Modeling of Polyolefin Polymerization in Semibatch Slurry Reactors: Experiments and Simulations

Abstract

In this work, a well‐known single‐particle model (the multigrain model) is applied to simulate size growth and morphology evolution of polyethylene particles obtained through a slurry phase, catalytic polymerization in semibatch reactor. The model explicitly accounts for diffusion limitations within the polymer particle and between the particle and the solvent, as well as void fraction variations due to the particle growth. Catalyst behavior is described assuming a two‐site model, a good compromise between accuracy and simplicity, along with a conventional kinetic scheme, whose kinetic parameters are determined by fitting the model predictions to the experimental data. Polymer molecular weight is evaluated from population balances, solved through the method of moments. The semibatch reactor is modeled by means of fundamental mass conservation equations. Simulations show that the proposed model is able to describe not only the polymer properties of interest but also the evolution of the average particle size, thus providing a comprehensive overview of the system behavior. The model is further validated through regression‐free simulation of additional sets of experimental data, including materials with a bimodal molar mass distribution: the good agreement also found in this case confirms the robustness of the model and the estimated parameter values.

The Effect of Residence Time Distribution on the Slurry‐Phase Catalytic Ethylene Polymerization: An Experimental and Computational Study

“…For every value of inlet particle size, the average molecular weight increases until an asymptotic value. This is due to the operating kinetic scheme, as discussed in a previous paper: the polymerization is mainly governed by the monomolecular termination mechanism (chain transfer) and the instantaneous degree of polymerization becomes constant in time. An example of time evolution of the normalized weight average molecular weight is shown in Figure b at unitary value of the initial normalized size: after an initial fast build‐up, the average molecular weight reaches the final plateau.…”

“…The first step of this work is the simulation of a single CSTR, whose operative conditions ranges are summarized in Table 4 , where H 2 /C 2 is the ratio between hydrogen and ethylene partial pressures in gas phase, ethylene partial pressure in gas phase, P the overall pressure of gas phase, and τ 1 the mean residence time. The values of C p0 and α have been set equal to 0.25 and 0.39, respectively …”

“…The selected values have been already discussed in the previous work and a brief description only will be here provided for the sake of completeness. Diffusion coefficients, diameter of catalyst crystal, initial porosity and tortuosity have been taken from the literature .…”

“…The values of the kinetic parameters, including the fraction of active sites of type k , have been estimated by fitting the model predictions to experimental data obtained through semibatch reactor setup . The final values are summarized in Table 3 ; propagation and chain transfer rate constants have been normalized with respect to the propagation rate constant of catalytic site 1.…”

“…The total fraction of catalytic sites which can be activated, defined as the sum of the contribution of each kind of catalytic site, is expressed aswhere C p0 is the overall fraction of active metal with catalytic activity and α the fraction of active sites of type 1. The values of C p0 and α have been obtained from fitting of experimental data from semibatch reactor as a function of hydrogen partial pressure, since catalyst activity strongly depends on the amount of hydrogen present in the system.…”