Polymerization Fluidized Bed Reactors Research Articles

Scale-up and thermal stability analysis of fluidized bed reactors for ethylene polymerization

A set of hydrodynamic similarity laws is applied to the scale-up of ethylene polymerization fluidized bed reactors (FBRs) under the condensed mode operation. The thermal stability of open-loop controlled FBRs is investigated by the homotopy continuation method. And the Hopf bifurcation point is selected as an index of the thermal stability similarity. The simulation results show the similarity in state variables, operation parameters, the space–time yield (STY), and the thermal stability of FBRs with different scales. Furthermore, the thermal stability behaviors and similarity of the closed-loop controlled FBRs with different scales are analyzed. The observed similar trend of Hopf bifurcation curves reveals the similarity in the thermal stability of closed-loop controlled FBRs with different scaling ratios. In general, the results of the thermal stability similarity confirm that the hydrodynamics scaling laws proposed in the work are applicable to the scale-up of FBRs under the condensed mode operation.

CFD-DEM study of reactive gas-solid flows with cohesive particles in a high temperature polymerization fluidized bed

The cohesive particles are commonly encountered in high temperature polymerization fluidized bed reactors. However, detailed studies on the effect of cohesiveness on the hydrodynamics, heat transfer, and reaction characteristics in such reactors have been rarely reported. In this work, a CFD-DEM approach coupled with solid bridge force model and polymerization reaction kinetics model is established to simulate the reactive gas–solid flows with cohesive particles. The results demonstrate that cohesive particle agglomeration significantly increases the inhomogeneity of concentration and velocity fields, and hinders particle circulation. The hindered particle circulation further results in pronounced temperature inhomogeneity. Moreover, the local hot spots are readily formed in the agglomerates, which expand and deteriorate rapidly if not controlled promptly. Besides, how the cohesion model parameters, namely contact time and maximum cohesive force limitation, affect the onset temperature and the severity of particle agglomeration is revealed. The established model extends the particle-scale study of cohesive gas–solid flows to reactive systems.

A computational fluid dynamics model coupled with ethylene polymerization kinetics for fluidized bed polyethylene reactor

This work aims at exploring the effect of macro operating conditions on the distribution of temperature field and chain microstructures of polymer in a gas phase ethylene polymerization fluidized bed reactor (FBR) by coupling polymerization kinetics with computational fluid dynamics (CFD). An ethylene polymerization kinetic model in terms of the method of moments was introduced to CFD model. In the modeling framework, the Flory's distribution and custom field functions were used to predict the molecular weight distribution (MWD). The results show that the average molecular weight decreases with the increasing of reaction temperature, and MWD becomes wider because of the increase of temperature gradient. With the increase of hydrogen concentration, the average molecular weight and MWD becomes smaller and narrower, respectively. Unlike the hydrogen concentration, an increase in the ethylene concentration leads to an increase in the average molecular weight, polydispersity distribution index (PDI), and temperature. An increase of gas velocity improves the heat transfer of FBR but results in a very wide MWD when gas velocity exceeds the optimum fluidizing velocity. This model is helpful to understand how the operating conditions affect the performance of FBR and can provide theoretical guidance for the operation and design of polyolefin FBR.

CFD modeling of droplet permeability in fluidized beds

The deposition of droplets of size [5–22] μm on the surface of particles in a gas–solid fluidized bed has been investigated employing a one-way coupled CFD-DEM modeling. In this approach the gas phase is solved as a continuum and the free-droplets are solved as Lagrangian objects, while solid particles are assumed to be stationary and act as an obstacle for the fluid flow.In this way, it was possible to calculate the deposition factor, defined as the ratio of droplets deposited on the surface of the particles to the number of droplets injected into the domain as a function of the droplet Stokes number and particle Reynolds number. An empirical correlation was developed that describes the deposition factor as a function of the operating conditions, which is valid for the inertial regime where effects of Brownian motion can be neglected.The final goal of this investigation is to employ the developed correlation in a larger scale CFD-DEM model describing a polymerization fluidized bed reactor operated in condensed mode, i.e. by injecting liquid into the gas–solid suspension through the gas distributor plate or via nozzles in order to improve the heat management. In this kind of modeling the free-droplets phase is described as a continuum and the deposition rate of droplets determines the amount of liquid on the particle surface, which affects for the hydrodynamic (i.e. formation of liquid bridges between particles) and thermal behavior (i.e. liquid evaporation, kinetic and subsequent cooling effect) of the system.

Triboelectric effects in catalyst feeders of fluidized-bed polymerization reactors

Solid material in fluidized-bed polymerization reactors consists of two components with different triboelectric properties. The main material is a granular polymer with the particle size from ∼100 to >1000 μm. The second component, very small in volume, is fresh catalyst, spherical particles with the diameter from 20 to 40 μm. Small loads of catalyst particles are periodically fed into the reactors through narrow metal tubes with a gas flow (nitrogen or monomer).This catalyst injection process leads to accumulation of a significant electric charge on the catalyst particles. This paper describes results of laboratory experiments aimed at determining sources of electrostatics generation in spherical catalyst particles passing through narrow tubes, a possible mechanism of the charging and methods of its mitigation.

Prediction of Temperature and Concentration Profiles in an Industrial Polymerization Fluidized Bed Reactor under Condensed-Mode Operation

Condensed mode operation of gas-phase ethylene polymerization adds more complexity to the hydrodynamics of fluidized bed reactors (FBRs). This study aims at the development of an industrial ethylen...

High production circulating fluidized bed polymerization reactors

This paper is an extension of the Gidaspow invention relating to fluidized bed polymerization reactors, such as the UNIPOL reactor or the later reactors containing cooling tubes. It shows that the polymer production rate can be increased by two or more orders of magnitude over that obtained in the UNIPOL reactor due to the orders of magnitude decreased adiabatic temperature rise. The CFD design of several size polyethylene reactors is described. In all cases the small catalyst particle concentrations are high at the walls showing that electrostatics is not the only mechanism for sheet formation.

Optimal Grade Transitions in a Gas-phase Polymerization Fluidized Bed Reactor

Abstract Industrial polymerization plants experience frequent changes of products, driven by end-use properties to meet various market requirements. Good transition policies are essential to save time and materials. In this study, the gas-phase catalytic polymerization is modeled in a fluidized bed reactor by a single-phase model and dynamic optimization is implemented to determine optimal operating sequences for grade changes. Two optimization formulations, a single-stage and a multi-stage formulation, are introduced and compared. The superiority of the multi-stage formulation is concluded owing to better control at each stage during the transition and a further reduction of off-grade time. Subsequently, an on-line optimal control framework is established by incorporating a shrinking horizon nonlinear model predictive control with an expanding horizon weighted least-squares estimator for process states and unknown parameters. The results of a case study indicate the designed framework is able to overcome certain levels of uncertainty, while reducing the transition time.

Heat transfer effect in scaling-up a fluidized bed reactor for propylene polymerization.

The effects of operating conditions and scaling-up on reactor temperature control and performance in propylene polymerization fluidized bed reactors were studied by phenomenological and CFD models. A phenomenological model with CFD hydrodynamics parameters predicts average information, while a CFD-based reactor model provides local information. Results suggest improved productivity and reactor temperature control by cautiously increasing catalyst feed rate, operating temperature, reactor size and superficial velocity, with consideration of hot spots and catalyst deactivation. High catalyst loading increases productivity but involves risk with regards to the control of oscillating temperature and hot spots. The model identifies an operating window to improve productivity and temperature control and to study operation details. Mixing effect is important to heat transfer but not to propylene conversion. Scaling-up cannot provide similarity of heat transfer. Keeping the same temperature when scaling up from 0.2 to 4 m in diameter requires heat transfer area multiplying factors of 2.43 to 5.26 or lowering the wall temperature by 7 to 18 K. Hot spots are detected with a temperature variation of 10 to 14 K. The results are useful for analyses of laboratory and industrial scale reactors and provide information on scale up.

Computational Fluid Dynamics Modeling of Temperature Distribution in Fluidized Bed Polymerization Reactor for Polypropylene Production

Propylene polymerization process is one of the chemical processes that take place in the fluidized bed reactor. The gas monomer is fed through the distributor to react with the solid catalyst particles inside the reactor. The polymer particles are produced from the heterogeneous exothermic polymerization reaction. Although the fluidized bed polymerization reactor has high heat and mass transfer rate, the thermal efficiency is usually limited due to the formation of hot spots and improper mixing. In this research, a computational fluid dynamics (CFD) model of the fluidized bed polymerization reactor for polypropylene production is developed in order to study the temperature distribution within the reactor. The polymer growth rate and particle size distribution are included in the CFD model development. The heat is generated from highly exothermic polymerization reaction and removed in the heat exchanger. By proper adjusting the monomer feed velocity, the uniform temperature distribution inside the reactor is obtained in axial and radial directions. Additionally, the temperature of the outlet gas stream can be reduced, thus reducing the external cooling duty.

Optimization of Energy Consumption in Gas-Phase Polymerization Process for Linear Low Density Polyethylene Production

Gas-phase polymerization is a method that is widely used in the polymer production process. The heterogeneous reaction between monomer gas and solid catalyst takes place in the fluidized bed reactor to produce the polymer product. The design and production planning of this process is a difficult task due to the complicated fluid behavior and polymerization reaction inside the fluidized bed reactor. In this paper, a model of the fluidized bed polymerization reactor for linear low density polyethylene (LLDPE) production is developed using the Aspen Custom Modeler (ACM) to predict the polymer characteristics and flow behavior. The developed model is then integrated to the main process flow simulation in order to optimize the energy consumption in the polymer production process. The results show that the total energy requirement is reduced while maintaining the same polymer characteristics.

Particle agglomeration and control of gas-solid fluidized bed reactor with liquid bridge and solid bridge coupling actions

Fluidized beds reactors with liquid bridge and solid bridge coupling actions have been widely used in industry and thus studies of particle agglomerations under such fluidization conditions are of great significance. This work proposed an experimental apparatus combining electro-magnetic induction heating system with fluidized bed to simulate the real conditions for particle reaction heat release and heat transfer in industrial polymerization fluidized bed reactors. The effects of liquid content on agglomeration behavior of wax/graphite composite particles under different fluidization gas temperatures, gas velocities and heating powers have been studied. Through a force balance analysis of particles with liquid bridge and solid bridge coupling actions, relative solid bridge force was taken as a key parameter to demonstrate the effects of solid bridge force and other forces on agglomerations. Results showed that as the relative solid bridge force varied in different ranges, the agglomeration mass presented three different variation trends with liquid increases, namely monotonic increasing, non-monotonic changing and monotonic decreasing, due to different dominating agglomerating mechanisms (liquid evaporation and liquid bridge) during fluidization. According to the results of this study, the proposed criterion based on relative solid bridge force can be used to guide the regulation and control of particle agglomerations in fluidization with liquid bridge and solid bridge coupling actions.

Discrete element modeling of electrostatic charging of polyethylene powder particles

The contacts of particles with a device wall as well as mutual particle collisions cause the redistribution of charge in systems with powder particles. This, usually undesirable phenomenon known as triboelectric charging, leads to operational problems and safety risks in industrial systems, e.g., in fluidized-bed polymerization reactors. We focus on the charging of polyethylene powder particles caused by the particle-particle interactions. Our model utilizes the Discrete Element Method (DEM) for the description of the particle motion and involves the balance of transferable charged species to quantify the triboelectric charging of particles in contact. The model predicts the charging dynamics of polyethylene powder particles in systems of various particle size distributions (PSD) and addresses the influence of segregation of particles on charging. Our simulation results are in agreement with two general trends in charging that are often observed in both industrial and naturally-occurring particulate systems: (i) Charging is most pronounced in systems with wide PSD and (ii) small particles charge mostly negatively, whereas big particles typically carry positive charge. Subsequently, our results suggest that particle segregation plays an important role in the charging dynamics of particulate systems, as segregation of particles of different sizes can alter PSD locally. It is important to include electrostatic forces in charging models, especially in systems that exhibit significant charging. Specifically, electrostatic attraction among unlike charged particles leads to the ‘orbiting motion’ of particles that is followed by the formation of particle aggregates.

Online measurement of particle charge density in a gas-solid bubbling fluidized bed through electrostatic and pressure sensing

Electrostatic phenomena widely exist in various olefin polymerization fluidized bed reactors, due to the continuous collision and friction of dielectric particles and the low-humidity reaction environment. The measurement of particle charge density is essential for monitoring and controlling the electrostatic level as well as probing the hydrodynamic characteristics in a fluidized bed. In this work, a non-intrusive online measurement system, employing ring-shaped sensing electrodes and a pressure transducer, is designed and implemented on a fluidized bed test rig. A modified model considering the influence of spatial sensitivity of the electrodes is proposed to predict the particle charge density in the fluidized bed, based on the measured induced electrostatic current and pressure drop signals. Experimental results have demonstrated that the induced electrostatic current and pressure drop signals show remarkable similarity, with the two PSDs both mainly distributed within 0.5–5Hz, which indicates close relationship between the two signals. The predicted particle charge density increases with the superficial gas velocity and decreases with the injection content of liquid anti-static agent (LAA), showing the same variation tendencies as that measured through a Faraday cup. When the electrode width is 20mm, the mean absolute relative errors for the direct method, area method and envelope method are 12.7%, 14.7% and 13.5%, respectively, indicating that the direct method is the most reliable signal processing approach proposed in this work.

Discrete element method modeling of the triboelectric charging of polyethylene particles: Can particle size distribution and segregation reduce the charging?

Polyethylene particles of various sizes are present in industrial gas-dispersion reactors and downstream processing units. The contact of the particles with a device wall as well as the mutual particle collisions cause electrons on the particle surface to redistribute in the system. The undesirable triboelectric charging results in several operational problems and safety risks in industrial systems, for example in the fluidized-bed polymerization reactor. We studied the charging of polyethylene particles caused by the particle-particle interactions in gas. Our model employs the Discrete Element Method (DEM) describing the particle dynamics and incorporates the ‘Trapped Electron Approach’ as the physical basis for the considered charging mechanism. The model predicts the particle charge distribution for systems with various particle size distributions and various level of segregation. Simulation results are in a qualitative agreement with experimental observations of similar particulate systems specifically in two aspects: 1) Big particles tend to gain positive charge and small particles the negative one. 2) The wider the particle size distribution is, the more pronounced is the charging process. Our results suggest that not only the size distribution, but also the effect of the spatial segregation of the polyethylene particles significantly influence the resulting charge distribution ‘generated’ in the system. The level of particle segregation as well as the particle size distribution of polyethylene particles can be in practice adjusted by the choice of supported catalysts, by the conditions in the fluidized-bed polymerization reactor and by the fluid dynamics. We also attempt to predict how the reactor temperature affects the triboelectric charging of particles.

2D CFD-PBM simulation of hydrodynamic and particle growth in an industrial gas phase fluidized bed polymerization reactor

In an industrial fluidized bed polymerization reactor, particle size distribution (PSD) plays a significant role in the reactor efficiency evaluation. The computational fluid dynamic (CFD) models coupled with population balance model (CFD-PBM) have been extensively employed to highlight its potential to analyze the industrial-scale gas phase polymerization reactor (FBRs) utilizing ANSYS Fluent software. The predicted results reveal an acceptable agreement with the observed industrial data in terms of pressure drop and bed height. Courant number independent study has been carried out to record the mesh and time step independent results for large scale FBRs. Furthermore, the minimum fluidization velocity (Umf) and size-dependent particle growth rate have been assessed to emphasize the impact of PSD along the reactor. The results show transient regime in the case of minimum fluidization velocity. The simulation results signify that in order to improve the polymerization yield, the amount of gas velocity can be increased without change in the fluidization regime, i.e. segregation. Hence, the 2D CFD-PBM/DQMOM coupled model can be used as a reliable tool for analyzing and improving the design and operation of the gas phase polymerization FBRs.

Feature extraction and early warning of agglomeration in fluidized bed reactors based on an acoustic approach

The potential use of audible acoustic emissions for monitoring particle agglomeration in ethylene polymerization fluidized bed reactors was investigated by the authors and reported in this paper. The offset in the power spectral centroid between normal and agglomeration signals was compared, and the energy distribution of the acoustic signals was confirmed to change under agglomeration conditions. On this basis, the acoustic signals were decomposed by wavelet packet decomposition (WPD) and the energy ratios of every sub-band were set as the voiceprint. Subsequently, principal component analysis (PCA) was introduced to reduce the dimensionality of the feature vector. Furthermore, based on normal signals, an agglomeration warning model could be created by support vector data description (SVDD) to avoid the decrease in description accuracy caused by the lack of agglomeration samples. Finally, a proper alarm rate (AR) parameter was designed to solve the problem of false alarms caused by the lack of repeatability and haphazardness of agglomeration in polymerization. According to the experimental results in a pilot plant, the proposed early-warning approach for agglomeration could provide a warning 20–50min in advance of the traditional pressure and temperature monitoring methods.

CFD prediction of scale-up effect on the hydrodynamic behaviors of a pilot-plant fluidized bed reactor and preliminary exploration of its application for non-pelletizing polyethylene process

This work aims to explore the scale-up effect on the hydrodynamic natures in a pilot-plant gas phase ethylene polymerization fluidized bed reactor (FBR) based on two-dimensional (2D) transient Eulerian model integrating the kinetic theory of granular flow (KTGF). Unlike lab scale FBR, significant differences in the core–annulus structures caused by the scale-up effect are revealed. The core region in pilot-plant scale FBR is much broader, and the distribution is nearly flat for two phases. The annulus structure is thin, and exhibits a very small particle velocity fluctuation. The bed expanding section could reduce the solid velocity and improve the flow patterns of gas and polymer particles. Simulations are also performed to assess the effects of polymer particle size and gas velocity on the hydrodynamics for the non-pelletizing polyethylene process. With the increase of polymer particle diameter (446μm to 1338μm), the superficial gas velocity should be increased from 0.60m/s to 0.90m/s to achieve better steady fluidized state. The results are helpful for understanding how the local mean two phases' velocities and volume fractions vary in pilot-plant reactor for PE production caused by scale-up effect.

CFD-PBM modeling polydisperse polymerization FBRs with simultaneous particle growth and aggregation: The effect of the method of moments

Real solid systems in polymerization fluidized bed reactors (FBRs) are characterized by wide particle size distributions that change continuously due to particle micro-behavior, growth and aggregation, etc. Simulations of such gas–solid flow hydrodynamics require the solution of a coupled computational fluid dynamics (CFD)-population balance equation (PBE) model, i.e. the CFD-PBM. Therefore, the analysis of the existing numerical methods for solving the PBE is important for the ability of the numerical prediction of the coupled model. Three representatively numerical moment-based methods, namely the quadrature method of moments (QMOM), the direct quadrature method of moments (DQMOM) and the fixed pivot quadrature method of moments (FPQMOM), were used to solve the PBE for evaluating the effect of numerical method. Comparative results demonstrated the suitability of the FPQMOM for modeling polymerization FBRs with simultaneous polymerization particle growth and aggregation.

CFD modeling and simulation of industrial scale olefin polymerization fluidized bed reactors

A two-fluid model for the numerical simulation of industrial scale olefin polymerization fluidized bed reactors is presented. However, a fully resolved simulation of industrial scale reactor is still nearly unfeasible. We, therefore, use sub-grid models (Schneiderbauer and Pirker, 2014) for the interphase drag and the solids stresses to account for the effect of the small unresolved structures on large resolved scales when using coarse grids. The sub-grid correction for the drag force is modified to consider the wide particle size distribution of high density polyethylene (HDPE). Furthermore, the sub-grid modification for the solids stresses is adapted to include the rheological properties of the polymer. On the one hand, the presented model is validated in the case of the coarse grid simulation of lab-scale bubbling fluidized bed by comparing bed expansion, bubble size and bubble rise velocities with experimental data. On the other hand, the model is applied to the coarse grid simulation of an industrial scale fluidized bed – moving bed reactor assembly. The numerical results demonstrate that our model reveals fairly good agreement with experimental data of average bed voidage, bubble diameters and bubble rise velocities. Finally, the impact of a barrier gas injection is studied, which is aimed to separate the fluidization gas from the gas in the moving bed reactor.