Patent ID: 6939991

Claim:
A process for preparing acrylic acid by generating an acrylic acid-containing product gas mixture by heterogeneously catalyzed gas phase partial oxidation of at least one C 3 precursor of acrylic acid with molecular oxygen over catalysts in the solid state at elevated temperature, initially reducing the temperature of a hot acrylic acid-containing product gas mixture by direct cooling with a quench liquid 1 and subsequently passing the cooled product gas mixture which may possibly contain portions of evaporated quench liquid 1 into a condensation column equipped with separating internals, allowing said product gas mixture to rise within the condensation column, thus fractionally condensing it, and withdrawing crude acrylic acid as the target product from the condensation column in a sidestream and withdrawing bottom liquid comprising acrylic acid oligomers from the bottom of the condensation column or removing high boiler fraction comprising acrylic acid oligomers or a mixture of such bottom liquid comprising acrylic acid oligomers and said high boiler fraction from the condensation column via a sidestream takeoff disposed below the sidestream takeoff for the crude acrylic acid and using it as a quench liquid 1, recirculating the portion of the quench liquid 1 which is not evaporated on cooling the product gas mixture, optionally via the bottom or via the high boiler takeoff of the condensation column or via both and also optionally via a heat exchanger, discharging a portion of the quench liquid 1 from a circuit of the quench liquid 1 as a bleed stream and feeding it to a dissociation vessel and dissociating therein the acrylic acid oligomers contained in the bleed stream of the quench liquid 1 at elevated temperature to acrylic acid, and recycling dissociation gases comprising acrylic acid and escaping from the liquid phase in gaseous form into said circuit of the quench liquid 1 or into the condensation column or into both said circuit of the quench liquid 1 and into the condensation column, either in gaseous, condensed or partially condensed form, which comprises subjecting the dissociation gases to a countercurrent rectification before they are recycled.