Polymerization rate modeling of ethylene polymerization with supported chromium oxide catalysts

Supported chromium oxide catalysts have long been used for the polymerization of ethylene to high‐density polyethylene. Unlike Ziegler–Natta catalysts, chromium oxide catalyst systems do not require cocatalysts to obtain high polymerization activity. One of the most distinctive characteristics of the chromium oxide catalyst is the presence of an induction period during the initial period of polymerization. The duration of induction period is dependent on many factors such as catalyst preparation, activation procedures, and polymerization conditions. After the induction period, the polymerization rate increases steadily with reaction time until it reaches a stationary value. Although chromium oxide catalysts have been used for years in industrial processes, there is still a dearth of literature concerning the kinetic modeling of chromium oxide catalyzed polymerization processes. In this article, a kinetic model is developed for the chromium oxide catalyzed ethylene polymerization in gas and slurry phases. In this model, it is proposed that the reaction byproduct generated by the reduction of hexavalent chromium species to Cr2+ by ethylene poisons the active sites. With the desorption of the poison species, the deactivated sites are transformed to active sites to polymerize ethylene. The validity of the proposed model is illustrated by comparing the model calculations with the experimental data obtained from gas phase and liquid slurry polymerization experiments.