The most common production method of PE is through slurry or gas phase processing, and a few are produced by solution phase processing. All of these processes are exothermic reactions involving ethylene monomer, α-olefin monomer, catalyst system (may be more than one compound) and various types of diluents. Hydrogen and some catalysts are used to control molecular weight. The slurry reactor is generally a stirred tank or a more commonly used large-scale loop reactor in which the slurry can be stirred in circulation. When ethylene and comonomer (as needed) come into contact with the catalyst, polyethylene particles are formed.
After removing the diluent; polyethylene granules or powder granules are dried and added with additives according to the dosage to produce pellets. A modern production line with large reactors with twin-screw extruders can produce more than 40,000 pounds of PE per hour. The development of new catalysts contributes to improving the performance of new grades of HDPE. The two most commonly used types of catalysts are Philips' chromium oxide-based catalysts and titanium compound-alkyl aluminum catalysts. The HDPE produced by the Phillips catalyst has a medium-width molecular weight distribution; the titanium-alkyl aluminum catalyst has a narrow molecular weight distribution. The catalyst used in the production of narrow MDW polymers in a dual reactor can also be used to produce wide MDW grades. For example, two series reactors producing significantly different molecular weight products can produce bimodal molecular weight polymers that have a full range of molecular weight distribution.
