Process for polymerizing olefin in a gas phase reactor with improved thermal homogeneity

1. A process for polymerizing at least one olefin in gas phase in a fluidized bed in a polymerization reactor having a top zone of a generally conical shape as such that the equivalent cross-sectional diameter is monotonically decreasing with respect to the flow direction of the fluidization gas, a middle zone in direct contact with and below said top zone of a generally cylindrical shape and a bottom zone in direct contact and below said middle zone of a generally conical shape as such that the equivalent cross-sectional diameter is monotonically increasing with respect to the flow direction of the fluidization gas, comprising the steps of:

a) introducing a first stream of fluidization gas into the bottom zone;

b) polymerizing at least one olefin in the presence of a polymerization catalyst in the fluidized bed formed by particles of a polymer of the at least one olefin suspended in an upwards flowing stream of the fluidization gas in the middle zone;

c) withdrawing a second stream comprising the fluidization gas and optionally particles of a polymer of the at least one olefin from the top zone;

characterized in that

the temperature of the particles of the polymer of the at least one olefin in the fluidized bed (T PP ) does not exceed 120% of the operating temperature set point (T S ) of the polymerization reactor, wherein T PP and T S are both given in ° C., wherein the superficial gas velocity of the fluidization gas in the middle zone of the polymerization reactor is in the range of from 0.45 to 1.0 m/s, the polymerization in the fluidized bed is preceded by prior polymerization stages and the particles of the polymer of the at least one olefin in the fluidized bed of the polymerization reactor have a particle size d 90 of from 1000 μm to 2500 μm and a particle size d 50 of from 400 μm to 1500 μm.

2. The process according to claim 1 , further comprising the steps of:

d) directing the second stream comprising the fluidization gas and optionally particles of the polymer of the at least one olefin into a separating unit comprising at least one cyclone;

e) withdrawing from the separating unit a third stream of fluidization gas depleted from particles of the polymer of the at least one olefin;

f) reintroducing said third stream of fluidization gas into the bottom zone of the polymerization reactor as first stream of fluidization gas; and

g) withdrawing from the separating unit a fourth stream enriched with particles of the polymer of the at least one olefin.

3. The process according to claim 2 , further comprising the step of:

h) reintroducing the fourth stream enriched with particles of the polymer of the at least one olefin into the middle zone of the polymerization reactor.

4. The process according to claim 1 , wherein the particles of the polymer of the at least one olefin in the second stream have a particle size d 50 of less than 150 μm.

5. The process according to claim 1 , further comprising the steps of:

i) withdrawing a fifth stream comprising fluidization gas and agglomerates of the polymer of the at least one olefin from the bottom zone of the polymerization reactor;

j) separating the agglomerates of the polymer of the at least one olefin from the fifth stream to obtain a sixth stream of fluidization gas depleted from agglomerates of the polymer of the at least one olefin; and

k) optionally reintroducing the sixth stream of fluidization gas into the bottom zone of the polymerization reactor together with the first stream of fluidization gas.

6. The process according to claim 1 , wherein the superficial gas velocity of the fluidization gas in the middle zone of the polymerization reactor as determined as dimensionless number N Br is in the range of from 2.5 to 7.

7. The process according to claim 1 , wherein the particles of the polymer of the at least one olefin in the fluidized bed of the polymerization reactor have a particle size d 10 of from 100 μm to 500 μm.

8. The process according to claim 1 , wherein the particles of the polymer of the at least one olefin in the fluidized bed of the polymerization reactor have a span of particle size distribution (PSD), being the ratio of (d 90 −d 10 )/d 50 , of from 1.0 to 2.0.

9. The process according to claim 1 , wherein the at least one olefin is selected from at least one of ethylene and alpha-olefins having from 3 to 12 carbon atoms.

10. The process according to claim 1 , wherein the polymerization reactor does not comprise a fluidization grid.

11. The process according to claim 1 , wherein the process is a in a multi-stage polymerization process in which at least two polymerization stages are connected in series and the last polymerization stage is said process for polymerizing at least one olefin in gas phase in a fluidized bed as defined in claim 1 .

12. The process according to claim 11 , wherein in the first polymerization stage of the multistage process, which is not a pre-polymerization stage, the catalyst activity is at least 15 kg/gcat/h.

13. The process according to claim 1 for polymerizing an olefin homo- or copolymer having a narrow particle size distribution.

14. The process according to claim 13 , wherein the olefin homo- or copolymer has a span of particle size distribution (PSD), being the ratio of (D90−D10)/D50, of from 1.0 to 2.0.