Patent ID: 8431743

Claim:
A process for preparing acrylic acid by heterogeneously catalyzed partial gas phase oxidation of propylene, comprising: a) in a first reaction stage, subjecting propane to a heterogeneously catalyzed dehydrogenation in the presence of and/or with exclusion of oxygen to obtain a product gas mixture 1 comprising propane and propylene, and b) optionally, removing and/or converting to other compounds a portion of the constituents, other than propane and propylene, present in the product gas mixture 1formed in the first reaction stage to obtain a product gas mixture 1′ from product gas mixture 1, and c) in a second reaction stage, subjecting product gas mixture 1 and/or product gas mixture 1′, as a constituent of a starting reaction gas mixture (2- a ) which comprises molecular oxygen, molecular nitrogen, and propylene, wherein the molar ratio of O 2 :C 3 H 6 is ≧1 and the molar ratio of N 2 : O 2 ranges from 3 to 6, to a heterogeneously catalyzed partial gas phase oxidation of propylene present in product gas mixture 1 and/or product gas mixture 1′ to acrolein in a second reaction stage charged with a fixed catalyst bed (2- b ) whose catalysts have at least one multimetal oxide comprising the elements Mo, Fe and Bi as an active composition to obtain a product gas mixture (2- c ), and d) optionally, lowering the temperature of the product gas mixture (2- c ) leaving the second reaction stage by indirect and/or direct cooling and optionally, adding molecular oxygen and/or inert gas to product gas mixture (2- c ), and then e) in a third reaction stage, subjecting product gas mixture (2- c ), as a starting reaction gas mixture (3- a ) which comprises acrolein, molecular oxygen and at least one inert gas and comprises molecular oxygen and acrolein in a molar O 2 :C 3 H 4 O ratio of ≧0.5, to a heterogeneously catalyzed partial gas phase oxidation of acrolein present in starting reaction gas mixture (3- a ) to acrylic acid in a third reaction stage charged with a fixed catalyst bed (3- b ) whose catalysts have at least one multimetal oxide comprising the elements Mo and V as an active composition to obtain a product gas mixture (3- c ), and f) removing acrylic acid in a separating zone A from product gas mixture (3- c ) and recycling at least the unconverted propane and propylene present in product gas mixture (3- c ) to an extent of in each case at least 90 mol % based on the particular amount present in product gas mixture (3- c ) into at least the first of the three reaction stages, wherein the conversion C P of propylene in the second reaction stage, based on a single pass through it, ranges from 90 to 98 mol %, and the process has at least one separate discharge for constituents other than propane and propene and the content of propane and propene in that discharge is <1% by volume and the conversion C A of acrolein in the third reaction stage, based on a single pass through it, ranges from 99 to 99.9 mol %.