Run a sour‑water stripper even slightly off spec and ammonium salts will drop out, trays will blind, and downtime will spike. A tight program of pretreatment, temperature control, and wet chemical cleaning keeps the column online — and prevents pyrophoric surprises.
Sour‑water strippers (SWS) remove hydrogen sulfide (H₂S) and ammonia (NH₃) from refinery wastewaters; the feeds are a rough mix of organic acids (phenols), ammonia, H₂S, and sometimes oxygenates or corrosion inhibitors. The service is highly fouling — operators classify SWS as “fouling service” (digitalrefining.com).
Five foulant families dominate: hydrocarbons (heavy oils or naphthas) agglomerate into sticky black sludge, especially in exchangers; particulates (dust, coke fines, corrosion particles) plug trays or packing; salt scales (inorganic CaCO₃, Mg(OH)₂, plus acid–gas salts such as ammonium sulfide/bisulfide, NH₄HS, and ammonium chloride) precipitate when local concentrations or temperatures favor solid formation; elemental sulfur deposits as fine powder; and polymers form when oxygen meets “phenolic” sour water from coker/visbreaker effluent, where dissolved phenols resinize (digitalrefining.com; patents.google.com).
The effects are immediate and expensive. One refinery watched a packed‑column stripper’s performance “steeply decreasing” within days due to fouling, forcing an emergency clean‑out on a five‑day turnaround (chemengonline.com). Exchanger and reboiler fouling drives up temperature differentials and pressure drops; fouled trays lose mass‑transfer contact efficiency — often 15–45% Murphree efficiency (a common tray efficiency metric) (Scribd).
The spec pain shows up fast: delayed cleaning leaves strip‑effluent ammonia far above limits — often >100 ppm NH₃ versus a 10–20 ppm target when bottoms are reused in the crude desalter (digitalrefining.com). Worse, iron sulfide (FeS) scale is pyrophoric; dry FeS exposed to air has ignited around ≈2000 °F, warping a column (chemengonline.com).
Fouling mechanisms and operating risk
Hydrocarbon solids, particulates, salt scales, elemental sulfur, and oxygen‑driven organic polymers all strip heat‑ and mass‑transfer efficiency and force steam duty higher (digitalrefining.com; digitalrefining.com; patents.google.com). Inorganic CaCO₃ and Mg(OH)₂ and acid–gas salts (NH₄HS, NH₄Cl) drop out anywhere cold spots or high local concentrations exist (digitalrefining.com).
Feed pretreatment and oxygen control
Foulant precursors are the first target. Packed or fine‑tray towers are vulnerable to debris; rigorous filtration or cyclones are advised (digitalrefining.com). Free hydrocarbons should be skimmed in the sour‑water tank because entrained oil drives sticky sludge fouling (digitalrefining.com; digitalrefining.com).
Hardness must be stripped out — Ca²⁺/Mg²⁺ are cut via softening or Demin so CaCO₃/Mg(OH)₂ scale never forms; a standard water‑quality check is “no appreciable hardness” (digitalrefining.com). Plants commonly deploy softening in tandem with Demin to meet that target.
Caustic‑rich wastes are excluded: SWS should not receive industrial caustic effluent because it precipitates salts; strong acid slugs create issues as well (digitalrefining.com). Where sour water carries CaCO₃ (such as coal‑gasification water), pH is held ~6.0–6.5 using dilute HCl to stay below CaCO₃ solubility; strong acids, especially H₂SO₄, can interfere with ammonia stripping (digitalrefining.com).
Oxygen drives polymeric fouling in phenolic sour water. Deaeration and tank blanketing are essential; oxygen scavengers (e.g., sodium sulfite) are applied to keep conditions reducing (patents.google.com). Many programs formalize this with oxygen scavengers to suppress phenol polymerization. Corrosion inhibitor carryover (triazine, nitrites) is minimized, and antioxidant dosing under reducing conditions suppresses organic agglomeration.
Column design and thermal profile
Tray choice matters. Trays are preferred to random packing in fouling service, and sieve trays with high vertical velocity sweep liquid off tray bottoms, inhibiting deposit adhesion (MDPI; chemengonline.com). Case guidance is explicit: “trays should be preferred… because trays are easier to clean and less prone to metal fire” (chemengonline.com).
Fouling‑resistant trays and multiple parallel exchangers with isolation valves (bypasses) allow online cleaning without a full shutdown (digitalrefining.com). Slight oversizing (extra trays) increases runtime between cleanings.
Temperature uniformity prevents salt drop‑out. Bottoms and overheads are kept hot, with full steam reboil maintained during upsets or shutdown purges (digitalrefining.com). Ammonium carbonate/bicarbonate (NH₄)₂CO₃/NH₄HCO₃ sublime at 55–75 °C; overhead lines and fittings are kept ≥~85 °C — steam‑traced and insulated — so salts do not crystallize (digitalrefining.com). Vapor condensation from hydrocarbon or steam spikes and thermal shocks is avoided.
Liquid hold‑up and monitoring
Ample liquid hold‑up keeps ammonium salts in solution. Industry practice maintains about 20% liquid volume remaining so NH₄⁺ salts do not supersaturate; operators inject clean water or condensate into the feed, or use a steam/mist inlet, to flush salts continuously (AFPM). Western Refining guidance targets enough wash water so ~20% remains liquid, with ~5 wt% NH₄HS in solution to stay well below saturation (AFPM).
Exchangers often receive a scheduled “water‑wash” (molten wash) 1–2 hours, roughly twice per week (AFPM). Continuous pH, conductivity, and ammonia monitoring provides early warning of salt saturation trends.
Effluent quality and stripper performance
Targets align to reuse. If stripper bottoms feed the crude desalter, ammonia is typically ~10–20 mg/L (ppm) in the effluent; results above ~100 ppm signal inadequate stripping, often from fouling (digitalrefining.com). Best practice holds >98% NH₃ removal, with steam rate and stripping pressure tuned to avoid gas “shortcuts” that leave NH₃ or H₂S unstripped.
Process control to prevent ammonium salts
Pressure/temperature balance is central. Higher pressure increases NH₃ solubility but can over‑cool overheads by condensation, promoting NH₄HS drop‑out; bottoms are held around 100–120 °C and overheads at ≥85 °C with sufficient steam (digitalrefining.com; digitalrefining.com).
Dewpoint management avoids crystallization. Identified dew points for NH₄HS/NH₄Cl guide where not to over‑cool; control valves and exchangers are tuned to prevent vapor temperatures dropping below those points. A slight positive pressure (purge gas if needed) helps ensure complete acid‑gas removal (digitalrefining.com; digitalrefining.com).
Chemistry monitoring shapes the feed. Chloride, NH₃, and H₂S are tracked; if high chloride or acid‑gas triggers arise (e.g., from amine tail gas or hydrochlorination units), operators adjust via dilution or pH shift, or apply corrosion inhibitors upstream to steer chemistry. Where HCl is present, controlled ammonia injection can form NH₄Cl in a known liquid zone and wash it out; hydrocracker operators do this routinely at liquid flash points, with a water wash afterward (AFPM). If excess NH₃ drives NH₄HS precipitation, steam is adjusted or weak acid (CO₂) is added for balance.
Caustic or acid spikes are avoided. A caustic slug precipitates carbonate solids; strong acid swings also form salts and are less controllable with H₂SO₄ reactivity. Sour water is typically kept near pH ~7, and “never feed stray caustic to the SWS” is a common rule (digitalrefining.com).
Cycle discipline matters. Even small leaks (e.g., slightly alkaline boiler feedwater) can tip NH₄HCO₃ into precipitation; tight drains/vents and filtered reclaim water prevent drift. Entrainment zones are monitored so salts do not concentrate in tray bottoms or exchanger sumps; level control prevents puddling. Some units apply short steam‑blow pulses through bottom trays or a small bottom blowdown to purge buildup. Online analyzers flag rising NH₃/H₂S in overhead or effluent — a cue to raise temperature or briefly flush exchangers — and advanced control strategies hold margin from ammonium‑salt dewpoints by adjusting steam or reflux.
Shutdown chemical cleaning workflow
Pre‑shutdown purge (safety): overheads are routed to flare or Claus and the sour‑water feed is drawn down slowly. Steam stripping continues until perchloric H₂S and NH₃ in the water are nearly eliminated (off‑gas monitored); valves are positioned to keep steam at the reboiler bottoms so H₂S/NH₃ are driven to flare (digitalrefining.com; digitalrefining.com). When H₂S/NH₃ are low (typically 1–2 hours of continued steam), overhead blowdown is isolated and the column cooled slowly. Air ingress is prevented until fully washed — oxygen contacting hot, dry FeS will ignite — so towers are pressed with nitrogen (or floated on flare pressure) and kept wet with continuous spray (digitalrefining.com; chemengonline.com). Hydrocarbon “blankets” are skimmed from the sour tank to avoid foaming and carryover (digitalrefining.com).
Mechanical draining and flushing: after inerting, the column, exchangers, and piping are drained (nitrogen push‑out as needed), then flushed with warm demineralized water to dissolve soluble salts and keep FeS wet; flush water is collected for treatment. Plants often rely on demineralized water for this step.
Chemical soaks follow. A reducing agent (e.g., sodium bisulfite or metabisulfite) is circulated to convert residual H₂S into nonvolatile sulfite/bisulfite, while ammonia is neutralized with mild acid (acetic or sulfamic) or CO₂ to ammonium ion; in‑situ neutralization is cited as key to “rapid elimination” of H₂S/NH₃ hazards (fqechemicals.com). Petroleum/organic removal uses a hot alkaline detergent solution — e.g., 2–4% NaOH with surfactant for several hours — followed by a water rinse. Pyrophoric FeS is passivated with oxidizers (a few percent hydrogen peroxide or stabilized sodium hypochlorite formulations) to convert FeS to ferric hydroxides; one case cites “PYROPHORIC” chemicals rendering iron sulfide inert (fqechemicals.com). Mineral scale is removed with a dilute HCl soak (1–5%) or chelating solutions (EDTA, NTA, proprietary “scale‑solvers”) at 40–60 °C for several hours to dissolve CaCO₃, FeCO₃, siderite, and residual FeS; specialty formulas are designed for “water‑born scaling” removal (fqechemicals.com). Thorough flushing to neutral pH closes the cycle.
Throughout, internals remain wet — dry trays or packing exposed to air have led to catastrophic FeS ignition — and inert purges supplement water coverage (chemengonline.com; chemengonline.com). Specialized oxidizing/neutralizing agents are part of standard chemical cleaning programs.
Inspection and mechanical cleaning: manways are opened to inspect trays, packing, reboiler tubes, and exchanger channels; stubborn solids are scraped or pressure‑washed, and trays or rings are removed for hand‑cleaning if needed. Trays, favored in case literature, also clean faster and are less prone to metal fire (chemengonline.com). Corrosion/FeS pits are identified for repair.
Final flush and dry: circuits are flushed with fresh water to acceptable residual levels, then the vessel is fully dried and inerted (or brought up under nitrogen sweep) before any air entry. Some plants conduct an “air introduction test” with gas analyzers to confirm no pyrophoric reaction.
Post‑clean outcomes and verification
Chemical decontamination cuts downtime. One case reported a 50% reduction in pre‑inspection cleaning time when modern oxidizing/neutralizing agents replaced permanganate‑based washes (fqechemicals.com). Industry guidance calls a well‑executed chemical decontamination the “most efficient way” to clean an SWS (fqechemicals.com).
After cleaning, performance is verified by NH₃/H₂S in the effluent; with fresh packing or trays, operators restore outlet targets (e.g., <20 ppm NH₃ where required) and confirm steam duty has returned to baseline.
Operational takeaway
Preventing ammonium‑salt fouling is proactive: temperature above crystallization thresholds, composition held away from supersaturation, and steady flows so salts never reach their saturation point. Maintaining ~20% liquid hold‑up — plus thorough pretreatment and careful shutdown procedures — keeps precipitation at bay and the stripper running clean (AFPM; digitalrefining.com).
