Process for producing purified terephthalic acid

In a process for producing purified terephthalic acid which comprises catalytically oxidizing p-xylene in liquid phase to produce crude terephthalic acid containing 4-carboxybenzaldehyde as a main impurity therein, and treating the crude terephthalic acid with hydrogen in the presence of a palladium, ruthenium, rhodium, osmium, iridium, platinum, platinum black, palladium black, iron, or cobalt-nickel hydrogenation catalyst in a reaction vessel, thereby to produce purified terephthalic acid containing 4-carboxybenzaldehyde in an amount of fixed range in a stationary manner at a temperature of 255.degree.-300.degree. C. and under a pressure of 10-110 Kg/cm.sup.2 with a partial pressure of hydrogen being 0.5-20 Kg/cm.sup.2, the improvement comprising exchanging the hydrogenation catalyst in the reaction vessel in part for a new one when the catalyst is deactivated, and thereafter feeding into the reactor crude terephthalic acid which contains 4-carboxybenzaldehyde in an amount larger than that in the stationary state of the treatment for restarting the treatment of the crude terephthalic acid, and treating the terephthalic acid until the treatment reaches the stationary state.