Jiaxing Petrochemical (Tongkun) PTA is a purified terephthalic acid grade manufactured by Tongkun Group’s Jiaxing Petrochemical platform. The monomer is benzene-1,4-dicarboxylic acid with CAS 100-21-0 and molecular weight 166.13 g/mol. In commercial fiber and bottle-grade polyethylene terephthalate production, the solid is charged as a monoethylene glycol slurry; this distinguishes it from dimethyl terephthalate transesterification routes. The oxidation and catalytic hydrogenation purification sequence removes the primary chain-terminating impurity 4-carboxybenzaldehyde. Superior-grade material evaluated under GB/T 32685-2016 should meet total acidity purity not less than 99.5 wt%, 4-carboxybenzaldehyde not more than 25 mg/kg, p-toluic acid not more than 150 mg/kg, ash not more than 15 mg/kg, and moisture not more than 0.20 wt%. The powder is controlled for yellowness; fiber-grade b* is typically not greater than 1.2. The product is not identified by a universal public model code; the binding definition is the certificate of analysis issued for the specific fiber-grade or bottle-grade lot. Some downstream contracts distinguish a lower-4-CBA bottle-grade designation from standard fiber-grade PTA, but published lot-specific data for the Jiaxing Tongkun line are limited and should be confirmed against shipment documentation.

At the monomer production stage, p-xylene is oxidized with air in acetic acid over a cobalt-manganese-bromine catalyst system; the crude acid is then hydrogenated in aqueous solution over a palladium-on-carbon fixed bed to convert residual 4-carboxybenzaldehyde to p-toluic acid. The purified slurry is crystallized, filtered, and dried. This sequence reduces the visible color body and metal content that would otherwise appear in the final polyester. The resulting product is stored under closed conditions; prolonged exposure to dust or moisture degrades the specification.

Which release limits apply to superior-grade Tongkun PTA?

The release limits below are aligned with Chinese PTA standard grades and are used to qualify direct esterification feedstock. The controlling limit for 4-carboxybenzaldehyde is not arbitrary: each 4-carboxybenzaldehyde molecule acts as a monofunctional chain terminator during polycondensation. At 25 mg/kg, the chain-stopper molar contribution is approximately 0.166 mmol/kg, based on the 150.13 g/mol molecular weight of 4-carboxybenzaldehyde. This is significant when targeting a number-average molecular weight above 20,000 g/mol.

In a continuous polyester train, Tongkun PTA is slurried with monoethylene glycol at a molar ratio of EG:PTA between 1.08:1 and 1.15:1. The slurry solids content is normally 70–75 wt%, and slurry temperature is maintained at 60–80 °C to control viscosity. Two-phase feed stability depends on particle size distribution; if fines below 45 µm exceed a producer-specific threshold, mud-like viscosity excursions can reduce pump suction consistency and cause cavitation in centrifugal feed pumps. Conversely, particles retained above 250 µm slow dissolution and can accumulate in esterification reactor inlet screens. Esterification is conducted in a first reactor at 260–270 °C and 0.15–0.25 MPa gauge, generating water that is removed through a rectification column. The oligomer passes to polycondensation reactors operating from 270 °C to 290 °C with vacuum progressively reduced to 50–150 Pa absolute in the finisher. Intrinsic viscosity of the amorphous melt is tracked at 0.63–0.68 dL/g for textile fiber and 0.80–0.85 dL/g for bottle-grade preform resin when measured per ASTM D4603-18. These values are not unique to Tongkun PTA; they are the operating window in which the impurity profile and particle morphology remain process-compatible.

Catalyst and Additive Compatibility Boundaries in Direct Esterification

Tongkun PTA is compatible with antimony trioxide, antimony acetate, titanium alkoxide, and germanium dioxide polycondensation catalysts used in continuous PET lines. Antimony trioxide is typically added at 200–300 mg/kg based on polymer yield; titanium catalysts are used at lower metal loading, commonly 5–15 mg/kg, because of higher activity. The acid value of PTA is derived from carboxylic acid ends rather than free mineral acidity, so no pre-neutralization is required. However, residual p-toluic acid at 150 mg/kg contributes a monofunctional end that can reduce the theoretical degree of polymerization; increased finisher vacuum or extended residence time may be required when upstream 4-carboxybenzaldehyde and p-toluic acid drift toward the specification limit. Phosphorus-based thermal stabilizers, such as triphenyl phosphate or phosphoric acid, are used at 10–80 mg/kg P to chelate catalyst metals, but overdosing shifts the melt viscosity and can increase sublimation fouling in vacuum lines. PTA containing excessive ash above 15 mg/kg interacts with these stabilizers and can form filter-blocking residues in 20–40 µm polymer melt filters. Because direct esterification generates water, the vapor system must be constructed of corrosion-resistant alloys such as 316L or higher; chloride contamination in the PTA feed above 1 mg/kg is a known stress-corrosion risk in stainless steel esterification condensers. Published chloride data for this specific Tongkun product should be confirmed on the certificate of analysis.

Production-scale batch-to-batch variance in PTA bulk density affects weigh-feeder calibration in slurry preparation. A shift from 0.85 g/cm³ to 1.05 g/cm³ alters the volumetric feed factor by approximately 19% at constant screw speed; therefore gravimetric metering is required rather than screw-speed-based control. Pneumatic conveying of PTA should comply with combustible dust management procedures under NFPA 652. PTA dust is combustible; dust hazard analysis is applied to conveyance, storage, and bag dumping stations. Published dust-cloud explosibility data for PTA are specific to particle size and moisture, so site-specific Kst and minimum ignition energy measurements are required before inerting design. Moisture ingress above 0.20 wt% lowers electrostatic charging but promotes cohesiveness and bridging in storage silos; dry air purge or desiccant drying is applied when warehouse relative humidity exceeds 60%.

When Bottle-Grade SSP Feedstock Requires Tighter 4-CBA and Color Boundaries

For bottle-grade preforms and hot-fill containers, the amorphous melt is first polymerized to an intrinsic viscosity near 0.60–0.62 dL/g, pelletized, crystallized, and then upgraded in solid-state polycondensation to 0.80–0.85 dL/g. Because solid-state kinetics depend on carboxyl end-group concentration and diffusion, the same PTA lot that is acceptable for textile fiber may be marginal for high-IV resin if 4-carboxybenzaldehyde is near 25 mg/kg and esterification is run at low EG ratio. 4-carboxybenzaldehyde is hydrogenated to p-toluic acid in purified PTA; p-toluic acid is less chromophoric but still a chain terminator. The b* value of the PTA feed is amplified in bottle preform sidewalls because melt residence time in injection molding can exceed 5 min, and acetaldehyde generation is a function of thermal history. Tongkun PTA with b* ≤1.2 is generally suitable for water bottles, but carbonated soft drink and edible oil applications often require melt-phase filtration at 25 µm and SSP temperature control below 220 °C. For aseptic filling or long-shelf-life beverages, resin producers may request 4-carboxybenzaldehyde ≤15 mg/kg and p-toluic acid ≤100 mg/kg, although such tightened limits are supply-chain specifications rather than mandatory values under GB/T 32685-2016. Carboxyl end-group concentration in the finished resin is controlled below 30 mmol/kg when measured per ASTM D7409-15. Published data for the specific Tongkun lot distribution in bottle-grade campaigns is limited; qualification should use a pilot SSP trial with the intended line’s crystallizer and shaft dryer.

MTA, QTA, DMT, and rPET Flake Are Not Monomer Equivalents

Medium-purity terephthalic acid and crude terephthalic acid contain higher 4-carboxybenzaldehyde and metal catalyst residues. Direct substitution into a continuous PET line that was qualified on Tongkun PTA can shift esterification color, increase diethylene glycol formation, and shorten filtration cycle life. Dimethyl terephthalate is a purified diester that enters polymer via transesterification with ethylene glycol at 160–220 °C, releasing methanol rather than water; it is not interchangeable with PTA in direct esterification because the reactor metallurgy, vent recovery, and catalyst sequence are different. Recycled PET flake is not a monomer but a post-consumer polymer with variable intrinsic viscosity, comonomer content, and contaminants; it is introduced after melt polymerization or as a separate extrusion stream, not as a PTA replacement in the paste mixing section.

Storage boundaries for Tongkun PTA are defined by moisture and contamination control. The product should be kept in closed silos or bulk bags away from amines, strong oxidizers, and open flames. Amine-based additives are incompatible because they neutralize carboxyl end groups and can disturb esterification stoichiometry. If stored above 25 °C and 60% RH, moisture regain can exceed 0.05 wt% within 8 h in unsealed hoppers; use dry air purge or install hopper dryers for long-term feed stability. In pneumatic conveying, line velocity should be maintained above 18 m/s to prevent saltation and below 35 m/s to limit particle attrition and fine generation. Attrition raises fines that complicate slurry rheology and dust handling. These operational constraints, rather than single-purity values, determine whether Tongkun PTA performs as an interchangeable feedstock in a given PET asset.