Process for the preparation of terephthalic acid by the hydrolysis of intermediate stage crude dimethyl terephthalate

A process for the preparation of fiber-grade terephthalic acid from an intermediate stage crude dimethyl terephthalate obtained from a crude ester mixture produced by the oxidation of p-xylene and/or methyl p-toluate with oxygen-containing gases in the presence of a heavy-metal-containing oxidation catalyst at an elevated temperature and under an elevated pressure;

• and by the esterification of the oxidation mixture with methanol at elevated temperature and elevated pressure, which comprises separating the resulting crude ester mixture by distillation into a methyl p-toluate-enriched fraction, a residual high boiling fraction, and a crude dimethyl terephthalate having a limited content of intermediate oxidation products including terephthalaldehydic-acid methyl ester and other by-products, the content of terephthalaldehydic acid methyl ester being limited up to 0.1% by weight based on the weight of the crude dimethyl terephthalate;
• subsequently hydrolyzing the crude dimethyl terephthalate with water at a weight ratio of the crude dimethyl terephthalate to water of between 3:1 and 0.1:1, and at temperatures of between 140.degree. C. and 350.degree. C. and under the pressure required to maintain the liquid phase to produce a reaction mixture containing crystalline fiber-grade terephthalic acid and then recovering the terephthalic acid from the reaction mixture;
• said hydrolysis of the crude dimethyl terephthalate being conducted in at least two stages, wherein in the first hydrolysis stage, a mixture of the crude dimethyl terephthalate and water is passed cocurrently through a first reactor column and a portion of terephthalic acid formed by hydrolysis is crystallized from the reaction mixture at temperatures of between 300.degree. and 150.degree. C., a suspension of terephthalic acid crystals in a mother liquid is separated from the reaction mixture and introduced into the second hydrolysis stage and the remaining portion of the reaction mixture containing methanol and water is discharged from the first hydrolysis stage;
• and in the second hydrolysis stage demineralized water is passed countercurrently to the suspension of terephthalic acid crystals in the mother liquor in a second reactor column, whereby the mother liquor is constantly diluted, replaced by the demineralized water, the motor liquor is withdrawn from the top of the second reactor column, and additional terephthalic acid is formed and crystallized within the length of the second reactor column during the continual hydrolysis.