Reactor having a heat exchange medium circulation

A reactor comprising:

• a bundle of catalyst tubes through which a reaction mixture is passed, and wherein a heat exchange medium is passed through a space surrounding the catalyst tubes;
• ring lines at both of reactor ends with jacket orifices for feeding in and removing the heat exchange medium by at least one pump, the heat exchange medium being fed to the upper ring line and being sucked in via the lower ring line by the at least one pump having at least one vertical pump shaft mounted and driven at its end, the pump configured to transport the heat exchange medium through the pump upwardly,
• wherein the at least one pump comprises a diagonal rotor and a leak-permeable restrictor gap, which allows a leakage stream of the heat exchange medium to pass through, upwardly in the longitudinal direction of the at least one pump shaft, within the heat exchange medium, on a pressure side of the at least one pump for sealing and mounting the at least one pump shaft and reducing axial shear of the diagonal rotor.