Abstract: The solids will first of all reach a first container under atmospheric pressure and then reach a second container of variable pressure, which is disposed thereunder, before they are introduced into the pressure vessel. The first and the second container each have a lower outlet passage and a movable shutter cooperating with the outlet passage. The outlet end of the outlet passage is disposed 20 to 400 mm above the shutter in the closed position, in the closed position the shutter forms the bottom of a chamber at least partly filled with solids. The chamber is connected with the outlet passage in a gastight way, and there is no gastightness between the chamber and the shutter. In the closed position, the shutter carries a solid bed, a vertical solid column having a height of at least 1 m is present in the outlet passage and in the container. In the closed position, seal gas is pressed into the chamber and into the solid column from the outside.