Acrylonitrile (prop-2-enenitrile, C₃H₃N, molecular weight 53.06 g/mol) is a liquid monomer with a normal boiling point of 77.3 °C at 101.3 kPa, density 0.806 g/cm³ at 20 °C, lower flammability limit in air of 3.1 vol%, and upper flammability limit of 17 vol%. The material is classified for transport as UN 1093, Class 3, Packing Group I. Commercial production is dominated by vapour-phase propylene ammoxidation over bismuth molybdate-based fluidised-bed catalysts at 400–510 °C and 1–2 bar, with ammonia and oxygen as co-reactants. Reactor effluent passes through water quench, absorption, and extractive distillation to give monomer of at least 99.5 wt% purity. Acrylonitrile is stabilised with 35–50 mg/kg 4-methoxyphenol (MEHQ) and maintained under oxygen to prevent radical polymerisation during storage. Polyacrylonitrile (PAN): The Acrylic-Fibre Chain That Consumes the Most Acrylonitrile is the largest single derivative, absorbing an estimated 55–60% of global acrylonitrile production, principally as acrylic staple fibre and filament. The nitrile group that imparts solvent resistance and fibre-forming capacity also creates downstream constraints in polymerisation, spinning, thermal treatment, and storage.

Reactor effluent from propylene ammoxidation is a dilute gas stream containing acrylonitrile, acetonitrile, hydrogen cyanide, and unconverted propylene. The quench column contacts the gas with dilute sulfuric acid to remove unreacted ammonia as ammonium sulfate, while heavy nitriles and acrylic acid are rejected in the absorber and recovery column. Extractive distillation with water or solvent separates acrylonitrile from acetonitrile; the product then passes through a finishing column to remove oxazole and other light impurities. Standard commercial specification includes acrylonitrile 99.5 wt% minimum, water 0.25–0.50 wt%, acetone 100 mg/kg maximum, and MEHQ 35–50 mg/kg. Process licensors report acrylonitrile carbon yield of 80–84%, with byproduct acetonitrile yields of 2–4 wt% and hydrogen cyanide yields of 3–6 wt%; actual values depend on catalyst age and air-to-ammonia ratio. The fluidised-bed reactor uses in-bed cooling coils to remove reaction heat, which can reach 600–700 kJ/mol of acrylonitrile produced. The main side reactions are ammoxidation of propylene to acetonitrile and oxidation to oxides of carbon.

Does PAN Polymerization Route Dictate Monomer Utilization Efficiency?

Monomer utilisation in the PAN chain depends less on downstream fibre spinning than on the polymerisation and monomer-recovery loop. Homopolymer PAN is not melt-processable because its literature dry melting endotherm near 317 °C is immediately followed by degradation; therefore, acrylic fibre production uses solvent-assisted or aqueous heterogeneous polymerisation. In solution polymerisation, acrylonitrile, methyl acrylate at 5–10 wt% of monomer feed, and itaconic acid or sodium styrene sulfonate at 1–2 wt% are charged into dimethylacetamide, dimethylformamide, or aqueous sodium thiocyanate. The polymerisation is run as a continuous stirred-tank train with residence times of 20–90 min and temperature maintained at 50–65 °C by jacket cooling and reflux condensers. Conversion per pass is deliberately limited to 70–85% to control viscosity and molecular weight distribution; unreacted acrylonitrile is recovered by vacuum stripping at 60–70 °C, condensed, and returned to the monomer feed. Residual acrylonitrile in the dope after devolatilisation is typically controlled to below 50 mg/kg in spinning lines that comply with fibre contact requirements. Weight-average molecular weight for textile acrylic fibre is normally 90,000–140,000 g/mol, measured by gel permeation chromatography against polymethyl methacrylate standards. Aqueous suspension polymerisation offers lower solvent viscosity and easier powder handling, but requires post-reactor filtration, washing to remove oligomers, and drying at 60–80 °C before dissolution in a solvent. Published data for exact residual monomer limits in suspension-grade PAN varies by licensor; the critical process conflict is balancing high conversion against chain-transfer reactions that reduce molecular weight and increase gel defects.

Commercial wet-spinning of PAN fibre converts a filtered, degassed dope into coagulated filaments through spinneret capillaries of 30–70 µm diameter. Dope filtration is performed with depth filters rated at 1–5 µm absolute retention to remove gel particles that would otherwise cause filament breaks. The spinneret assembly is immersed in a coagulation bath; solvent type determines coagulation rate, skin formation, and downstream drawing behaviour. In the sodium thiocyanate route, PAN dope at 10–12 wt% solids enters a bath containing 10–12 wt% NaSCN at 0–10 °C, where counter-diffusion of solvent and coagulant produces a gel filament. In the dimethylformamide route, dope solids are 20–25 wt% and the bath is 20–40 °C with 40–60 wt% DMF. Positive or negative jet stretch is applied between spinneret and take-up rolls to orient the coagulated gel; typical jet stretch values range from 0.8 to 2.0 depending on solvent and bath temperature. The filaments are then washed countercurrently, drawn in hot water or steam at 90–98 °C, given a secondary dry draw at 120–150 °C, and annealed to control shrinkage. Total draw ratios for textile acrylic fibre range from 6× to 18×, producing final linear densities of 0.8–1.7 dtex for staple and 1.0–1.3 dtex for carbon-fibre precursor. Fibre tensile properties are tested under ISO 5079 or ASTM D3822; typical conditioned tenacity for textile acrylic fibre is 0.28–0.35 N/tex, elongation at break 25–45%, and hot-water shrinkage can exceed 10% unless annealed. The limiting operational parameter is not monomer purity alone, but the solvent/coagulant balance: excessive solvent in the bath collapses the filament surface, while insufficient solvent creates a dense skin that restricts subsequent drawing.

Oxidative Stabilization Before Carbonization in PAN-Based Precursor

Where PAN is used as carbon-fibre precursor, the acrylic-fibre chain that consumes the most acrylonitrile must pass through an oxidative stabilisation step before carbonization. Stabilisation converts the thermoplastic PAN structure into a thermally stable ladder polymer through cyclization, dehydrogenation, and oxygen uptake. Commercial oxidation ovens are divided into multiple temperature zones, typically starting at 180–220 °C and ending at 260–290 °C, with total residence times of 60–120 min for 1.0 dtex precursor tows. The heating rate in the early zone is held below 3 °C/min because the cyclization reaction is strongly exothermic; differential scanning calorimetry of PAN precursor at 10 °C/min under nitrogen commonly shows an exothermic peak onset near 240 °C and peak maximum near 280 °C, with energy release that can exceed 1000 J/g. If centre-line temperature in the tow exceeds the oven setpoint by more than 10–15 °C, autocatalytic degradation can cause filament fusion and tow breakage. Tension control is applied across the oxidation line at 0.5–1.5 MPa stress on the tow bundle to maintain molecular orientation and prevent physical shrinkage. The resulting oxidised PAN fibre has a density of 1.35–1.45 g/cm³, oxygen content of 8–12 wt%, and an elongation at break below 5%, tested under ASTM D4018 for carbon-fibre precursor tow. Incompatibility with amine-based finish oils is noted because amines can accelerate cyclization before the oxidation oven, producing gel spots and poor tow handleability. The processing window for successful stabilisation is therefore narrow: low temperature leads to long residence times and low carbon yield, while high temperature leads to thermal runaway. Published data for some high-speed carbonization lines is limited by licensor confidentiality, but the key variable remains the oxygen partial pressure in the oven atmosphere, typically 18–21 vol% in air with forced circulation.

Beyond PAN fibre, acrylonitrile enters styrene-acrylonitrile copolymers, ABS resins, and nitrile rubber. In bulk or continuous mass polymerisation of styrene-acrylonitrile resin, acrylonitrile content is 24–32 wt%; the copolymerisation is carried out at 120–160 °C in a reactor train with devolatilisation at 230–250 °C under vacuum. In ABS compounding, styrene-acrylonitrile matrix is melt-blended with polybutadiene graft rubber in a twin-screw extruder with length-to-diameter ratio of 32:1 to 44:1; melt temperature at the die is controlled to 220–250 °C. Moisture in the polybutadiene impact modifier must be kept below 0.1 wt%; pre-drying at 80 °C for 4 h is required when stored above 60% relative humidity. Nitrile rubber consumes acrylonitrile through emulsion polymerisation at 5–15 °C for cold NBR or 30–50 °C for hot NBR; bound acrylonitrile content ranges from 18% to 50%. Mooney viscosity ML(1+4) at 100 °C is measured under ISO 289-1 or ASTM D1646, with typical commercial NBR grades spanning 30–90 MU. These non-fibre uses are smaller than PAN but require careful acrylonitrile handling because residual monomer in the latex must be reduced to below 10 mg/kg by steam stripping or pH-adjusted post-polymerisation treatment to meet workplace and food-contact requirements where applicable.

When MEHQ Inhibition Fails During Acrylonitrile Storage

Acrylonitrile storage is a bonded inventory operation because the monomer is both flammable and polymerisable. Bulk storage tanks are typically fabricated from carbon steel or stainless steel, blanketed with an oxygen-containing gas, and maintained at temperatures below 25 °C. MEHQ inhibitor content is monitored by high-performance liquid chromatography and maintained at 35–50 mg/kg; oxygen is required as a co-inhibitor because MEHQ is a phenolic radical trap that operates in the presence of dissolved oxygen. If the oxygen blanket falls below approximately 0.1 vol% oxygen in the vapour space, inhibition can fail even with MEHQ within specification, leading to exothermic polymerisation and potential runaway. Refrigerated storage at 5–15 °C reduces vapour pressure and extends safe holding time; normal vapour pressure at 20 °C is 11.7 kPa. The lower flammability limit of 3.1 vol% in air imposes strict degassing and inerting requirements. Acrylonitrile is also subject to cyanide release during combustion, so storage areas require gas detection and water deluge. In transfer lines, low-point drains and dead legs must be avoided because stagnant monomer can polymerise even at ambient temperature; polymer deposits have caused blockages in flow meters and pump seals. For quality control, monomer purity is verified by gas chromatography using ASTM D1078 for distillation range and Karl Fischer titration by ASTM E203 for water content, with water typically below 0.5 wt% in dry acrylonitrile. The product must be used within its inhibitor lifetime; supplier certificates define retest intervals of 3–6 months depending on storage temperature and oxygen control. Published data for very large refrigerated storage tanks is limited to operator safety reports, but the critical operational boundary is the combined MEHQ-oxygen-inhibition system, not monomer purity alone.