(118d) Development of a Multi-Scale, Multi-Phase, Multi-Zone Dynamic Model for the Prediction of Particle Segregation in Catalytic Olefin Polymerization Fbrs

Fluidized bed solid catalyzed olefin polymerization has long been recognized as one of the main roads for producing high-density and linear low-density polyethylenes (HDPE and LLDPE). In such a process, catalyst particles, in the size range of 20-80 ?Ým, are continuously fed into the reactor at a point above the gas distributor and react with the incoming fluidizing monomer(s) to form a broad distribution of polymer particles (e.g., 100-5000 £gm). The particulate polyolefin product is continuously withdrawn from the reactor at a point, preferably, close to the bottom of the bed. The polymerization heat is removed via the unreacted gas mixture exiting the bed, which is subsequently cooled and recycled back to the reactor. Industrial fluidized bed reactors typically operate at temperatures of 70-110 0C and pressures of 20-40 bar. The superficial gas velocity can vary from 3 to 6 times the minimum fluidization velocity. The catalyst feed rate can vary from 0.01 to 0.5 g/s, depending on the catalyst activity and reactor capacity. Commonly, the polymer particles in the bed are assumed to be well-mixed. However, in large industrial fluidized bed reactors, particle segregation can occasionally occur. This means that the PSD will vary with respect to the bed height. Particle segregation can appear in beds, operated at low gas flow velocities, in the presence of large particle size or/and particle density differences. In the present study, a multi-scale, multi-phase, multi-zone dynamic model is developed for the prediction of distributed properties (i.e., particle size distribution, molecular weight distribution) and the extent of particle segregation in a catalytic gas-phase olefin copolymerization FBR. Accordingly, the bed is divided into a number of reaction zones. To account for the multi-phase behaviour of the FBR, each zone is assumed to comprise two compartments, namely, an emulsion phase compartment (i.e., containing a well-mixed, dense gas-solids mixture) and a bubble-wake compartment (consisting of a pure gas phase (bubble) and a gas-solid mixture in the wake of the bubble). The multi-scale description of the FBR utilizes models at four different length scales, namely, a kinetic model, a single particle model, a population balance model and a multi-phase, multi-zone reactor mixing model. At the molecular level, a two-site catalytic kinetic model is employed to describe the copolymerization of ethylene with propylene over a heterogeneous Ziegler-Natta (Z-N) catalyst. To calculate the particle growth and the spatial monomer and temperature profiles in a particle, the random pore polymeric flow model (RPPFM) is utilized. The RPPFM is solved together with a dynamic particle population balance model, to calculate the dynamic evolution of PSD in each compartment of the mutli-compartment reactor configuration. Moreover, the molecular properties of the polymer (e.g., molecular weight distribution) in each zone are calculated by solving simultaneously the single particle growth and population balance models using the postulated two-site Z-N kinetic scheme. Finally, to calculate the monomer(s) concentration(s) and temperature profiles along the reactor height, the overall dynamic mass and energy balances are simultaneously solved for all the compartments of the multi-zone FBR configuration. Extensive numerical simulations are carried out to investigate the effect of reactor operating conditions (i.e., fluidization gas velocity, catalyst feed) on the dynamic evolution of the molecular (MWD) and morphological (PSD) polymer properties, particle segregation, spatial temperature and monomer(s) concentration(s) profiles in the FBR.