Acrylonitrile (CAS 107-13-1) is manufactured by the vapor-phase ammoxidation of propylene over bismuth molybdate or iron antimonate catalysts in fluidized-bed reactors. Typical reaction temperature ranges from 400°C to 510°C, with reactor pressure held at 50–200 kPa gauge. The quench and absorber train produces an aqueous purge containing acrylonitrile, acetonitrile, hydrocyanic acid, heavy nitriles, and dissolved ammonium sulfate. Because the process generates cyanide in both the liquid and gas phases, Cyanide Removal from Acrylonitrile Wastewater and Off-Gas must be designed as an integrated treatment sequence. Off-gas from the absorber contains residual hydrogen cyanide, unreacted ammonia, carbon monoxide, and volatile nitriles. Wastewater streams contain free cyanide, weak-acid-dissociable cyanide complexes, and thiocyanate, with total cyanide concentrations that vary across the quench bottoms, acetonitrile separator purge, and recovery column condensate.

Hydrogen cyanide exhibits a pKa of 9.2 at 25°C, meaning pH management controls the liquid–vapour partition. At pH below 8.5, cyanide is present largely as molecular HCN and can be stripped into off-gas or workplace air; at pH above 11.0, the cyanide ion dominates and remains in the aqueous phase. Closed drains, nitrogen-blanketed equalization tanks, and continuous gas detection in pump pits are therefore specified for acrylonitrile wastewater handling areas. Integrated plants commonly recover acetonitrile and HCN in a dedicated recovery column. The aqueous bottoms from this column are steam-stripped to reduce total cyanide before discharge, while the non-condensable overhead stream is routed to the off-gas incinerator or a caustic scrubber. Quench column internals are normally constructed of high-alloy stainless steel because free cyanide and organic acids at low pH can promote stress corrosion cracking. Fouling by acrylonitrile polymer and heavy nitriles in quench loop exchangers may require throughput reduction for chemical cleaning on intervals of 6–12 months, depending on feed purity and polymerization inhibitor dosing.

What Cyanide Species Are Found in Acrylonitrile Quench Condensate and Absorber Off-Gas?

Three operationally defined fractions are monitored in the aqueous effluent: free cyanide, weak-acid-dissociable cyanide, and total cyanide. Free cyanide includes CN⁻ and HCN. Weak-acid-dissociable cyanide releases hydrogen cyanide at pH 4.5 and room temperature and includes cadmium, copper, nickel, and zinc complexes. Total cyanide includes iron cyanide complexes that require distillation with strong acid and a catalyst. In acrylonitrile wastewater, the total cyanide fraction is normally the regulatory driver because thermal and ultraviolet exposure can release cyanide from stable iron complexes. Off-gas characterization focuses on hydrogen cyanide vapour, which is typically sampled with sorbent tubes or impingers and analyzed by ion chromatography or visible spectrophotometry.

Published process data for individual acrylonitrile plants vary widely. Total cyanide in quench bottoms can range from 10 mg/L to more than 500 mg/L depending on the acetonitrile recovery configuration and the degree of steam stripping. Absorber off-gas HCN concentrations depend on reactor conversion and quench temperature, with lower quench temperatures increasing HCN absorption efficiency but increasing refrigeration duty. The off-gas HCN fraction that is not recovered or incinerated must be reduced before venting. In plants that operate a wet quench rather than a direct-contact cooler, the cyanide distribution shifts toward the aqueous phase; however, steam stripping of the quench bottoms can re-volatilize HCN unless the pH is maintained above 11.0.

Flow-proportional composite samples are preserved with sodium hydroxide to pH 12–13 and held at 4°C before analysis. Total cyanide may be determined by ISO 14403-2:2017 using flow injection analysis or by US EPA Method 335.4 using semi-automated colorimetry after distillation. Free cyanide can be measured by ISO 17690:2015 or ASTM D7237-18. Hydrogen cyanide in workplace air and source atmospheres is analyzed by NIOSH 7904 using ion chromatography, or by EPA Method 320 Fourier-transform infrared spectroscopy for continuous stack gas monitoring. Sampling of off-gas requires heated lines maintained above 120°C to prevent water and ammonium salt condensation.

Alkaline Chlorination Design Thresholds for Free Cyanide Oxidation

The first-stage alkaline chlorination reactor is designed to oxidize cyanide to cyanate at pH 10.5–11.5. Sodium hypochlorite is metered under automatic ORP control with a setpoint of 350–400 mV against a silver/silver chloride reference electrode. The reaction requires rapid dispersion; inline static mixers or high-speed mixers with a velocity gradient G of 300–600 s⁻¹ are used at the reagent injection point, followed by a baffled holding tank with a liquid residence time of 30–60 min at peak flow. Stoichiometrically, 1 mol of cyanide consumes 1 mol of hypochlorite in the first stage. Industrial units dose 1.2–1.5 times the stoichiometric requirement to overcome competing reactions with ammonia, unsaturated nitriles, and bisulfite.

Second-stage oxidation converts cyanate to carbon dioxide and nitrogen at pH 7.5–8.5 and an ORP setpoint of 600–700 mV. The second stage requires an additional 1.5–2.0 mol of hypochlorite per mole of cyanide and is carried out in a separate reaction basin to prevent the pH-dependent back-reaction of cyanate to cyanide. Mixed liquor from the first stage is acidified with sulfuric acid to the second-stage pH, and the vessel is fitted with gas-tight venting to a caustic scrubber because any residual HCN released during pH adjustment must be captured.

Alkaline chlorination does not reliably oxidize iron cyanide complexes, and its reaction with organic nitrogen compounds can form chloramines and trihalomethanes. In acrylonitrile wastewater containing high concentrations of unsaturated nitriles, the total organic chlorine by-product load may exceed the discharge permit for adsorbable organic halogens if no downstream granular activated carbon is provided. For this reason, the hypochlorite route is normally limited to streams where the ratio of free cyanide to total cyanide is greater than 0.8 and the AOX formation potential is below 1.0 kg/m³ of wastewater treated. When these limits are not met, wet air oxidation or sulfur-based conversion to thiocyanate is selected instead.

Sulfur-based treatment using sodium thiosulfate or sodium polysulfide converts cyanide to thiocyanate at pH 9.0–10.0 and 60–80°C in 15–30 min. The resulting thiocyanate is less toxic but still contributes to nitrogen demand and cannot be discharged without subsequent biological nitrification. Therefore, the sulfur route is used as a pre-treatment for biological polishing, not as a standalone cyanide compliance step.

When Wet Air Oxidation Replaces Hypochlorite for High-AOX Nitrile Wastewater

Wet air oxidation becomes technically preferable when cyanide is accompanied by high concentrations of acrylonitrile oligomers, heavy nitriles, and ammonium sulfate, because the hypochlorite demand and organic chlorine by-product formation increase sharply with COD. Commercial WAO systems operate in the temperature range 200–320°C and total pressure 2–15 MPa, using air or oxygen-enriched air as the oxidant. Liquid residence time in the reactor tube or bubble column is maintained between 30 min and 120 min. Cyanide is oxidized to carbon dioxide and nitrate or nitrogen, while organic nitriles are hydrolyzed and oxidized. The treated liquid leaves the reactor through a pressure letdown valve, and the off-gas is quenched and routed to a thermal oxidizer because it can contain carbon monoxide, ammonia, and trace volatile nitriles.

WAO is the preferred technology where acrylonitrile wastewater contains total cyanide above 100 mg/L and COD above 20,000 mg/L, because alkaline chlorination at such loadings can produce unacceptable trihalomethane and chloroamine concentrations. The capital cost is higher than hypochlorite oxidation, and the reactor materials require high-alloy stainless steel or titanium-clad construction to resist stress corrosion cracking in the presence of chloride and sulfate at high temperature. Published performance data for WAO applied specifically to acrylonitrile wastewater is limited, but engineering designs for comparable petrochemical cyanide streams specify a destruction efficiency of 99% for total cyanide and 70–95% for COD.

Operational boundaries for WAO include the need to preheat the raw wastewater to at least 180°C before oxidation becomes self-sustaining, and the risk of salts precipitation in the reactor when ammonium sulfate is present above 5 wt%. The purge stream from WAO off-gas condensation typically contains ammonia and residual carbonate, which must be returned to the biological treatment train. The combination of WAO with downstream biological denitrification is therefore documented in petrochemical wastewater treatment schemes, but published data for acrylonitrile-specific wastewater is limited.

Thermal Oxidizer and Caustic Scrubber Coupling for HCN-Bearing Off-Gas

Absorber off-gas containing hydrogen cyanide is collected from the absorber overhead, acetonitrile recovery column vents, and process tank blanketing systems. The combined gas stream is compressed or drawn by a blower and sent to a refractory-lined thermal oxidizer. Destruction of HCN requires oxidation at 850–1100°C with a residence time of 0.5–2.0 s and excess oxygen maintained above 3 mol% on a dry basis. Natural gas burners are used to maintain the combustion chamber temperature, and a waste heat boiler or quench column cools the flue gas to below 200°C before it enters a caustic scrubber.

The caustic scrubber following thermal oxidation removes residual acid gases and any unconverted hydrogen cyanide by absorption in sodium hydroxide solution at pH 10.0–11.0. Scrubbed liquor is periodically purged to the wastewater treatment system, where residual cyanide is measured by ISO 14403-2:2017. The stack gas is monitored for hydrogen cyanide by FTIR or by periodic sorbent-tube sampling with ion chromatography. Continuous emission monitoring systems for nitrogen oxides and carbon monoxide are required at the stack because thermal destruction of HCN and nitriles increases NOx formation. Selective non-catalytic reduction with aqueous ammonia or urea injection at 850–1100°C is used to control NOx, but the ammonia slip must be limited to avoid forming ammonium salts in downstream ductwork.

Where thermal oxidation is not permitted because of NOx allocation limits, cyanide-bearing off-gas can be routed to a packed caustic scrubber without prior combustion. In this configuration, hydrogen cyanide is absorbed at pH 11.0–12.0 using recirculated sodium hydroxide, and the spent scrubber liquor is treated with hypochlorite or WAO. However, carbon monoxide, ammonia, and volatile organic compounds pass through the scrubber and require separate control. The choice between direct caustic scrubbing and thermal oxidation is determined by the combined HCN, CO, and VOC load, the available steam system, and the NOx permit limit.