Process for the introduction of a polymerisation catalyst into a gas-phase fluidised bed

1. A process for the introduction of a metallocene polymerisation catalyst in solid form into a gas-phase fluidised bed reactor, which process comprises using an injection device having an inner tube of internal cross-sectional area of 10 to 100 mm 2 and an outer tube forming an annulus around said inner tube with a cross-sectional area of 2 to 10 times the internal cross-sectional area of the inner tube, and passing said polymerisation catalyst and a carrier gas through the inner tube and into the gas-phase fluidised bed at a linear velocity of said carrier gas of 4 to 20 m/s and at a mass flow rate of carrier gas in the range 10-35 kg/h, and passing a shielding gas through the outer tube and into the gas-phase fluidised bed reactor at a linear velocity of said shielding gas of 1 to 10 times the linear velocity of the carrier gas through the inner tube and at a mass flow rate of the shielding gas in the range 100-500 kg/h, wherein the shielding gas comprises ethylene and hydrogen in a molar ratio of hydrogen to ethylene which is higher than the molar ratio of hydrogen to ethylene at the gas outlet of the fluidised bed reactor.

2. A process according to claim 1 wherein the carrier gas is selected from a group consisting of olefins, inert gases and mixtures thereof.

3. A process according to claim 2 wherein the carrier gas is selected from a group consisting of ethylene and nitrogen.

4. A process according to claim 1 wherein the shielding gas is passed through the outer tube and into the gas-phase fluidised bed at a linear velocity of said shielding gas of 3 to 8 times the linear velocity of the carrier gas through the inner tube.

5. A process according to claim 1 wherein the cross-sectional area of the annulus is less than 500 mm 2 .

6. A process according to claim 1 wherein the linear velocity of the shielding gas is less than 50 m/s.

7. A process according to claim 1 wherein the mass flow rate of the shielding gas is less than 400 kg/h.

8. A process according to claim 1 wherein the ratio of the mass flow rate of the shielding gas to the mass flow rate of the carrier gas is less than 20.

9. A process according to claim 1 wherein the shielding gas has less than 10% inert gases by volume.

10. A process according to claim 9 wherein the shielding gas comprises less than 1% inert gases by volume.

11. A process according to claim 1 wherein the shielding gas comprises at least 90% ethylene and hydrogen by volume.

12. A process according to claim 11 wherein the shielding gas consists essentially of ethylene and hydrogen.

13. A process according to claim 5 wherein the cross-sectional area of the annulus is less than 250 mm 2 .

14. A process according to claim 5 wherein the cross-sectional area of the annulus is in the range 50 to 150 mm 2 .

15. A process according to claim 6 wherein the linear velocity of the shielding gas is 20 to 50 m/s.

16. A process according to claim 7 wherein the mass flow rate of the shielding gas is in the range 100-300 kg/h.

17. A process according to claim 8 wherein the ratio of the mass flow rate of the shielding gas to the mass flow rate of the carrier gas is less than 15.

18. A process according to claim 8 wherein the ratio of the mass flow rate of the shielding gas to the mass flow rate of the carrier gas is in the range 5 to 15.

19. A process according to claim 11 wherein the shielding gas comprises at least 95% ethylene and hydrogen by volume.

20. A process according to claim 3 wherein the carrier gas is nitrogen.