Polyethylene(PE) and Polypropylene(PP) are produced through processes that polymerize ethylene and propylene molecules into long molecular chains, mostly by using catalysts. Small amounts of comonomers are often added to give the PE/PP unique physical properties. A number of different production processes available to make PE/PP with some general similarities:
1) Raw materials including feedstock monomers, comonomers and catalysts have to be purified or refined to eliminate minute amounts of impurities that affect the process, such as water, oxygen, carbon monoxide, carbon dioxide, carbonyl sulfide and the like.
2) The polymerization takes place either in gas phase, liquid phase or in a high pressure environment and forms polymer products which are then separated and pelletized.
3) The monomers and diluents left are recovered and fed again to the process of polymerization.
The simple design of gas-phase technology leads to lower capital costs and lower operating costs and accommodates a large variety of catalysts. It is evolving to be the most common process in modern PE/PP production plants, and therefore based on which the control of process gas is profiled in the section below.