A pressure vessel and a drum rotatably arranged inside which has an inner space for material that is introduced into the pressure vessel. In various embodiments, a drive mechanism rotates the drum in relation to the pressure vessel. The drive mechanism can include a motor that can be located inside the pressure vessel in an interspace between the drum and the pressure vessel. Some embodiments include a door to close an opening of the pressure vessel and a part of the door can extend into an open end of the drum when the door is closed to keep the material in the drum and out of the interspace. In particular embodiments, a steam supply conduit extends into the drum at the closed end of the pressure vessel. In some embodiments, a helical agitation blade in the drum moves the material away from or toward the opening.