Purified Terephthalic Acid from p-Xylene (BP Technology)

The process under analysis comprises three major sections: (1) Oxidation; (2) Crude terephthalic acid recovery; and (3) Purification.

Oxidation. p-Xylene is mixed with acetic acid solvent and a catalyst solution and then fed to the oxidation reactor. In the reactor, p-xylene is oxidized to terephthalic acid and most of the product generated is precipitated. Unreacted p-xylene, acetic acid and water formed are partially vaporized and sent to a high pressure column downstream for the recovery of the acetic acid and p-xylene. The residual gas from the column is sent to a oxidation unit, where volatile organic compounds are oxidized before disposal.

Crude terephthalic acid recovery. The crude terephthalic acid (CTA) slurry from reactor passes through a crystallization step and then, it is filtered in a band filter. Solvent recovered from CTA filter is fed to a stripper column for purification before it is recycled to the CTA crystallizers. Catalysts are also recovered from the stripper column. The catalyst solution is mixed with fresh catalyst is recycled to the oxidation reaction.

Purification. The CTA is dissolved with hot water and then sent to a hydrogenation step. During the hydrogenation, the undesired impurities are converted into soluble compounds, which remains dissolved in the mother liquor in a solid-liquid separation step downstream. The effluent from hydrogenation passes through a second crystallization step and subsequently filtered for the separation of PTA crystals from mother liquor. Part of the mother liquor is then recycled back to the oxidation reaction, in such a way that the dissolved hydrogenated compounds are converted to terephthalic acid. Finally, the PTA wet cake is dried and PTA product is obtained.