Process for stably operating a continuous preparation process for obtaining acrolein or acrylic acid or a mixture thereof from propane

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.