Flow diagram of an HDPE loop

The fluidised bed reactor is a vertical pressure vessel with a total height of up to 40 m. A fluidised bed of polymer particles in gaseous ethylene is maintained by a recycle compressor. The ethylene recycling gas enters the reactor through a distributor plate at the bottom to achieve an even gas flow over the entire cross-section and to hold the particles when the gas flow is turned off. In the characteristically conical upper part of the reactor, the gas velocity decreases with the increasing diameter of the reactor to keep the particles in the fluidised bed. The gas leaves the reactor at the top. It is cleaned from entrained particles by a eyelone, the reaction heat is removed by a recycle gas cooler and the gas is then routed back to the bottom reactor inlet. 

Feed ethylene is normally delivered to the plant by a pipeline grid or directly from a cracker on the same site. As the process is highly sensitive to impurities, sulphin compoimds, acetylene and other impurities are removed from the feed ethylene by purification beds. The cleaned feed ethylene is then compressed to the required reaction pressure and enters the reactor loop at the bottom of the reactor. A metal oxide catalyst, alinniniinn alkyl in hydrocarbon as a cocatalyst, lower olefins as comonomers and other auxihary chemicals are fed directly into tire reactor loop. Typically, different product types can be produced by selecting the catalyst system, the comonomers and the reaction conditions. 

The reaction temperature is 80 to 105 °C and the reaction pressure of 0.7 to 2 MPa is controlled by the ethylene feed compressor. Polymer and gas are extracted from the fluidised bed by valves at the bottom section of the reactor and expanded to a lower pressure of approx. 0.15 MPa in a degassing vessel to separate polymer particles from monomers. 

The degassed polymer granules are fed to a melting extmder and pelletised in an underwater pelletiser. If required, additives can be added to the melted polymer. The pelletiser water is recycled a small side-stream has to be discharged. The resulting waste water contains a small amount of hydrocarbons due to the direct contact with the pellets. 

The process, set up with a loop reactor part and a gas phase reactor part, is similar to corresponding single reactor processes. The catalyst is mixed with propane diluent and fed into a pre-polymerisation reactor. Cocatalyst, ethylene, comonomer and hydrogen are also fed into this reactor. The pre-polymerised slurry, together with the main feeds, is then fed into the slurry loop reactor, designed for supercritical conditions and typically operated at 85 - 100 C and 5.5 - 6.5 MPa. This reactor produces a low molecular weight, high density product. The reactor content is sent to a flash tank where diluent and unreacted components are separated from the polymer produced in the loop reactor. The diluent is condensed and recycled back into the loop reactor. 

---