Scavenger of Catalyst Poisons

In commercial polyethylene operations, poisons may enter the process as trace (ppm) contaminants in ethylene, comonomer, hydrogen (CTA), nitrogen (used as inert gas), solvents and other raw materials. These poisons reduce catalyst activity. Most damaging are oxygen and water. However, CO, CO, alcohols, acetylenics, dienes, sulfur-containing compounds and other protic and polar contaminants can also lower catalyst performance. With the exception of CO, aluminum alkyls react with contaminants converting them to alkylaluminum derivatives that are less harmful to catalyst performance. Illustrative reactions of contaminants with triethylaluminum are provided in eq 4.9-4.11  

Products from eq 4.9-4.11 may undergo additional reactions to form other alkylaluminum compounds. Since CO is not reactive with aluminum alkyls, it must be removed by conversion to CO in fixed beds. 

As mentioned in section 4.2.2, Ziegler-Natta catalyst systems used in the polyethylene industry typically employ high ratios of A1 to transition metal in the polymerization reactor. Ratios of 30 are common. Hence, there is a large excess of aluminum alkyl to achieve the roles depicted in sections 4.2.2 and 4.2.3 and to scavenge poisons. 

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