Rigorous Dynamic Model of the High-Pressure Polymerization of Ethylene in Tubular Reactors Able to Predict the Full Molecular Weight Distribution

High-pressure polymerization of ethylene in tubular reactors is a widely employed industrial process. It allows obtaining branched low density polyethylene (LDPE) with characteristics that have not been reproduced by the more modern low or medium pressure polymerizations. The process is carried out under rigorous conditions. For instance, the reactor is operated at very high pressure, between 1300 to 3000 bar at the inlet. The temperature rises from about 50 oC at the reactor inlet to 330 oC at the hottest points due to the exothermic polymerization. Axial velocities are also high, usually around 10 m/s. Besides, the reactor configuration is complex. The main feed usually consists of ethylene, inert components, transfer agents to control the molecular weight, and eventually oxygen as initiator. Most of the polymerization reaction, however, is initiated by peroxides that are fed to the reactor through lateral injections. Monomer and/or transfer agents can also be fed through side injections. The polymerization takes place in short reaction zones following the peroxides injections, exhibiting high heat generation and steep temperature profiles. The rest of the reactor is mainly used as a heat exchanger, heating or cooling the reaction mixture in order to reach appropriate temperatures for peroxides addition or for downstream units. The control of the reaction mixture temperature is achieved by circulating vapor or liquid water through different jacket zones, which may be interconnected or not. Moreover, some of these reactors are provided with a let-down valve located at the reactor exit that is periodically opened to produce a pressure pulse which sweeps out the polymer from the walls. Besides, the interaction between the different reacting species is very complex, and there is an intricate connection between polymer quality and process conditions.