Method, an arrangement and use of an arrangement of preparing polymer

A method of producing polymer particles wherein the method comprises

• polymerizing in a fluidized bed polymerization reactor comprising a fluidized bed in the reactor and the reactor having a top zone having a generally conical shape, a middle zone in direct contact with 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 thereby polymerizing at least one olefin, optionally at least one comonomer and optionally hydrogen, in the presence of a polymerization catalyst and fluidization gas to obtain
• (i) a first stream comprising fluidization gas and olefin polymer particles,
• (ii) a second stream comprising fluidization gas and agglomerates of olefin polymer,
• (iii) a third olefin polymer product stream,
• directing the first stream comprising fluidization gas and olefin polymer particles to a series of at least three cyclones connected to the fluidized bed polymerization reactor thereby obtaining from a last cyclone in the series a final stream of fluidization gas depleted of olefin polymer particles and from the at least three cyclones in the series a final stream of olefin polymer particles separated from the fluidization gas,
• separating agglomerates of olefin polymer from the second stream comprising fluidization gas and agglomerates of olefin polymer and removing the separated agglomerates from the bottom zone of the fluidized bed polymerization reactor, and
• withdrawing from the fluidized bed polymerization reactor the third olefin polymer product stream.