Product

Crude Terephthalic Acid (CTA) is the intermediate solid product formed by the liquid-phase catalytic oxidation of para-xylene, serving as the direct precursor to Purified Terephthalic Acid (PTA) — it is never sold as a final commercial product, but rather processed on-site (or in an integrated plant) through hydrogenation purification.

Chemical Identity

CTA is chemically identical in its base backbone to terephthalic acid (benzene-1,4-dicarboxylic acid) — the difference from PTA lies entirely in impurity content, not molecular structure. It appears as an off-white to yellowish/tan crystalline solid or powder, in contrast to the pure white powder appearance of finished PTA

Formation and Chemistry

CTA is produced via liquid-phase air oxidation of para-xylene in an acetic acid solvent medium, catalyzed by a homogeneous cobalt/manganese/bromine (Co/Mn/Br) catalyst system, typically at 150–220°C and 15–30 atm (1500–3000 kPa) in an agitated bubble-column or stirred-tank reactor. The oxidation proceeds via the intermediate p-toluic acid, and the reaction inevitably produces measurable levels of impurities carried through into the solid CTA crystal — most significantly 4-carboxybenzaldehyde (4-CBA), along with unreacted p-toluic acid, p-tolualdehyde, and trace colored oxidation byproducts.

Key Distinguishing Impurity Profile (CTA vs. PTA)

CTA is unsuitable for direct use in polyester (PET) production because residual 4-CBA acts as a chain-terminating impurity, degrading polymer molecular weight and imparting undesirable color to the finished resin — this is precisely why the purification/hydrogenation step exists.

Process Role: CTA Recovery and Handling

Following oxidation, the CTA-in-acetic-acid slurry undergoes staged flash crystallization (pressure/temperature letdown), then solid-liquid separation (historically centrifuge or belt filter; in current-generation KTS P8++ technology, a rotary pressure filter performing solvent interchange), where acetic acid mother liquor is exchanged for an aqueous wash medium. In some plant configurations, CTA is dried and stored/transported as an intermediate solid prior to purification; in fully integrated modern plants, the CTA slurry is transferred directly (without drying) into the purification section as an aqueous suspension, per the "aliphatic carboxylic acid mother liquor" filtration approach.

Purification Pathway (CTA → PTA)

1. CTA is reslurried/dissolved in an aqueous medium at 250–350°C

2. The resulting aqueous CTA solution is catalytically hydrogenated (typically over a palladium-on-carbon fixed-bed or trickle-bed catalyst at 250–350°C, ~20 atm) to convert 4-CBA into the more soluble and separable p-toluic acid

3. The purified solution is crystallized (cooling/depressurization) to precipitate PTA crystals

4. Crystals are separated, washed, and dried to yield finished PTA, typically specified at ≥99.98% purity for polyester-grade applications

Economic and Industrial Note

Historically, some producers operated separate, standalone CTA production units feeding downstream, sometimes third-party, PTA purification facilities — a business model that existed in the industry's earlier development, though most modern world-scale plants integrate oxidation and purification into a single continuous flowsheet, eliminating the need to dry, store, or transport CTA as an independent commodity.