Process for preparing acrylic acid from methanol and acetic acid

A process for preparing acrylic acid from methanol and acetic acid, which comprises the following measures:

• a stream of a reaction gas input mixture A comprising the methanol and molecular oxygen reactants and at least one inert diluent gas other than steam is conducted through a first reaction zone A charged with at least one oxidation catalyst A and, in the course of passage through reaction zone A, methanol present in the reaction gas input mixture A is oxidized under heterogeneous catalysis to formaldehyde and steam so as to form a product gas mixture A comprising formaldehyde, steam and at least one inert diluent gas other than steam, with or without excess molecular oxygen, and a stream of product gas mixture A leaves reaction zone A, it optionally being possible to supply further molecular oxygen and/or further inert diluent gas to the reaction gas mixture A flowing through reaction zone A on its way through reaction zone A,
• optionally, the stream of product gas mixture A leaving reaction zone A is fed to a separation zone T* and any unconverted methanol still present in product gas mixture A in separation zone T* is removed from product gas mixture A to leave a formaldehyde-comprising product gas mixture A*, and a stream of product gas mixture A* leaves reaction zone A,
• a stream of a reaction gas input mixture B which comprises acetic acid, steam, at least one inert diluent gas other than steam and formaldehyde, with or without molecular oxygen, and in which the molar amount nHAc of acetic acid present is greater than the molar amount nFd of formaldehyde present therein is obtained from the stream of product gas mixture A or from the stream of product gas mixture A* and at least one further stream comprising acetic acid,
• the stream of reaction gas input mixture B is passed through a second reaction zone B charged with at least one aldol condensation catalyst B and formaldehyde present in reaction gas input mixture B, as it flows through reaction zone B, is condensed with acetic acid present in reaction gas input mixture B under heterogeneous catalysis to give acrylic acid and H2O, so as to form a product gas mixture B comprising acrylic acid, acetic acid, steam and at least one inert diluent gas other than steam, with or without molecular oxygen, and a stream of product gas mixture B leaves reaction zone B, it optionally being possible to supply further molecular oxygen and/or further inert diluent gas to the reaction gas mixture B flowing through reaction zone B on its way through reaction zone B,
• the stream of product gas mixture B leaving reaction zone B is fed to a separation zone T and separated in separation zone T into at least three streams X, Y and Z,
• the acrylic acid flow present in stream X being greater than the acrylic acid flow present in streams Y and Z together,
• the acetic acid flow present in stream Y being greater than the acetic acid flow present in streams X and Z together,
• the flow of inert diluent gas other than steam present in stream Z being greater than the flow of inert diluent gas other than steam present in streams X and Y together, and
• stream Y is recycled into reaction zone B and used to obtain reaction gas input mixture B.