Cyclohexane (CAS 110-82-7) is a saturated alicyclic hydrocarbon with the molecular formula C₆H₁₂ and a molar mass of 84.16 g/mol. At 101.325 kPa, the normal boiling point is 80.7 °C, the freezing point is 6.5 °C, and the closed-cup flash point is -18 °C. The liquid density is 0.779 g/cm³ at 20 °C, and the vapor pressure reaches approximately 13.0 kPa at 25 °C. The flammable envelope in air extends from 1.3 vol% to 8.4 vol%. As a low-polarity solvent, cyclohexane exhibits a Hildebrand solubility parameter near 16.8 MPa^1/2 and a dipole moment below 0.3 D; it dissolves hydrocarbon resins and nonpolar process oils but is immiscible with water, with an aqueous solubility of about 55 mg/L at 25 °C. Commercial benzene hydrogenation followed by extractive or solvent-assisted distillation yields a 99.9% minimum purity grade with benzene below 50 mg/kg, sulfur below 1 mg/kg, and water below 50 mg/kg when a finishing column and molecular sieve dryer are used.

Pure Cyclohexane 99.9% Solvent: Caprolactam & Adipic Acid Synthesis

The solvent grade designated as Pure Cyclohexane 99.9% Solvent: Caprolactam & Adipic Acid Synthesis enters the caprolactam and adipic acid routes primarily as a feedstock to liquid-phase air oxidation rather than as an inert diluent. In the oxidation train, cyclohexane is converted to a cyclohexanol/cyclohexanone mixture known as KA oil, and the unreacted cyclohexane is recovered by condensation and recycled. The 99.9% minimum assay is not a rhetorical purity claim; it limits oxygenated and sulfur-bearing impurities that alter the radical-chain oxidation sequence. Residual benzene above 100 mg/kg competes for hydroxyl radical attack and produces phenolic oxygenates that increase downstream distillation fouling. Sulfur above 1 mg/kg suppresses the cobalt(II) carboxylate initiation step. A production-scale bubble column with internal cooling coils and a liquid height-to-diameter ratio of 3:1 to 6:1 typically operates at 4–6% conversion per pass to keep KA oil selectivity at 78–85%. The specification frame for the 99.9% solvent is summarized below.

Parameter	Unit	Typical limit	Test method
Purity, GC normalized	% m/m	≥99.9%	ASTM D5309-17
Benzene	mg/kg	≤50	GC-FID per ASTM D5309-17
Water	mg/kg	≤50	ASTM D6304
Total sulfur	mg/kg	≤1	ASTM D5453
Non-volatile residue	mg/100 mL	≤5	ASTM D1353
Color, Pt-Co	units	≤10	ASTM D1209
Distillation range	°C	80.0–81.0	ASTM D1078

Fresh cyclohexane feed with water below 50 mg/kg is heated through a shell-and-tube feed/effluent exchanger and introduced into the oxidation reactor, where the off-gas is cooled to recover entrained hydrocarbon. The crude oxidate contains cyclohexanol, cyclohexanone, unreacted cyclohexane, water, and C₁–C₆ mono- and dicarboxylic acids. Saponification and vacuum distillation separate KA oil from high-boiling acid esters. In a phenol-free caprolactam plant, cyclohexanol is dehydrogenated over a copper–zinc catalyst at 250–300 °C and atmospheric pressure to generate additional cyclohexanone. Cyclohexanone is then condensed with hydroxylamine sulfate at pH 6–8 and 80–90 °C to form cyclohexanone oxime. Recycled cyclohexane from the oxidate distillation is caustic-washed and monitored for methylcyclopentane, because that impurity can be dehydrogenated to methylcyclopentanone and behaves similarly to cyclohexanone in the oximation stage, degrading caprolactam purity.

What Limits Cyclohexane Conversion Per Pass in the Air Oxidation Reactor?

The upper bound of conversion is set by selectivity, not by equilibrium. Cyclohexane is abstracted to a cyclohexyl radical and then oxidized to cyclohexyl hydroperoxide. Above 170 °C, the hydroperoxide decomposes bimolecularly to cyclohexanone and water, but ring-opening to adipic, glutaric, and succinic acids also accelerates. Therefore, commercial oxidation reactors deliberately operate at 4–6% conversion per pass and 155–165 °C to hold KA oil selectivity at 78–85%. The vessel is a bubble column or stirred gas-liquid reactor; in a bubble column, superficial gas velocity is typically 0.02–0.06 m/s, and the liquid height-to-diameter ratio is 3:1 to 6:1. Internal cooling coils using tempered water at 120–140 °C remove the exothermic reaction heat, with published heat removal duties reported in the order of 1,500–2,000 kJ/kg of KA oil produced. Oxygen concentration in the off-gas is controlled below 2.5 vol% after hydrocarbon condensation to remain outside the flammable envelope. Trace cobalt deactivation by organic acids becomes pronounced when the acid number of the reactor bottoms exceeds 30 mg KOH/g; continuous withdrawal of a small purge prevents acid accumulation. Published data for this specific configuration is limited at the production scale; the numerical windows are representative operating envelopes reported in open engineering literature.

Process step	Key feed quality	Operating window	Conversion/selectivity metric
Benzene hydrogenation	benzene min 99.0%	Ni/Al2O3, 150–250 °C, 1.5–3.0 MPa	cyclohexane yield above 99.8 mol%
Cyclohexane oxidation	cyclohexane ≥99.9%, sulfur <1 mg/kg	Co/Mn carboxylate, 155–165 °C, 0.9–1.2 MPa	conversion 4–6%, KA oil selectivity 78–85%
Cyclohexanol dehydrogenation	KA oil cyclohexanol cut	Cu/Zn, 250–300 °C, atmospheric pressure	cyclohexanone selectivity above 98%
Oximation	cyclohexanone ≥99.8%	hydroxylamine sulfate, pH 6–8, 80–90 °C	oxime yield above 98%
KA oil nitric acid oxidation	KA oil, water <1,000 mg/kg	50–60 wt% HNO3, Cu/V catalyst, 60–90 °C	adipic acid yield 92–94%

The adipic acid route from the same KA oil intermediate begins with nitric acid oxidation in a stirred reactor under reflux at 60–90 °C. The cyclohexanol/cyclohexanone feed is added to 50–60 wt% nitric acid containing dissolved copper and vanadium salts; the molar excess of nitric acid drives ring cleavage while suppressing glutaric and succinic acid byproducts. The vent gas contains N₂O, NO_x, and CO₂. In modern plants, N₂O is routed to a thermal decomposition or catalytic reduction unit. Published plant-specific N₂O destruction efficiency data vary with catalyst age; a fresh catalyst bed in tail-gas abatement typically achieves 95–99% N₂O removal, but the metric declines as the catalyst surface is fouled by nitric acid mist. Crude adipic acid is crystallized, washed with demineralized water, and dried to a water content below 0.1 wt% before storage.

When Make-Up Cyclohexane Purity Falls Below 99.5%, Downstream Distillation and Oximation Units Carry Higher Loads

At 99.5% assay, the difference from 99.9% is not in the bulk cyclohexane molecule but in the accumulation of hydrogenation byproducts and saturated isomers. Methylcyclopentane, methylcyclohexane, and n-hexane are the principal impurities. Methylcyclopentane increases the load on the KA oil distillation column, where methylcyclopentanone and cyclohexanone boil within a narrow range; a rise of methylcyclopentane in fresh feed from 100 mg/kg to 500 mg/kg increases the reflux ratio required to hold cyclohexanone overhead purity at 99.8%. Methylcyclohexane is less reactive in the radical oxidation but accumulates in the recycle loop; above 0.5 wt% in the reactor feed, it lowers cyclohexane partial pressure and can reduce KA oil productivity by 2–4% relative to clean feed. In the oximation reactor, methylcyclopentanone competes for hydroxylamine, forming the corresponding oxime that co-crystallizes with caprolactam and depresses the melting point. Published data for this specific configuration is limited; however, batch-to-batch variance in recycled cyclohexane has been observed on production lines as an increase in the cyclohexanol-to-cyclohexanone ratio in the oxidate when the paraffinic impurity fraction rises. To limit these effects, the finishing hydrogenation catalyst is operated with a hydrogen-to-benzene molar ratio above 3.0, and the stabilizer column removes light ends so that methylcyclopentane plus n-hexane remains below 200 mg/kg in the 99.9% solvent grade.

Closing of the solvent day tank is required because the closed-cup flash point is -18 °C and the vapor pressure at ambient temperature is sufficient to form a flammable mixture in the headspace. Nitrogen blanketing at 5–10 kPa gauge, flame arrestors, and bonded grounding are standard on production lines. The solvent must not be stored in contact with strong oxidizers, especially nitric acid above 40 wt%, because condensed-phase runaway oxidation can occur. If cyclohexane is used as a process solvent outside the oxidation train, moisture-sensitive reactions require pre-drying through a 3 Å molecular sieve column to a water content below 10 mg/kg. Prolonged exposure to air in translucent containers can generate trace hydroperoxides; these are removed by passing the solvent through activated alumina before charging into catalyst-sensitive hydrogenation or polymerization steps.