Cyclohexanone (CAS 108-94-1) is a six-carbon cyclic ketone with molecular formula C₆H₁₀O and molar mass 98.15 g/mol. The compound is recovered from cyclohexane oxidation and phenol hydrogenation routes, and it is consumed in caprolactam, adipic acid, and solvent applications. Distillation, storage, and water-washing operations are constrained by boiling point, flash point, and water solubility boundaries. The atmospheric-pressure boiling point is 155.6 °C at 101.325 kPa; the closed-cup flash point is 44 °C; and the water solubility is 87 g/L at 20 °C. These values are used for process mass balances and safety calculations. The table below records the physical property set referenced across supplier certificates of analysis and engineering specification sheets.

Commercial cyclohexanone may contain cyclohexanol, cyclohexene, and low-boiling aliphatic ketones. The distillation range under ASTM D1078 generally opens below 155 °C and concludes at 156 °C for the 95 vol% recovery point when the material is dry. Water in the feed alters the apparent boiling range because cyclohexanone and water form a minimum-boiling azeotrope. In plant practice, water-wet condensate from the column is routed to a decanter; the upper organic layer returns to the column and the lower aqueous layer is sent to solvent recovery. The presence of 0.5–2.0 wt% water is sufficient to require a pre-drying step when the material is intended for moisture-sensitive polyurethane or epoxy formulations.

What Process Conditions Shift Cyclohexanone’s Boiling Point from Published Values?

Absolute pressure is the primary variable that shifts the boiling point from the atmospheric value. At 20 kPa absolute, the vapor-liquid equilibrium temperature is approximately 104 °C; at 5 kPa absolute, it is approximately 68 °C. These estimates are derived from the normal boiling point and vapor pressure curve; reboiler settings and condenser cooling-water limits are selected accordingly. In continuous KA-oil distillation, the cyclohexanone-enriched side draw is taken from the vacuum column at a tray temperature between 85 °C and 95 °C, depending on top pressure and residual cyclohexanol content. The separation from cyclohexanol, which boils at 161 °C at atmospheric pressure, is the chief energy cost. If the reboiler wall temperature exceeds 130 °C, cyclohexanone undergoes acid- or base-catalyzed aldol condensation to high-boiling dimers, increasing pressure drop and fouling structured packing. Published data for specific column temperature profiles is limited because tray efficiency varies with feed ratio, column diameter, and fouling history. Process control therefore relies on top-pressure compensation and side-draw composition analysis rather than a single boiling-point setpoint.

Cyclohexanone dissolves in water at 87 g/L at 20 °C. Solubility is depressed by electrolyte addition; sodium chloride or sodium sulfate is sometimes used in workup to reduce the organic content of the aqueous phase and improve decantation. The aqueous stream leaving a cyclohexanone wash contains enough dissolved ketone to exceed biological treatment inhibition thresholds if not recovered. The octanol/water partition coefficient of 0.81 indicates only moderate extraction efficiency into toluene or cyclohexane; multistage countercurrent extraction with at least 3 theoretical stages is required to reduce water-phase concentration below 1 wt%. Steam stripping is technically feasible but energy-intensive because the Henry’s law constant is relatively low. Adsorption on macroreticular resin is used for low-concentration streams; regeneration produces a ketone-water mixture that must be decanted and redistilled.

Flash Point Control in Ketone Storage and Transfer

The closed-cup flash point of 44 °C places cyclohexanone in GHS flammable liquid category 3 with hazard statement H226 and in NFPA 30 Class II combustible liquid storage. Although the flash point is above ambient in temperate climates, an uninsulated tank in direct sun can reach a headspace temperature above 44 °C; fixed storage vessels are therefore equipped with nitrogen blanketing and pressure/vacuum conservation vents. At the flash point, the vapor pressure is approximately 1.1 kPa, which corresponds to the lower explosible limit concentration in air at atmospheric pressure. Transfer lines and drums are bonded and grounded to a resistance below <1 × 10⁶ Ω in accordance with NFPA 77. Centrifugal pumps with single mechanical seals are avoided in manned process areas because shaft leakage at discharge temperatures above 60 °C can produce a flammable atmosphere near the pump casing. Closed-loop sampling with a spring-loaded cylinder and vapor return is specified over open-container sampling. Lower and upper explosible limits in air are 1.1 vol% and 9.4 vol%; the autoignition temperature is 420 °C, which influences area classification and heat transfer fluid selection.

In vinyl chloride/vinyl acetate copolymer coatings, cyclohexanone is used as a true solvent at addition levels of 10–40 wt%. Dissolution is conducted at 60–80 °C in a low-shear turbine agitator. After cooling to 25 °C, a 20 wt% resin solution commonly exhibits a Brookfield viscosity of 500–3,000 mPa·s, depending on copolymer molecular weight and acetate content. The vapor pressure of cyclohexanone at 25 °C is 0.47 kPa, compared with 12.6 kPa for methyl ethyl ketone; this lower vapor pressure retards skinning in spray-applied lacquers and allows flow-out in high-humidity conditions. For nitrocellulose and acrylic lacquers, cyclohexanone is blended with aromatic hydrocarbons or n-butyl acetate. The aromatic content is limited by resin solubility; excessive toluene or xylene addition causes resin precipitation and film whiteness. In polyurethane leather finishes, moisture content is controlled below <0.05 wt% by Karl Fischer titration (ASTM E203) to prevent reaction with isocyanate hardeners.

When Reduced-Pressure Distillation Shifts Boiling Point and Peroxide Accumulation

Cyclohexanone exposed to air at 25 °C slowly autoxidizes to hydroperoxide species and acidic degradation products. Ultraviolet light and dissolved transition metals, particularly copper and iron above 1 ppm, accelerate autoxidation. Recovered solvent stored longer than 90 days should be tested by iodometric titration, such as ASTM E298, and kept below 30 mg/kg active oxygen before distillation. Distillation residues are not stripped to dryness; a minimum pot volume of 5–10 vol% is held to avoid concentrating nonvolatile peroxides. Vacuum operation at 20 kPa or 5 kPa keeps the reboiler temperature below 130 °C, reducing the likelihood of exothermic peroxide decomposition. In a wiped-film evaporator used for high-boiler removal, the jacket is maintained at 100–120 °C and 5 kPa absolute; distillate is condensed at 5–10 °C and sent to a water-ketone decanter. The vacuum pump discharge is protected by a flame arrestor and routed to thermal oxidation or activated carbon.

Captive use of cyclohexanone in caprolactam production consumes the largest share of output. The ketone reacts with hydroxylamine sulfate at 80–90 °C and pH 4–6 to form cyclohexanone oxime; the oxime is rearranged in oleum at 90–120 °C. In adipic acid synthesis, cyclohexanone is oxidized by nitric acid in a continuous stirred-tank reactor with a copper-vanadium catalyst at 60–90 °C. The off-gas contains nitrogen oxides and is recovered as nitric acid. Dicarboxylic acid by-products, including glutaric and succinic acids, are separated by crystallization; crystallizer temperature and pH determine the yield loss. Seals in these processes are normally polytetrafluoroethylene or perfluoroelastomer because cyclohexanone swells nitrile rubber and EPDM in continuous contact.

Gas chromatographic analysis of commercial cyclohexanone commonly uses a polyethylene glycol capillary column, such as a 30 m × 0.53 mm × 1.0 µm WAX column, with flame ionization detection. The oven program starts at 60 °C and rises to 240 °C at 10 °C/min. Principal impurities quantified include cyclohexanol, cyclohexene, 2-cyclohexen-1-one, and high-boiling dimers. This method is used for lot release, process troubleshooting, and recovery column feed analysis. For water-containing streams, azeotropic distillation or Karl Fischer moisture analysis is performed before gas chromatography because water affects the polar stationary phase and alters retention time reproducibility.

Under CLP Regulation (EC) No 1272/2008, cyclohexanone carries Flam. Liq. 3 H226, Acute Tox. 4 H332, Skin Irrit. 2 H315, Eye Irrit. 2 H319, and STOT SE 3 H335. The material is not classified as mutagenic, carcinogenic, or reproductive toxic under Annex VI. Wastewater releases are controlled under the Industrial Emissions Directive as a volatile organic compound. Abatement is normally by thermal oxidation at 750–850 °C with a residence time above 0.5 s. Activated carbon adsorption is used for lower-concentration vent streams, but ketone desorption requires steam at 110–130 °C; admission of air to a hot carbon bed creates a fire risk and must be avoided until cooling below 40 °C.

Material compatibility data for cyclohexanone indicate severe swelling of nitrile and EPDM elastomers after 72 h immersion at 25 °C. Gaskets and O-rings in long-term service are specified in polytetrafluoroethylene or fluoroelastomer grades. High-density polyethylene may be used for short-term storage, but lower-molecular-weight grades can stress-crack; crosslinked polyethylene and fluorinated polymers are preferred for long-term wetted surfaces. Stainless steel 316L is acceptable for storage and distillation, but carbon steel should be passivated or lined if rust contamination is to be kept below 1 ppm iron because dissolved iron accelerates color formation and autoxidation.