Process for producing polyolefins

A process of producing a polymer of at least one olefin in two consecutive reactors in gas phase in the presence of an olefin polymerisation catalyst comprising the steps of:

• polymerising an olefin in a first polymerization reactor in the presence of an olefin polymerisation catalyst and a first reaction gas mixture to form a fluidized bed comprising an olefin polymer and said first reaction gas mixture;
• continuously or intermittently withdrawing said first reaction gas mixture together with said olefin polymer comprising said olefin polymerisation catalyst dispersed therein from the first polymerisation reactor to form a first product stream;
• directing said first product stream into a separation vessel to form a bed of olefin polymer in said separation vessel;
• withdrawing a stream of said first reaction gas mixture from said separation vessel and returning it into said first polymerization reactor;
• continuously or intermittently withdrawing said olefin polymer from said separation vessel to form a second product stream;
• and
• directing said second product stream into a second polymerization reactor, characterised in that the pressure in said separation vessel is within 5% of the pressure in said first reaction stage at the point from where said first product stream is withdrawn and that said stream of the first reaction gas mixture is returned into said first polymerization stage to a point where the pressure is lower than in said separation vessel.