Process for producing terephthalic acid

摘要

Improved process for producing terephthalic acid by liquid-phase catalytic oxidation of p-xylene with oxygen at a temperature of at least about 160 DEG C. and a pressure of at least about 5 kg/cm2G under specific recycling condition of the mother liquor, which comprises (A) performing said liquid-phase catalytic oxidation at a temperature (T) within the range of about 170 DEG C. to about 230 DEG C. to form an oxidation reaction mixture containing crude terephthalic acid having a 4-carboxybenzaldehyde content of 500 to 3,000 ppm and an optical density, measured at 340 m mu , of not more than 0.3, (B) separating about 60 to about 98% by weight of the total mother liquor from the resulting oxidation reaction mixture at a temperaure of from (T-30) DEG C. to (T+10) DEG C., and recycling the separated mother liquor to the oxidation zone, (C) adding hot acetic acid to the remainder of the oxidation reaction mixture so that the weight ratio of acetic acid to crude terephthalic acid in the product is 2:1 to 10:1 to form a suspension of the crude terephthalic acid in acetic acid kept at a temperature ranging from the reaction temperature (T) to about 240 DEG C., (D) stirring the suspension at a temperature of from T DEG C. to about 240 DEG C. to perform a primary leaching treatment, (E) stirring the treated suspension at a temperature of 150 DEG C. to about 220 DEG C., said temperature being at least 10 DEG C. lower than the temperature of the primary leaching treatment, thereby to perform a secondary leaching treatment, and (F) recovering terephthalic acid from the suspension subjected to the secondary leaching treatment.