An adaptable model to predict the induction time of the Phillips catalyst

The Phillips catalyst, which accounts for a large fraction of the world’s linear polyethylene, is known for an induction period without appreciable ethylene consumption, followed by a period of rising activity, which together can last an hour or two. In this investigation, the effect of trace amounts of a ubiquitous contaminant, molecular oxygen, on the length of the induction period is determined. Experiments were conducted in a tubular flow reactor with a packed bed of 1 or 3 wt% Cr on SiO₂. The protocol was to first treat Cr(VI)/SiO₂ at a temperature of 600 °C in 50 % O₂, then cool and adjust the residual O₂ concentration at the reactor inlet to between about 50 to 500 ppm by an inert gas purge, with optional application of 1-hexene as reductant. Polymerization was conducted at a temperature of 100 °C and ethylene partial pressures of 10 to 18 kPa. The length of the induction period and the time to reach maximum activity correlated with the O₂ concentration at the inlet. The observations were qualitatively confirmed by high-pressure (3.79 MPa) experiments with slurried catalyst, which also demonstrated that the effect of O₂ can be transient, and the catalyst reaches its full polymerization activity. An adaptable model of competing reduction and reoxidation rates in the packed-bed reactor is presented that accurately predicts the length of the induction time based on O₂ level and the amount of catalyst loaded into the reactor.

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