GB908011A - Process for the separation of isophthalic and terephthalic acids - Google Patents

Abstract

Isophthalic and terephthalic acid are separated from mixtures containing them by forming a solution of them, supersaturating the solution with respect to one of these acids by controlled cooling or concentration so that nucleation is established without substantial precipitation of crystals from the solution, thereafter cooling or concentrating the solution to precipitate the acids as a crystalline mass, and separating from this mass a relatively large-crystal fraction enriched in the nucleated acid, and a relatively small-crystal fraction enriched in the other acid. Preferably the solution is supersaturated with establishment of nucleation by controlled cooling at a slow rate and the subsequent crystallisation is effected by cooling at a relatively rapid rate. The solvent may be water, methanol, ethanol, acetic acid, dimethyl formamide, or dimethyl sulphoxide. The iso- and tere-phthalic acids may be dissolved in hot water under super-atmospheric pressure and the nucleation and crystallisation steps may be effected by evaporatively cooling the solution by reduction in pressure. The treatment may be applied to a mixture of phthalic acids obtained by oxidation of a mixture of xylenes, or other alkyl benzenes or intermediate oxidation products thereof. Such acid mixture may be subjected to solvent extraction to remove orthophthalic acid, and the residue consisting of a mixture of iso- and tere-phthalic acids containing terephthalic acid in excess of that corresponding to the constant solute ratio may be leached with a solvent to form the solution for the separation treatment, leaving a solid residue of terephthalic acid. In an example, water is used as the solvent. A solution of the mixed acids is prepared in an autoclave at 337 DEG F. and is concentrated to effect supersaturation by evaporative cooling. The temperature is lowered slowly to 327 DEG F. and then over a period of 2 hours to 300 DEG F. It is then lowered more rapidly by reducing the pressure to atmospheric pressure over a period of 1/2 to 1 hour. The crystalline mass which is formed is graded by screening using screens of mesh sizes of 35-325. In this way isophthalic acid crystals of 98-94% purity are separated from the smaller crystals in which terephthalic acid predominates. Instead of screening, other methods of grading the crystals may be used e.g. settling, elutriation, and centrifuging. The fractions obtained may be further purified by re-crystallisation, or re-running. Specification 807,091 is referred to.