Process and device for decomposing fibrous material in an installation including a drum having an inner wall, a first end, and a second end, a displacer having an outer wall positioned inside the drum, and a processing channel formed between at least a portion of the outer wall and at least a portion of the inner wall. The process includes pouring fibrous material to be decomposed into the drum, relatively moving the outer wall relative to the inner wall in a peripheral direction of the drum, and mechanically processing the fibrous material in the processing channel.