cresols).

Step-by-Step Solution:

Verification / Alternative check:
Training materials for caustic extraction/sweetening list cresylic acids or phenol as solutizers to enhance mercaptan conversion.

Why Other Options Are Wrong:

• Methanol: Not the canonical solutizer answer in exam context.
• Sodium plumbite: Doctor solution reagent, different function.
• Potassium isobutyrate: Not a standard additive for this purpose.

Common Pitfalls:
Mixing up odorant/Doctor solutions with sweetening catalysts and solutizers; each has a distinct role.

Final Answer:
Phenol