The reactor device comprises a horizontally-disposed housing (1), at one end of which there is disposed an inlet (4) for supply of the medium, and at the other end of which there is an outlet (5) for the reaction products. Within the housing (1) of the reactor device a rotor (1) is rotatably mounted. The rotor (8) has a hollow cylinder (9), which is provided with openings (10) and annular members (12, 13) for moving and transporting the medium. The hollow cylinder (9) is partly immersed in the free-flowing medium, the openings (10) in the hollow cylinder (9) ensuring additional admixture of the medium.