Process for producing polyolefins

1. A process of producing a polymer of at least one olefin in two consecutive reactors in gas phase in the presence of an olefin polymerisation catalyst comprising the steps of:

polymerising an olefin in a first polymerization reactor in the presence of an olefin polymerisation catalyst and a first reaction gas mixture to form a fluidized bed comprising an olefin polymer and said first reaction gas mixture;

continuously or intermittently withdrawing said first reaction gas mixture together with said olefin polymer comprising said olefin polymerisation catalyst dispersed therein from the first polymerisation reactor to form a first product stream;

directing said first product stream into a separation vessel to form a bed of olefin polymer in said separation vessel;

withdrawing a stream of said first reaction gas mixture from said separation vessel and returning it into said first polymerization reactor;

continuously or intermittently withdrawing said olefin polymer from said separation vessel to form a second product stream; and

directing said second product stream into a second polymerization reactor, characterised in that the pressure in said separation vessel is within 5% of the pressure in said first reaction stage at the point from where said first product stream is withdrawn and that said stream of the first reaction gas mixture is returned into said first polymerization stage to a point where the pressure is lower than in said separation vessel.

2. The process according to claim 1 , wherein said stream of said first reaction gas mixture is withdrawn from above the level of said olefin polymer bed in said separation vessel.

3. The process according to claim 1 , wherein said first product stream is withdrawn continuously from said first polymerization reactor.

4. The process according to claim 1 wherein the olefin polymer forms a settled downward moving bed of olefin polymer in said separation vessel.

5. The process according to claim 1 wherein said second product stream is withdrawn continuously from said separation vessel.

6. The process according to claim 1 wherein said second polymerization reactor contains a second reaction gas mixture which differs in terms of at least one component from the composition of said first reaction gas mixture characterized in that a second gas which is different from said first reaction gas mixture is introduced into said separation vessel.

7. The process according to claim 6 wherein said second gas is introduced into the lower part of said separation vessel.

8. The process according to claim 6 characterized in that the ratio of the volumetric flow rate of said second gas to the volumetric flow rate of said first reaction gas mixture is from 0.5 to 2.

9. The process according to claim 6 , characterized in that the average residence time of the olefin polymer in the separation vessel is from 30 seconds to 5 minutes.

10. The process according to claim 6 wherein at least a part of said second gas is introduced in liquid form.

11. The process according to claim 6 wherein said second gas has the same composition as said second gas mixture.

12. The process according to claim 6 wherein said second gas is propylene.

13. The process according to claim 1 wherein said at least one olefin monomer is selected from the group consisting of ethylene, propylene, 1-butene, 1-hexene, 4-methyl-1-pentene or 1-octene, 1-decene and their mixtures.

14. The process according to claim 13 wherein said at least one olefin monomer is selected from the group consisting of ethylene, propylene and their mixtures.

15. The process according to claim 7 wherein said second gas is introduced into the lower part of said separation vessel so that y≦l/2, wherein

l is the height of said separation vessel and

y is the distance from bottom of the separation vessel to the point from which the second gas is introduced into the separation vessel.