MODELING MOLECULAR-WEIGHT DEVELOPMENT OF GAS-PHASE ALPHA-OLEFIN COPOLYMERIZATION

A comprehensive kinetic model developed for molecular weight calculati ons of ethylene and alpha-olefin copolymerizations in the context of a terminal model accounts for multiple-type active centers of the catal yst, detailed elementary chemical reactions, and catalyst composition. The moments of copolymer chain distributions are defined considering molecular weights of comonomer units so that copolymer molecular weigh t averages can be directly calculated from those moments. A double Z-t ransformation is introduced for the derivation of differential equatio ns of the moments. Model simulations are carried out based on ethylene and 1-butene copolymerizations in a gas-phase fluidized-bed reactor. Polydispersity of accumulated copolymer depends on catalyst compositio n and kinetic characteristics of the catalyst For a catalyst with spec ified kinetic characteristics, the polydispersity depends on the mole fraction of each type of active center. For a catalyst with two types of active centers, the maximum polydispersity of copolymer occurs at 5 0 wt. % of the total copolymer if polydispersities of the copolymers g enerated at each active site ave the same. Polydispersity of accumulat ed copolymer is sensitive to propagation reactions and chain transfer to hydrogen reactions. Differences in chain transfer to cocatalyst and monomers and in spontaneous deactivation rates for different types of active centers may play minor roles in controlling molecular weight d evelopment in the presence of hydrogen. This model can be used for cat alyst composition design, simulation of commercial olefin copolymeriza tion processes, and kinetic parameter estimation.