A reaction chamber for a chemical reactor comprises a casing (100) of the reaction chamber, a floor (200) of the reaction chamber having an opening (300) located in the floor, an agitator shaft (400) located in the chamber and having at least one agitator element (500), connected thereto, wherein the agitator shaft (400), seen in the longitudinal direction, has a beginning (600) and an end (700). In the opening (300) of the floor (200) a removable sleeve (800) is provided, which projects out of the reaction chamber. The sleeve (800) is arranged in alignment with the axis of rotation of the agitator shaft (400). The internal diameter of the sleeve (800) is greater than the diameter of the agitator shaft (400) and the agitator shaft (400), at the beginning (600) thereof and/or at the end (700) thereof, is adapted to absorb reversibly a torque provided by means of a further shaft and/or to transmit a torque to a further shaft. Using such a reaction chamber, it is possible to build up modular chemical reactors having decreased backmixing.