Process for performing an endothermic reaction in a reactor containing catalyst tubes, the catalyst tubes containing a catalyst promoting the endothermic reaction, the process comprising the steps of, a. contacting the catalyst contained in the catalyst tubes with a feed flow passing through the channels from an entrance end to an exit end, b. contacting an outer surface of the catalyst tubes with a flow of a heating medium having an initial heating temperature and flowing co-currently with the flow of feeds to heat the surface by convection, c. mixing at least part of the heating medium after having been contacted with the catalyst tubes with a flow of fresh heating medium having a start temperature higher than the initial heating temperature to form the co-current heating medium having the initial heating temperature and reactor for carrying out the process.