Studies On The Separation And Purification Of Aromatic Carboxylic Acids Mixtures

Terephthalic acid and isophthalic acid are important starting materies of polyester industry. In the producing process of these aromatic carboxylic acids, a great number of solid wastes were generated, which contain not only many organics like benzoic acid, terephthalic acid, isophthalic acid, p-toluic acid and so on, but also metals like cobalt (Co) and manganese (Mn). In industry, these solid wastes are usually treated with burning and burying methods, which not only cause serious environmental problems but also waste a great number of non-renewable resources.In this paper, pure terephthalic acid (PTA) residue was taken as the experimental subject to develop a method for separating and purifying this kind of mixed aromatic carboxylic acids. The experimental results showed that solvent extraction was an efficient way to separate the organics and toluene-water system and p-xylene-water system were appropriate systems. After the liquid-liquid extraction, more than80%trimellitic acid (TMA) and90%o-phthalic acid (OPA) were dissolved in the water phase, more than80%benzoic acid and p-toluic acid were dissolved in the oil phase, while more than90%terephthalic acid and isophthalic acid were enriched in the solid phase. In addition, relevant phase equilibrium data was measured, including the solubilities of benzoic acid, p-toluic acid, terephathalic acid and isophthalic acid in toluene and p-xylene and the partition coefficients of benzoic acid and p-toluic acid in water-toluene system. The experimental data was fitted with the NRTL equations and the correlation thermodynamics parameters were determined. Beyond that, the demetalization and purification method of the mixed dicarboxylic acids, terephthalic acid and isophthalic acid, were also studied and the optimum reaction systems and operating conditions were determined. After the hydrolysis reaction with the hydrogen bromide, the metal content in the mixed dicarboxylic acids could be reduced to less than lOppm. The mixed terephthalic acid and isophthalic acid products with high purity could be obtained after the hydro fining by the presence of Palladium Carbon Catalysts.