Refiners’ low‑ppm obsession: how settlers and wash towers strip acid from alkylate
Modern H₂SO₄ alkylation units are turning a frothy, 50/50 acid–hydrocarbon emulsion into “clear and bright” product by pairing engineered settlers with simple water or caustic washes—and by running the settler under tight, data‑driven conditions. The result: acid in the alkylate measured in the low‑ppm range.
In sulfuric‑acid (H₂SO₄) alkylation—where isobutane and olefins are combined in the presence of a strong acid catalyst to make high‑octane alkylate—the reactor effluent exits as a roughly 50/50 by‑volume acid/hydrocarbon emulsion (toc.123docz.net). That emulsion heads straight to a settler, where the heavier acid drops out to the bottom for recycle and the lighter hydrocarbon overflows to downstream processing (toc.123docz.net).
Operators are squeezing acid carryover to single‑digit parts per million (ppm, a concentration unit) by upgrading that settler from a quiescent tank to a coalescing machine, then polishing the product with water or caustic washes. The playbook is backed by DuPont/STRATCO and UOP designs—and by a stark comparison: ~10–15 vol ppm acid with coalescing stages versus 50–100 vol ppm by gravity alone (toc.123docz.net).
Settler internals for phase coalescence
Modern settlers embed coalescing internals—trays and packed beds—to agglomerate fine acid droplets for faster separation. UOP’s patented design uses multiple perforated trays with packed beds (examples cited include Pall rings, saddles, and structured packing) in series (freepatentsonline.com). DuPont/STRATCO data show that converting a simple gravity settler into a two‑stage coalescer system yields a ∼90 vol% H₂SO₄ recycle stream and cuts acid carryover in the alkylate to only ~10–15 vol ppm—roughly one‑third the 50–100 vol ppm typical of plain settling (toc.123docz.net).
In parallel multi‑reactor units, acid flows in series through staged reactors so that fresh and intermediate acids feed each stage, maintaining a consistent optimal acid strength throughout the unit—a hydraulic choice that also stabilizes the settler’s phase split (toc.123docz.net). The settler itself is sized and piped to maintain about a 50:50 by‑volume acid/hydrocarbon ratio in reactor effluent (for STRATCO) (toc.123docz.net).
Pressure control, weirs, and clean phase split
Settlers are run at steady pressure to avoid flashing and emulsions. STRATCO units hold about 60 psig (4.2 kg/cm²; psig is pressure relative to atmosphere) via a back‑pressure valve to prevent flashing of hydrocarbons in the settler; downstream, the lighter‑phase overflow is throttled (to ~5 psig) and flashed to cool the effluent before fractionation (toc.123docz.net). Proper weir geometry plus coalescing media promote fast agglomeration of acid droplets so they settle instead of entraining—a combination that, in practice, leaves only a few ppm of acid in the hydrocarbon leaving the settler (toc.123docz.net).
Water and caustic washing downstream
Any acid that survives the settler is typically removed by downstream wash steps. In HF‑alkylation units (HF is hydrogen fluoride), the liquid alkylate is caustic‑washed: aqueous NaOH or Na₂CO₃ dissolves residual HF, neutralizing it to fluoride salts. In sulfuric units, a plain water wash can extract soluble acid or acid esters. These washes are followed by liquid/liquid coalescers or electrostatic precipitators to separate the wash water from the product (pall.com). Pall notes that “in the alkylation process, downstream of the reactor, a caustic wash is typically done on the alkylate to remove any remaining acid,” and that high‑efficiency liquid/liquid coalescers are used to meet strict “clear and bright” specifications (pall.com).
Quantitatively, the impact can be dramatic. University of Kansas research reported that without washing, acid remains as ~3 µm droplets stable in alkylate; with a single water wash, virtually all water‑soluble sulfates transfer to the aqueous phase (digitalrefining.com). Industry practice is often to use two‑stage washes (acid wash and then water wash) together with coalescing filters (paperzz.com) (pall.com).
In practice, a well‑designed wash tower plus coalescer train can reduce acid traces essentially to zero. Treating spent alkylate with caustic results in a two‑phase effluent; the organic phase typically exits at near‑neutral pH and <1–10 ppm fluoride, while the alkaline water is sent to recovery or disposal (pall.com). Although acid/water washing generates waste water that must be treated, it typically achieves the “clear and bright” product required by fuel specs (e.g., <0.0015% residual acid by volume) (toc.123docz.net). Accurate chemical addition during these washes is a practical consideration; plants commonly deploy an accurate chemical dosing pump to control NaOH or Na₂CO₃ injection.
Facilities still need to handle the aqueous stream from washing. Given the paper’s note that washing “generates waste water that must be treated,” refineries typically route this to suitable primary treatment; options include waste-water physical separation systems that address free oil and suspended solids prior to downstream handling.
Interface control and operating conditions
Clean separation hinges on level and interface control. The liquid interface between acid and hydrocarbon is monitored (often by a float or guided‑wave radar) to keep each phase in its lane. Ametek (Orion) warns: “Interface level control of the settler is required – product/acid cross‑contamination causes efficiency problems and hazards. Inaccurate level indication is a frequent cause of … acid carryover in the reactor effluent” (ametek-measurement.com).
Acid strength and circulation rate must be in range. Very high acid concentration, while favoring reaction, increases viscosity and can aggravate carryover. Caton et al. (DuPont) note that reactor tubing often fouls when acid carryover occurs from high‑strength acid, since thicker acid films stick inside tubes (paperzz.com). Operators typically run acid in the ~80–90 wt% range, replenishing spent acid continuously to avoid stagnation and excessive polymerization in the settler. In multi‑contactors, staged acid flow (series flow between reactors) keeps acid strength uniform, stabilizing the hydrocarbon/acid interface (toc.123docz.net).
Temperature control matters too. The STRATCO design uses refrigerated effluent to keep hydrocarbons liquid and prevent emulsions, and turbulence in the settler is minimized so droplets collide and merge. The upshot of residence time (minutes), engineered internals, and automated controls: a “clean break” of phases—typically reducing acid in the alkylate to single‑digit ppm by volume (toc.123docz.net) (pall.com).
Taken together, data‑driven settler design (e.g., two‑stage coalescing settler) and rigorous interface control translate directly into measurable outcomes—such as a 3× reduction in acid carryover (toc.123docz.net)—and avoidance of downstream corrosion.
Sources and references
Technical design manuals and studies for sulphuric‑acid alkylation units (toc.123docz.net) (toc.123docz.net); DuPont/STRATCO engineering papers (paperzz.com) (paperzz.com); industry (Pall) and instrumentation guidance (pall.com) (pall.com) and Ametek/Orion Instruments (ametek-measurement.com); recent research on acid droplets (digitalrefining.com) (digitalrefining.com); and UOP LLC, US Patent 7,935,320 (2011) (freepatentsonline.com).