Purified terephthalic acid marketed under the Sichuan Energy Investment PTA designation is a commodity polyester-grade aromatic dicarboxylic acid manufactured by continuous air oxidation of p-xylene, followed by hydrogenation, crystallization, and drying. The product carries CAS 100-21-0, molecular formula C8H6O4, and a molecular weight of 166.13 g/mol. No differentiated sub-model code is published in trade documents; the material is supplied as a single PTA grade intended for melt-phase polyester synthesis. A key difference from technical-grade terephthalic acid and medium-purity streams is the control of 4-carboxybenzaldehyde and p-toluic acid, which are the major oxidized intermediates affecting esterification stoichiometry and polymer color. The manufacturing train is located in Sichuan Province, China, and uses a continuous oxidation section with acetic acid solvent and a post-oxidation hydrogenation reactor; published data for this specific site configuration is limited, so certificate-of-analysis values should be obtained for batch-specific release.

At ambient temperature, the dry product is a free-flowing white crystalline powder with a crystal density near 1.52 g/cm³ and a bulk density in the range of 800–1,000 kg/m³. Terephthalic acid does not exhibit a practical atmospheric melting point; it sublimes above approximately 300 °C. Because the first and second carboxylic acid dissociation constants are near 3.5 and 4.8, the material is a weak organic acid and remains largely insoluble in water at neutral pH, but it is readily converted to the bis(2-hydroxyethyl) terephthalate monomer during direct esterification with monoethylene glycol. Moisture uptake above 0.2 wt% can promote caking in silo storage and should be prevented with closed-loop drying air or nitrogen blanketing.

The oxidation route is continuous and uses a cobalt-manganese-bromide catalyst system in acetic acid solvent. After oxidation, the crude terephthalic acid is washed and passed through a hydrogenation reactor where residual 4-carboxybenzaldehyde is catalytically reduced to p-toluic acid. The hydrogenation step is critical because it lowers the aldehyde content without removing the mono-acid fully; p-toluic acid remains as the main organic impurity and is controlled by crystallization and washing. This process route is common to multiple large-scale PTA producers, but differences in dryer temperature, crystallizer residence time, and screening efficiency create variation in particle size, moisture, and fines that downstream polycondensation units can detect as changes in feeding behaviour and melt clarity.

What quality envelope applies after hydrogenation and crystallization?

The hydrogenation step reduces 4-carboxybenzaldehyde to p-toluic acid. In polyester-grade PTA, the residual 4-CBA content is typically limited to ≤ 25 mg/kg, while p-toluic acid is controlled to ≤ 150 mg/kg. These values are consistent with the reference envelope in GB/T 32685-2016 for purified terephthalic acid intended for polyester production. Acid number is maintained at 675 ±2 mg KOH/g, which corresponds to the theoretical dicarboxylic acid content and is determined by non-aqueous titration. The certificate of analysis also commonly reports moisture, ash, and optical properties. The following reference ranges are typical for polyester-grade PTA trade in Asia; site-specific certified limits should be confirmed with the manufacturer.

The parameter that most directly differentiates the product from technical-grade acid is 4-CBA. During PET polycondensation, residual 4-CBA can participate in color-forming side reactions. The 25 mg/kg ceiling is widely used in continuous bottle-grade PET lines because higher values reduce L* and increase b* in preforms when measured by transmission spectrophotometry. The p-toluic acid content acts as a monofunctional chain stopper; at 150 mg/kg, the effect on the carboxyl end-group balance is minor in standard fiber-grade formulations but may require molar compensation in high-viscosity grades above 0.80 dL/g. Acid number outside the 675 ±2 mg KOH/g window can indicate contamination by isophthalic acid, other aromatic acids, or residual solvent, and is therefore used as a release check.

Laboratory verification includes HPLC-UV at 254 nm for 4-CBA and p-toluic acid; the sample is dissolved in a buffered aqueous or polar organic mobile phase. Metal analysis is performed by ICP-OES after microwave acid digestion. Moisture is preferably determined by Karl Fischer coulometry rather than oven drying because the powder can retain acetic acid traces that volatilize and bias weight-loss methods. These test choices reduce batch-release uncertainty and allow direct comparison against the reference limits.

Typical downstream usage includes continuous PET polycondensation for bottle resin, film, and textile-grade fiber. The product is fed as a dry solid into a slurry mix tank with monoethylene glycol at a typical molar ratio between 1.15 and 1.40 MEG:PTA. The resulting slurry is pumped through an esterification reactor where water is removed under vacuum. For polyester fiber applications, the same PTA grade can be used without grade changes; the differentiation occurs in the downstream polycondensation recipe and catalyst package rather than in the PTA itself. In polybutylene terephthalate production, the product is esterified with 1,4-butanediol instead of monoethylene glycol; the low 4-CBA and metal profile is also relevant because PBT melt polymerization is sensitive to color formation and oligomer oxidation.

Catalyst Residue, Particle Geometry, and Feeding Behaviour

Trace metal residues from the oxidation catalyst are controlled because they influence esterification and polycondensation side reactions. Typical polyester-grade PTA specifications include iron ≤ 1 mg/kg, cobalt ≤ 2 mg/kg, and manganese ≤ 1 mg/kg, with total metals often below 10 mg/kg. These limits reduce the risk of catalyst poisoning in antimony- and titanium-catalyzed PET systems and minimize insoluble particulates that could bypass melt filtration. Ash at ≤ 15 mg/kg is an aggregate indicator of metal oxides and siliceous contaminants. In practice, ash and metal limits are critical for film-grade PET, where gel formation and surface defects are monitored by optical inspection on production winders.

Particle geometry affects handling in continuous dense-phase conveying and loss-in-weight feeding. A D50 in the 100–160 µm range with a low sub-45 µm fines fraction is preferred because coarse particles can segregate in hoppers and fines can increase dusting. On production-scale PET lines equipped with a co-rotating twin-screw extruder with 44:1 L/D, feedstock with excessive fines may generate melt pressure variation at the discharge die. Published data for this specific configuration is limited, but equipment manufacturers recommend maintaining a narrow particle-size distribution to avoid feed bridging and to keep the extruder specific mechanical energy input stable. The product is therefore dried and screened at the PTA plant to reduce batch-to-batch variation in bulk density and particle size.

During continuous slurry preparation, fluctuations in bulk density alter the mass flow calibration of loss-in-weight feeders. A bulk density drop below 800 kg/m³ can reduce the volumetric feed rate for the same screw speed and create an off-molar esterification charge. Silo aeration with dried air at a dew point below -40 °C is used in high-humidity sites to prevent moisture uptake. These operating constraints are not unique to the Sichuan Energy Investment product but are common to all dry PTA powders.

When the product is compared with medium-purity terephthalic acid and recycled monomers

The main processing distinction is not chemical identity but impurity profile. Medium-purity terephthalic acid, often used in alkyd resins, plasticizers, and non-polyester applications, may carry 4-CBA levels from 100 mg/kg to 500 mg/kg or higher, which makes it unsuitable for controlled PET melt polymerization without additional purification. Recycled terephthalate monomers and glycolysis products can contain isophthalic acid, colored degradation products, and variable oligomer distributions, which shift the melting point and crystallization kinetics of the resulting polyester. In contrast, Sichuan Energy Investment PTA is sold with a narrow mono-acid byproduct specification and low particulate burden, allowing use in clear bottle resin and high-speed melt-spun fiber without a separate purification step.

Comparison should be made on the full impurity profile rather than 4-CBA alone. A recycled stream with 25 mg/kg apparent 4-CBA may still require color correction because oxidation products and metals are not controlled to virgin PTA limits. Similarly, medium-purity TPA may have an acid number below 660 mg KOH/g due to non-terephthalic organic content; use in PET would shift the carboxyl-hydroxyl balance and require recipe correction. These differences explain why the product is specified for direct esterification rather than for solvent-borne coatings or plasticizer synthesis where the impurity tolerance is wider.

Standard compliance in trade documentation for PTA typically references ISO 9001:2015 for quality management and the relevant product specification under GB/T 32685-2016. The material is transported as bulk powder in dedicated hopper trucks or in 1,000 kg flexible intermediate bulk containers with moisture-barrier liners. Contact surfaces should be stainless steel or aluminum; carbon steel is not recommended because acid pick-up can occur over prolonged contact and can discolor the powder. Store the product in closed silos under dry air or nitrogen. Inventory rotation should follow first-in-first-out practice to limit moisture adsorption in humid regions where ambient relative humidity exceeds 60 %.

From a polymer-quality perspective, the product should be evaluated not only by certificate-of-analysis parameters but also by melt-phase trials on the intended line. The esterification reaction is acid-catalyzed, and the first carboxylic acid group reacts rapidly with monoethylene glycol, while the second esterification step is slower. For a continuous line, the esterification temperature is typically held between 250 °C and 270 °C; the water removal rate provides an indirect on-line indication of acid number and particle reactivity. Published data for this specific configuration is limited, so users should characterize the product under their own reactor residence time and agitation intensity.

4-CBA control is not the sole discriminator of melt performance.

Melt performance also depends on acid number, moisture, fines content, and trace metal profile. A batch with acceptable 4-CBA but high moisture can produce esterification pressure fluctuations and prolong the heat-up cycle in a continuous slurry reactor. A batch with acceptable 4-CBA but elevated fines can blind melt filters and reduce extruder throughput on cast-film lines. For this reason, the product is specified as a system of interdependent parameters rather than a single impurity limit. Users should compare the full certificate of analysis against the narrow window required by their polymerization process; the product is not formulated for direct use in non-polyester chemistry without evaluating the acid number and particle behavior.

Operational boundaries include pre-drying or humidity-controlled storage when ambient relative humidity exceeds 60 %. The product should not be blended with basic additives prior to melting because exothermic acid-base reactions can create localized powder clumps. Avoid contact with strong reducing agents and open flames in dusty environments, because organic powders can form combustible dust clouds. The product is intended only for industrial processing; material safety and exposure limits should be confirmed from the current safety data sheet.