(345a) Prediction of Dynamic Particle Size Distribution in Industrial Slurry-Phase Olefin Catalytic Polymerization Loop Reactors

Continuous slurry-phase polymerization, in the presence of a heterogeneous Ziegler?Natta catalyst, is one of the most commonly employed processes in the production of polyolefins, including high-density polyethylene (HDPE), isotactic polypropylene (IPP) as well as their copolymers with higher olefins.

An industrial slurry-phase catalytic olefin polymerization process usually consists of two jacketed loop reactors in series. The first reactor of the series is continuously fed with monomer, co-monomer, hydrogen, diluent and catalyst. During polymerization, the polymer solids are gradually collected in the settling legs placed at the lower part of the loop reactor. The settling legs periodically open to remove the highly concentrated slurry (i.e., consisting of polymer solids and a fraction of the liquid phase). The product stream leaving the first loop reactor is fed to the second reactor of the series together with fresh monomer(s), diluent and hydrogen. After the concentrated slurry is removed from the reactor, the polymer particles are separated from the unreacted monomer(s) and the solvent by hot flashing. The solvent is completely recovered due to the high monomer(s) conversion while there is no need for monomer(s) recovery. Finally, the polymer product is dried and pelletized.

In the present study, a multi-scale, multi-phase, dynamic model is developed for the determination of the distributed properties (i.e., particle size distribution (PSD), molecular weight distribution (MWD)) of polyolefins produced in industrial slurry-phase olefin polymerization reactors. The polymer MWD is determined by employing a generalized multi-site, Ziegler-Natta (Z-N) kinetic scheme (including activation, propagation, deactivation and site transfer reactions) in conjunction with the well-known method of moments. All the thermodynamic calculations are carried out using the Sanchez-Lacombe Equation of State (S-L EOS). To calculate the dynamic evolution of PSD in the slurry-phase olefin polymerization reactor, a dynamic population balance model needs to be solved together with the differential equations describing the radial monomer(s) concentration and temperature profiles in a single polymer particle. Numerical simulations are carried out to investigate the effect of initial catalyst size distribution, catalyst activity, reactor feed policy, temperature and other key process variables on the molecular and morphological properties of the produced polymer.