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

The present invention relates to a process for polymerisation in a gas-phase fluidised bed, and in particular to a process for the introduction of a polymerisation catalyst in solid form into a gas-phase fluidised bed, which process comprises using an injection device having an inner tube of internal cross-sectional area of 10 to 100 mm and an outer tube forming an annulus around said inner tube with a cross-sectional area of 1 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 14 m/s and at a mass flow rate of earner gas in the range of 10 to 35 kg/h, and passing a shielding gas through the outer tube and into the gas-phase fluidised bed 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 to 500 kg/h