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Growth of polymer particle

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Catalytic polymerizations of olefins in the gas-dispersion (e.g., fluidized bed) and liquid-dispersion (slurry) reactors are industrially important processes. The polymer is typically produced on supported catalysts fed into the reactor in the form of particles. Three spatial levels are considered in the description of liquid and gas phase catalytic polymerization reactors: 

  • Micro-scale: Complex reaction kinetics, the presence of comonomers (dienes), several types of active catalyst sites, branching of polymer chains. The molecular architecture of polymer product can be predicted by population balances of polymer chains (method of polymer moments).
  • Meso-scale: Polymer particle growth and morphology, fragmentation of catalyst, phase equilibria in the polymer particle, heat and mass transport in the particle.
  • Macro-scale: Mass, heat, momentum and energy balances in the reactor, particle size distribution, residence time distribution, heat removal, polymer fouling, macro- and micro-mixing, non-uniform concentration and temperature fields in reactors.

 

The growth of polymer particles starts from small catalyst particles (0.010-0.050 mm in diameter) which are fed into the reactor containing monomer, diluent and other species. At the beginning of the polymerization the polymer fills up the porous structure of the catalyst particle. The catalyst particle is quickly broken into small microfragments that are typically between 1 nm and 20 nm in diameter. However, these microfragments are still kept together by the produced polymer. The process of the catalyst particle break-up often determines the final particle morphology. Monomer and other components (e.g. cocatalyst, chain transfer agents, etc.) must diffuse through the porous polymer particle to the catalyst surface where the reaction takes place. The polymer particle continuously exchanges mass and heat with its surroundings during polymerization as the growth of the particle proceeds. The final polymer particle is typically 10-50 times as large (measured as a diameter) as the original catalyst particle. The development of the polymer particle from the supported porous catalyst is schematically shown in Figure 2

 

A number of mathematical models have been developed in our research group to describe the growth and morphogenesis of the polymer particle. Modeling at the meso-scale enhances the understanding of effects of various chemical and physical phenomena on the particle dynamics and on the final properties of the polymer product. 

 

Following effects are considered in the models: 

  • Complex reaction kinetics - multi-site reaction kinetics, catalyst activation and deactivation, non-uniform distribution of catalyst sites.
  • Population balances of polymer chains - MWD.
  • Intraparticle mass and heat transport - in the pores of the particle and in the polymer phase.
  • Heat and mass transfer at the particle surface.
  • Gas-liquid equilibrium.
  • Heat removal by vaporization of volatile species.
  • Polymer thermodynamics and sorption kinetics, polymer swelling.
  • Crystallization and partial melting.
  • Particle morphology, e.g., multi-grain, dendritic or partially melted.
  • Convective flow in the polymer particle.

 

Problems investigated by effective-scale models: 

  • particle overheating,
  • broadening of the MWD and non-uniform polymer composition due to intra-particle mass transport resistance,
  • sorption and transport of low molecular species in the polymer phase
  • effects of catalyst activation and deactivation
  • interactions of liquid phase with porous particles

 

 

Published Results:

Horáčková B., Grof Z., Kosek J.: Dynamics of fragmentation of catalyst carriers in catalytic polymerization of olefins. Chemical Enginnering Science, 2007, 62 (18-20), 5264 – 5270.

Grof Z., Kosek J., Marek M.: Modeling of morphogenesis of growing polyolefin particles. AICHE Journal, 2005, 51 (7), 2048 – 2067.

Grof Z., Kosek J., Marek M.: Principles of the morphogenesis of polyolefin particles. Industrial & Engineering Chemistry Research, 2005, 44 (8), 2389 – 2404.

Grof Z., Kosek J., Marek M., Adler P.M.: Modeling of morphogenesis of polyolefin particles: Catalyst fragmentation. AICHE Journal, 2003, 49 (4), 1002 – 1013.

Kosek J., Grof Z., Novák A., Štěpánek F., Marek M.: Dynamics of particle growth and overheating in gas-phase polymerization reactors. Chemical Engineering Science, 2001, 56 (13), 3951 – 3977.

 

 

Contact

Juraj Kosek
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+420 220 443 296

University of Chemistry and Technology Prague
Department of Chemical Engineering
Technicka 5
166 28 Prague 6
Czech Republic

University of West Bohemia
New Technologies Research Centre (NTC)
Univerzitní 8
306 14 Pilsen
Czech Republic