Process for separating acrylic acid present as a main constituent and glyoxal present as a by-product in a product gas mixture of a partial heterogeneously catalyzed gas phase oxidation of a C3 precursor compound of acrylic acid

A process for separating acrylic acid present as a main product and glyoxal present as a by-product in a product gas mixture of a partial heterogeneously catalyzed gas phase oxidation of a C3 precursor compound of acrylic acid, in which a liquid phase P is obtained which comprises acrylic acid to an extent of at least 70% of its weight and, based on the molar amount of acrylic acid present therein, comprises at least 200 molar ppm of glyoxal, which comprises separating the glyoxal from the acrylic acid in the liquid phase P by crystallization, the acrylic acid being enriched in the crystals formed and the glyoxal in the mother liquor which remains in the course of crystallization.