Multistage process for the polymerization of olefins

A process for polymerizing olefins at temperatures of from 30° C. to 140° C. and pressures of from 1.0 MPa to 10 MPa in the presence of a polymerization catalyst in a multistage polymerization of olefins in at least two serially connected gas-phase polymerization reactors, the process for transferring polyolefin particles from a first gas-phase polymerization reactor to a second gas-phase polymerization reactor comprising the steps of:

• a) discharging polyolefin particles from the first gas-phase polymerization reactor into a separation chamber wherein the polyolefin particles are separated from concomitantly discharged reaction gas, the separation chamber being at a lower pressure than the pressure in the first gas-phase polymerization reactor;
• b) transferring the polyolefin particles within the separation chamber into a lower part of the separation chamber comprising a bed of polyolefin particles which moves from top to bottom of this part of the separation chamber and into which a fluid is introduced in an amount capable of producing an upward stream of the fluid in the bed of polyolefin particles above the fluid introduction point;
• c) withdrawing polyolefin particles from the lower end of the lower part of the separation chamber and transferring the polyolefin particles to at least one lock hopper operating intermittently in parallel;
• and
• d) simultaneously pressurizing one or more lock hoppers that do not comprise the polyolefin particles and operating intermittently by means of a gas comprising reaction gas coming from the second gas-phase polymerization reactor.