A process for preparing acrolein or acrylic acid or a mixture thereof from
propane, in which propane is dehydrogenated under heterogeneous catalysis
in a reaction zone A and the propylene formed is partially oxidized
heterogeneously in a reaction zone, reaction zone A being operated in
loop mode and reaction gas mixture input stream A fed to the reaction
zone A comprising added molecular hydrogen which is added to this input
stream as late as possible.