A multi-bed catalytic reactor with a cylindrical shape comprising a mixing device mounted between two catalyst beds in said reactor, said mixing device has a circular outer rim adapted to fit within the inner wall of the reactor, the mixing device comprises; collecting means disposed in a collecting section for collecting fluid from an up-stream catalytic bed, mixing means disposed in a mixing section comprising a floor, a ceiling and inner and outer walls for mixing the collected fluid, and discharging means disposed in a discharging section for discharging the mixed fluid to a down-stream catalytic bed; wherein the mixing device has a donut shape and the collecting section, the mixing section and the discharging section are disposed outside the centre of the circular cross-section of the reactor and wherein said mixing means comprise guide vanes with a first and a second end and guide ramps with a first and a second end, and wherein the collecting section, the mixing section and the discharging section are disposed outside the centre one third of the circular cross-sectional area of the reactor, preferably outside the centre half of the circular cross-sectional area of the reactor, whereby the centre of the mixer is free space which can be used for service and maintenance of the reactor internals and the mixer itself, and wherein the floor of said mixing section comprises guide vanes, the outer wall of said mixing section comprises guide ramps, said guide vanes have an inwards progression when seen in the flow direction from the collecting section to the discharging section and the inwards progression relative to the length of the guide vanes is less than 1 to 2 and said guide ramps have an upwards progression when seen in the flow direction from the collecting section to the discharging section and the upwards progression relative to the length of the guide vanes is less than 1 to 2. collecting means disposed in a collecting section for collecting fluid from an up-stream catalytic bed, mixing means disposed in a mixing section comprising a floor, a ceiling and inner and outer walls for mixing the collected fluid, and discharging means disposed in a discharging section for discharging the mixed fluid to a down-stream catalytic bed; wherein the mixing device has a donut shape and the collecting section, the mixing section and the discharging section are disposed outside the centre of the circular cross-section of the reactor and wherein said mixing means comprise guide vanes with a first and a second end and guide ramps with a first and a second end, and wherein the collecting section, the mixing section and the discharging section are disposed outside the centre one third of the circular cross-sectional area of the reactor, preferably outside the centre half of the circular cross-sectional area of the reactor, whereby the centre of the mixer is free space which can be used for service and maintenance of the reactor internals and the mixer itself, and wherein the floor of said mixing section comprises guide vanes, the outer wall of said mixing section comprises guide ramps, said guide vanes have an inwards progression when seen in the flow direction from the collecting section to the discharging section and the inwards progression relative to the length of the guide vanes is less than 1 to 2 and said guide ramps have an upwards progression when seen in the flow direction from the collecting section to the discharging section and the upwards progression relative to the length of the guide vanes is less than 1 to 2.