Acrylonitrile is a vinyl monomer with CAS 107-13-1, IUPAC identity prop-2-enenitrile, molecular formula C₃H₃N, and molar mass 53.06 g/mol. At atmospheric pressure it is a clear, mobile liquid; commercial material is typically stabilized with hydroquinone monomethyl ether at 35–45 ppm and appears colourless to pale yellow. The compound is produced primarily by propylene ammoxidation and is consumed in polyacrylonitrile, acrylonitrile-butadiene-styrene resins, and nitrile rubber. The physical property profile is dominated by a normal boiling point of 77 °C, a liquid density below that of water, a vapour pressure sufficient to form flammable and toxic atmospheres at common storage temperatures, and partial water miscibility. These parameters influence monomer recovery, storage, transfer, and air emission control. Regulatory identifiers include UN 1093, EC 203-466-5, and the OSHA chemical-specific standard 29 CFR 1910.1045. The following reference data form the basis for process design and hazard analysis.

Why the 77 °C Normal Boiling Point Constrains Recovery-Distillation Operation

Anhydrous acrylonitrile boils at 77.3 °C at 101.3 kPa absolute; the rounded operational limit is 77 °C. This places the monomer among the volatile components of ammoxidation effluent, ahead of acetonitrile at 81.6 °C and water at 100 °C, but behind hydrogen cyanide at 25.7 °C. The close boiling interval between acrylonitrile and acetonitrile is only 4.3 K, which makes sharp separation by simple atmospheric distillation difficult. Commercial recovery therefore combines absorption, extractive distillation, and decanting rather than a single distillate cut. Along the purification train, the reboiler liquid cannot be held at elevated temperature for extended periods. Published data for specific proprietary absorber-to-distillation configurations is limited, but the thermal stability boundary is commonly placed at reboiler skin temperatures below 120 °C because homopolymer formation and nitrile hydrolysis increase rapidly above that threshold. Hydrolysis produces acrylamide and acrylic acid, while self-polymerization fouls trays and reboiler tubes. The overhead condenser can operate with normal cooling water at 30–35 °C, but noncondensables containing hydrogen cyanide must be routed to a thermal oxidizer. The atmospheric reflux drum is conventionally set at 70–75 °C to keep acrylonitrile liquid while allowing nitrogen and low boilers to vent.

A water azeotrope forms at approximately 70.6 °C, with an organic-rich layer near 88 wt% acrylonitrile. This azeotrope prevents simple dehydration of wet acrylonitrile by atmospheric distillation and requires decanting or extractive separation. Low-pressure or vacuum operation is sometimes applied to reduce reboiler temperature in monomer recovery, but reduced pressure also increases vapour volume and condenser size. The trade-off is evaluated against the 77 °C normal boiling point and the requirement to keep inhibitor distributed throughout the column. Storage and distillation stability are not a single phase-equilibrium issue. The monomer can undergo base-induced polymerization; therefore, pH in the water-rich portions of recovery is maintained neutral or mildly acidic. In addition, hydroquinone monomethyl ether requires dissolved oxygen for full inhibiting activity. During distillation, air ingress is excluded, so inhibitor concentration at the reboiler may be lower than the storage specification. Process operators monitor reflux inhibitor level to avoid runaway polymerization at hot metal surfaces. These constraints arise directly from the 77 °C boiling point: the boiling point is low enough to permit atmospheric distillation with cooling water but high enough that long hot residence times degrade product quality.

When Vapour Pressure Reaches 11.1 kPa at 20 °C in Fixed-Roof Storage Headspace

The vapour pressure of acrylonitrile at 20 °C is approximately 11.1 kPa (83 mm Hg), and the value at 25 °C is approximately 14.5 kPa. At the normal boiling point of 77.3 °C, vapour pressure equals atmospheric pressure, 101.3 kPa. A saturated headspace at 20 °C therefore contains approximately 110,000 ppm by volume, calculated as 11.1 kPa / 101.3 kPa × 10⁶. This saturated concentration is far above the OSHA 29 CFR 1910.1045 permissible exposure limit of 2 ppm 8-hour TWA and the 10 ppm short-term exposure limit. Even at 0 °C, vapour pressure remains near 4 kPa, producing a saturated concentration above 40,000 ppm. Fixed-roof tanks, railcars, and isotainers therefore must be inerted or connected to vapour recovery.

The closed-cup flash point is −1 °C; flammable limits are 3.0–17.0 vol% in air. At 20 °C, the saturated vapour concentration is 11 vol%, above the lower flammable limit and below the upper limit, so a tank headspace without inerting is explosible. Vapour density relative to air is 1.83; released vapour accumulates in low points and drains rather than rising promptly. Electrical area classification follows EN 60079-10-1, and bulk loading racks use closed vapour-balance lines and bottom loading. Pressure-rated storage systems are typically designed for a maximum allowable working pressure of at least 5 kPa gauge, but vapour pressure at 40 °C exceeds 25 kPa absolute. If a fixed-roof tank is not insulated, solar radiation can raise headspace pressure and emissions. Padding with nitrogen at 10–15 kPa gauge suppresses flammable atmosphere formation but does not eliminate toxic exposure during sampling. Conservation vents are sized by thermal inbreathing and outbreathing calculations according to API 2000; the relevant vapour-pressure swing between minimum night temperature and maximum day temperature can be several kilopascals. A 10 K rise from 20 °C to 30 °C increases acrylonitrile vapour pressure from 11.1 kPa to roughly 18 kPa. Sampling points on tank nozzles are equipped with spring-loaded closed couplings and purged with nitrogen; open-loop sampling is not used. The relationship between vapour pressure and temperature can be approximated by the Clausius-Clapeyron equation using a heat of vaporization of approximately 33 kJ/mol, derived from the reference points 20 °C and 77.3 °C. This thermodynamic basis allows engineering estimation of emissions and relief sizing without proprietary software.

For liquid density, the reference value at 20 °C is 0.806 g/cm³, equivalent to 806 kg/m³; at 25 °C the value is approximately 0.805 g/cm³. This is measured by ASTM D4052 digital density meter. Because the liquid density is lower than water, spills on stationary water form a floating organic layer, and firefighting water spray can spread burning monomer if direct application is attempted. Alcohol-resistant foam or dry chemical is preferred; water may be used to cool exposed tanks. In continuous decanters, the density difference between the aqueous bottom layer and the organic top layer is normally 0.19–0.20 g/cm³, providing phase disengagement if the feed is not emulsified by high shear. The pump hydraulic design must account for low static head from the low-density liquid: a 3 m liquid level of acrylonitrile contributes only 23.7 kPa static head. Canned motor pumps or magnetically driven sealless pumps are used in transfer service to reduce emissions; a standard ANSI centrifugal pump with single mechanical seal is generally not sufficient for UN 1093 acrylonitrile service without a closed seal-support system. For custody transfer, mass is preferred over volume because density decreases measurably with temperature; temperature compensation is required if volumetric meters are used.

In aqueous systems, solubility is moderate. At 20 °C, acrylonitrile dissolves in water at approximately 7.3 g/100 g; water dissolves in acrylonitrile at approximately 3.1 g/100 g. The system is not fully miscible and forms a minimum-boiling azeotrope at 70.6 °C. This partial miscibility is exploited in the recovery decanter: the light organic phase is sent to dehydration and product distillation, while the heavy aqueous phase is returned to the absorber or stripper. Elevated pH must be avoided because dissolved acrylonitrile hydrolyzes to acrylamide and acrylic acid; the rate is pH- and temperature-dependent and is accelerated by ammonia in ammoxidation quench water. Neutralization of quench water with caustic is therefore controlled to avoid localized high pH. In wastewater treatment, the water solubility prevents effective removal by simple settling; steam stripping or biotreatment is required to meet discharge limits. Dissolved salts can alter the organic content of the aqueous phase; sodium sulfate from acid-gas scrubbing tends to reduce acrylonitrile solubility by salting-out. In analytical practice, acrylonitrile is readily diluted with acetone, ethanol, toluene, and diethyl ether; however, aqueous calibration standards must be prepared below the solubility limit and stored cold in sealed glass vials to avoid partition losses. In polymer washing and latex processes, residual monomer partitions between polymer, serum, and air; stripping columns use the water solubility and vapour pressure data to size steam flow and vacuum level.