APPARATUS AND PROCESS FOR THE PREPARATION OF TEREPHTHALIC ACID

The method and apparatus for manufacturing terephthalic acid (TA) are capable of producing TA having a closely controlled, desired amount of contaminant, 4-carboxy-benazldehyde (4-CBA), therein and/or a closely controlled optical density. In the method, a feed of para-xylene or similar feedstock mixed with catalyst components and solvent system is fed to a reactor stage comprising one or more reactors with oxygen for carrying out an exothermic oxidation reaction in the reactor stage, and an output slurry from the reactor stage is fed with oxygen to a first crystallizer of a crystallizing stage. The method comprises the steps of: determining empirical relationships between (1) the desired level of contaminant in the TA output product from the crystallizing stage and the oxygen uptake in the first crystallizer and (2) the oxygen uptake in the first crystallizer and the CO2 in the vent gas from the reactor stage; monitoring the CO2 in the vent gas from the reactor stage; and, adjusting operating variables of the reactor stage within minimum and maximum constraints thereof to adjust the reactor vent CO2 to a desired level. Also, the method includes the steps of: determining empirically a mathematical relationship between the oxygen in the vent gas from the reactor stage and the opticals or optical density of the TA output product; monitoring the oxygen in the vent gases; and, adjusting the air intake to the reactor stage until the oxygen in the reactor vent gas is at a desired level to provide TA output product with a desired optical density. The apparatus includes one or more reactors, one or more crystallizers, and control devices for carrying out the various steps of the method.