Sour water stripping is getting a precision upgrade: advanced controls and online analyzers are pushing effluent ammonia into the 5–10 mg/L band and hydrogen sulfide to nondetect—while cutting steam duty by as much as ~17%.
Refinery sour water—think overheads from crude columns and cokers—arrives loaded with hydrogen sulfide (H₂S) and ammonia (NH₃). To reuse it as process wash or meet discharge permits, plants need the stripper to push NH₃ into the overhead vapor and H₂S into the acid gas, leaving the effluent almost clean. Targets are tight: non‑phenolic sour water destined for reuse as desalting or hydrotreater wash typically must hold NH₃ below 10–20 ppmw (parts per million by weight), roughly ≪20 mg/L, according to MDPI, and essentially zero H₂S, per Metrohm.
Those reuse specs often exceed local wastewater rules. Indonesian limits, for example, call for NH₃–N ≤10 mg/L and dissolved H₂S ≤1.0 mg/L in industrial effluent (regulatory presentation). Hitting “very low contaminant” levels therefore implies virtually complete NH₃ removal (>99%) and near‑total H₂S stripping (~100%), achievable only with aggressive steam and pH control and high column efficiency. Many plants now aim to consistently hold NH₃ at < 5–10 mg/L and H₂S at nondetect.
Effluent quality targets and drivers
The playbook blends process design and operation. Most refineries rely on a single steam‑stripped column, elevating pH with caustic. pH control is pivotal: H₂S strips best below pH 5, while NH₃ strips only above pH 10, notes Yokogawa. In practice, sites hold a compromise near pH ~8 by injecting NaOH at the column bottom to convert ammonium (NH₄⁺) to gaseous ammonia (NH₃)—and, critically, to avoid unstrippable NH₄⁺—as emphasized by Metrohm (“keeping pH above 8, facilitating NH₃ gas formation”).
Where NaOH dosing is used for that control, refineries often specify precision chemical metering via a dosing pump to hold pH steady under feed swings. Some plants go further with dual‑stage stripping (an acid‑gas tower plus a separate NH₃ stripper), but most optimize one column. That means adequate tray/packing count; reflux or pump‑around heat exchange to limit liquid entrainment and corrosion (MDPI); and a tuned pressure/temperature profile.
Thermal design and energy leverage
On the hot side, operators maximize reboiler duty (steam rate) up to the point where NH₃ just meets spec—then stop. Newer twists like mechanical vapor recompression (MVR, a method that recycles vapor energy) or split‑flow regeneration can trim steam by roughly 15–20% without sacrificing removal, per MDPI. In one study, a single‑column sour water stripper re‑optimized via simulation—adjusting feed split, draw tray, pressure and temperatures—cut energy duty by ~17% (MDPI).
Advanced control and soft sensing
Fixed steam‑to‑feed ratios are giving way to feedforward/feedback schemes under advanced process control (APC). Ho et al. (2021) modeled a stripper and showed that redistributing control to a sensitive internal tray temperature—e.g., stage 29—beats simple ratio control. Their “FFT” loop infers the required reboiler heat duty from estimated feed composition and adjusts steam flow accordingly, then closes the loop with an effluent NH₃ feedback (accepting a few‑minute analyzer delay). Simulations showed the best stability with this combined approach (MDPI).
In plant service, “soft sensors” (models that infer composition from readily measured signals) estimate sour‑water feed H₂S and NH₃ using online pH, density, and temperature; this has been demonstrated for real‑time control (MDPI). A practical cascade can stack a feed pH/flow feedforward, a tower temperature controller, and a secondary NH₃ valve or steam‑trim loop. In tests, the FFT scheme with 3‑min feedback sharply reduced integrated ammonia deviation and steam overshoot versus constant ratio control (MDPI). Even without full model predictive control, tuned cascade/PID (proportional–integral–derivative) loops targeting the tray where NH₃ begins to slip cut transient ammonia peaks, shortening settling time and reducing steady‑state error (MDPI).
Online analyzers and closed‑loop stripping
Feedback only works as fast as the data. Continuous analyzers—wet‑chemical titration units or ion‑selective probes—track NH₃ (as N or NH₄⁺) and sulfide (S²⁻) in ranges tailored to refinery effluent. Metrohm’s 2060 Process Analyzer, for example, can simultaneously titrate NH₃ in the 0–200 mg/L range and H₂S in the 0–50 mg/L range and stream results to the control system (Metrohm). The payoff is real‑time trim: if NH₃ creeps up, the system automatically nudges steam or caustic.
Vendor application notes argue that live NH₃/S²⁻ analysis “guarantees stripping efficiency” and significantly increases it (Metrohm; Metrohm). Operators can safely lean out steam until the analyzer barely meets the ammonia setpoint—“increasing the ‘stripper efficiency’ of the SWS, leading to significant steam reduction and increased energy savings,” one note states (Metrohm).
Because the instruments log trends, they also flag trouble early. A slow rise in NH₃ can hint at a fouled reboiler or a leaking feed tank. Plants set alarms—e.g., NH₃ > 5 mg/L—to prompt operator response before permits are at risk. While specific case studies remain proprietary, industry reports say analyzers can pay back fast by avoiding fines or downstream treatment. One refinery reported that continuous NH₃/H₂S monitoring let them cut steam usage by ~10% while reliably staying under spec (Metrohm).
Energy, compliance, and reuse impact
Pull these levers together and the gains stack up. Simulation studies show up to ~17% steam‑duty reductions after process re‑optimization (MDPI). Advanced control schemes halve the integrated NH₃ concentration deviation in disturbance tests (MDPI). With online analyzers in the loop, refineries routinely hold NH₃ in stripped water at single‑digit ppm and H₂S at nondetectable levels. Lower chemical carryover reduces corrosion and emissions, cutting maintenance costs.
In water‑scarce regions—from Indonesia to the Middle East—squeezing maximum reuse from the stripper (often >90% H₂O recovery) matters; every 1% increment recycles millions of liters a year. The upshot: design measures (adequate trays, pump‑arounds, caustic dosing) set the baseline, but APC loops and online NH₃/H₂S analyzers lock in performance, letting operators run precisely between under‑ and over‑strip while meeting environmental limits (Metrohm; Metrohm).
Sources and regulatory references
Ho et al. (2021) on control schemes (MDPI), Zhang et al. (2022) on stripping efficiency optimization (MDPI), and Metrohm application notes on NH₃/H₂S analyzers (Metrohm; Metrohm) underpin the data cited. Indonesian effluent standards are drawn from environmental regulations (Scribd). pH trade‑offs for H₂S/NH₃ stripping are outlined by Yokogawa. Additional MDPI context on industrial SWS design and control appears here (MDPI; MDPI).
