CFD based evaluation of polymer particles heat transfer coefficient in gas phase polymerization reactors

Dehnavi, Mohammad Ali; Shahhosseini, Shahrokh; Hashemabadi, Seyed Hassan; Ghafelebashi, Seyed Mehdi

doi:10.1016/j.icheatmasstransfer.2008.07.017

“…Assuming a steady-state hypothesis and equating the rate of chemical initiation to that of termination, the overall growing polymer concentration is given by (Kolhapure and Fox, 1999): (4.3-5) where f is the initiator efficiency which stands for the fraction of initiator amount that has been consumed. The values of the reaction rate constants (Schmidt and Ray, 1981;Maschio and Moutier, 1989;Choi, 1986;Dehnavi et al, 2008) are listed in Table 4 K t,0 is the initial total termination rate constant as described below: (4.3-6) To account for the auto-acceleration phenomenon in the kinetics model, the approach of the free volume theory is considered. The free volume f of the solution polymerization system is expressed as (Schmidt and Ray, 1981):…”

Section: Reactive Source Termmentioning

confidence: 99%

Experimental and CFD investigation of the mixing of MMA emulsion polymerization in a stirred tank reactor

Roudsari¹

2021

Preprint

0

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Although significant advances have been achieved in emulsion polymerization in recent decades, the effect of mixing on this type of polymerization has not been fully delineated yet. In fact, mixing plays a significant role in the performance of an emulsion polymerization reaction. For instance, in case of a very low agitation rate, larger droplets are generated and phase separation, which limits the diffusion mechanism, may occur. In contrast, vigorous agitation can result in reduced nucleation of particles. Therefore, the main objective of this study is to investigate the impact of mixing parameters (e.g. impeller speed, impeller type, impeller number, and baffles) on the monomer conversion, the polymer average molecular weight, particle size and size distributions, transition glass temperature, and number of particles. To achieve this objective, the emulsion polymerization of methyl methacrylate (MMA) was carried out in a lab-scale reactor equipped with a top-entry agitator, 4 wall baffles, a U shaped cooling coil, and a temperature controller. To analyze the reactive flow inside the polymerization reactor, a novel computational fluid dynamics (CFD) model was developed. The multiple reference frames (MRF) technique, k-ε model, and mixture model approach were employed to model the impeller rotation, turbulence, and multiphase flow, respectively. The particle number density distribution within the reactor was also estimated by means of the population balance approach, which employs a discrete method to describe the nucleation and growth of the polymer particles. The experimental data and CFD results showed that the installation of the baffles enhanced the particle size and molecular weight but reduced the conversion and particle number. The number density achieved using the Rushton impeller was higher than that for the pitched blade impeller. The results revealed that the effect of the impeller speed on the characteristics of the polymer attained using the pitched-blade turbine was more prominent than that for the Rushton turbine. It was also found that the impact of the impeller speed on the polymer characteristics was much more pronounced for the double pitched-blade turbines rather than for the double Rushton turbines.

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“…Serra et al (2007) investigated CFD modeling of styrene free radical polymerization in a T-junction micro reactor and assumed isothermal conditions in a micro-scale reactor to calculate poly-dispersity index, degree of polymerization and conversion. CFD was also estimated to calculate convection heat transfer coefficient between particles and polymer fluids in an ethylene polymerization fluidized bed reactor (Dehnavi et al 2008). A study on CFD analysis of polymerization claimed that the gradients of temperature and concentration were only necessary for poorly designed agitation systems and perfect mixing was a good assumption (Poubel et al, 2010).…”

Section: Computational Fluid Dynamicsmentioning

confidence: 99%

Experimental and CFD investigation of the mixing of MMA emulsion polymerization in a stirred tank reactor

Roudsari¹

2021

Preprint

0

View full text Add to dashboard Cite

Although significant advances have been achieved in emulsion polymerization in recent decades, the effect of mixing on this type of polymerization has not been fully delineated yet. In fact, mixing plays a significant role in the performance of an emulsion polymerization reaction. For instance, in case of a very low agitation rate, larger droplets are generated and phase separation, which limits the diffusion mechanism, may occur. In contrast, vigorous agitation can result in reduced nucleation of particles. Therefore, the main objective of this study is to investigate the impact of mixing parameters (e.g. impeller speed, impeller type, impeller number, and baffles) on the monomer conversion, the polymer average molecular weight, particle size and size distributions, transition glass temperature, and number of particles. To achieve this objective, the emulsion polymerization of methyl methacrylate (MMA) was carried out in a lab-scale reactor equipped with a top-entry agitator, 4 wall baffles, a U shaped cooling coil, and a temperature controller. To analyze the reactive flow inside the polymerization reactor, a novel computational fluid dynamics (CFD) model was developed. The multiple reference frames (MRF) technique, k-ε model, and mixture model approach were employed to model the impeller rotation, turbulence, and multiphase flow, respectively. The particle number density distribution within the reactor was also estimated by means of the population balance approach, which employs a discrete method to describe the nucleation and growth of the polymer particles. The experimental data and CFD results showed that the installation of the baffles enhanced the particle size and molecular weight but reduced the conversion and particle number. The number density achieved using the Rushton impeller was higher than that for the pitched blade impeller. The results revealed that the effect of the impeller speed on the characteristics of the polymer attained using the pitched-blade turbine was more prominent than that for the Rushton turbine. It was also found that the impact of the impeller speed on the polymer characteristics was much more pronounced for the double pitched-blade turbines rather than for the double Rushton turbines.

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“…The general conclusions were that local hydrodynamic conditions can be important, that conduction played a role in heat transfer even in a turbulent fluidised bed, and that (as expected) the conductivity of a solid surface with which a polymerising particle is in contact, also plays a role in the development of temperature profiles inside the growing polymer 13, 14. In a very similar effort, another group used essentially the same approach to reconfirm that particle/particle interactions can influence the observed particle heat transfer coefficient, but no attempt was made to look in detail at particle/surface interactions 16, 17…”

Section: Introductionmentioning

confidence: 98%

Heat Transfer in Gas‐Phase Olefin Polymerisation: A Study of Particle/Surface Interactions

Eriksson

¹

,

Weickert

²

,

McKenna

³

2010

Macro Reaction Engineering

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This work focuses on the heat exchange between a polymer particle and the reactor wall for low gas velocities. The role of different wall materials (or heat transfer conditions) is investigated with an eye to understanding how this influences the likelihood of build up of wall sheeting via melting of particles. The temperature profiles inside growing polymer particles in the vicinity of the reactor wall when it is clean (steel) or covered with a layer of non‐reactive polymer are simulated. As a comparison, we have also simulated glass reactor walls to represent bench‐scale polymerisation equipment. The results can help to understand how how we can compare results from bench‐scale experiments to industrial scale units.

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CFD based evaluation of polymer particles heat transfer coefficient in gas phase polymerization reactors

Cited by 17 publications

References 16 publications

Experimental and CFD investigation of the mixing of MMA emulsion polymerization in a stirred tank reactor

Experimental and CFD investigation of the mixing of MMA emulsion polymerization in a stirred tank reactor

Experimental and CFD investigation of the mixing of MMA emulsion polymerization in a stirred tank reactor

Heat Transfer in Gas‐Phase Olefin Polymerisation: A Study of Particle/Surface Interactions

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