Process for treating polyolefin particles obtained by gas-phase polymerization

1. A process for treating polyolefin particles produced by a gas-phase polymerization of one or more olefins in the presence of a polymerization catalyst system and a C 3 -C 5 alkane as a polymerization diluent in a gas-phase polymerization reactor comprising:

a) discharging the polyolefin particles continuously or discontinuously from the gas-phase polymerization reactor and transferring the particles to a first degassing vessel;

b) contacting therein the polyolefin particles with a gaseous stream comprising at least 85 mol-% of C 3 -C 5 alkane while the polyolefin particles have an average residence time in the first degassing vessel of from 5 minutes to 5 hours;

c) transferring the polyolefin particles to a second degassing vessel;

d) contacting therein the polyolefin particles with a stream comprising nitrogen and steam while the polyolefin particles have an average residence time in the second degassing vessel of from 5 minutes to 2 hours, wherein the contacting is carried out using a stream comprising 2-20% by mol water vapor at conditions under which no condensation of water occurs;

e) transferring the polyolefin particles to a third degassing vessel;

f) contacting therein the polyolefin particles with a stream of nitrogen while the polyolefin particles have an average residence time in the third degassing vessel of from 5 minutes to 8 hours.

2. The process for treating polyolefin particles according to claim 1 , wherein, in step a), the polyolefin particles are discharged into a separation vessel in which they are separated from the major part of the concomitantly discharged reaction gas and then transferred from the separation vessel to the first degassing vessel.

3. The process for treating polyolefin particles according to claim 1 , wherein, in step a), the polyolefin particles are directly discharged into the first degassing vessel.

4. The process for treating polyolefin particles according to claim 1 , wherein the contacting of the polyolefin particles in steps b), d) and f) is carried out countercurrently.

5. The process for treating polyolefin particles according to claim 1 , wherein at least a part of the nitrogen for contacting the polyolefin particles in steps d) had previously been used for contacting the polyolefin particles in steps f).

6. The process for treating polyolefin particles according to claim 1 , wherein step e) is at least partly carried out by nitrogen which had previously been used for contacting the polyolefin particles in steps f).

7. The process for treating polyolefin particles according to claim 1 , wherein step b) is carried out at a pressure of from 0.1 MPa to 0.4 MPa and a temperature of from 50° C. to 120° C.

8. The process for treating polyolefin particles according to claim 1 , wherein step d) is carried out at a pressure of from 0.1 MPa to 0.35 MPa and a temperature of from 50° C. to 120° C.

9. The process for treating polyolefin particles according to claim 1 , wherein step f) is carried out at a pressure of from 0.1 MPa to 0.2 MPa and a temperature of from 50° C. to 120° C.

10. The process for preparing a polyolefin polymer according to claim 1 , wherein the polyolefin polymer is a homopolymer or copolymer of ethylene.