Method for returning polymer to a fluidised bed reactor

A process of polymerising at least one olefin in a fluidised bed in a fluidised bed polymerisation reactor comprising a top zone having a generally conical shape, a middle zone in direct contact and below said top zone having a generally cylindrical shape, a bottom zone in direct contact with and below the middle zone and having a generally conical shape and wherein a base of a fluidised bed is formed and wherein the reactor does not comprise a fluidisation grid, the process comprising the steps of

• (i) passing a first stream of fluidisation gas into the bottom zone;
• (ii) polymerising at least one olefin in the presence of a polymerisation catalyst in a fluidised bed formed by particles of a polymer of the at least one olefin suspended in an upwards flowing stream of the fluidisation gas in the middle zone;
• (iii) withdrawing a second stream comprising the fluidisation gas and particles of the polymer of the at least one olefin from the top zone;
• (iv) passing the second stream into a separation step;
• (v) withdrawing a third stream comprising the majority of the particles of the polymer of the at least one olefin from the separation step and returning the third stream to the polymerisation reactor through a transfer pipe,
• characterised in that the process comprises adding a support gas stream to the third stream downstream of the separation step into the transfer pipe, wherein entrainment flux of the particles of the polymer of the at least one olefin entrained from the polymerization reactor in the second stream, defined as the mass flow rate of the particles divided by the cross-sectional area of the pipe, is 0.5 to 100 kg/(s·m2).