Method for producing an aromatic dianhydride

A method for producing an aromatic dianhydride, the method comprising:

• reacting an aromatic diimide with a substituted or unsubstituted phthalic anhydride in an aqueous medium in the presence of an amine exchange catalyst under conditions effective to provide an aqueous reaction mixture comprising an N-substituted phthalimide, an aromatic tetraacid salt, and at least one of an aromatic triacid salt and an aromatic imide diacid salt, wherein the reacting is at a reaction temperature that is 140 to 250° C. and a reaction pressure of 1.13 to 2.16 MPa, (150 to 300 psig);
• removing the phthalimide from the aqueous reaction mixture by extracting the aqueous reaction mixture with an organic solvent;
• and
• converting the aromatic tetraacid salt to the corresponding aromatic dianhydride;
• wherein the extracting is in an extraction column comprising a metal packing material having a specific surface area of greater than 1500 square meters per cubic meter;
• wherein extracting the aqueous reaction mixture comprises passing a dispersed phase comprising the aqueous reaction mixture through a continuous phase comprising the organic solvent;
• and
• wherein an interface between the aqueous reaction mixture and the organic solvent in the extraction column is at a level that is 14 to 85% of the height of the extraction column.