(33b) The Impact of Ethylene and Propylene Impurities Upon Polyolefins Units

Ethylene units have traditionally supplied polymer grade ethylene and propylene monomers to polyethylene (PE) and polypropylene (PP) process units. Any additional purification of these monomers to remove catalyst poisons has typically been carried out in the polyolefin units.
PE units also require alpha olefins such as 1-Butene, 1-Hexene and 1-Octene to enhance product properties. These comonomer feedstocks must also meet polymer grade and other specifications.
Polyolefin units vary in the process technologies employed (i.e., gas, slurry or solution phase) and in the types of catalysts used (i.e., Ziegler Natta, advanced Ziegler Natta, Chromium Oxide (Phillips type) and more recently metallocene or single site). Catalyst can be used in heterogeneous or homogeneous forms.
Nearly all of the catalyst currently employed in polyolefin production are susceptible to poisoning from heteratoms (O, N and S). Also, the latest co-catalysts used with metallocene or simple site catalysts, such as modified aluminoxane (MAO), is also susceptible to poisoning.
Catalyst poisons consume catalyst and co-catalyst with the result that operating costs and the residual concentration of the catalysts in the final product increase.
Polyolefin catalysts and processes differ in their susceptibility to catalyst poison and in their purification requirements. In addition, polyolefin units must also purge impurities brought into the units by the feedstocks (methane, ethane, propane, etc.) and also generated within the polyolefin unit itself, in some cases from the deactivation of catalysts and co-catalysts. These purge streams can be managed internally within the polyolefin unit or recycled back to the ethylene unit.
The needs of polyolefin producers will be explored with respect to catalyst and process as used today and in the future so that ethylene units can be designed and integrated more effectively with downstream polyolefin units.