Method of starting up a reactor for the oxidative dehydrogenation of n-butenes

A process for preparing butadiene from n-butenes, which has a start-up phase and an operating phase and the operating phase of the process comprises the steps:

• A) provision of a feed gas stream a1 comprising n-butenes;
• B) introduction of the feed gas stream a1 comprising n-butenes, an oxygen-comprising gas stream a2 and an oxygen-comprising recycle gas stream d2 into at least one oxidative dehydrogenation zone and oxidative dehydrogenation of n-butenes to butadiene, giving a product gas stream b comprising butadiene, unreacted n-butenes, steam, oxygen, low-boiling hydrocarbons, high-boiling secondary components, possibly carbon oxides and possibly inert gases;
• C) cooling and compression of the product gas stream b and condensation of at least part of the high-boiling secondary components, giving at least one aqueous condensate stream c1 and a gas stream c2 comprising butadiene, n-butenes, steam, oxygen, low-boiling hydrocarbons, possibly carbon oxides and possibly inert gases;
• D) introduction of the gas stream c2 into an absorption zone and separation of incondensable and low-boiling gas constituents comprising oxygen, low-boiling hydrocarbons, possibly carbon oxides and possibly inert gases as gas stream d from the gas stream c2 by absorption of the C4 hydrocarbons comprising butadiene and n-butenes in an absorption medium, giving an absorption medium stream loaded with C4 hydrocarbons and the gas stream d, and recirculation, optionally after a purge gas stream p has been separated off, of the gas stream d as recycle gas stream d2 to the oxidative dehydrogenation zone,
• where the start-up phase comprises the steps, in the order i) to iv):
• i) introduction of a gas stream d2′ having a composition corresponding to the recycle gas stream d2 in the operating phase into the dehydrogenation zone and setting of the recycle gas stream d2 to at least 70% of the total volume flow in the operating phase;
• ii) optionally additional introduction of a steam stream a3 into the dehydrogenation zone;
• iii) additional introduction of the feed gas stream a1 comprising butenes at a lower volume flow than in the operating phase and raising of this volume flow until at least 50% of the volume flow of the feed gas stream a1 in the operating phase has been attained, with the total gas flow through the dehydrogenation zone corresponding to not more than 120% of the total gas flow during the operating phase;
• iv) additional introduction, when at least 50% of the volume flow of the feed gas stream a1 comprising butenes in the operating phase has been attained, of an oxygen-comprising stream a2 at a lower volume flow than in the operating phase and raising of the volume flows of the feed gas streams a1 and a2 until the volume flows in the operating phase have been attained, with the total gas flow through the dehydrogenation zone corresponding to not more than 120% of the total gas flow during the operating phase.