Abstract: Apparatus for carrying out physical and/or chemical processes has a lower chamber, an upper chamber, vertical riser tubes for the passage of a first fluid medium from the lower chamber to the upper chamber and an intermediate chamber through which the riser tubes exetend so that a second fluid medium can contact the riser tubes. A granular mass is fluidized in at least the riser tubes by the flow of the first medium. A vertical gravity tube for return passage of the granular mass from the upper to the lower chamber has an outlet below the inlets of the riser tubes. A distribution plate for the first medium is mounted in the lower chamber between the inlets of the riser tubes and the outlet of the gravity tube. To allow the granular mass to fall below the distribution plate for start-up of the apparatus the gravity tube has, at least one opening connecting to the lower chamber above the distribution plate.