Gas Phase Olefin Polymerization Research Articles

Multiscale modelling of multizone gas phase propylene (co)polymerization reactors—A comprehensive review

AbstractCatalysts and polymerization processes have evolved over the years. Such significant developments have allowed producers to broaden the range of polymer microstructure and process productivity, thereby making it possible to offer a wide range of end‐use properties at a reasonably low cost. However, these advantages in catalyst performance and reactor operation require that we understand as much as possible about reactor operation in the broadest sense. In addition to the fundamental experimental study of polymer chemistry, this means that one needs to develop complete, robust process models. The present paper provides a rapid overview of recent developments in various gas phase propylene (co)polymerization reactors in use today, concentrating on multizone gas phase polypropylene reactors: that is, multizone circulating reactor, fluidized bed reactor with internal circulation, fluidized bed reactor with external circulation, and horizontal stirred bed reactor. We then concentrate on the advances in multiscale modelling of gas phase propylene (co)polymerization reactors, from microscale kinetics at the active sites, to the mesoscale, including physical transport and thermodynamic modelling at the single‐particle level and its boundary layer, up to the macroscale reactor modelling. A systematic guideline used for the selection of appropriate thermodynamic models is proposed for gas phase olefin polymerization processes. Finally, current challenges and remaining issues related with the development of mathematical multiscale modelling are addressed.

Applied Thermodynamics for Process Modeling in Catalytic Gas Phase Olefin Polymerization Reactors

AbstractThe Sanchez–Lacombe Equation of State (SL EoS) is used to model the solubility of different industrial alkane penetrants in polyethylene to explain the importance of considering different diluents for different processes, and the impact that this choice can have on operating conditions, especially for the production of linear low density polyethylene (LLDPE). Extension of this approach to ternary (ethylene/penetrant/LLDPE) systems shows the effect of composition of penetrant/ethylene mixtures on the solubility of such mixtures in LLDPE and swelling of the polymer phase at conditions of industrial relevance. This analysis reveals that using a constant polymer density instead of that predicted by the SL EoS can result in erroneous calculations of the particle size distribution developments in an olefin polymerization reactor.

Advances in Mathematical Modeling of Gas-Phase Olefin Polymerization

Mathematical modeling of olefin polymerization processes has advanced significantly, driven by factors such as the need for higher-quality end products and more environmentally-friendly processes. The modeling studies have had a wide scope, from reactant and catalyst characterization and polymer synthesis to model validation with plant data. This article reviews mathematical models developed for olefin polymerization processes. Coordination and free-radical mechanisms occurring in different types of reactors, such as fluidized bed reactor (FBR), horizontal-stirred-bed reactor (HSBR), vertical-stirred-bed reactor (VSBR), and tubular reactor are reviewed. A guideline for the development of mathematical models of gas-phase olefin polymerization processes is presented.

Experimental and simulation study of heat transfer in fluidized beds with heat production

As a result of highly exothermic reactions during gas-phase olefin polymerization in fluidized bed reactors, difficulties with respect to the heat management play an important role in the optimization of these reactors. To obtain a better understanding of the particle temperature distribution in fluidized beds, a high speed infrared (IR) camera and a visual camera have been coupled to capture the hydrodynamic and thermal behavior of a pseudo-2D fluidized bed. The experimental data were subsequently used to validate an in-house developed computational fluid dynamics and discrete element model (CFD-DEM). In order to mimic the heat effect due to the exothermic polymerization reaction, a model system was used. In this model system, heat is released in zeolite 13X particles (1.8–2.0mm, Geldart D type) due to the adsorption of CO2. All key aspects of the adsorption process (kinetics, equilibrium and heat effect) were studied separately using Thermogravimetric Analysis (TGA) and Simultaneous Thermal Analysis (STA), and subsequently fluidized bed experiments were conducted, by feeding gas mixtures of CO2 and N2 with different CO2 concentrations to the bed, where the total heat of liberation could be controlled. The combined infrared/visual camera technique generated detailed information on the thermal behavior of the bed. Furthermore, the comparison of the spatial and temporal distributions of the particle temperature measured in the fluidized bed with the simulation results of CFD-DEM provides qualitative and quantitative validation of the CFD-DEM, in particular concerning the thermal aspects.

Hydrodynamics and particle mixing/segregation measurements in an industrial gas phase olefin polymerization reactor using image processing technique and CFD-PBM model

Particle size distribution (PSD) has a significant impact on the performance of fluidized bed reactors due to uneven distribution in the segregation and mixing phenomena. This paper develops a new method of digital image processing that investigates the hydrodynamics of an industrial gas phase olefin polymerization reactor and studies the fluidization structure of a wide range of particle size distribution in an industrial gas phase polymerization reactor by means of a CFD-PBM coupled model, where the direct quadrature method of moments (DQMOM) was implemented to solve the population balance model. It was shown that the applied parameter assumptions and closure laws were appropriately chosen to satisfactorily predict the available operational data in terms of pressure drop and bed height. The transient CFD-PBM/DQMOM coupled model and image analysis technique are then implemented extensively to analyze bubble fluidization structure and segregation phenomena at different velocities. The particle segregation indicates that the small bubbles present in the bed are unable to induce vigorous mixing at low superficial gas velocity while particle mixing improves at a velocity above the minimum fluidization velocity. Further, the predicted results show higher axial segregation phenomena when compared to the radial direction.

ChemInform Abstract: Supported Single‐Site Organometallic Catalysts for the Synthesis of High‐Performance Polyolefins

AbstractReview: 127 refs.

Supported Single-Site Organometallic Catalysts for the Synthesis of High-Performance Polyolefins

Single-site organometallic catalysts supported on solid inorganic or organic substrates are making an important contribution to heterogeneous catalysis. Early and late transition metal single-site catalysts have changed the polyolefin manufacturing industry and research with their ability to produce polymers with unique properties. Moreover, several of these catalysts have been commercialized on a large scale. Their heterogenization for slurry or gas phase olefin polymerization is important to produce polyolefin as beads and to avoid reactor fouling. The large majority of supports currently used in industry are inorganic materials (SiO2, Al2O3, MgCl2), with silica being the most important. Single-site supported catalysts are most commonly prepared by molecular-level anchoring/chemisorption, in which a molecular precursor undergoes reaction with the surface while maintaining most of the ligand sphere of the parent molecule. Chemisorption of discrete organometallic complexes on solid supports yields catalysts with well-defined active sites, greater thermal stability than the homogeneous analogues, and decreased reactor fouling versus the homogeneous analogues. This review presents a detailed account of the synthesis, characterization and polymerization properties of single-site catalysts supported on metal oxides and metal sulfated oxides, primarily carried out at Northwestern University. Early and late transition metal single-site catalysts supported on inorganic surfaces have changed the polyolefin research and production with their unique ability to produce polymeric materials with unique architectures. This review presents a detailed account of the synthesis, characterization, and polymerization properties of single-site catalysts supported on metal oxide surfaces.

A 2‐D observer to estimate the reaction rate in a stopped flow fixed bed reactor for gas phase olefin polymerization

A two‐dimensional high gain observer has been constructed to estimate the reaction rate parameters in a laboratory scale stopped flow fixed bed reactor for gas phase ethylene polymerization. The observer is based on the heat balances of a validated model of the reactor and the measured variable is the outlet temperature of the fixed bed. To create an observable system of equations, the polymerization rate is considered in two parts, an activation energy term and a lumped parameter. The effective radial conductivity in the fixed bed and the heat‐transfer coefficient at the wall are considered separately. The polymerization rate is calculated from the observer results and is in good agreement with the measured data and calculated values. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3511–3523, 2014

Decentralized Advanced Model Predictive Controller of Fluidized-Bed for Polymerization Process

The control of fluidized-bed operations processes is still one of the major areas of research due to the complexity of the process and the inherent nonlinearity and varying dynamics involved in its operation. There are varieties of problems in chemical engineering that can be formulated as NonLinear Programming (NLPs). The quality of the developed solution significantly affects the performance of such system. Controller design involves tuning the process controllers and implementing them to achieve certain performance of controlled variables by using Sequential Quadratic Programming (SQP) method to tackle the constrained high NLPs problem for modified mathematical model for gas phase olefin polymerization in fluidized-bed catalytic reactor. The objective of this work is to present a comparative study; PID control is compared to an advanced neural network based MPC decentralized controller and also, see the effect of SQP on the performance of controlled variables. The two control approached were evaluated for set point tracking and load rejection properties giving acceptable results.

Dynamic modelling of a stopped flow fixed bed reactor for gas phase olefin polymerisation

A heterogeneous 2D dynamic model of a stopped flow fixed bed reactor for gas phase ethylene polymerisation has been constructed and validated. The reactor contains two solid phases with accumulation of mass within the reactor bed and the experiments modelled are of very short duration (0.1–75s). There is a good fit between measured data and calculated values, and the model results allow us to interpret the experimentally observed temperature rises. Higher than expected catalyst temperatures are found towards the reactor centre and exit which are due to the initial intensity of the polymerisation reaction.

Packed‐bed reactor for short time gas phase olefin polymerization: Heat transfer study and reactor optimization

AbstractA specially conceived packed‐bed stopped flow minireactor (3 mL) suitable for short gas phase catalytic reactions has been used to study the start‐up of ethylene homopolymerization with a supported metallocene catalyst. Focus has been put on the heat transfer characteristics of the supported catalysts and on understanding the relationship between the initial rate and the relative gas/particle velocities and the influence of particle parameters in the packed bed. We performed a comprehensive study on the influence of various physical parameters on the heat transfer regime at start up conditions. The catalyst activity as well as the polymer morphology is shown to be dependent on heat transfer regime. The knowledge thus obtained is applicable to industrial problems like catalyst injection in fluidized beds and helps preventing experimental artifacts due to overheating in following studies. © 2011 American Institute of Chemical Engineers AIChE J, 2012

CFD simulation of hydrodynamics and heat transfer in gas phase ethylene polymerization reactors

The hydrodynamics and temperature of a two-dimensional gas–solid fluidized bed of gas phase olefin polymerization reactor had been studied. A two-fluid Eularian Computational Fluid Dynamics (CFD) model with closure relationships according to the kinetic theory of granular flow has been applied in order to simulate the gas–solid flow. Fluidization regime and gas–solid flow pattern were investigated using three different drag models. Model predictions of bed pressure drop were compared with corresponding experimental data reported in the literature to validate the model. The predicted values were in reasonable agreement with the experimental data. The temperature behavior of fluidized bed with various drag models was investigated. The temperature gradient in the primary section of the bed was much larger than the gradient in other sections and the effect of all drag models on temperature gradient along the bed was approximately similar.

Heat Transfer in Gas Phase Olefin Polymerisation

AbstractA fixed bed microreactor has been used to study heat transfer during the initial transient state of gas phase olefin polymerization on a supported catalyst. It has been shown that heat transfer during this stage of the polymerisation is critical, and under conditions found commercially problems can arise with hot spots and polymer melting. It is proven how the thermal properties of the gas mixture flowing on the catalytic bed exert great influence on heat dissipation reducing the sudden increase in temperature by as much as a factor of 5. Flow rate and especially the process gas composition are the key factors in controlling the bed temperature.

Heat Transfer of Polymer Particles in Gas-Phase Polymerization Reactors

In this paper the effects of particles configuration and particles distance on the heat transfer rate in a gas phase olefin polymerization reactor have been studied using the computational fluid dynamic (CFD) modeling approach. The goal was to determine the causes of particle overheating in this reactor. It has been shown that classic correlations such as Ranz-Marshall are sufficiently adequate when far away particles with no interactions are to be modeled. However, when particles are sufficiently close to having interactions, these correlations fail to satisfactorily predict the convective heat transfer coefficient. The results indicate an increase in particle distance leads to an increase in the Nusselt number on the particle surface. Therefore, for particles with a large distance and triangular or rotated square configurations, the local Nusselt number is closer to the Nusselt number for a single particle.

Heat Transfer and Nascent Polymerisation of Olefins on Supported Catalysts

AbstractAn improved gas phase reactor system has been designed and tested in order to study heat transfer in gas phase olefin polymerisation on supported catalysts. It is shown that temperature rise in a fixed bed configuration is highly dependent on the flow rate of gas, with higher outlet gas temperatures being observed at higher flow rates. In addition it is shown that the shape of the rate curve is dependent on the initial conditions of the reaction, suggesting that the mechanism of particle fragmentation depends to a certain extent on the start‐up of the reaction.

Modified mathematical model for gas phase olefin polymerization in fluidized-bed catalytic reactor

Abstract A modified model for the gas phase catalyzed olefin polymerization fluidized-bed reactors (FBR) using Ziegler–Natta catalyst is presented in this study. This mathematical model accounts for mass and heat transfer between the bubbles and the clouds without chemical reaction, between the clouds and the emulsion without chemical reaction, and between emulsion and solid with chemical reaction that occurs at the surface of the catalyst particles. The model accounts for the effect of catalyst particles type and porosity on the rate of reaction. In this work, the concentration and temperature profiles in the bubble, and emulsion phases are calculated and the effect of catalyst solid phase on the system is estimated. The effect of important reactor parameters such as superficial gas velocity, catalyst injection rate, and catalyst particle growth on the dynamic behavior of the FBR is investigated and the behavior of mathematical model is compared with the reported models for the constant bubble size model , well - mixed model and bubble growth model . Moreover, the results of the model are compared with the experimental data in terms of molecular weight distribution and polydispersity of the produced polymer at steady state. A good agreement is observed between our model prediction and the actual plant data.

Mathematical Model and Advanced Control for Gas-phase Olefin Polymerization in Fluidized-bed Catalytic Reactors

In this study, the developments in modeling gas-phase catalyzed olefin polymerization fluidized-bed reactors (FBR) using Ziegler-Natta catalyst is presented. The modified mathematical model to account for mass and heat transfer between the solid particles and the surrounding gas in the emulsion phase is developed in this work to include site activation reaction. This model developed in the present study is subsequently compared with well-known models, namely, the bubble-growth, well-mixed and die constant bubble size models for porous and non porous catalyst. The results we obtained from the model was very close to the constant bubble size model, well-mixed model and bubble growth model at the beginning of the reaction but its overall behavior changed and is closer to the well-mixed model compared with the bubble growth model and constant bubble size model after half an hour of operation. Neural-network based predictive controller are implemented to control the system and compared with the conventional PID controller, giving acceptable results.

Evaluation of the Internal Particle Morphology in Catalytic Gas-Phase Olefin Polymerization Reactors

An unsteady-state diffusion model is developed to calculate the transport of penetrant molecules in semicrystalline nonporous polyolefin films and porous powders in terms of the internal particle morphology (i.e., pore size distribution and crystallinity) of the polymer. To calculate the diffusion coefficient of penetrant molecules in a semicrystalline nonporous polymer matrix, the free volume theory is employed. Moreover, to calculate the diffusion coefficient of penetrant species in a porous polymer matrix, a dual diffusion model, accounting for the molecular diffusion of penetrant in the pores and the amorphous polymer phase, is developed. A novel experimental setup, comprising a gravimetric magnetic suspension microbalance connected in series with an optical view cell, is employed to carry out dynamic sorption/swelling experiments of ethylene and propylene in both nonporous films and porous high-density polyethylene (HDPE) powders. Experimental sorption measurements, along with the developed unsteady-state diffusion model, are then employed to calculate the diffusion coefficient of R-olefins in porous and nonporous semicrystalline HDPE as well as the overall porosity of HDPE powders. Finally, to reconstruct the pore size distribution in a porous polyolefin particle in terms of the estimated overall particle porosity and the size distribution of the microparticles, a diffusionlimited aggregation (DLA) model is developed. It is shown that the morphological characteristics of porous polyolefin particles (i.e., porosity and pore size distribution) can be described using the proposed DLA model in terms of the size distribution of the microparticles and the extent of microparticles fusion.

Assessment of Particle Agglomeration in Catalytic Olefin Polymerization Reactors Using Rheological Measurements

A comprehensive multi-scale modeling approach is described to predict the dynamic evolution of the particle size distribution (PSD), molecular weight distribution (MWD), and rheological properties of the polyolefin particles produced in a catalytic Ziegler−Natta gas-phase olefin polymerization reactor. The overall MWD of a single polymer particle is calculated by the weighted sum of the MWDs of all polyolefin fractions produced over the different active sites of the multi-site catalyst. Numerical simulations are carried out, using the multi-scale model, to investigate the effect of particle agglomeration on the distributed molecular polymer properties of the polyolefin. It is shown that agglomerated particles of less than 600 μm in diameter can exhibit different distributed molecular properties (i.e., MWD, CCD) as compared to those of the non-agglomerated particles of the same size. It is also shown that the agglomerated particles can exhibit bimodal MWDs even though the MWDs of the individual non-agglome...

OPTIMISATION OF GRADE TRANSITIONS IN AN INDUSTRIAL GAS-PHASE OLEFIN POLYMERIZATION FLUIDIZED BED REACTOR VIA NCO TRACKING

Gas-phase olefin polymerization fluidized-bed reactors are complex processes that are characterized by large time constants and slow transitions. Optimal grade changes can be computed off-line on the basis of a detailed process model and applied to the process in an open-loop mode. However, the presence of uncertainties in the form of process-model mismatch and disturbances make this approach clearly non-optimal. Fortunately, measurements can be used to improve the situation; this can be done either explicitly via process model refinement and repeated optimisation, or implicitly via direct input adjustment. This work considers the optimisation of grade-transition changes by enforcing on-line the necessary conditions of optimality (NCO tracking) using available measurements. The key steps are the generation of an adjustable model of the optimal solution and its adaptation using appropriate measurements or estimates of the controlled variables. A realistic simulation process model is employed to compare the performance of NCO tracking to that obtained via the open-loop application of nominal optimal trajectories.