Process for gas phase polymerization of olefins in a fluidized bed reactor

A process for gas phase polymerization of one or more olefins of formula CH2.dbd.CH--R in which R is hydrogen atom or an alkyl radical containing from 1 to 6 carbon atoms, in the presence of a solid catalyst based on chromium oxide, activated by heat treatment, or of a catalyst system of the Ziegler-Natta type comprising a solid catalyst consisting essentially of atoms of magnesium, of halogen and of at least one transition metal belonging to groups IV, V or VI of the Periodic Classification of the elements and a cocatalyst based on an organometallic compound of a metal belonging to groups II or III of this Classification, in which process a rising stream of a gas mixture containing the olefin(s) to be polymerized keeps the polyolefin particles being formed in the fluidized state in a fluidized bed reactor (1) comprising an upright reactor body (2) partially occupied by the fluidized bed and mounted under a disengagement chamber (3) of larger cross-sectional area than that of the reactor body (2), from which disengagement chamber there leaves a gas mixture entraining solid particles which are separated from the gas mixture in a dust separator and are reintroduced into the reactor body (2), the gas mixture being then cooled, compressed and recycled to the base of the reactor, and forming the recycle gas, the solid particles separated from the major part of the gas mixture leaving the reactor are put into contact with a carrier gas in a mixing device and are reintroduced mixed with this carrier gas into the reactor body, the process being characterized in that the particles mixed with the carrier gas are introduced into the reactor body through an opening in the side wall thereof at a position above the fluidized bed, but below the disengagement chamber and in that the velocity of the particles introduced into the reactor body is insufficient to cause them to be blown directly into the bed.