Purified Isophthalic Acid Production

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

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

Crude isophthalic acid recovery. The crude isophthalic acid (CIA) slurry from reactor passes through a crystallization step, then it is centrifuged and dried. Solvent streams recovered from the Scrubber and the CIA drying areas are fed to a solvent & water recovery step before it is recycled to the oxidation reactor. The dry CIA obtained is subjected to further purification steps, since it still contains impurities that make it a product not acceptable in the polyester industry.

Purification. The CIA 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 is subsequently centrifuged for the separation of PIA crystals from the mother liquor. Part of the liquor is reused as solvent in the hydrogenation step and part of it is sent to a distillation column to recover dissolved PIA, which is recycled back to the oxidation reactor along with the hydrogenated compounds, so that they are converted to isophthalic acid. Finally, the PIA wet cake is dried and PIA product is obtained.