Refineries Are Rethinking Spent Alkylation Acid: Regenerate Sulfuric, Neutralize HF, Comply Hard

Alkylation is everywhere in refining—and so is the waste acid that comes with it. The world runs about 2.8 million bbl/d of alkylation capacity (~120 million t/yr) (bcinsight.crugroup.com), and the balance of technologies is drifting toward sulfuric acid, now roughly 60:40 versus HF (bcinsight.crugroup.com).

That shift matters because “spent” acid doesn’t vanish. In sulfuric-acid (H₂SO₄) alkylation, the used acid still carries ~85–90% H₂SO₄ by weight—the rest is water and heavy organic polymers (patents.google.com). In HF (hydrofluoric acid) systems, the spent stream can be laced with acid soluble oils and dissolved AlCl₃, and it’s exceptionally hazardous.

The options diverge: sulfuric acid is routinely regenerated back to spec; hydrofluoric acid is not. For environmental managers, the playbook includes thermal regeneration, neutralization and precipitation, solids removal, and strict licensing—especially in Indonesia, where both acid types are classified as B3 (hazardous and toxic) waste and tightly regulated (alpha.rkcmpd-eria.org).

Sulfuric-acid regeneration units (ARU/SAR)

Acid Regeneration Units (ARUs), also called sulphuric acid regeneration (SAR) plants, turn spent acid into fresh product via high-temperature decomposition, gas cleanup, conversion, and absorption. In brief: atomize and burn the spent acid with fuel at ≈1000°C to decompose it to SO₂+H₂O; clean the gas; convert SO₂→SO₃; then absorb to form fresh H₂SO₄ (digitalrefining.com) (elessentct.com).

Performance is the headline: such units routinely recover acid up to ~99% purity; MECS® designs produce 93–99.2% H₂SO₄ from spent acid feed (digitalrefining.com). SAR plants range from a few tens of tonnes per day to ~1,800 STPD of product acid, often serving refinery clusters (digitalrefining.com).

Material balances back this up: regeneration typically recovers over 90% of the original acid content while removing ≥95% of water and organics (trea.com). That’s significant when alkylation units can generate ~5–15% of annual acid feed as “spent” waste—roughly 60,000 t/yr for a unit making 3–9 kt/y alkylate (returned to 98% H₂SO₄ via an ARU in the cited case) (patents.google.com).

HF acid neutralization and fluoride removal

Hydrofluoric acid alkylation relies on an anhydrous HF catalyst. Unlike H₂SO₄, it is not “regenerated.” Spent HF streams—potentially containing acid soluble oils and dissolved AlCl₃—must be treated, not dumped. The standard route is neutralization and precipitation of fluoride (finishing.com).

Practitioners add a calcium source (lime/Ca(OH)₂ or CaCl₂) to convert HF to insoluble CaF₂ (fluorspar), then adjust to about pH 9–10. Industry examples include dosing with ≈0.5 oz/gal CaCl₂ and then hydrated lime until pH≈9, precipitating ≥99% of F⁻ as CaF₂ (finishing.com). Caustic (NaOH) can neutralize HF if permits allow, but strict fluoride limits in many jurisdictions make calcium precipitation the preferred pathway (finishing.com).

Execution details matter. Accurate chemical dosing during neutralization supports pH control and reagent economy, which is why operators often specify a dosing pump for caustic or lime slurries. The resulting CaF₂ solids are chemically inert and suitable for secure landfill disposal; any off-gas is scrubbed to capture remaining HF/fluorides (per the same industry guidance). Where settling or microfiltration is recommended, facilities commonly complement precipitation with a clarifier for solids removal or a downstream cartridge filter to capture fines (finishing.com).

The bottom line is that little if any HF is recovered—virtually all fluoride ends up as calcium fluoride. Spent HF management is specialized treatment: neutralize the strong acid and remove fluoride completely before any disposal or release.

Regulatory requirements (Indonesia)

Indonesia treats spent H₂SO₄ and HF acid as B3 hazardous waste. Under Law 32/2009, discharging any hazardous waste without treatment and permits is prohibited (alpha.rkcmpd-eria.org). Government Regulation 101/2014, updated by GR 22/2021, strictly controls B3 disposal: only engineered facilities may accept such waste, including lined landfills or permitted incinerators/thermal processors (pdfcoffee.com).

Chapter IX of GR 101/2014 allows hazardous waste disposal only in a “final landfill facility” (class I–III) or dedicated waste-treatment plants (chemical-physical units, incineration/pyrolysis, etc.) (pdfcoffee.com). Dumping, such as open discharge to soil or sea, is generally forbidden without a special ministerial permit (alpha.rkcmpd-eria.org) (iea.org).

Licensing is detailed. Regulation of the Minister of Environment and Forestry (MAF) No. 6/2021 requires Technical Approval (PLB3 TA) and Operational Certification (SLO) for any entity handling B3 waste, including treatment or dumping (iea.org). Generators must characterize the spent acid (including TCLP tests if required; TCLP is a leaching test for hazardous characteristics), store it in bunded tanks, manifest shipments, and use only licensed B3 carriers/facilities. For spent H₂SO₄, the route can be an ARU (considered “recycling” under the law) or a B3 incinerator; for spent HF, on-site neutralization followed by landfill disposal of the CaF₂ precipitate or shipment to a B3-waste processor—all meeting effluent pH and fluoride limits and executed via licensed channels (pdfcoffee.com) (iea.org).

Scale, risk, and the operating choice

With millions of tonnes per year of spent acid at stake, acid regeneration yields above 90% shrink waste and curb purchases; conversely, HF hazards—highlighted in incidents such as the 2019 Philadelphia refinery accident—have tightened controls and prompted HF-to-H₂SO₄ conversions in some regions (bcinsight.crugroup.com). The Indonesian framework means the operational decision—ARU for sulfuric, neutralize-and-precipitate for HF—must be matched by certified handling from storage to final disposal.