Accounting for Temperature Effects When Predicting Molecular Weight and Composition Distribution for Gas-Phase Polyethylene Produced Using a Multi-Site Catalyst

Abstract

A dynamic model is developed for gas-phase ethylene/1-hexene polymerization with a three-site hafnocene catalyst. The model accurately predicts molecular weight and comonomer composition distributions for fifteen lab-scale copolymerization runs performed at different temperatures. The experimental runs used to fit this model were performed at temperatures between 60 and 85 °C. Gas-phase concentrations were measured every 2.7 minutes throughout each run. Predicted chain-length distributions are discretized to aid model development, keeping the number of ordinary differential equations manageable. Kinetic parameters at the reference temperature of 81 °C and activation energies are estimated. Using parameter subset selection techniques, it is determined that 53 of the 60 model parameters should be estimated using the product characterization and reactor data. An additional data set obtained at 85 °C is used for model validation, confirming the predictive power of the model. The proposed model and its parameter estimates will aid selection of operating conditions to achieve targeted polymer properties.