The conversion per pass is usually 15-20% and 20-30% per pass for single-zone (no addi tional injections of reactants) and multizone reactors, respectively. After leaving the reactor. polyethylene is removed from the reaction mixture by reducing the pressure, usually in two stages in series at 20-30 MPa and 0.5 MPa or less. Molten polyethylene is extruded, pelletized, and cooled. The recovered ethylene is cooled to allow waxes and oils (i,e., very low- molecular-weight polymer) to be separated prior to recycling of ethylene to the reactor. overall, ethylene polymerization is carried out as a bulk polymeriza e., no added sol tion vent or diluent) in spite of the inherent thermal and viscosity problems, Control s achieved miting conversion to no more than 30% per pass. The polymerization is effectively car- ried out as a solution or suspension process (with unreacted monomer acting as solvent or diluent) depending is above or below 200 MPa.