Process of degassing and pelletizing polyolefins

Abstract

The invention relates to a process for degassing and pelletizing polyolefin particles produced by a gas phase polymerization. The polyolefin particles are submitted to a primary degassing, preferably at the outlet of a reactor for catalytic polymerization in the gas phase. Then the polyolefin particles are subjected to a secondary degassing step comprising agitating the particles by a mechanical stirrer and converting the mechanical energy therefrom into heat such that the temperature of the particles is brought to between 80.degree. and 120.degree. C. which is close to minimum sintering temperature Ts so that the polyolefin particles are substantially freed from the gases accompanying them. The heated degassed polyolefin particles are then pelletized in an extruder of a pelleting unit. Optionally, during the secondary degassing step an inert gas such as nitrogen is circulated to facilitate evacuation of gases from the particles, and/or gas containing oxygen and/or water vapor is circulated to inactivate any catalyst in the particles. Furthermore, during the secondary degassing step the polyolefin particles heated by the conversion of mechanical energy may attain a temperature between Ts-5.degree. C. and Ts+5.degree. C., preferably between Ts-2.degree. C. and Ts+2.degree. C. The polyolefin particles can be polyethylene or copolymers of ethylene and of higher alpha-olefins having 3 to 12 carbon atoms.