surfaces of the mixing zone and of the distribution zone on the one hand and the facing surfaces of the mixing zone and of the reaction zone on the other are substantially equal these surfaces are those defined by a plane perpendicular to the flow direction or to the channels preferably they are substantially equal to the cross section of the reactor through a radial plane so that the whole surface of the reactor is used to full capacity fig2 b fig3 a 3b 3c and 3d show different embodiments of mixer 3 this latter in fact includes a plurality of cylindrical disks 8a 8b 8c of a diameter preferably equal to that of the reactor and of a thickness between 1 and 10 mm each disk 8 is provided with a mesh pattern of channels monolith of individual section between 0 0025 mm 2 and 100 mm 2 and which corresponds to the surface of a square at least of the dimensions of which is equal to the flame quenching distance the mesh pattern of each disk is offset in the directions x and y of the plane preferably by a 2 if a is the side of the square the disks are stacked and held in position by a bar for example not shown in the figure which is housed in a notch 14 provided for this purpose one thus end up with a progress of the layers of fluids shown in fig3 d which provides homogeneous mixing thereof while avoiding the risk of runaway and explosion of the oxidization.