Acid alkylation practice: Why is sulphuric acid concentration maintained between 90% and 98% in refinery alkylation units producing high-octane alkylate?

Introduction / Context:
Acid alkylation combines light isoparaffins (e.g., isobutane) with olefins (e.g., propylene, butylenes) to produce high-octane alkylate. Sulphuric acid (H2SO4) units require careful control of acid strength to keep desired alkylation pathways dominant and to suppress side reactions that harm yield and product quality.

Given Data / Assumptions:

• H2SO4 concentration is operationally kept near 90–98%.
• Main goal: favour carbocation pathways to form branched C7–C9 alkylate.
• Undesired side reactions include polymerisation and cracking.

Concept / Approach:
At too low acid strength (< 90%), the medium inadequately stabilises desired intermediates and promotes polymerisation/oligomerisation of olefins, increasing acid consumption and creating heavy ends. At too high acid strength (> 98%), excessive acidity and dehydration tendencies shift pathways toward cracking and other side reactions, lowering selectivity to high-quality alkylate. Therefore, the accepted operating window balances these effects.

Step-by-Step Solution:

Verification / Alternative check:
Operating manuals for H2SO4 alkylation specify tight acid strength control with regular acid regeneration to maintain optimal selectivity and manageable acid consumption rates.

Why Other Options Are Wrong:

Common Pitfalls:
Treating acid strength as “the higher the better”; in reality, too-high strengths increase side reactions and corrosion concerns.

Final Answer:
both (a) and (b)