Using the pressurized hot water extraction technique, benzoic acid and phthalic anhydride were selectively removed from solid petrochemical wastes (called molten phthalic anhydride). A statistical experimental design based on "central composite design" was applied for obtaining optimized conditions for the semi-continuous extraction of phthalic anhydride and benzoic acid from their mixture with benzoic acid, maleic acid, phthalic acid, phthalic anhydride, aldehydes, phthaldehyde, toluic acid, and some minor impurities. Variables such as pressure (60-220 bar), temperature (60-140 degrees C), dynamic time (5-45 min), and flow rate (0.2-1 mL/min) were used. (C) 2010 Elsevier B.V. All rights reserved.