Manufacture of formaldehyde

In a process for the manufacture of formaldehyde by oxidative dehydrogenation of methanol in the presence of a silver catalyst, at an elevated temperature, the improvement wherein the reaction is started at a lower throughput and a lower temperature comprising

• (a) a gaseous starting mixture consisting of methanol and steam, containing from 0.1 to 0.7 mole of water per mole of methanol, is passed through a catalyst bed of silver particles, heated to from 250 to 450.degree. C, at a throughput of from 0.04 to 1.0 tonne of methanol per square meter of catalyst bed cross-section per hour, thereafter
• (b) air corresponding to from 0.3 to 0.6 mole of oxygen per mole of methanol is thereafter added to the starting mixture at the above temperature, and the silver-catalyzed reaction is started,
• (c) at a point in time ranging from just after the start of said reaction to 0.5 hour after the start of said reaction, an off-gas taken from the same or another process for manufacture of formaldehyde by oxidative dehydrogenation of methanol and containing from 0.25 to 1.0 percent by volume of carbon monoxide, from 3.5 to 10 percent by volume of carbon dioxide, from 10 to 25 percent by volume of hydrogen and from 0.02 to 0.1 percent by volume of formaldehyde vapor, is admixed with the starting mixture, in amounts of from 90 to 3,500% by weight of off-gas, based on methanol, and is passed therewith through said catalyst bed,
• (d) the throughput of gaseous mixture fed to the catalyst, and the reaction temperature, are both increased and finally
• (e) the oxidative dehydrogenation reaction is carried out at from 550.degree. to 800.degree. C, at a throughput of from 1 to 3 tonnes of methanol per square meter of catalyst bed cross-section per hour, using from 0.2 to 0.7 mole of steam and from 0.3 to 0.8 mole of oxygen per mole of methanol, and from 90 to 180% by weight of off-gas, based on methanol.