A CFD-PBM model considering ethylene polymerization for the flow behaviors and particle size distribution of polyethylene in a pilot-plant fluidized bed reactor

This work is focused on the development and validation of a model accounting for the impact of the reactor residence time distribution in well‐stirred slurry‐phase catalytic polymerization of ethylene. Particle growth and morphology are described through the Multigrain model, adopting a two‐site model for the catalyst and a conventional kinetic scheme. Particle size distribution and polymer properties (average molecular weights and polydispersity) are computed as a function of particle size through a segregated model, assuming that neither breakage nor aggregation occur. Reactors are modeled by means of fundamental mass conservation equations. The model is applied to a system constituted by a series of two ideal continuous stirred tank reactors, where the synthesis of polyethylene with bimodal molecular weight distribution is performed, employing the initial catalyst size distribution as the only adjustable parameter. The model provides insights at the single particle scale for each specific size, thus highlighting the inhomogeneity which arises from the synergic effects of chemical kinetics and residence time distributions in both reactors. The satisfactory agreement between model results and experimental data, in terms of particle size distribution and average molecular weights, confirmed the suitability of the model and underlying assumptions.

Section: Introductionmentioning

confidence: 99%

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

Casalini

Visscher

Tamaddoni

et al. 2018

“…In continuous reactors, the impact of residence time distribution on particle size distribution can be accounted for; in addition, population balances and appropriate kernels are used to take into account particle breakage and aggregation. Recently, computational fluid dynamics has been applied for a detailed description of polymerization processes in fluidized bed reactors, accounting for polymerization kinetics, particle breakage/aggregation, and velocity field …”

Section: Introductionmentioning

confidence: 99%

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

Casalini

Visscher

Janssen

et al. 2016

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.

“…In the last 20 years modeling approaches using CFD simulations to describe polymer reactors have overcome this challenge by using either simplified kinetics, as refs. 15–18 and Fox et al,19–21 and/or “user defined function”, as the group of Luo22–24 to take polymerization reaction into account, which is still numerical demanding. On the contrary, axial dispersion models describe hydrodynamics on macro scale with only one or two parameters, but allow for the implementation of complex polymerization kinetics on micro scale.…”

Section: Introductionmentioning

confidence: 99%

Simulation of Polymer Reactors Using the Compartment Modeling Approach

Cremer-Bujara

Biessey

Grünewald

2019

A compartment modeling approach based on computational fluid dynamics (CFD) simulations is applied to a simplified static mixer geometry. Compartments are derived from velocity fields obtained from cold CFD simulations. This methodology is based on the definition of periodic flow zones (PFZ) derived from the recurrent flow profile within the static mixer. In general, PFZ can be characterized by two different compartments: flow zones with hydrodynamic behavior of a tubular reactor and dead zones exhibiting a more continuous stirred tank reactor‐like characteristic. In CFD studies the influence of changing fluid properties, for example viscosity, on flow profile due to polymerization progress is considered. In the deterministic compartment model, the continuous flow profile within the static mixer is transformed to basic reactor models interconnected via an exchange stream. To reduce model complexity and the number of model parameters, constant volumes of compartments are assumed. Changes in hydrodynamics are considered by a variable exchange flow rate as a function of Re manipulating residence time in compartments. Simulation studies show the influence of decreasing exchange flow rates with polymerization progress, as Re decreases, resulting in a greater increase of viscosity in dead zones. The reactor performance is qualitatively represented by the simulation results.