A catalytic reactor having a mixing section (10) connected to a downstream reaction section (12) containing a catalyst (44) to promote a reaction of oxygen and a hydrocarbon fed to the catalytic reactor. The mixing section (10) is provided with a flame arrestor (30) to prevent a stable flame from propagating should any reaction of oxygen and hydrocarbons occur during mixing. The flame arrestor (30) permits flow in both axial and radial directions to promote mixing. Baffle elements (32) and a downstream static mixer (34) can also be used. The catalyst (44) is preferably in the form of monolithic blocks (46) enclosed by a ceramic tube (48) that is maintained as a unitary catalyst assembly (44) that can be removed for replacement and installation of the catalyst (44) as a single unit.