Improved process for recovery of purified terephthalic acid

A process for the preparation of purified terephthalic acid containing p-toluic acid present in a concentration equal to or less than 200 parts per million by weight which process comprises:

• (a) introducing into a filter cell or a series of filter cells in a filtration zone at a temperature within the range of from about 38.degree. C. to about 205.degree. C. at a system pressure of from atmospheric to 235 psig an aqueous slurry comprising purified terephthalic acid present as crystals and p-toluic acid present as an aqueous solution and as a co-crystallized form with crystals of said purified terephthalic acid, said filter cell or series of filter cells maintained in suitable position whereby each filter cell develops a filter cake or filter cakes upon introduction of said slurry into each said cell;
• (b) transporting each said filter cell containing said filter cake from said filter zone to a wash zone;
• (c) introducing a water stream into each said filter cell to form a reservoir of water in each filter cell over said filter cake or filter cakes, wherein said water stream is at a pressure gradient of at least 0.5 psi over said system pressure and a temperature in the range of from about 38.degree. C. to about 205.degree. C.;
• (d) washing said filter cake with water for a period sufficient to reduce the concentration of p-toluic acid to equal to or less than 200 ppmw;
• (e) transporting each said filter cell containing washed filter cake to a pressure release zone wherein said system pressure is released to a range from atmospheric to about 90 psig to reduce temperature of said filter cake to a temperature equal to or less than 166.degree. C.;
• and
• (f) discharging said washed filter cake comprising purified terephthalic acid from each said filter cell wherein the concentration of p-toluic acid in said purified terephthalic acid is equal to or less than 200 ppmw.