A fixed bed reactor for cyclic, catalytic dehydrogenation of hydrocarbons, such as alkanes and a reactor-internal device for improving distribution of hydrocarbon feed into the fixed catalyst bed of the reactor. The device comprises a vertical deflector plate with multiple horizontal slits and a frustum cone with multiple perforations on the lateral surface, connected to the bottom end of the plate. The reactor includes a main horizontal reaction vessel containing a fixed catalyst bed and a cactus-shaped inlet assembly sub-divided into three inlets at the upper half portion to supply fluid streams, including hydrocarbon feed, to the catalyst bed. The distributor device is positioned inside a main central vertical arm of the inlet assembly form by the convergence of the three inlets. Various embodiments of the distributor device where the slit thicknesses, distance between slits, perforation diameter, distance between perforations are varied, are also provided.