the duct through the nozzle 6 if individual filters become clogged the top layer of these filters must be changed each filter can be shut down individually either by closing the related purified gas header 10 or by increasing the pressure in the purified gas header via ejector nozzles 19 or other devices in order to obtain a differential pressure on the raw gas side the differential pressure ensures that the gas stream flows upwards through the layer of bulk material at a low velocity which reduces the angle of repose when the angle becomes smaller than the angle of inclination of the layer in the chamber the laden top layer of the bed starts sliding downwards and falls into the outlet duct 13 when the filter is re connected the gas stream is reversed and the bulk material remains in the bed because the angle of repose is larger than the angle of inclination in the chamber vibrating or knocking devices may also be provided for the trays 8 in order to facilitate the removal of the laden layer however a device of this type is not shown in the figures the laden filter material can be withdrawn from the bottom section while the invention has been illustrated and described as embodied in a device with granulated filters for raw gas purification it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention without further analysis the foregoing will.