A gas processing vessel of cylindrical shape having a cylindrical shell of the vertical axis, a gas distribution device and a first particulate material, the distribution device being fixed in the vessel next to a gas inlet or outlet orifice recessed into a lower end wall, a cover being designed to be in contact without a weld with the lower end wall and having a diameter greater than or equal to one third of the diameter of the vessel, a second particulate material with an equivalent diameter De greater than or equal to 10 mm and greater than the dimensions of the openings in the cover, and a means for centering the cover above the inlet or outlet orifice, the means being fixed to the pipe that is connected to the orifice.