P-Xylene

Liquid-phase aerobic oxidation of p-xylene to crude terephthalic acid is the first large-volume conversion step in polyester supply chains. The oxidation-grade p-xylene specification typically requires 99.7 wt% minimum p-xylene, ethylbenzene below 0.10 wt%, and nonaromatics below 0.05 wt%. The reactor feed is blended with acetic acid at a solvent-to-xylene mass ratio of 3:1 to 6:1 and with a homogeneous cobalt-manganese-bromide catalyst package. The Mid-Century/Amoco chemistry operates at 175–225 °C and 1.5–3.0 MPa air partial pressure. The stoichiometry consumes 3 mol O₂ per 1 mol p-xylene. Commercial units deliberately limit p-xylene conversion to 95–99% to keep 4-carboxybenzaldehyde below downstream withdrawal thresholds. Published catalyst loading ranges are 150–450 mg/kg cobalt, 250–600 mg/kg manganese, and 400–1200 mg/kg bromide relative to the acetic acid stream. Co:Mn:Br molar ratios vary from 1:0.8:0.6 to 1:2.0:1.2 depending on licensor balance. The oxidation vessel is a titanium-lined bubble column or continuous stirred tank reactor with air sparging. On production-scale bubble column lines, gas hold-up is held at 10–30%, and excessive superficial gas velocity above design point increases vapour-phase acetic acid losses and methyl bromide slip. Higher bromide concentration accelerates conversion and reduces p-toluic acid, but it also increases corrosive hydrogen bromide load in the overhead and raises methyl bromide emissions. Lower bromide levels improve off-gas compliance but raise 4-carboxybenzaldehyde and colour body formation. The off-gas train is not optional. Methyl bromide and carbon monoxide are incinerated at 850–950 °C, and vent gas is scrubbed before atmospheric discharge. Emissions compliance in the United States follows the hazardous air pollutant requirements established for methyl bromide and acetic acid under the Clean Air Act. REACH registration data cover acetic acid, cobalt acetate, manganese acetate, and hydrobromic acid handling at the oxidation and catalyst charging stations. After oxidation, the crude terephthalic acid slurry is let down through a three-stage crystalliser train that reduces temperature from reactor conditions to 95–110 °C through staged atmospheric and vacuum flash vessels. The slurry is then washed and filtered on rotary vacuum filters, reslurried, and dried in nitrogen-blanketed rotary dryers to a residual moisture specification below 0.10 wt%. Mean particle size after drying is controlled between 110 μm and 125 μm with span below 1.4 to prevent downstream hopper bridging and pneumatic conveying pressure drop on continuous PTA transfer lines. The powder is pneumatically conveyed to silos and subsequently to PET or DMT plants. The resulting purified terephthalic acid is specified under ASTM D8062 reporting criteria for the merchant polyester chain.

What Still Justifies DMT Isolation When PTA Is the Dominant Terephthalate Feedstock?

Dimethyl terephthalate remains a strategic intermediate for copolyester producers that require low free-acid chain ends or process flexibility in polyester polyol and polymer modification. In the Witten/Katzschmann route, p-xylene is first oxidised to p-toluic acid over cobalt naphthenate at 140–180 °C and 0.5–1.0 MPa. The first oxidation is stopped at p-toluic acid to avoid uncontrolled diacid formation. The p-toluic acid is esterified with methanol at a 2:1 molar excess. The resulting methyl p-toluate is oxidised in a second oxidation stage to monomethyl terephthalate using the same cobalt catalyst chemistry, followed by a second methanol esterification to crude DMT. Purification is carried out by vacuum distillation and multiple crystallisation stages. Final DMT flake or briquette is specified by acid number below 0.03 mg KOH/g under ISO 2114 and saponification value in the range 575–582 mg KOH/g. DMT melts at 140–142 °C, which gives a narrow but industrially useful melt crystallisation window. The DMT route is preferred in some PBT and polyester polyol plants because methanol removal is simpler than water removal and because the monomer stream is easier to handle in remote compounding locations. Terminal applications include polyester polyols, high-brightness PBT, resin-grade copolyesters, and sulfonated intermediates. Merchant DMT in the EU is registered under REACH for CAS 120-61-6, and the registration covers the exposure scenarios for molten transfer and flake dust control.

Melt-phase PET production directly receives oxidation-grade PTA and monoethylene glycol in a paste feed ratio of 1:1.15 to 1:1.20 on a molar basis. The first esterification stage operates at 240–270 °C and 0.10–0.30 MPa with continuous water removal through the column overheads. Esterification conversion is judged by end-group acid value below 30 mg KOH/g before the prepolymer enters the finisher. Polycondensation takes place in a horizontal disc-ring reactor or cage reactor under vacuum. Absolute pressure is pulled from 10–20 mbar in the first finisher to 0.5–1.5 mbar in the final finisher at 275–285 °C. Bottle-grade formulations include isophthalic acid at 1.5–2.5 wt% of total aromatic acid, diethylene glycol controlled to 1.0–1.4 wt%, antimony trioxide at 150–350 mg Sb/kg, and cobalt acetate or iron chloride toner at 10–50 mg/kg when blue extinction is required. The melt reaches intrinsic viscosity 0.60–0.65 dL/g in the melt phase and is pelletised through an underwater pelletizer with die plate hole diameters 2.5–3.5 mm and water temperature 20–35 °C. Solid-state polycondensation then raises bottle-grade IV to 0.80–0.85 dL/g under nitrogen at 210–230 °C with residence times of 14–20 h. Acetaldehyde is stripped in a concurrent hot-air de-aldehyde unit to below 3 mg/kg. Food-contact compliance is anchored to 21 CFR 177.1630 and EU 10/2011. Moulded preforms are tested under ASTM D4603 for intrinsic viscosity and under ASTM F2013 for residual acetaldehyde. The resulting amorphous chips feed injection-stretch-blow-moulded containers, heat-set bottles, and returnable water bottles.

Polyester Fibre Spinning Windows for POY and Staple Grades

Textile-grade polyester melt is fed directly from continuous polycondensation or chipped and remelted in single-screw extruders with L/D ratios of 25:1 to 30:1. Spinning lines for partially oriented yarn use melt temperatures of 285–295 °C, metering pump delivery at 0.15–0.30 g/min/hole, and spinneret hole diameters of 0.16–0.25 mm. Titanium dioxide is added as a delustrant at 0.30–0.50 wt%. Quench air temperature is held at 18–22 °C with airflow of 0.40–0.70 m/s. Take-up speed is 2500–4500 m/min for POY and 4500–6000 m/min for fully drawn yarn. Draw texturing of POY uses draw ratios of 1.50–1.80 and primary heater temperatures of 180–220 °C. Staple fibre lines draw at 2.5–4.5, with tow crimping applied at 60–120 crimps per 100 mm. Spin finish migration is a critical limitation. Finish below 0.30 wt% raises filament friction and break rate, while finish above 0.60 wt% contaminates draw rolls and texturing discs. The resulting POY, DTY, fully drawn yarn, and staple fibre enter apparel, nonwovens, and industrial textiles. Chemical compliance for textile grades follows REACH SVHC limits and Oeko-Tex Standard 100 Annex 6 thresholds for antimony and extractable metals.

During biaxially oriented PET film production, the polymer is formulated with 1–3 wt% isophthalic acid to weaken crystallisation kinetics and 0.8–1.5 wt% diethylene glycol. The resin is dried below 50 ppm moisture content and extruded through a slot die at 270–285 °C. The melt is quenched on a chill roll at 20–40 °C. The resulting sheet is preheated to 95–115 °C, then stretched in machine direction at 3.0–4.0 and transverse direction at 3.5–4.5. Heat setting under tenter-frame restraint at 210–230 °C locks in crystallinity and thermal shrinkage. Capacitor-grade film demands shrinkage below 1.5% at 200 °C. Silica antiblock is dosed at 500–3000 mg/kg. Below 500 mg/kg, film-to-film slip is poor and mill roll telescoping occurs on rewinders. Above 3000 mg/kg, haze increases above 3%, creating a direct conflict with optical-grade film specifications. Production lines experience oligomer deposition on tenter clips and ID rolls after 24–48 h of continuous operation. Clip cleaning intervals are set by cyclic trimer migration. Food-contact films must comply with 21 CFR 177.1630 and EU 10/2011 migration limits. End products include polyester packaging film, metalized barrier film, and dielectric capacitor film under IEC 60674.

When PTA Is Esterified with 1,4-Butanediol for PBT Moulding Resins

Polybutylene terephthalate production uses either DMT transesterification or direct esterification of PTA with 1,4-butanediol. The direct route feeds a diol-to-diacid molar ratio of 1.30:1 and tetrabutyl titanate catalyst at 50–200 mg Ti/kg. Esterification temperature is maintained at 225–250 °C. The by-product tetrahydrofuran formation is controlled by keeping the esterification stage below 250 °C and by avoiding diol starvation in the loop reactor. Published engineering data for this specific direct PTA-to-PBT configuration is limited, but licensors commonly set the BDO-to-PTA ratio at 1.30:1 to avoid excessive THF loss. Polycondensation follows at 250 °C and <1 mbar abs until intrinsic viscosity reaches 0.85–1.20 dL/g for injection moulding grades. The polymer is then compounded on a co-rotating twin-screw extruder with L/D ratio 40:1 and screw speed 300–600 rpm. Glass fibre reinforcement at 30 wt% is fed downstream after polymer melting. Hydrolysis resistance is improved by addition of 0.1–0.5 wt% epoxy chain extender, but this reduces melt flow rate and must be balanced against mould fill pressure. Compliance for electrical and electronic components is verified under UL 94 at the intended thickness, ISO 527-1 for tensile properties, and RoHS hazardous substance limits. Terminal products include automotive connectors, sensor housings, relay bobbins, and appliance components exposed to under-hood heat.

Terephthaloyl Chloride Conversion in Para-Aramid Supply Chains

In para-aramid precursor synthesis, purified terephthalic acid derived from p-xylene is chlorinated with thionyl chloride in the presence of N,N-dimethylformamide. The batch reaction is run at 75–85 °C with a thionyl chloride-to-PTA molar ratio of 2.2:1. The reaction releases sulfur dioxide and hydrogen chloride, which are routed to caustic scrubbers. The crude terephthaloyl chloride is vacuum-distilled to a hydrolyzable chloride specification below 50 mg/kg and a freezing point above 79 °C to limit monofunctional acid chloride. The distilled TPC is then reacted with p-phenylenediamine in N-methyl-2-pyrrolidone containing 6–8 wt% calcium chloride at 5–15 °C. The diacid chloride-to-diamine molar ratio is held at 1.000:1.000 with a deviation limit of ±0.002. Excess of either monomer caps the growing chain and drops inherent viscosity below spinnable values. The resulting poly(p-phenylene terephthalamide) has inherent viscosity in concentrated sulfuric acid of 5.0–6.5 dL/g. Dry-jet wet spinning extrudes the anisotropic dope through an air gap of 5–15 mm into a water/NMP coagulation bath maintained at 0–10 °C. Water washing and drying under tension produce para-aramid filament yarns with linear density from 55 dtex to 1670 dtex. Tensile properties for industrial yarns are tested under ASTM D7269. Ballistic packs are evaluated under NIJ 0101.06. Compliance for the chlorinating stage is managed under REACH for thionyl chloride and TPC. End products include cut-resistant gloves, ropes, tyre cord, and ballistic panels.