HF vs Sulfuric Acid Alkylation: Selection & Risk Trade-Offs

The Choice in One Paragraph

For a greenfield US refinery in 2025+, the answer is almost always sulfuric acid (H₂SO₄). HF alkylation has lower capex and slightly better octane, but the regulatory and community-acceptance burden has shifted decisively against it after Torrance (2015), Philadelphia Energy Solutions (2019), and tightening RMP (Risk Management Plan) requirements. For a brownfield revamp of an existing HF unit, the decision becomes whether to mitigate (modified HF additive, ROSE), convert (ISOALKY ionic liquid, STRATCO), or shut down. This guide gives you the framework.

Side-by-Side Technical Comparison

Safety: The Decisive Factor

HF: The Real Hazard

Anhydrous HF at 105°F has a vapor pressure of about 16 psig. A breach in piping or vessel doesn't produce a pool - it produces an aerosol cloud that can travel miles before dispersing below the IDLH (30 ppm). The 1986 Goldfish trials in Nevada confirmed that an HF release behaves more like chlorine than like gasoline.

Modified HF (additive of organic amines or sulfones) reduces aerosol formation by 70-90% in lab tests, but field validation has been limited and not all refineries adopted it.

H₂SO₄: The Manageable Hazard

Concentrated sulfuric is a contact hazard, not an inhalation hazard at process conditions. A leak forms a pool that can be diked, neutralized with lime, and managed with conventional spill response. Worker exposure is real (skin/eye burns) but bounded.

The historical issue with sulfuric is the regenerator - SO₂/SO₃ handling has its own scrubbing challenges, and SO₂ emissions are tightly regulated under NAAQS and CSAPR.

Regulatory Landscape (US)

• EPA RMP (40 CFR 68) lists HF at 1,000 lb threshold quantity. Sulfuric acid is not RMP-regulated (mist is, but bulk acid is not).
• OSHA PSM (29 CFR 1910.119) applies to both, but HF has additional reporting and mitigation requirements after the 2017 final rule.
• CalEPA AB 617 community emissions monitoring effectively ended HF expansion in California.
• City of Philadelphia banned new HF after the 2019 PES fire/explosion.

Decision Tree

For a greenfield project

1. Choose H₂SO₄ unless you have a strong technical reason and a regulatory pathway. The capex/opex penalty is real but the risk profile justifies it for any unit near a population center.
2. Consider ionic liquid (ISOALKY, Honeywell-Chevron) for new units - lower acid consumption, no SO₂, comparable octane, smaller footprint. Operating at >5 commercial units as of 2025; reasonable maturity for new builds.
3. Consider solid acid (AlkyClean, CDAlky) for small (<5,000 bpd) units where regeneration economics work. Limited commercial deployment - higher technology risk.

For a brownfield HF revamp

1. Modified HF (ReVAP, Alkad) is the cheapest option but does not eliminate aerosol risk - it reduces it. Some communities will not accept this as sufficient mitigation.
2. Convert to ionic liquid (ISOALKY) reuses much of the HF unit infrastructure (deisobutanizer, depropanizer, fractionation). This is the path PBF/Salt Lake City took. Capital is meaningful but less than greenfield sulfuric.
3. Convert to sulfuric requires building a refrigeration train and regenerator. Largest capex, but proven and supported by both major licensors (DuPont/STRATCO and EMRE).
4. Shut down if the unit is small and the refinery has access to merchant alkylate or octane via reformate/MTBE/iso-octane. This is increasingly the answer for end-of-life HF units.

Process Heuristics

• Iso-butane to olefin ratio matters more than catalyst choice. Both technologies need I/O of 5:1 to 12:1 to suppress polymerization and oligomerization. If you can't deliver enough iC₄, neither acid will save you.
• Olefin feed must be free of butadiene (<0.3% in HF, <0.1% in sulfuric). Both technologies form acid-soluble oils (ASO) on butadiene; sulfuric tolerates less but recovers more easily.
• Sulfuric runs cold for a reason. Above 60°F, polymerization and ester formation accelerate, dropping octane and consuming acid. Refrigeration is not optional.
• HF runs warm because viscosity matters. Below 70°F, HF emulsion mixing is poor and conversion drops.

References

• API RP 751 - Safe Operation of Hydrofluoric Acid Alkylation Units, 5th Ed (2024)
• API RP 754 - Process Safety Performance Indicators for the Refining and Petrochemical Industries
• EPA RMP Final Rule, 40 CFR 68 (Amended 2024)
• UOP HF Alkylation, Technology Description Manual (proprietary)
• STRATCO Sulfuric Acid Alkylation Process (DuPont, now Elessent Clean Technologies)
• Honeywell-Chevron ISOALKY Process - Hydrocarbon Processing 2018, 2022 articles
• CSB Investigation Report - Philadelphia Energy Solutions Refinery Fire and Explosions, June 2019
• CSB Investigation Report - ExxonMobil Torrance Refinery ESP Explosion, Feb 2015