Gas phase polymerization process

1. An olefin polymerization process comprising polymerizing at least one olefin in gas phase in a fluidized bed in the presence of an olefin polymerization catalyst in a polymerization reactor having a vertical body; a generally conical downwards tapering bottom zone; a generally cylindrical middle zone having a height to diameter ratio L/D of at least 4, above and connected to said bottom zone; and a generally conical upwards tapering top zone above and connected to said middle zone wherein (i) fluidization gas is introduced to the bottom zone of the reactor from where it passes upwards through the reactor; (ii) the fluidization gas is withdrawn from the top zone of the reactor, compressed , cooled and returned into the bottom zone of the reactor; (iii) a fluidized bed is formed within the reactor where the growing polymer particles are suspended in the upwards rising gas stream; and (iv) there is no fluidization grid in the reactor; characterized in that the gas velocity is maintained in the reactor such that N Br is within the range of from 2.5 to 7 wherein

N

Br

=

d

90

-

d

10

d

50

U

s

U

t

,

wherein d 90 represents the smallest equivalent particle diameter so that 90% of the particles have a smaller equivalent diameter than d 90 ; d 10 represents the smallest equivalent particle diameter so that 10% of the particles have a smaller equivalent diameter than d 10 ; d 50 represents the median equivalent particle diameter; U s is the superficial gas velocity within the middle zone; and U t is the terminal velocity of the particles within the reactor.

2. The process according to claim 1 wherein N Br is within the range of from 2.5 to 5.

3. The process according to claim 1 wherein the ratio L/D is from 4 to 15.

4. The process according to claim 3 wherein the ratio L/D is from 5 to 10.

5. The process according to claim 1 comprising the step of removing polymer from the fluidization gas which has been withdrawn from the top zone of the reactor before the compression and cooling steps.

6. The process according to claim 5 wherein the polymer removed from the fluidization gas is recovered and passed to further processing.

7. The process according to claim 5 wherein the polymer removed from the fluidization gas is returned to the polymerization reactor.

8. The process according to claim 1 wherein polymer is withdrawn from the reactor through an outlet located in the middle zone.

9. The process according to claim 8 wherein the polymer is withdrawn from the reactor continuously.

10. The process according to claim 8 wherein polymer or polymer agglomerates are withdrawn from the bottom zone of the reactor.

11. The process according to claim 1 wherein the fluidization gas is filtered before being passed to the compressor.

12. The process according to claim 1 wherein the bottom zone comprises multiple conical segments having different cone angles.

13. The process according to claim 1 further comprising the steps of (v) determining d 90 , d 10 and d 50 of the polymer powder in the reactor; (vi) determining the values of U s and U t in the reactor; (vii) calculating the corresponding value of N Br ; and (viii) adjusting U s such that the value of N Br is within the range of from 2.5 to 7.

14. The process according to claim 13 wherein the value of N Br is adjusted to be within the range of from 2.5 to 5.

15. The process according to claim 13 comprising the steps of defining a set point for N Br ; calculating the actual value of N Br within the reactor; and adjusting U s such that the difference between said set point and said actual value comes to a minimum.

16. The process according to claim 9 wherein polymer or polymer agglomerates are withdrawn from the bottom zone of the reactor.