Patent ID: 7396956

Claim:
A process for stably operating a continuous preparation process for obtaining acrolein or acrylic acid or a mixture thereof from propane, in which A) in a first reaction zone A, propane is subjected in the presence of molecular oxygen to a heterogeneously catalyzed dehydrogenation to obtain a product gas mixture A comprising propane and propylene, B) if appropriate, product gas mixture A is conducted into a first separation zone A in order to remove a portion or more of constituents other than propane and propylene present in product gas mixture A therefrom and thus to obtain a remaining product gas mixture A′ comprising propane and propylene, C) product gas mixture A or product gas mixture A′ is used in a second reaction zone B to charge at least one oxidation reactor and, in the at least one oxidation reactor, propylene present in product gas mixture A or in product gas mixture A′ is subjected to a selective heterogeneously catalyzed partial gas phase oxidation with molecular oxygen to give a product gas mixture B comprising acrolein or acrylic acid or a mixture thereof as the target product, unconverted propane, excess molecular oxygen and any unconverted propylene, D) in a second separation zone B, target product present in product gas mixture B is removed and, from the remaining residual gas comprising propane, molecular oxygen and any unconverted propylene, at least a portion comprising unconverted propane, molecular oxygen and any unconverted propylene is recycled as cycle gas 1 comprising molecular oxygen into reaction zone A in cycle gas mode, E) fresh propane is fed to the continuous preparation process via at least one of the zones from the group comprising reaction zone A, separation zone A, reaction zone B and separation zone B at a feed rate which has a predefined steady-state value in the stable operating state of the preparation process, and F) the content of molecular oxygen in product gas mixture B is determined continuously and compared to the steady-state target content of molecular oxygen in product gas mixture B predefined for the stable operating state of the preparation process, wherein in the case that the content of molecular oxygen determined at one time in product gas mixture B is greater than the steady-state target content, after this time, fresh propane is fed to the preparation process at a feed rate which is greater than its steady-state value, and in the case that the content of molecular oxygen determined at one time in product gas mixture B is less than the steady-state target content, after this time, fresh propane is fed to the preparation process at a feed rate which is less than its steady-state value.