Propylene (propene; CAS 115-07-1; EC number 204-062-1; molar mass 42.081 g mol⁻¹) is a light olefin with a normal boiling point of -47.6 °C at 1 atm, a critical temperature of 92.4 °C, a critical pressure of 4.60 MPa, and a vapour pressure of approximately 1.15 MPa at 25 °C. The product is transported as liquefied gas under UN 1075 and is supplied in three commercial grades: refinery-grade, chemical-grade, and polymer-grade. Grade designation is determined by the molar fraction of propylene and by the concentrations of propane, ethylene, butylene, methylacetylene, propadiene, oxygen, carbon monoxide, carbon dioxide, sulfur compounds, and water. Refinery-grade fluid catalytic cracking product typically contains 50–70 mol% propylene with propane and C4 olefins as co-products; chemical-grade material is concentrated to 95 mol% or higher for selective oxidation, ammoxidation, alkylation, and hydroformylation; polymer-grade propylene is refined to 99.5 mol% or higher because residual polar compounds act as catalyst poisons in Ziegler-Natta and metallocene polymerisation.

Industrial production routes include naphtha and ethane/propane steam cracking, fluid catalytic cracking, propane dehydrogenation over chromia or platinum catalysts, metathesis of ethylene and 2-butene over tungsten or rhenium catalysts, and methanol-to-olefins on SAPO-34 molecular sieve. Steam cracking severity shifts the propylene-to-ethylene mass ratio from roughly 0.45–0.60 for naphtha feed to lower values for ethane feed; the exact ratio depends on coil outlet temperature and residence time. Fluid catalytic cracking propylene yield is increased by adding ZSM-5 additive at 3–10 wt% of the fresh catalyst inventory, but this also increases dry gas and naphtha aromatics. Major derivative outlets by mass include polypropylene, propylene oxide, acrylonitrile, acrylic acid, cumene, and oxo alcohols; the product grade selected depends on the poison tolerance of the downstream catalyst and the value of avoided purification. The distinct commercial models are therefore not separate molecules but common propylene that has passed through different separation and finishing trains.

What Impurity Limits Dictate Polymer-Grade Propylene Suitability?

The limiting specification for polymer-grade propylene is not total propylene content but the combined concentration of methylacetylene and propadiene (MAPD), water, carbon monoxide, carbon dioxide, total sulfur, and oxygen. In a gas-phase fluidised-bed polypropylene reactor operating at 60–90 °C and 2.0–3.5 MPa, water and carbon monoxide bind competitively to titanium or zirconium active sites and reduce the apparent propagation rate. Carbon dioxide inhibition is partially reversible by increasing propylene partial pressure, whereas sulfur compounds such as carbonyl sulfide and hydrogen sulfide are irreversible poisons for most Ziegler-Natta systems. The poisoning severity follows an approximate order: CO > H2O > CO2 > sulfur species for supported titanium catalysts, although the exact ranking changes with donor chemistry and aluminium alkyl cocatalyst concentration. MAPD at 5 µmol mol⁻¹ can be hydrogenated or copolymerised competitively, and the resulting pendant unsaturation can alter stereorigidity and lower the melt temperature of the isotactic polypropylene fraction.

Commercial polymer-grade propylene commonly specifies MAPD at <5 µmol mol⁻¹, water at <5 mg kg⁻¹, total sulfur at <1 mg kg⁻¹, and oxygen at 1–2 mg kg⁻¹. The analytical basis for hydrocarbon impurities is ASTM D2712-20, which uses capillary gas chromatography with flame ionisation detection; noncondensable gases in C3 hydrocarbon products are determined by ASTM D2504 using gas chromatography with appropriate detectors. Carbon monoxide and carbon dioxide are typically measured by gas chromatography with a methaniser and flame ionisation detector or by discharge ionisation detector. A trace oxygen analyzer based on electrochemical cells or wavelength-scanned optical absorption is calibrated with certified gas blends; sample-line materials must be purged with high-purity nitrogen to prevent atmospheric contamination. These values represent a typical specification envelope across polypropylene licensors; published data for specific producer grades may be lower or modified by site-specific purification steps.

Typical commercial grade specifications are compiled in the following matrix. The values are representative ranges from publicly available product-data summaries and should not be used as contractual limits without a supplier certificate of analysis. For polymer-grade material, specifications are often reported on a dry basis, and the analytical result is sensitive to sample cylinder conditioning.

Sample handling modifies the reported purity. If a liquid propylene sample is withdrawn from a non-homogenised cylinder without a vaporising regulator, volatile C2 impurities can concentrate in the headspace and heavier C4+ impurities can remain in the liquid heel, producing biases in the assay. The ASTM D2712-20 method applies a capillary column with a polarity appropriate for C1–C4 separation and uses an internal standard or multipoint calibration to report mol% levels. Repeatability for the propylene main peak at 99.5 mol% is commonly better than 0.1 mol%, but trace impurity uncertainty increases below 10 µmol mol⁻¹. Total sulfur in propylene depends on production route: fluid catalytic cracking derived material may contain mercaptans and sulfides at levels requiring caustic extraction or fixed-bed adsorption; steam-cracked material may be comparatively lower. Water determination by Karl Fischer titration or quartz-crystal hygrometry requires pre-drying of sample lines when ambient relative humidity exceeds 60% because plastic or elastomer tubing can adsorb and release moisture.

MAPD reduction in raw C3 streams is performed in a fixed-bed selective hydrogenation unit using palladium on alumina promoted with silver or bismuth. The inlet temperature is typically 30–80 °C, and hydrogen is added at a molar ratio to MAPD of 1.0–1.5 to avoid saturating propylene to propane. Excessive bed temperature above 120 °C can initiate oligomerization and green oil fouling, which increases pressure drop across the reactor. The catalyst cycle length is determined by the inlet MAPD concentration and the H2S breakthrough from upstream caustic washing; a residual MAPD specification of <5 µmol mol⁻¹ is economically achievable with a two-bed lead-lag arrangement but requires continuous hydrogen analyser on the outlet.

Propylene/propane separation uses a high-purity C3 splitter operated at 1.8–2.1 MPa overhead pressure. The relative volatility of propylene to propane at these conditions is 1.10–1.20, so the theoretical tray requirement for a 99.5 mol% overhead propylene and 5 mol% bottoms propylene may exceed 150 theoretical stages; commercial columns are designed with 90–130 actual trays or structured packing with a total height of 80–120 m. A heat-pump splitter compresses the overhead vapour to 1.8–2.2 MPa above the column pressure and condenses it in a reboiler; the compressor discharge temperature is maintained below 90 °C to limit fouling. The reboiler duty for a 1,000 t/d splitter can exceed 50 MW, which explains why adsorption or membrane hybrids are introduced only where low-grade heat is available or where feed propane content is above 10 mol%.

When Propylene Is Compared with Ethylene and Butylene in Steam Cracker Product Slates

Propylene differs from ethylene and butylene in three operationally important properties: boiling point, allylic reactivity, and acid-catalysed carbocation stability. Propylene condenses at -47.6 °C at 1 atm, whereas ethylene requires -103.7 °C or high pressure for condensation, and 1-butene condenses at -6.3 °C. This makes propylene storage feasible as a pressurised liquefied gas at ambient temperature in horizontal cylindrical vessels designed for 1.8–2.5 MPa, while ethylene is typically stored in cryogenic spheres or high-pressure bullets and butylenes may be stored as liquids at near atmospheric pressure under slight cooling.

The allylic hydrogens of propylene support selective oxidation and ammoxidation to acrolein, acrylic acid, and acrylonitrile; ethylene lacks this site and is instead oxidised to ethylene oxide over silver-based catalysts at high selectivity. Under acid catalysis, protonation of propylene yields a secondary carbocation, which is more stable than the primary carbocation from ethylene and less prone to internal isomerisation than the carbocation from butylene; this behaviour directs propylene to alkylation, oligomerisation, and hydration with fewer by-product isomers than butylene feed. In C3 splitter distillation, the propylene-propane relative volatility is approximately 1.10–1.20 at 1.8–2.1 MPa, requiring high reflux ratios and staged separation; the ethane-ethylene system has a higher relative volatility around 1.4–1.6 at similar conditions, while the butylene-isobutane system can be more difficult depending on isomer distribution and may require extractive distillation. Propylene also differs from propane by the presence of the double bond, which produces a density and dipole moment difference that is exploited in adsorption separation, but the dominant large-scale separation remains distillation because the heat of adsorption is often too high for simple pressure-swing adsorption at refinery scale.

Polypropylene polymerisation consumes polymer-grade propylene in bulk loop reactors and gas-phase fluidised-bed reactors. In a bulk loop process, liquid propylene is circulated at 3.5–4.5 MPa and 65–80 °C; polymer powder is separated from unreacted propylene in a flash tank, and the residual propylene is recovered through a membrane or distillation sequence. The catalyst system is typically a fourth- or fifth-generation Ziegler-Natta catalyst with an internal phthalate or diether donor and an external alkoxysilane donor; productivity is sensitive to the water and CO content of the recovered propylene, which is why polymer-grade feed is dried over molecular sieve 3A or alumina beds to less than 1 µg g⁻¹ water before injection. In gas-phase fluidised-bed reactors, condensing mode operation injects a recycled liquid-propylene stream to remove heat at production rates that exceed the dry-bed limitation; the dew point margin is controlled by the concentration of propane and ethane in the recycle loop, making propane accumulation a production-capacity constraint rather than a simple inert impurity.

In the propylene oxide chain, chemical-grade propylene is fed to a chlorohydrin or hydroperoxide process; the chlorohydrin route tolerates propane diluent but produces chlorinated brine, whereas the hydroperoxide route requires low sulfur to avoid catalyst decomposition. Propylene oxide via the hydroperoxide route is run in liquid-phase epoxidation at 90–130 °C and 2–4 MPa over a molybdenum or titanium catalyst. Single-pass propylene conversion is often limited to below 20–30% to reduce ring-opening to propylene glycol and polyether by-products; the unreacted propylene is recycled after light-ends removal. In the chlorohydrin route, propylene, chlorine, and water form propylene chlorohydrin at 20–50 °C, and the subsequent lime saponification produces calcium chloride brine as a waste stream. Acrylonitrile production uses chemical-grade propylene, ammonia, and air over bismuth molybdate catalysts in a fluidised-bed reactor; the ammonia-to-propylene molar ratio is controlled in the range 0.9–1.1 to reduce acrolein by-product and to control heat release. Acrylonitrile recovery uses water absorption and extractive distillation; the reactor effluent contains acrylonitrile, acetonitrile, hydrogen cyanide, and unconverted propylene. Acrylic acid is produced by two-stage oxidation of propylene over Mo-Bi oxide and Mo-V oxide catalysts; a first fixed bed operates at 300–370 °C and a second fixed bed at 260–300 °C, with steam injection to maintain selective oxidation and to limit hot spots. Propane in the feed is inert but consumes compression energy and lowers partial-pressure efficiency. Cumene synthesis via benzene alkylation over zeolite or phosphoric acid catalysts can use chemical-grade propylene, but C4 olefins must be limited because butylbenzene impurities are difficult to reject in downstream phenol. Cumene alkylation over zeolite catalysts uses a benzene-to-propylene molar ratio of 6–10 at 100–180 °C to limit oligomerization; propane in the feed passes through the reactor and is recovered from the flash gas. Hydroformylation of propylene to butyraldehydes with syngas over rhodium or cobalt catalysts occurs at 80–160 °C and 1–6 MPa, depending on rhodium triphenylphosphine or cobalt carbonyl catalyst. Sulfur compounds above catalyst thresholds degrade rhodium-ligand activity; cobalt processes are more sulfur-tolerant than rhodium-ligand systems. For proprietary propylene derivative configurations, published data for exact poison tolerances are limited because licensors provide site-specific guarantees rather than generic public thresholds.

Storage, Transfer, and Metallic Poisoning Constraints in Propylene Systems

Propylene is stored as a liquefied gas under its own vapour pressure. Carbon steel is acceptable for dry propylene, but trace water can hydrolyse to acidic species and accelerate stress corrosion cracking; specification-grade propylene is therefore dried and contact with copper, silver, and their alloys is avoided because acetylides may form in the presence of acetylene or methylacetylene. Oxygen ingress is controlled below 1–2 mg kg⁻¹ because oxygen promotes peroxidation and may form gums in vaporiser lines. Pressure relief systems are sized for the fire case using the UN 1075 transport classification; lower and upper flammability limits at ambient pressure are 2.0 vol% and 11.1 vol%, and the autoignition temperature is approximately 455 °C. The product is not combined with oxidising agents, chlorine, or nitrogen oxides in storage or transfer headers; for unloading from tank trucks, a closed-loop vapour return is used to prevent moisture ingress. Polymeric fouling in propylene vaporisers can occur when trace dienes exceed 5–10 µmol mol⁻¹ and surface temperature exceeds 120 °C; therefore hot water or low-pressure steam heating is preferred over direct flame or high-pressure steam. Published data for long-term stability of propylene in refinery-fuel gas service are limited due to variable co-product composition.