Cyclohexanone (CAS 108-94-1) is manufactured principally by the air oxidation of cyclohexane to a cyclohexanone–cyclohexanol mixture, followed by dehydrogenation of the cyclohexanol, or by direct phenol hydrogenation. The compound has a normal boiling point of 155.6 °C, a flash point of 44 °C, a water solubility of approximately 8.7 g/100 mL at 20 °C, and a vapor pressure near 0.5 kPa at 20 °C. These properties define the boundaries of cyclohexanone wastewater & emission control: the ketone is volatile enough to require vapor controls on storage and transfer equipment, yet water-soluble enough to appear in aqueous process streams at milligram-per-liter to gram-per-liter concentrations. Regulatory control is framed by the hazardous organic NESHAP under 40 CFR Part 63 in the United States and by Directive 2010/75/EU in the European Union, with monitoring methods drawn from EPA, EN, and ISO series.

What Aqueous and Vent Streams Require Segregation in a Cyclohexanone Production Train?

In the cyclohexane oxidation route, the reactor operates in the liquid phase at approximately 150–170 °C and 0.8–1.2 MPa. The product stream contains unconverted cyclohexane, cyclohexanone, cyclohexanol, and a range of C1–C6 carboxylic acids produced by ring-opening side reactions. Water washing of the oxidation effluent extracts these polar acids and a portion of the ketone; the resulting aqueous phase is acidic and, on full-scale units, is neutralised to pH 7.0–8.5 with sodium hydroxide or soda ash before recovery. Neutralisation is not a compliance step alone but a fouling-control requirement: acidic condensate promotes acid-catalysed aldol condensation of cyclohexanone, which deposits on reboiler tubes and stripper packing.

Distillation overheads from the cyclohexanone purification train form a second aqueous stream. Because cyclohexanone forms a minimum-boiling azeotrope with water at approximately 94–95 °C, condensed overhead water is saturated with the ketone and cannot be discharged as non-contact cooling water. Tank vents and loading operations are the main atmospheric release points. Internal floating-roof tanks are specified for cyclohexanone storage because the vapor pressure at 20 °C exceeds common volatile-liquid thresholds, and the flash point of 44 °C places the stored liquid in flammable-liquid service.

The vacuum system serving the purification columns is another source of contaminated water. Liquid-ring vacuum pumps or steam ejectors discharge a mixed-phase stream containing cyclohexanone, water, and organic acids. Full-scale units route this stream to a hotwell separator where the organic phase is decanted; the water phase is returned to the steam stripper. Where a single hotwell is used, the pH of the recirculating water is kept above 7.0 to reduce corrosion and suppress acid-catalysed condensation. The blowdown rate is set by dissolved solids buildup rather than by ketone concentration. Published data for optimum hotwell blowdown rates in cyclohexanone service is limited.

When cyclohexanone is produced by phenol hydrogenation instead of cyclohexane oxidation, the wastewater profile shifts toward unconverted phenol and cyclohexanol rather than carboxylic acids. The hydrogenation reactor is typically operated in the vapor phase over a supported platinum or palladium catalyst. Aqueous condensates from this route are lower in organic acids but can contain phenol at concentrations requiring recovery before biological treatment. Steam stripping of phenol is strongly pH-dependent; at pH 10–11 phenol exists primarily as phenolate, which has lower volatility than molecular phenol and therefore reduces stripping efficiency. For this reason, solvent extraction of phenol from the acidic or neutral condensate is generally installed before the end-of-pipe stripping and biotreatment train. The extracted raffinate is then combined with the oxidation-route wastewater only after phase separation and pH adjustment.

Water bottoms from cyclohexanone storage tanks are drained periodically and constitute a small but concentrated waste stream. The tank mixture separates into an organic layer and a saturated water layer. This water layer is not discharged directly; it is pumped to a slop oil/water separator and then to the wastewater stripper. Because the tank bottom water is saturated at ambient temperature, the concentration of cyclohexanone can approach the aqueous solubility limit, producing a load spike in the downstream treatment train. Online pH and conductivity cells are installed in the transfer line to detect phase carryover and to stop the pump before organic phase enters the biological unit.

Steam Stripping, Carbon Adsorption, and Aerobic Polishing

Steam stripping removes cyclohexanone from wastewater at feed temperatures of 95–105 °C and column pressures near atmospheric, using the cyclohexanone–water azeotrope at 94–95 °C to carry the ketone into the overhead vapor. The overhead is condensed and separated into an organic-rich layer and an aqueous layer; the organic layer is returned to the process, while the aqueous layer may be refluxed or sent forward with the stripped feed. Packed columns used in this service are designed with liquid redistribution at intervals of 3–5 m of bed depth to avoid wall channeling. A limitation of steam stripping is its inability to remove nonvolatile carboxylic acids; the stripped bottoms therefore still contain acetic, propionic, and adipic acids and residual COD.

Granular activated carbon is applied to clarified water after stripping when discharge limits for cyclohexanone are below 0.1–1.0 mg/L. Carbon beds operating at 20–40 °C remove the ketone by physical adsorption, but acidic feed must be avoided because cyclohexanone can undergo acid-catalysed condensation on the carbon surface, increasing pressure drop and reducing working capacity. Published data for the working capacity of granular activated carbon for cyclohexanone in real stripped wastewater is limited; design is therefore based on site-specific isotherm and rapid small-scale column testing. Aerobic polishing in activated sludge or moving-bed biofilm reactors removes residual biodegradable organics. Full-scale units treating KA oil wastewater commonly maintain dissolved oxygen at 2–4 mg/L and mixed-liquor suspended solids in the range of 3,000–5,000 mg/L; reported removal efficiencies for cyclohexanone are not constant across plants because of acclimation differences and variable salt loads.

Where discharge limits for cyclohexanone are below the practical detection threshold of biological treatment, advanced oxidation processes are installed after the stripper. Hydrogen peroxide with ferrous ion at pH 2.5–3.5 and a peroxide-to-COD mass ratio of 0.5–1.5 is used to oxidise residual ketone and aromatic intermediates; however, the process generates ferric hydroxide sludge that must be dewatered. Ozone with hydrogen peroxide at pH 7–9 avoids sludge but requires an off-gas destructor catalyst to prevent ozone release. Published data for cyclohexanone-specific Fenton oxidation in real wastewater is limited; treatability studies are required because carboxylic acid alkalinity consumes hydroxyl radicals and fluctuates with batch oxidation severity.

When Biological Treatment Is Applied to Carboxylic Acid-Laden Condensates

Biological treatment is the final barrier for soluble organic carbon after physical recovery. The carboxylic acid-laden condensate from cyclohexanone purification has a low raw pH and high chemical oxygen demand, so equalisation and nutrient addition are required before biotreatment. A sequencing batch reactor or membrane bioreactor is used where total nitrogen limits below 10 mg/L apply; the membrane bioreactor retains slow-growing ketone-degrading biomass at solids retention times of 20–40 days. Respirometric studies and full-scale trials indicate that cyclohexanone is aerobically biodegradable under the activated sludge simulation test described in ISO 11733:2004, but published degradation rates vary by more than an order of magnitude. That variability imposes a requirement for confirmatory pilot testing rather than direct adoption of literature kinetic constants.

Nitrification inhibition can occur when the acetic and propionic acid load creates a food-to-microorganism ratio above 0.25 kg BOD/kg MLSS·d. In such cases, the dissolved oxygen setpoint is raised to 3–4 mg/L, and alkalinity is supplemented to offset the acidity released by nitrification. Full-scale activated sludge units receiving unstripped wastewater have reported poor settleability and sludge bulking when influent cyclohexanone transiently exceeds 200 mg/L. Equalisation basins are therefore sized for 8–12 h of hydraulic retention at design flow, and online COD analyzers trim the feed to the biological reactor. Wastewater discharge compliance is typically monitored as total organic carbon or chemical oxygen demand under EN 1484:1997 or ISO 6060:1989, with cyclohexanone-specific verification by gas chromatography–mass spectrometry after liquid-liquid extraction.

Caustic wet scrubbers are used for low-volume continuous vents where a thermal oxidizer is uneconomical. A packed scrubber with 2–4 m of structured packing and liquid superficial velocity of 10–30 m³/m²·h transfers cyclohexanone from gas to water at pH 8–10. The scrubbing liquid is cooled to 10–20 °C to reduce the equilibrium vapor concentration; blowdown is returned to the wastewater stripper. Scrubber efficiency decreases when the gas stream contains cyclohexane or other poorly soluble hydrocarbons, because they pass through the absorber and require a downstream carbon canister. In such mixed streams, the carbon canister is often specified with a working capacity for total VOC of 5–15 g VOC/kg carbon and a maximum bed temperature rise of 20 °C to avoid exothermic reactions on the carbon. Wet scrubbers are not suitable for high-flow intermittent loading operations because the turndown ratio of the packing is limited; liquid distributor turndown below 30% of design causes dry spots and gas bypass.

Thermal oxidation remains the reference control for storage-tank and loading vent gases

Vent streams from storage tanks, loading racks, and stripper overhead receivers contain cyclohexanone at concentrations that fluctuate with tank level, ambient temperature, and transfer operations. The reference destruction route is a thermal oxidizer operated at 1 050–1 150 °C and 0.5–1.0 s residence time. Cyclohexanone destruction and removal efficiency is generally specified at 99% or greater, with stack verification conducted by total organic carbon measurement according to EN 12619:2013 or by EPA Method 18 for specific VOCs. Catalytic oxidation at 300–450 °C is technically feasible for continuous low-concentration vents, but catalyst poisoning by sulfur, phosphorus, or siloxanes requires upstream knockout drums, mesh-pad mist elimination, and periodic catalyst replacement.

Condensers operating at −10 °C to 5 °C recover cyclohexanone from concentrated vent gas, but condensation alone is rarely adequate for a 20 mg C/m³ stack limit because the vapor pressure of the ketone at condenser outlet conditions remains above the target. Wet scrubbing with water or dilute alkali at pH 8–10 can be applied where the water solubility of 8.7 g/100 mL at 20 °C permits efficient transfer from vapor to aqueous phase. Scrubber blowdown must be routed to the wastewater stripper rather than to a stormwater sewer.

Pumps, valves, and connectors in cyclohexanone service are subject to leak detection and repair under 40 CFR Part 63 Subpart H or equivalent state programs. A plant with 2,000–5,000 regulated components screens at 500 ppmv instrument response using EPA Method 21. Full-scale LDAR records show pump seals and open-ended lines as the dominant leak sources. Pumps in cyclohexanone service are generally specified with dual mechanical seals and a barrier fluid because the ketone softens single elastomer seals, while shaft seal leaks are collected in closed drain systems. Loading arms are equipped with vapor return lines to the carbon bed or thermal oxidizer; bottom loading without vapor recovery is not used where national VOC rules apply.