Production of terephthalic acid from p-xylene at integrated petrochemical complexes is dominated by the catalytic aerobic oxidation known as the Mid-Century or AMOCO process. In this process, p-xylene of greater than 99.5 wt% purity is oxidized with compressed air in a homogeneous cobalt-manganese-bromine catalyst system dissolved in acetic acid. The overall stoichiometry is C₈H₁₀ + 3 O₂ → C₈H₆O₄ + 2 H₂O, but the reaction proceeds through p-toluic acid and 4-carboxybenzaldehyde intermediates, and the oxidation of the second methyl group is substantially slower than the first. Commercial oxidation trains operate at 175–225 °C and total pressure 1.5–3.0 MPa, with acetic acid as the reaction solvent and water at 2–15 wt% as a catalyst activity modifier. Single-pass p-xylene conversion exceeds 99 mol% under stable liquid inventory and air feed control, while selectivity to crude terephthalic acid is typically 94–97 mol% with carbon oxides, benzoic acid, p-toluic acid, and high molecular weight chromophores as combustion and coupling byproducts. Heat removal is accomplished by vaporization of acetic acid and water, overhead condensation, and return of condensate to the reactor, with the reactor vapor line routed to a thermal oxidizer after oxygen monitoring. The oxidation reactor is never operated with vent oxygen in the flammable envelope; continuous paramagnetic oxygen analyzers maintain vent oxygen at 2–4 vol%, below the limiting oxygen concentration for acetic acid-air mixtures at process temperature and pressure.
| Parameter | Typical industrial range | Measurement or control point |
|---|---|---|
| Reactor temperature | 175–225 °C | Liquid-phase reactor inventory |
| Reactor total pressure | 1.5–3.0 MPa | Reactor head space |
| Acetic acid solvent water content | 2–15 wt% | Condensate return line |
| Vent oxygen concentration | 2–4 vol% | Overhead vapor line after condenser |
| Crude terephthalic acid 4-carboxybenzaldehyde | 2000–5000 mg/kg | Crude product filter cake |
| Purified terephthalic acid 4-carboxybenzaldehyde | maximum 25 mg/kg | Final dry product silo |
| p-Xylene conversion | greater than 99 mol% | Reactor liquid effluent |
| Selectivity to crude terephthalic acid | 94–97 mol% | Reactor liquid effluent and offgas balance |
Continuous oxidation reactors at integrated purified terephthalic acid complexes are usually titanium-clad bubble columns with an internal or external circulation loop, a bottom air sparger, and an overhead condenser system. The gas-liquid mass transfer of oxygen from dispersed air into the acetic acid phase is frequently the overall rate-limiting transport step because the dissolved p-xylene concentration in the reactive liquid remains low. The reaction rate is therefore governed by oxygen delivery rather than by p-xylene feed rate once the catalyst is in its oxidized state. In commercial units, superficial gas velocity is held in the homogeneous-to-heterogeneous bubble flow transition, and the air sparger is designed with sufficient open area to avoid jetting and localized oxygen-rich zones. Sparger fouling by cobalt and manganese acetate salts, precipitated terephthalic acid, and corrosion products is a known production bottleneck, requiring scheduled reactor entry and sparger replacement at turnaround intervals that depend on feedstock purity and bromide circulation. Mechanical agitation is avoided in some large trains because titanium-clad impellers and seals add capital cost and leak-inspection burden; circulation is instead provided by external pumps with double mechanical seals and flush systems using process solvent. The reactor offgas system includes mist eliminators, condenser trains, and oxygen analyzers; excursions above 4 vol% trigger automatic reduction in air flow or increase in nitrogen purge. Temperature control is achieved by adjusting reactor pressure and condensate reflux, because the latent heat of vaporization of acetic acid-water mixtures provides a high-capacity heat sink. The liquid level is maintained by overflow of slurry containing crude terephthalic acid crystals, dissolved catalyst, and intermediate byproducts. The solid loading in the reactor slurry is typically 20–35 wt%, and the slurry is discharged to a series of pressure letdown crystallizers before filtration. The selection of titanium Grade 2 or Grade 7 for wetted surfaces is driven by acetic acid and bromide corrosivity; stainless steel 316L is limited to low-bromide utility service.
Oxidation of the first p-xylene methyl group produces p-toluic acid with only minor accumulation of p-methylbenzyl alcohol and p-tolualdehyde under normal catalyst activity. The second methyl oxidation is retarded by the electron-withdrawing carboxylic acid substituent, and the steady-state liquid in the reactor contains measurable p-toluic acid and 4-carboxybenzaldehyde even when p-xylene conversion is nearly complete. Bromine radicals abstract benzylic hydrogen to form alkyl radicals; Co(III) regenerates bromide radicals and Mn(II) shuttles electrons to maintain cobalt in the active trivalent oxidation state. Water concentration in the acetic acid solvent is controlled between 2 wt% and 15 wt% because excessive water suppresses hydrocarbon solubility and increases carbon dioxide selectivity, while insufficient water reduces catalyst solubility and promotes cobalt bromide precipitation. Acetic acid is not an inert diluent; it participates in radical termination and esterification side reactions, forming methyl acetate and benzyl acetates that must be recovered or decomposed in the solvent dehydration column. The oxidation reactor liquid inventory contains dissolved cobalt and manganese at total metal concentrations in the range 0.1–0.3 wt% on a solvent basis; bromide is present as hydrogen bromide or sodium bromide in a molar ratio to metals that is tuned to feedstock impurities and reactor temperature. Published kinetic data indicate that the first methyl oxidation is fast and oxygen-limited, while the conversion of 4-carboxybenzaldehyde to terephthalic acid is activated and sensitive to catalyst redox potential, water content, and reaction temperature. Because the impurity profile of crude terephthalic acid depends on the residence-time distribution in the bubble column, backmixing in the liquid phase broadens the distribution of partially oxidized intermediates, and staged oxidation with plug-flow characteristics is sometimes incorporated in modern designs.
4-Carboxybenzaldehyde persists because the oxidation of the second p-xylene methyl group proceeds through an aldehyde intermediate that is less reactive than the first methyl oxidation products. The carbonyl group in 4-carboxybenzaldehyde is susceptible to further oxidation only when a sufficient steady-state concentration of active bromide and cobalt(III) is maintained in the liquid phase; otherwise the aldehyde remains occluded in the crude terephthalic acid crystal lattice and in residual mother liquor. Typical crude terephthalic acid from the Mid-Century process contains 2000–5000 mg/kg of 4-carboxybenzaldehyde and 500–2000 mg/kg of p-toluic acid as principal organic impurities. These values are not acceptable for polyester-grade purified terephthalic acid because 4-carboxybenzaldehyde acts as a chain termination agent during ethylene glycol polycondensation and contributes to yellow color, elevated optical density, and reduced intrinsic viscosity. The persistence of 4-carboxybenzaldehyde is also affected by crystal size and morphology: rapid precipitation of terephthalic acid occludes mother liquor rich in intermediates, and washing alone does not remove these occluded impurities. Therefore, purification must occur by dissolution and chemical conversion rather than by mechanical washing. The hydrogenation step converts 4-carboxybenzaldehyde to p-toluic acid, which is more water-soluble and is separated in the mother liquor after recrystallization. The residual 4-carboxybenzaldehyde specification for purified terephthalic acid in polyester applications is typically maximum 25 mg/kg, and premium fiber grades may require maximum 15 mg/kg. Optical reflectance color, expressed as CIE b*, is controlled below 1.0 for bottle resin precursors because the yellowness index of the final PET correlates with residual aldehyde and metal residues.
Purified terephthalic acid is obtained by continuous aqueous-phase hydrogenation followed by crystallization, filtration, and drying. Crude terephthalic acid crystals are reslurried in demineralized water, heated to 260–290 °C under pressure 6.5–9.0 MPa, and fed as a single-phase aqueous solution to a fixed-bed reactor containing palladium on activated carbon. Palladium loading is typically 0.3–0.5 wt%, and the reactor operates at a liquid hourly space velocity of 4–12 h⁻¹; hydrogen partial pressure is maintained by a high-purity hydrogen supply and a vent gas recovery system. Under these conditions, 4-carboxybenzaldehyde is selectively reduced to p-toluic acid, while terephthalic acid aromatic ring hydrogenation is negligible. The purified solution then passes through a series of crystallizers in which pressure is reduced stepwise, cooling the solution and precipitating purified terephthalic acid as crystalline solids. Crystal growth is controlled by residence time and agitation intensity to produce a mean particle size in the range 100–200 µm and a narrow particle size distribution suitable for continuous feeding to paste mixing with ethylene glycol. The slurry is filtered in pressure filters, washed with hot demineralized water to remove p-toluic acid and soluble metal residues, and dried with hot air or inert gas to a moisture content below 0.5 wt%. The final PTA powder has a bulk density of 0.80–1.10 g/cm³ and is conveyed pneumatically under nitrogen to storage silos. PTA dust is combustible; pneumatic transfer systems are designed with explosion venting and reduced oxygen concentration.
Hydrogenation catalyst performance is governed by residual bromide, cobalt, and manganese species carried from the oxidation section. Bromide is a documented palladium poison that accelerates crystallite sintering and reduces the active surface area of the carbon-supported catalyst. Therefore, the crude terephthalic acid is washed and reslurried before hydrogenation, and the hydrogenation reactor feed is monitored for total halides and transition metals. Palladium on carbon under hydrothermal conditions also undergoes physical degradation if the carbon support is not steam-stable; support methane formation and carbon gasification are side reactions that become measurable during temperature excursions above the normal operating window. Fixed-bed reactors for this service use a bed length-to-diameter ratio in the range 5:1–12:1 to distribute liquid flow and avoid channeling. Pressure drop across the catalyst bed is monitored continuously because bed compaction and crystal fines accumulation increase pressure drop and reduce effective contact time. The hydrogenation catalyst is eventually deactivated by trace sulfur from plant utilities, by bromide breakthrough during upstream upset conditions, and by irreversible palladium sintering after repeated shutdown and startup cycles. Published data for the exact deactivation rate in specific commercial configurations is limited because catalyst life is proprietary to licensors; however, operating experience shows that catalyst replacement is normally required after 12–36 months of continuous service. Regeneration off-site is possible but limited by carbon support oxidation and residual organic fouling. The purification reactor effluent is filtered downstream to separate any fine carbon particles, and the filter is designed to meet pressure drop and solids loading specifications that protect crystallizer surfaces from carbon contamination.
Purified terephthalic acid is stored and conveyed as a free-flowing powder with controlled moisture, particle size, and b* color. In polyester manufacture, PTA and ethylene glycol are mixed into a paste with a molar ratio of ethylene glycol to terephthalic acid in the range 1.05–1.20, and the paste is fed to an esterification reactor operated at 240–260 °C and 0.1–0.3 MPa. Direct esterification of PTA with ethylene glycol produces water and bis(2-hydroxyethyl) terephthalate oligomers without the methanol condensation required in dimethyl terephthalate routes. The esterification reaction is autocatalytic due to the carboxylic acid end groups of PTA, and the addition of external acid catalysts is unnecessary. However, the reaction rate depends on particle size and dissolution; PTA with a mean particle size above 200 µm may dissolve slowly and form unreacted cores that degrade fiber spinning or film clarity. The esterification water is removed through a distillation column, and ethylene glycol is recovered and returned to the paste mixer. Diethylene glycol formation is a parallel side reaction that must be controlled because DEG acts as a comonomer and depresses the crystalline melting point and tensile strength of the resulting polyester. In industrial practice, DEG in the final polymer is controlled below 1.5 wt% for fiber and below 1.0 wt% for bottle resin.
PET bottle resin produced from purified terephthalic acid and ethylene glycol is subjected to solid-state polymerization to increase intrinsic viscosity and reduce residual acetaldehyde. Melt-phase polycondensation typically produces an intrinsic viscosity of 0.60–0.65 dL/g; solid-state polymerization raises the intrinsic viscosity to 0.80–0.84 dL/g at temperatures 200–215 °C under vacuum or inert gas flow over 12–18 h. Acetaldehyde is generated by thermal degradation of ethylene glycol and vinyl ester end groups during preform injection molding; preform barrel temperatures are normally 270–285 °C, and screw designs with L/D ratios of 24:1–28:1 are used to minimize residence-time distribution. The acetaldehyde concentration in bottle wall after blowing is controlled below 1 ppm for water and sensitive beverage applications, because higher levels cause off-taste. This requirement imposes limits on the carboxylic end group concentration of the resin, typically 20–35 meq/kg, and on the residual metal catalyst content from PTA. Injection molding of PET preforms uses clamp forces in the range 3000–5000 kN for high-cavitation molds, depending on preform weight and cavity count. Resin feed to preform molding must be dried to below 50 ppm moisture to prevent hydrolytic molecular weight loss. Standards applied to PET bottle resin include ASTM D4603-18 for intrinsic viscosity, ISO 1133-1:2022 for melt mass-flow rate, and FDA 21 CFR 177.1630 for food-contact PET. European food-contact compliance is established under EU 10/2011 with specific migration testing for antimony, cobalt, and manganese.
Staple fiber and filament yarn manufacturing from PTA uses the same direct esterification and melt polycondensation chemistry but stops at lower intrinsic viscosity, typically 0.62–0.68 dL/g for partially oriented yarn and 0.64–0.68 dL/g for staple fiber. The polymer is melt-spun at 285–295 °C through spinnerets with holes of 0.15–0.35 mm diameter, and the filaments are quenched with laminar air, finished, and wound at speeds of 3000–6000 m/min for partially oriented yarn. The high winding speed imposes tight limits on PTA metal residues and gel particles because spin pack pressure rise and filament breaks are sensitive to contamination. Polycondensation catalysts used in fiber production include antimony trioxide at 150–300 mg/kg antimony in polymer, titanium alkoxides at 5–50 mg/kg titanium, and germanium oxide at 20–60 mg/kg germanium for selected film and fiber grades. Antimony trioxide can be reduced to metallic antimony under low oxygen conditions, causing gray discoloration; this is a known processing boundary in melt spinning. The yarn is drawn and textured to impart strength and elongation; tenacity values for polyester industrial yarn exceed 0.6 N/tex, and elongation at break is controlled according to the downstream textile process. Fiber-grade PTA must meet maximum 25 mg/kg 4-carboxybenzaldehyde, maximum 150 mg/kg p-toluic acid, and maximum 15 mg/kg ash, with b* color below 1.0. Moisture in PTA feed to paste mixing is limited to below 0.5 wt% because excess water disturbs the esterification mass balance and increases ethylene glycol inventory in the water removal column.
Polycondensation of bis(2-hydroxyethyl) terephthalate to PET is a step-growth equilibrium reaction with ethylene glycol as the byproduct. The melt polycondensation stage is carried out at 270–290 °C and absolute pressure below 1 mbar in a series of finishing reactors with specialized agitators that provide high surface renewal. Intrinsic viscosity increases as ethylene glycol is removed; the equilibrium constant is near unity, and the final degree of polymerization is controlled by the vacuum level and residence time rather than by stoichiometric imbalance alone. Diethylene glycol is formed by dehydration of ethylene glycol during esterification and early polycondensation; DEG incorporation into the polymer chain reduces the glass transition temperature, lowers crystallinity, and influences drawability. The DEG content of PET is measured by gas chromatography after transesterification and is specified below 1.5 wt% for textile fiber and below 1.0 wt% for bottle resin. Esterification reactors are designed with sufficient ethylene glycol reflux and water removal to suppress DEG formation; high-temperature operation above 265 °C in the esterification stage increases DEG generation and must be avoided unless catalyst selection compensates. Additives such as phosphoric acid, polyphosphoric acid, or triethyl phosphate are introduced after esterification to stabilize the antimony catalyst and prevent further DEG formation during polycondensation. The resulting amorphous PET is extruded through a die plate, cooled in a water bath, and pelletized to uniform granules. Pellet intrinsic viscosity is measured by solution viscometry according to ASTM D4603-18, and melt viscosity is checked by ISO 1133-1:2022. The carboxyl end group concentration is determined by titration and controlled between 20 and 35 meq/kg. Narrow residence-time distribution in the finishing reactor is critical because prolonged high-temperature exposure causes thermal degradation, gel formation, and acetaldehyde generation, which impose a lower melt viscosity and a higher color value on the final resin.
Terephthalic acid is also the preferred monomer for polybutylene terephthalate and certain liquid-crystalline polyesters when cost and thermal performance require an aromatic dicarboxylic acid. In PBT manufacture, purified terephthalic acid reacts with 1,4-butanediol in an esterification step at 150–170 °C; unlike PET, the butanediol esterification is accompanied by tetrahydrofuran formation, which must be removed and purified. The polycondensation is catalyzed by tetrabutyl titanate or other titanium compounds, and melt-phase finishing is conducted at 250–260 °C and reduced pressure to reach a melt-volume flow rate appropriate for injection molding. PBT compounds are processed in injection molding machines with barrel temperatures 230–260 °C, mold temperatures 40–80 °C, and clamp forces depending on part geometry; typical connector housings and sensor encapsulants require clamp forces from 800–3200 kN. The compound may be filled with glass fiber at 15–30 wt% and flame retardants to meet UL 94 V-0, with heat deflection temperature measured according to ASTM D648-18. PTA-based PBT is sensitive to moisture during melt processing; pellets are dried to below 50 ppm moisture before injection molding to prevent hydrolytic degradation and loss of mechanical strength. Tensile strength and elongation at break are determined according to ISO 527-2, flexural modulus according to ISO 178:2019, and Charpy impact according to ISO 179-1:2023. The same PTA quality constraints—low 4-carboxybenzaldehyde, low p-toluic acid, and low ash—apply because residual oxidation intermediates reduce esterification selectivity and color in PBT and liquid-crystalline polyesters. In high-temperature engineering polymer applications, PTA-based liquid-crystalline polyesters are processed in twin-screw extrusion with L/D ratios of 32:1–44:1 and vacuum venting to remove acetic acid and other volatiles.
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