N-Propanol Boiling Point, Density, Formula and Key Properties
n-Propanol (CH3CH2CH2OH; CAS 71-23-8; molar mass 60.095 g/mol) is a linear primary alkanol manufactured by hydroformylation of ethylene followed by hydrogenation of propionaldehyde, or obtained as a minor product in selected oxo-alcohol streams. The molecule has a terminal hydroxyl group on a three-carbon saturated chain, giving a refractive index of 1.3856 at 20 °C and a dielectric constant of 20.1 at 25 °C. The boiling point at standard atmospheric pressure is 97.2 °C when tested by ASTM D1078-21 or ISO 3405:2019; density is 0.8034 g/cm³ at 20 °C by ASTM D4052-22; closed-cup flash point is 22 °C by ASTM D56-21 or ISO 3679:2015; autoignition temperature is 371 °C by ASTM E659-78; vapor pressure is 1.99 kPa at 20 °C; dynamic viscosity is 2.26 mPa·s at 25 °C; surface tension is 23.8 mN/m at 20 °C; melting point is -126.1 °C. The octanol-water partition coefficient log P is 0.25, which indicates that n-propanol partitions appreciably into both polar and nonpolar phases and therefore functions as a boundary solvent in separation processes. The compound is fully miscible with water at 20 °C, but the water-propanol mixture is strongly nonideal and forms a minimum-boiling azeotrope. In regulatory data sets, n-propanol is classified under EC number 200-746-9 and is listed in REACH as a registered substance with a harmonized classification under Regulation (EC) No 1272/2008. These constants are used as design inputs for distillation, condensation, pumping, storage, and vapor-recovery systems, but the numerical values cannot be applied without correcting for water content, temperature, and measurement method.
What Thermodynamic Constraints Govern Vapor-Liquid Equilibrium?
The vapor-liquid equilibrium of n-propanol at 101.325 kPa is governed by the terminal hydroxyl group, which produces extensive hydrogen bonding and a higher boiling point than nonpolar molecules of comparable molar mass. The accepted atmospheric boiling point of 97.2 °C is measured with a distillation-range apparatus conforming to ASTM D1078-21; the initial boiling point for an anhydrous technical-grade stream should not fall below 96.0 °C and the dry point should not exceed 98.5 °C. In the presence of water, n-propanol forms a binary minimum-boiling azeotrope at 87.7 °C with a composition of approximately 71.7 wt% alcohol at atmospheric pressure. This azeotrope imposes a hard constraint on ordinary distillation: a single atmospheric column cannot separate wet n-propanol to dryness. Industrial drying is instead accomplished by azeotropic distillation with added entrainers, by extractive distillation with glycols, or by pressure-swing distillation that exploits the shift in azeotropic composition as pressure is changed. The Clausius-Clapeyron slope for the vapor pressure over liquid n-propanol is consistent with a molar enthalpy of vaporization near 41.4 kJ/mol at the normal boiling point; this value is used in reboiler duty calculations for solvent-recovery columns. Condensers serving n-propanol distillation columns are typically designed for a condensation temperature of 85–90 °C on the water side and a vapor-phase pressure drop below 2 kPa to avoid flooding of structured packing. The thermodynamic data package required for rigorous simulation should include NRTL or UNIQUAC binary interaction parameters regressed against experimental vapor-liquid equilibrium data; using Raoult’s law with an ideal gas phase underestimates the water content in the distillate by more than 5 mol% at atmospheric pressure. At reduced pressure, the boiling point drops to approximately 70 °C at 40 kPa, which is a common operating condition for heat-sensitive downstream separations. A detailed simulation of n-propanol-water VLE using the NRTL activity coefficient model should therefore be validated against isobaric VLE data obtained at the actual column pressure rather than relying on atmospheric data alone.
Density is a second-order control variable in n-propanol process design because mass flow, pump power, and hydrostatic head calculations all require temperature-compensated density rather than a single handbook value. The density at 20 °C is 0.8034 g/cm³ by ASTM D4052-22 or ISO 12185:1996; at 25 °C the value is 0.7995 g/cm³, and the average thermal expansion coefficient between 15 °C and 30 °C is approximately 0.00095 K-1. A process stream at 60 °C has a density near 0.773 g/cm³, which is 3.8% lower than the 20 °C value and must be reflected in the flow-totalizer input of a Coriolis mass-flow meter. The uncertainty contribution of online density measurement in n-propanol transfer lines is commonly taken as ±0.0002 g/cm³ for a vibrating-tube meter calibrated with air and water according to reference standards. In centrifugal pump hydraulics, the required net positive suction head for a given installation rises when the suction temperature approaches the boiling point; manufacturer performance curves for standard ISO 2858 process pumps show that a 10 °C rise in n-propanol feed temperature reduces the available NPSH margin by approximately 0.3 m because the vapor pressure increases from 1.99 kPa to 3.67 kPa. Diaphragm pump suppliers specify acceleration head based on liquid density and vapor pressure; using the 20 °C density without correction at 45 °C underpredicts acceleration head by approximately 4% and can result in partial cavitation. Density stratification is not observed in n-propanol storage because the pure liquid is a single phase above its melting point, but water contamination can create a separate aqueous phase under cold conditions because the n-propanol-water system is not ideal at all compositions.
When n-Propanol Is Processed Through Centrifugal Pumps: Flash Point and Viscosity Boundaries
The closed-cup flash point of 22 °C determined by ASTM D56-21 or ISO 3679:2015 places n-propanol in GHS flammable liquid Category 2 with hazard statement H225. The explosive range in air is 2.2 vol% to 13.7 vol% when measured according to ASTM E681-04. The autoignition temperature of 371 °C measured by ASTM E659-78 means that hot pump bearings, steam tracing, or welded surfaces above this temperature can ignite vapors even without an open flame. The dynamic viscosity of 2.26 mPa·s at 25 °C and a corresponding kinematic viscosity of 2.83 mm²/s by ASTM D445-21 are low enough that positive displacement and ball-bearing centrifugal pumps operate without heated suction lines, but the low viscosity reduces the hydrodynamic film thickness in mechanical seals; a dual mechanical seal with a barrier-fluid pressure 0.15 MPa above the stuffing-box pressure is the minimum configuration recommended by pump manufacturers for 50 Hz operation. The surface tension of 23.8 mN/m at 20 °C is too low to suppress mist formation in high-shear pump zones, so local exhaust ventilation of 8–10 air changes per hour is specified in enclosed process buildings. Electrical conductivity of n-propanol is typically below 1 μS/cm, so flow through nonconductive hoses can accumulate static charge; transfer systems must be bonded and grounded in accordance with NFPA 77 and the piping velocity should be limited to 1 m/s during initial tank filling until the receiving vessel is inerted or the fill pipe is submerged. Vapor pressure at 20 °C of 1.99 kPa creates a combustible vapor-air mixture near the liquid surface inside fixed-roof tanks; therefore API 2000 venting calculations use a flash point of 22 °C and a vapor molecular weight of 60.095 g/mol to determine emission rates. The lower explosive limit corresponds to a gas concentration of approximately 55 g/m³ at 20 °C, which is attained rapidly in quiescent air because the density of n-propanol vapor is 2.08 times that of air. These numerical boundaries require that pump seals, vents, and instruments be classified for Class I, Division 1 or Zone 1 electrical service in accordance with IEC 60079 and NEC Article 500.
| Measured or classified property | Value or class | Test method or standard |
|---|---|---|
| Boiling point at 101.325 kPa | 97.2 °C | ASTM D1078-21, ISO 3405:2019 |
| Density at 20 °C | 0.8034 g/cm³ | ASTM D4052-22, ISO 12185:1996 |
| Closed-cup flash point | 22 °C | ASTM D56-21, ISO 3679:2015 |
| Autoignition temperature | 371 °C | ASTM E659-78 |
| Explosive range | 2.2 vol%–13.7 vol% | ASTM E681-04 |
| Dynamic viscosity at 25 °C | 2.26 mPa·s | ASTM D445-21 |
| GHS flammability classification | Flam. Liq. 2, H225 | Regulation (EC) No 1272/2008 |
Solvency of n-propanol in industrial coating and cleaning operations is derived from its ability to dissolve both polar resins and moderately nonpolar oils. The 0.25 octanol-water partition coefficient and 20.1 dielectric constant place it between ethanol and ethyl acetate in extraction selectivity. Hildebrand solubility parameter is cited in coating-technology references as 24.3 MPa1/2; the three-component Hansen coordinate set is typically reported as δd 16.0 MPa1/2, δp 6.8 MPa1/2, and δh 17.4 MPa1/2. These parameters indicate strong hydrogen-bonding character and moderate dipole character. In nitrocellulose lacquers, n-propanol alone produces sufficient active-solvent character to reduce blushing under high relative humidity; however, formulations containing more than 15 wt% n-propanol in the letdown solvent may require adjustment of the ester fraction to maintain resin solvency as measured by the dilution-ratio test in ASTM D1720-12. In pharmaceutical crystallization, the anti-solvent action is controlled by maintaining a constant refractive index of 1.3856 at 20 °C during addition; deviations greater than ±0.0005 correspond to off-specification solvent composition. The solvent recovery loop of a coating line operates at 30–50 kPa to keep the distillate temperature below 80 °C and to minimize ester hydrolysis. Published data for membrane-assisted vapor permeation of n-propanol-water mixtures in long-term industrial service are limited, and distillation or adsorption is therefore specified when recoveries above 95% are required. The use of n-propanol in polyurethane systems is not advised in reactive component lines because the terminal hydroxyl group competes with the polyol for isocyanate groups, shifting the NCO:OH stoichiometry and causing soft films unless the amount is included in the equivalent-weight calculation.
Azeotropic Compositions and Solvent Recovery Specifications
The n-propanol-water azeotrope is not a simple single-parameter property; its composition and boiling temperature shift with pressure. At atmospheric pressure, the azeotrope boils at 87.7 °C and contains approximately 71.7 wt% n-propanol, which means that wet solvent recovered from a printing press or extraction vessel cannot be dried past this composition in a single conventional column. A column operating at 30 kPa exhibits a different azeotropic composition and a correspondingly lower boiling point, but the pressure-swing sequence requires two columns with an interstage water-rich recycle stream. Extractive distillation with ethylene glycol has been described in peer-reviewed separation literature; achieved overhead purities above 99.9 wt% are conditional on a solvent-to-feed mass ratio above 0.8:1, a column of at least 30 theoretical stages, and a reflux ratio near 3, though published data for commercial-scale n-propanol-water extractive distillation are limited. In practical solvent-recovery specifications, the recovered n-propanol is required to meet water content below 0.1 wt% by ASTM E203-21 and a distillation range within 96.0–98.5 °C by ASTM D1078-21 before reuse in moisture-sensitive coating formulations. Vapor-liquid equilibrium data used for the NRTL regression must include the binary azeotrope and at least five isobaric data points across the composition range; the use of ideal VLE routines is unacceptable for column sizing because it fails to predict the azeotrope entirely. In storage, water uptake from humid air can move an anhydrous n-propanol inventory toward the azeotropic composition if the tank breathes freely; a desiccant vent dryer using silica gel with a dew-point guarantee of -40 °C is one common engineering control. The distillation energy demand is dominated by the heat of vaporization of water and n-propanol; at 97.2 °C, the n-propanol heat of vaporization near 41.4 kJ/mol must be combined with the water heat of vaporization of 40.7 kJ/mol to estimate reboiler duty for a binary azeotropic feed. A recovery plant operating above 90% solvent efficiency must also account for the formation of minor ternary azeotropes with ester impurities; if butyl acetate or propyl acetate is present in the feed, the overhead composition shifts and the recovered solvent can carry low levels of ester, requiring an additional decanter or adsorption step.
Stored n-propanol is not intrinsically prone to peroxide formation to the same degree as ethers, but prolonged contact with air under oxygen-enriched conditions leads to slow autoxidation at the α-carbon, producing propionaldehyde and propionic acid. The peroxides that form are polar and have low volatility; they can concentrate in distillation bottoms and create explosive residues if a distillation campaign is operated to dryness. Storage tanks should therefore be inerted with nitrogen to an oxygen concentration below 5 vol%, and the peroxide content should be monitored periodically by iodometric titration; published data for long-term n-propanol peroxide stability under ambient warehouse conditions are limited. In storage and handling, n-propanol is a flammable liquid with a closed-cup flash point of 22 °C and a boiling point of 97.2 °C; the GHS classification is Flam. Liq. 2, H225. Steel drums and stainless-steel tanks are compatible; unlined carbon steel is acceptable for anhydrous material but may contribute iron contamination in water-containing streams. Materials of construction for pump seals should avoid Buna-N in high-temperature service because n-propanol can extract plasticizer and cause seal swelling; EPDM or PTFE-wrapped gaskets are preferred. Flexible transfer hoses should be constructed of conductive PTFE or stainless-steel braided chemical hose. In two-component polyurethane systems, n-propanol must not be introduced into the isocyanate line because the hydroxyl group is an active-hydrogen source; the reaction with an isocyanate releases heat and shifts the NCO:OH ratio. Storage buildings handling n-propanol must be provided with explosion-relief panels certified to NFPA 68 and mechanical ventilation of 8–10 air changes per hour for normal atmospheric dilution. Spill containment should be sized for 110% of the largest tank volume according to EPA 40 CFR 112. Regulatory acceptance in food-contact applications must be verified against the relevant national additive or extraction limits; published data for n-propanol in direct food-contact coatings are limited.
