Method and apparatus for increasing conversion efficiency and reducing power costs for oxidation of an aromatic alkyl to an aromatic carboxylic acid

In a reactor suitable for the oxidation of an aromatic alkyl comprising a vertically disposed elongated vessel having a bottom and a substantially cylindrical side wall, said reactor having reactant inlet means and solids-containing reaction product outlet means, oxidizing gas inlet means at at least one location within the vessel and vent means, and an agitator adapted for rotation within the vessel on a shaft at about the axis of the vessel, the agitator having an upper mixing element and a lowermost mixing element, the improvement wherein the upper mixing element is an outwardly impelling element comprising a flat blade turbine provided with a plurality of blades substantially evenly spaced from each other and each in a plane radial to the shaft with each blade extending substantially to the agitator shaft, the flat blade turbine having a diameter about 0.45 to 0.55 times the inside diameter of the vessel, and wherein the lowermost mixing element is an outwardly impelling element comprising a pitched blade turbine having a diameter of about 0.5 to about 0.55 times the inside diameter of the vessel and provided with a plurality of pitched blades substantially evenly spaced from each other, each of the pitched blades having a leading edge and a trailing edge and a pitch in a range of about 30.degree. to about 60.degree., the lowermost element being located so that a distance from its mid-level to the bottom of the vessel is about 0.25 to about 0.33 times the inside diameter of the vessel and the upper and lowermost mixing elements being spaced from one another at a distance sufficient to provide substantially uniform mixing of the entire content of said vessel upon rotation of the upper and lowermost mixing elements.