Process for producing terephthalic acid by catalytic liquid phase oxidation of para-xylene

In a process for producing terephthalic acid by the catalytic liquid phase oxidation of para-xylene with either molecular oxygen or a gas containing molecular oxygen in the presence of acetic acid and a cobalt-containing catalyst at a temperature (T.sub.1) of 150.degree. to 250.degree. C. and a pressure of 4 to 50 Kg/cm.sup.2.G and recovering terephthalic acid from the liquid phase oxidation reaction product and directly recycling, without distillation, the catalyst-containing mother liquor remaining after recovery of terephthalic acid to the oxidation reaction zone, the improvement comprising

• 1. recovering terephthalic acid at a temperature (T.sub.2) which satisfies the following equation
• (T.sub.1 - 150.degree.).sup. 1.9 /150 + 150.degree. .ltoreq. T.sub.2 .ltoreq. T.sub.1
• wherein (T.sub.1) is the temperature of the above liquid phase oxidation,
• 2. recycling a part or whole of the catalyst-containing mother liquor remaining after the recovery of terephthalic acid without distillation to the catalytic liquid phase oxidation while maintaining it at a temperature (T.sub.3) which satisfies the following equation
• T.sub.2 .ltoreq. T.sub.3 .ltoreq. T.sub.1
• wherein (T.sub.2) is the temperature employed in the recovery of terephthalic acid, and
• 3. passing the resulting gaseous oxidation reaction product through a fractionating zone in which the acetic acid vapor is condensed while the water vapor is not condensed, recycling the condensed acetic acid to the oxidation reaction zone, and removing the water vapor from the reaction system together with the non-condensable gas.