Patent ID: 7524987

Claim:
A process for preparing acrolein, or acrylic acid or a mixture thereof from propane, in which A) at least two gaseous feed streams comprising propane, at least one of which comprises fresh propane, are fed to a first reaction zone A to form a reaction gas A, in reaction zone A, reaction gas A is conducted through at least one catalyst bed in which partial heterogeneously catalyzed dehydrogenation of the propane forms molecular hydrogen and propylene, molecular oxygen which oxidizes molecular hydrogen present in reaction gas A in reaction zone A to steam is fed to reaction zone A, and product gas A which comprises molecular hydrogen, steam, propylene and propane is withdrawn from reaction zone A, B) in a reaction zone B, the product gas A withdrawn from reaction zone A, with feeding of molecular oxygen, is used to charge at least one oxidation reactor with a reaction gas B comprising molecular hydrogen, steam, propane, propylene and molecular oxygen, and the propylene present therein is subjected to a heterogeneously catalyzed partial gas phase oxidation to give a product gas B comprising acrolein, or acrylic acid or a mixture thereof as a target product, unconverted propane, molecular hydrogen, steam, carbon dioxide as a by-product, and also other secondary components having lower and higher boiling points than water, C) product gas B is conducted out of reaction zone B, and target product, water and secondary components having a higher boiling point than water present therein are removed therefrom in a first separation zone I to leave a residual gas I which comprises unconverted propane, carbon dioxide, molecular hydrogen, secondary components having a lower boiling point than water and any propylene unconverted in reaction zone B and any unconverted molecular oxygen, D) as an aftertreatment measure 1, carbon dioxide present in residual gas I is scrubbed out and any water still present in residual gas I is optionally condensed out in a second separation zone II, as an aftertreatment measure 2, a portion of residual gas I is discharged, optionally, as an aftertreatment measure 3, molecular hydrogen present in residual gas I is removed by means of a separating membrane in a third separation zone III and optionally, as an aftertreatment measure 4, any molecular oxygen present in residual gas I is reduced chemically, the sequence of use of aftertreatment measures 1 to 4 being as desired, and E) aftertreated residual gas I which comprises unconverted propane and remains after use of aftertreatment measures 1 and 2 and, optionally 3 and/or 4 is recycled into reaction zone A as at least one of the two feed streams comprising propane, wherein an amount M of molecular hydrogen which is at least 5 mol % but not more than 95 mol % of the total amount of molecular hydrogen produced in reaction zone A and, optionally, fed to reaction zone A is oxidized to steam in reaction zone A.