Process for starting-up a heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid or of methacrolein to methacrylic acid

A process for starting-up a heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid or of methacrolein to methacrylic acid in a fixed catalyst bed which is disposed in a tube bundle reactor in the reaction tubes of a vertical bundle of reaction tubes surrounded by a reactor jacket, both ends of the individual reaction tubes being open and the upper end of each reaction tube ending sealed into a passage orifice of an upper tube plate sealed at the top into the reactor jacket and the lower end ending sealed into a passage orifice of a lower tube plate sealed at the bottom into the reactor jacket, the exterior of the reaction tubes, the upper and the lower tube plate and the reactor jacket together delimiting the reaction tube surrounding space, and each of the two tube plates being spanned by a reactor hood having at least one orifice, in which, in order to begin the startup, a reaction gas entry mixture comprising ≧3% by volume of acrolein or methacrolein and also molecular oxygen is fed to the reaction tubes of the tube bundle reactor via the at least one orifice, referred to hereinafter as E, in one of the two reactor hoods, and the product gas mixture which results through partial gas phase oxidation of acrolein or methacrolein to acrylic acid or methacrylic acid in the course of passage through the fixed catalyst bed disposed in the reaction tubes and comprises acrylic acid or methacrylic acid is removed via the at least one orifice of the other reactor hood, while, on the jacket side of the tube bundle reactor, at least one liquid heat exchange medium is conducted around the reaction tubes such that each of the two surfaces of the two tube plates facing one another are wetted by liquid heat exchange medium and the at least one liquid heat exchange medium is conducted into the reaction tube surrounding space with the temperature THin and is conducted out of the reaction tube surrounding space again with the temperature THout, wherein, at the time at which, in order to begin the startup, the reaction gas entry mixture comprising ≧3% by volume of acrolein or methacrolein enters the reactor hood through the at least one orifice E,

• the temperature THin of the at least one liquid heat exchange medium which wets the tube plate spanned by the reactor hood having the at least one orifice E, referred to hereinafter as reactor plate E, is at least 290° C.,
• the reaction gas entry mixture which enters the at least one orifice E has a temperature of ≦285° C., and
• the temperature of the surface of the reactor plate E facing the reactor hood having the at least one orifice E, referred to hereinafter as reactor plate surface E, has a value of ≦285° C.