Gas-phase Ethylene Polymerization Research Articles

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.

Effects of Spatial Distributions of Active Sites in a Silica-Supported Metallocene Catalyst on Particle Fragmentation and Reaction in Gas-Phase Ethylene Polymerization

Effects of Spatial Distributions of Active Sites in a Silica-Supported Metallocene Catalyst on Particle Fragmentation and Reaction in Gas-Phase Ethylene Polymerization

Influence of Process Parameters on the Gas Phase Polymerization of Ethylene: RSM or ANN Statistical Methods?

AbstractThe direct and interactive effects of process variables, including the use of iso‐pentane as an inert condensing agent, on the rate of polymerization and main physical properties of linear low‐density polyethylene are studied using two different statistical methods. Response surface methodology (RSM) based on a three level five factor Box–Behnken design is used to generate the number of experimental runs. The generated dataset is used to develop the RSM and artificial neural network (ANN) models. The effect of the input on the dependent variables is studied using the 3D response surface of the developed RSM model. The developed models are statistically analyzed and compared to determine their performance capability. The ANN model marginally outperforms the RSM model based on the computed statistical parameters but provides less information on the variable interactions.

X-ray nanotomography uncovers morphological heterogeneity in a polymerization catalyst at multiple reaction stages

During olefin polymerization on supported catalysts, the controlled morphological evolution of the catalyst particle is vital for ensuring optimal product properties and high catalyst activity. We employed non-destructive hard X-ray holotomography to quantitatively assess the 3D morphology of multiple silica-supported hafnocene-based catalyst particles during the early stages of gas-phase ethylene polymerization. Image processing and pore network modeling revealed clear variations in the dimensions and interconnectivity of pristine particles' macropore networks. This, together with apparent differences in the fragmentation behavior of pre-polymerized particles, suggests that the reactivity and morphological evolution of individual particles are largely dictated by their unique support and pore space architectures. By minimizing the structural heterogeneity among pristine catalyst particles, more uniform particle morphologies may be obtained. Significant polymerization activity, observed in the particles' interiors, further implies that appropriate polymerization conditions and catalyst kinetics can guarantee sufficiently high particle accessibilities and thus more homogeneous support fragmentation. • 3D X-ray nanotomography on metallocene-based polymerization catalyst particles • Inter- and intraparticle heterogeneity observed at an advanced pre-polymerization stage • Pristine catalyst support structure strongly affects morphological evolution • Significant involvement of the layer-by-layer fragmentation mechanism postulated Our society's demand for polyolefins, such as polyethylene, is steadily rising. To improve the physicochemical properties of these high-performance materials, strict morphological control is required during the polymerization of related catalysts, especially at reaction onset. By assessing the 3D morphology of multiple catalyst particles during the early stages of ethylene polymerization, we were able to identify and correlate morphological differences to the initial catalyst particle support structure. Based on this, we believe that the implementation of structurally more uniform catalyst supports, in combination with controlled pre-polymerization procedures, may help optimize the performance of relevant catalysts. Our multi-particle approach also paves the way for other industrial heterogeneous catalysts, where the spatial distribution of catalytically relevant phases and/or carbon-based products is to be investigated in a representative number of catalyst particles. The morphology of an industrial-grade ethylene polymerization catalyst was investigated at multiple reaction stages using X-ray holotomography, a high-resolution 3D imaging technique. Image processing, radial phase distribution analysis, and pore network modeling revealed significant inter- and intraparticle heterogeneity in both pristine and pre-polymerized particles. Differences in the fragmentation and reactivity of individual catalyst particles can be attributed to the particles' unique support architectures. Mild reaction conditions ensured a high accessibility of the catalyst particles' interiors and more homogeneous support fragmentation.

Correlating the Morphological Evolution of Individual Catalyst Particles to the Kinetic Behavior of Metallocene-Based Ethylene Polymerization Catalysts.

Kinetics-based differences in the early stage fragmentation of two structurally analogous silica-supported hafnocene- and zirconocene-based catalysts were observed during gas-phase ethylene polymerization at low pressures. A combination of focused ion beam-scanning electron microscopy (FIB-SEM) and nanoscale infrared photoinduced force microscopy (IR PiFM) revealed notable differences in the distribution of the support, polymer, and composite phases between the two catalyst materials. By means of time-resolved probe molecule infrared spectroscopy, correlations between this divergence in morphology and the kinetic behavior of the catalysts’ active sites were established. The rate of polymer formation, a property that is inherently related to a catalyst’s kinetics and the applied reaction conditions, ultimately governs mass transfer and thus the degree of homogeneity achieved during support fragmentation. In the absence of strong mass transfer limitations, a layer-by-layer mechanism dominates at the level of the individual catalyst support domains under the given experimental conditions.

A Single Particle Model to Predict the Impact of Induced Condensing Agents on Polymerizing Particles During the Gas Phase Polymerization of Ethylene

AbstractA single particle model for homopolymerization of ethylene in the presence of hydrogen is used to predict the impact of induced condensing agents on the evolution of concentration profiles in the growing particle, the rate of homopolymerization, and the evolution of the molecular weight distribution. The model formulation is based on the random‐pore polymer flow model associated with a kinetic model using the method of moments to describe the chain length. The Sanchez–Lacombe equation of state is applied to calculate the equilibrium concentration of each penetrant in a mixture of gases in the amorphous polymer phase. The free volume model from Vrentas and Duda is employed to calculate the diffusivity coefficients of penetrants in a semi‐crystalline polymer. The model confirms that the increased rates of production and increased molecular weights observed in previously published experimental studies are due in large part to the co‐solubility and co‐diffusivity effects.

Gas‐Phase Polyethylene Reactors—A Critical Review of Modeling Approaches

AbstractDifferent approaches to modeling the gas phase polymerization of ethylene that have been considered in the literature are reviewed. It is shown that while simple, well‐mixed models can give an adequate representation of the average performance of a given polymerization reactor, they do not allow one to analyze certain modeling problems such as the existence of temperature gradients or particle segregation, nor can they be used to treat operational modes such as condensed cooling where there can be up to three different phases in different parts of the reactor. For this, more complex models are required. These can take the form of compartmentalized models or even models based on computation fluid dynamics. However, long computational times can be required to fully exploit these models. Furthermore, significant progress needs to be made if one needs to address important phenomena such as agglomeration or particle attrition during polymerization.

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...

Gas-phase polymerization of ethylene over Ti-based Ziegler–Natta catalysts prepared from different magnesium sources

This study focuses on gas-phase polymerization of ethylene using the titanium-based Ziegler–Natta catalysts prepared from different magnesium sources including MgCl2 (Cat A), magnesium powder (Cat B), and Mg(OEt)2 (Cat C). During polymerization, different cocatalysts were also used. It was found that Cat C with triethylaluminum as a cocatalyst exhibited the highest activity. This was likely attributed to optimal distribution of active sites on the catalyst surface. It can be observed by increased temperature in the reactor due to highly exothermic reaction during polymerization. By the way, the morphologies of the polymer obtained from this catalyst were spherical, which is more preferable. Besides the catalytic activity, crystallinity and morphology were also affected by the different magnesium sources used to prepare the catalysts.

A Self-Supporting Strategy for Gas-Phase and Slurry-Phase Ethylene Polymerization using Late-Transition-Metal Catalysts.

The polyolefin industry is dominated by gas-phase and slurry-phase polymerization using heterogeneous catalysts. In contrast, academic research is focused on homogeneous systems, especially for late-transition-metal catalysts. The heterogenization of homogeneous catalysts is a general strategy to provide catalyst solutions for existing industrial polyolefin synthesis. Herein, we report an alternative, potentially general strategy for using homogeneous late-transition-metal catalysts in gas-phase and slurry-phase polymerization. In this self-supporting strategy, catalysts with moderate chain-walking capabilities produced porous polymer supports during gas-phase ethylene polymerization. Chain walking, in which the metal center can move up and down the polymer chain during polymerization, ensures that the metal center can travel along the polymer chain to find suitable sites for ethylene enchainment. This strategy enables simple heterogenization of catalysts on solid supports for slurry-phase polymerization. Most importantly, various branched ultra-high-molecular-weight polyethylenes can be prepared under various polymerization conditions with proper catalyst selection.

Gas phase polymerization of ethylene towards UHMWPE.

For the first time, ultra-high molecular weight polyethylene (UHMWPE) was produced in gas phase process with a new fluidized bed concept where the solids are dispersed phase and the gas is bulk phase as opposed to conventional fluidized bed reactors (FBRs). With this concept, UHMWPE with average molecular weights about 1-6,9 × 106 g mole-1 were produced with a commercial supported Ziegler-Natta catalyst by using a gas phase mini semibatch reactor system. Additionally, optimum conditions of gas phase polymerization for the best results of productivity, catalyst activity, molecular weight and crystallinity were determined by Taguchi experimental design and catalyst stability at the optimum condition was tested by video microscopy polymerization. The characterization of products was carried out experimentally by TGA, DSC, FTIR, and NMR.

Gas-Phase Ethylene Polymerization by Single-Site Cr Centers in a Metal–Organic Framework

We report a systematic study on the gas-phase polymerization of ethylene by a metal–organic framework (MOF) catalyst. Cr3+-exchanged MFU-4l (Cr(III)-MFU-4l, MFU-4l = Zn5Cl4(BTDD)3, H2BTDD = bis(1H-...

Condensed Mode Cooling for Ethylene Polymerization: Part V—Reduction of the Crystallization Rate of HDPE in the Presence of Induced Condensing Agents

AbstractThe crystallization of high‐density polyethylene (HDPE) alone, and in the presence of n‐hexane (a common induced condensing agent [ICA]) is studied using differential scanning calorimetry. The presence of a noncrystallizable ICA which is partially soluble in the amorphous phase of HDPE reduces the rate of crystallization. This is reflected by a shifting of the crystallization and melting peaks of HDPE to lower temperatures when the ICA concentration in the medium increases. It is also observed that the rate of crystallization of HDPE can be very slow when the ratio of ICA to HDPE increases from zero. This behavior is useful to better understand the physical effects that can potentially occur at the beginning of the gas phase polymerization of ethylene, and suggests that the crystallization of the nascent polymer, and thus the properties of the polymer in the reactor will be quite different from those of the powder at later stages of the reaction when running in condensed mode.

Dynamic simulation and control of two-series industrial reactors producing linear low-density polyethylene

One of the leading routes for producing polyolefins is through gas-phase catalytic fluidized bed reactors. In this study, the industrial gas-phase ethylene polymerization reactor series of Jam Petrochemical Company has been dynamically analyzed, modeled and controlled. The copolymerization of ethylene with 1-butene is defined on Zeigler–Natta catalyst, assuming a double active site mechanism. To serve this purpose, pseudo-kinetic rate constants and the method of moments have been employed. The proposed model is capable of predicting the unsteady-state behavior of each reactor in addition to the properties of the product such as melt flow index (MFI), dispersion index, and molecular weight distribution (MWD). The verification of the model has been conducted with plant data to prove the accuracy of the model-estimated MWD and MFI. The controllability of the process control configuration has been examined through analyzing the dynamic behavior of the process under conventional feedback PID controllers. It has been observed that the control structure delivers a convincing performance for disturbance rejection.

Two-stage modeling strategy for industrial fluidized bed reactors in gas-phase ethylene polymerization processes

Fluidized bed reactor is one of the main units in gas-phase ethylene polymerization processes. Due to the complexity in the polymerization kinetics, hydrodynamic behavior and heat transfer, the modeling and simulation have played an important role in the real operations of industrial polymerization reactors. In this paper, a two-stage modeling strategy for industrial fluidized bed reactors in gas-phase ethylene polymerization processes is proposed. The kinetics of polymerization reactions is computed in the first stage based on a comprehensive kinetics model while the hydrodynamic behavior and heat transfer in the industrial fluidized bed reactors are computed in the second stage based on a set of partial differential equations (PDEs). The polymerization kinetics, hydrodynamic behavior and heat transfer are integrated into the modeling and simulation of industrial fluidized bed reactors in gas-phase ethylene polymerization processes.

Quality control for Ziegler-Natta catalysis via spectroscopic fingerprinting

Commercial olefin polymerization catalysts are typically produced at a manufacturing site before transport to production facilities for storage and eventual use. During transport and storage, catalysts can deteriorate resulting in decreased catalytic performance due to contact with environmental factors. In this work, a spectroscopic toolbox was developed for quality assurance purposes of a third generation Ziegler-Natta catalyst for ethylene polymerization. A pre-activated, industrial Ziegler-Natta catalyst was exposed singly to heat, dry air, and moisture to study the specific environmental factors. Activity tests were performed with the polymer morphology inspected by SEM and image analysis. Catalyst characterization was conducted using Fourier Transform Infrared spectroscopy with CO and Diffuse Reflectance UV–Vis spectroscopy to relate unique spectroscopic fingerprints to different environmental effects. Reactivity towards gas-phase ethylene polymerization was tested using Diffuse Reflectance Infrared Fourier Transform spectroscopy. This work demonstrates the development of a new spectroscopic methodology useful for quality control in Ziegler-Natta catalysis.

Modeling Condensed Mode Operation for Ethylene Polymerization: Part III. Mass and Heat Transfer

The polymer flow model is employed to predict the transient behavior of the concentration and temperature profiles within the growing particles during gas-phase ethylene polymerization in the absence and presence of n-hexane as the induced condensing agent. It is demonstrated that by accurate estimation of the model parameters, one can precisely describe the impact of gas phase composition on the reaction rate. The simulation results show that the experimentally observed change in the polymerization rate under the various operating conditions is the direct result of alteration in the average concentration of ethylene in the particle; the average ethylene concentration not only depends on its equilibrium concentration but is also controlled by the significance of the mass transfer resistance through the growing particles. This is while the effect of partial pressure of n-hexane and ethylene on the thermal behavior of the particle is found to be insignificant for the studied experiments.

High performance gas-phase ethylene polymerization metallocene catalyst investigation in a pilot-plant fluidized bed reactor (1) experimental analysis

This work aims to explore the scaling up process of lab developed heterogeneous metallocene catalyst in a pilot-plant scale fluidized bed reactor in comparison of benchmark catalyst producing polyethylene. The catalyst performance has been evaluated through copolymerization behaviors and polymerization kinetics. The produced polymers are analyzed in revealing the scale up effect of the catalyst. It was found that the catalyst particle size distribution shows very similar size distribution compared to the industrial catalyst and good particle morphology, which will benefit the gas phase fluidized bed operation. Compared to the benchmark HP-100, PME-18 shows better catalytic parameters in the ethylene/1-hexene polymerization behavior and reaction kinetics. The final product demonstrates very similar particle size distribution and polymer properties, which will obviously influence the product quality and production state.

Modeling Condensed Mode Operation for Ethylene Polymerization: Part I. Thermodynamics of Sorption

A process model based on the thermodynamic models of Sanchez–Lacombe and PC-SAFT EoS is developed to simulate and analyze the impact of vaporized n-hexane as an induced condensing agent on the rate of gas phase ethylene polymerization on supported catalyst. The simulation results of the process model indicate that the cosolubility phenomenon (i.e., the enhancement in the equilibrium concentration of ethylene in the amorphous phase of polyethylene in the presence of n-hexane) cannot be the sole reason for the experimentally observed increase in the polymerization rate seen over the entire duration of reaction. At the beginning, the rate of ethylene polymerization is enhanced much more strongly than would be expected simply from the cosolubility effect alone. However, as the reaction proceeds, the enhancement magnitude gradually decreases and reaches a steady-state value corresponding to the promotion magnitude in the equilibrium concentration of ethylene predicted by the two thermodynamic models.

Mechanism of particle build-up on gas-solid fluidization column wall due to electrostatic charge generation

Particle build-up on gas-solid fluidization column wall due to electrostatic charging causes significant operational challenges including reactor shutdown in industrial processes such as gas-phase ethylene polymerization to produce polyethylene. It is well acknowledged that in fluidization process electrostatic charges are generated as a result of continuous particle-particle and particle-vessel wall contacts. However, the mechanism of charged particle attraction towards the fluidization column wall and their adhesion has received minimal attention. This work proposes a mechanism for particle build-up on the column wall by experimentally investigating the fouled particles charge distribution using a charged particle separator apparatus. The experiments were carried with two types of linear low-density polyethylene resins in a pressurized pilot-scale gas-solid fluidization system. Experimental results showed that the polyethylene layer built-up on the column wall contained both positively and negatively charged particles. A mechanism was proposed indicating that charged particle migration towards the metallic column wall was due to the image and electrostatic forces. The image forces were attributed to the particle-wall contacts while the electrostatic forces were between the charged particles fouled on the column wall and those oppositely charged within the bulk of the bed. In addition, the net space charge of the bulk particles contributed to the migration of the particles towards the column wall.