Reaction engineering
Ali Ebrahimi; Saeid Ahmadjo; Mohsen Mohammadi; Mohammad-Mahdi Mortazavi; Mostafa Ahmadi
Abstract
Coordinative chain transfer polymerization (CCTP) has opened a new path for the development of novel products like olefin block copolymers and chain-end functional polyolefins. However, conflicting results are frequently reported on the catalyst performance including activity and comonomer selectivity ...
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Coordinative chain transfer polymerization (CCTP) has opened a new path for the development of novel products like olefin block copolymers and chain-end functional polyolefins. However, conflicting results are frequently reported on the catalyst performance including activity and comonomer selectivity under CCTP conditions. Here we have selected two catalysts including rac-ethylenebis(1-η5-indenyl)zirconocene and bis(imino) pyridine iron, with drastically different comonomer affinities. The effect of diethyl zinc as the chain transfer agent (CTA) on their individual performances is evaluated at different 1–hexene concentrations, in copolymerization with ethylene. Combined thermal fractionation and GPC results confirm that not all chains experience the reversible transfer reaction. Nevertheless, the metallocene catalyst shows twice activity and about 30% lower comonomer incorporation in the presence of CTA. Conversely, the late transition metal catalyst demonstrates lower activity and remains comonomer irresponsive. It could be concluded that, in addition to establishing a reversible transfer reaction, CTA affects the nature of active centers. This finding can help designing olefin copolymers with a more defined chemical composition based on CCTP reaction.
Reaction engineering
Mehrsa Emami; Mahmoud Parvazinia; Hossein Abedini
Abstract
Gas phase polymerization of propylene was carried out in a semi-batch minireactor using a commercially supported Ziegler–Natta (ZN) catalyst. The influence of variables including monomer partial pressure, external electron donor, reaction temperature and time on the particle morphology and size ...
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Gas phase polymerization of propylene was carried out in a semi-batch minireactor using a commercially supported Ziegler–Natta (ZN) catalyst. The influence of variables including monomer partial pressure, external electron donor, reaction temperature and time on the particle morphology and size distribution was investigated. Generally, more uniform fragmentation and particle densities were obtained at lower reaction rates. Monomer partial pressure showed a significant role of particle size and its distribution, the higher the monomer partial pressure, the broader particle size distribution was obtained. Polymerization pressure had a significant role on the morphology of particles. Wider cracks and more porosity were resulted from the polymerizations at higher pressures. Furthermore, a broader particle size distribution was obtained from the polymerization at higher pressures. The particle size analysis revealed the monomer partial pressure as the most effective parameter on the distribution of particles. The SEM images showed that three different steps could be distinguished in the development of particle morphology within the particle, showing the initiation and development of cracks and appearance of fragments inside the particle.
Reaction engineering
Nickolay Ostrovskii; Ladislav Fekete
Abstract
The olefins polymerization process in a slurry reactor is discussed. The reaction rate dynamics was analyzed and the contributions of feed flow, gas-liquid mass transfer, polymerization reaction, and catalyst deactivation were estimated. The propylene solubility in a solvent mixture “heptane” ...
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The olefins polymerization process in a slurry reactor is discussed. The reaction rate dynamics was analyzed and the contributions of feed flow, gas-liquid mass transfer, polymerization reaction, and catalyst deactivation were estimated. The propylene solubility in a solvent mixture “heptane” was calculated using Soave-Redlich-Kwong equation of state. These data were then approximated by Henry-like equation and the results were verified in experiments. The influence of propylene dissolving in ”heptane which was examined in special experiments without catalyst has provided the independent estimation of gas-liquid mass transfer coefficient. It has been shown that the reaction rate during the first 20-30 min of test is much lower (or higher) than total monomer consumption, depending on reactant addition sequence. The method of kinetic experiments interpretation and corresponding mathematical model are proposed. The method enables to estimate the kinetic parameter of monomer dissolution, the reaction rate constant of polymerization, as well as the parameters of active centers transformation – activation, deactivation and self-regeneration. An adequacy of model was proved by the description of experiments at two different pressures but with the same parameters values.
Reaction engineering
Marzieh Nouri; Mahmoud Parvazinia; Hassan Arabi; Mohsen Najafi
Abstract
A two-dimensional (2D) single particle model for the copolymerization of propylene-ethylene with heterogeneous Ziegler-Natta catalyst is developed. The model accounts for the effects of the initial shape of the catalyst and carck/ pore patterns on the copolymer composition, polymerization rate and the ...
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A two-dimensional (2D) single particle model for the copolymerization of propylene-ethylene with heterogeneous Ziegler-Natta catalyst is developed. The model accounts for the effects of the initial shape of the catalyst and carck/ pore patterns on the copolymer composition, polymerization rate and the average molecular weight properties. The spherical and oblate ellipsoidal shapes of catalyst particle and four different pattern distributions of cracks and pores in a growing particle are studied in this simulation. It is assumed that the diffusion coefficient of monomers in the cracks/pores is 10 times higher than the compact zone of the particle.In other word, the cracks are distinguished from parts with higher monomer diffusion coefficient.The dynamic 2D monomer diffusion-reaction equation is solved together with a two-site catalyst kinetic mechanism using the finite element method. Simulation results indicate that the initial shape of catalyst changes the average copolymer composition only in the early stage of polymerization, but the crack/pore patterns in the growing particle have a strong impact on the copolymer composition in the polymer particles due to the change ofmass transfer limitations.
Reaction engineering
Mohsen Najafi; Mahmoud Parvazinia; Mir Hamid Reza Ghoreishy
Abstract
A two-dimensional single particle finite element model was used to examine the effects of particle fragmental pattern on the average molecular weights, polymerization rate and particle overheating in heterogeneous Ziegler-Natta olefin polymerization. A two-site catalyst kinetic mechanism was employed ...
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A two-dimensional single particle finite element model was used to examine the effects of particle fragmental pattern on the average molecular weights, polymerization rate and particle overheating in heterogeneous Ziegler-Natta olefin polymerization. A two-site catalyst kinetic mechanism was employed together with a dynamic two-dimensional molecular species in diffusion-reaction equation. The initial catalyst active sites distribution was assumed to be uniform, while the monomer diffusion coefficient was considered to be different inside the fragments and cracks. In other words, the cracks were distinguished from fragments with higher monomer diffusion coefficient. To model the particle temperature a lumped heat transfer model was used. The fragmentation pattern was considered to remain unchanged during the polymerization. A Galerkin finite element method was used to solve the resulting two-dimensional (2-D) moving boundary value, diffusion-reaction problem. A two-dimensional polymeric flow model (PFM) was implemented on the finite element meshes. The simulation results showed that the fragmentation pattern had effects on the molecular properties, reaction rate and the particle temperature at early stages of polymerization.
