A kinetic model for industrial gas-phase ethylene copolymerization (original) (raw)
A dynamic model for ethylene copolymerization in an industrial Fluidized-Bed Reactor (FBR) is developed to describe its behavior and calculate the properties of polyethylene. The presented model considers particle entrainment and polymerization reaction in two phases. Two-site kinetic and hydrodynamic models in combination, provide a comprehensive model for the gas phase fluidized-bed polyethylene production reactor. The governing moment and hydrodynamic differential equations were solved simultaneously and the results compared with a similar work, as well as industrial data. The dynamic model showed accurate results for predicting Polydispersity Index (PDI), Molecular Weight Distribution (MWD), reactor temperature and polymer production rate.
e-Polymers, 2008
In the present work, ethylene polymerization kinetics was modeled using moment equations. According to the results obtained, the molecular weight distribution of each active center follows a Schultz-Flory distribution. However, the molecular weight distribution of polymer produced is much broader than a Schultz- Flory distribution. Besides, the order of polymerization with regard to monomer concentration is higher than unity. Moreover, the catalyst active centers deteriorate in the presence of hydrogen and consequently catalyst yield drops; nevertheless, polymerization kinetics is not mainly affected by hydrogen. Hydrogen also reduces polymer molecular weight since it is a strong transfer agent in olefin polymerizations. Notwithstanding, it does not affect polydispersity index. Last of all, although increasing cocatalyst concentration does not influence the activity of active centers, it lessens the molecular weight as a transfer agent
Industrial & Engineering Chemistry Research, 2012
A packed bed stopped flow minireactor (3 mL) suitable for performing gas phase polymerizations of olefins has been used to study the initial phases of ethylene homo-and copolymerization with two supported metallocene catalysts. The reactor can be used to perform gas phase polymerizations at times as short as 100 ms under industrially relevant conditions. It has been used to follow the evolution of the rate of polymerization, the gas phase temperature (and indirectly the particle temperature), and the polymer properties (molecular weight distribution, melting temperature, and crystallinity) for the two catalysts. It is shown that polymerization activity during the first 2−5 s of reaction can be up to 20 times higher than what is measured at longer polymerization times. The main consequence is the release of a significant amount of heat due to the rapid reaction that has to be efficiently evacuated in order to avoid particle overheating and melting. It has been seen that insufficient heat removal can strongly influence the behavior of the active sites, eventually leading to uncontrolled transfer reactions and polymers with unusually broad molecular weight distributions (MWD). It is also observed that the kinetic behavior of the two types of catalyst is similar at short times. Finally, some influence of particle size on reaction rate and molecular weight is observed between the largest and smallest catalyst particle cuts.
Journal of Applied Polymer Science, 2002
A method for quantitative evaluation of kinetic constants in Ziegler-Natta and metallocene olefin polymerizations presented previously Pinto, J. C. J Appl Polym Sci 2001, 79, 2076 is adapted to allow the estimation of kinetic constants for bulk propylene polymerizations by using a conventional fourth-generation high-activity Ziegler-Natta catalyst (HAC). In this particular case, reaction rate profiles are not available, so that estimation of kinetic data must rely on average polymer yields. The method comprises some fundamental steps, including the initial design of a statistical experimental plan, the execution of the designed experiments, the development of simple mathematical models to describe the polymeriza-tion, and the estimation of kinetic parameters from available yields, gel permeation chromatography (GPC), and nuclear magnetic resonance (NMR) data. It is shown that the proposed method allows the successful interpretation of experimental olefin polymerization data and the quantitative evaluation of kinetic parameters, which can be inserted into a process simulator to provide an accurate picture of actual industrial plant behavior.
Reactor Modeling of Gas‐Phase Polymerization of Ethylene
Chemical …, 2004
A model is developed for evaluating the performance of industrial-scale gas-phase polyethylene production reactors. This model is able to predict the properties of the produced polymer for both linear low-density and high-density polyethylene grades. A pseudo-homogeneous state was assumed in the fluidized bed reactor based on negligible heat and mass transfer resistances between the bubble and emulsion phases. The nonideal flow pattern in the fluidized bed reactor was described by the tanks-in-series model based on the information obtained in the literature. The kinetic model used in this work allows to predict the properties of the produced polymer. The presented model was compared with the actual data in terms of melt index and density and it was shown that there is a good agreement between the actual and calculated properties of the polymer. New correlations were developed to predict the melt index and density of polyethylene based on the operating conditions of the reactor and composition of the reactants in feed.
Kinetic modeling of propylene homopolymerization in a gas-phase fluidized-bed reactor
Chemical Engineering …, 2010
A comprehensive mechanistic model describing gas-phase propylene polymerization is developed. The kinetics of polymerization is based on a multiple active site for Ziegler-Natta catalyst. The model considers the polymerization reaction to take place in both bubble and emulsion phases. The developed model was used to predict polymer production rate, number and weight average molecular weights, polydispersity index (PDI) and melt flow index (MFI). Results showed that by increasing the superficial gas velocity from 0.1 to 0.7 m/s the proportion of the polymer produced in the bubble phase increases from 7.92% to 13.14% which highlights the importance of considering the existence of catalyst in the bubble phase. Comparing the developed model with published models of the same reactor revealed that the polymer productivity will be higher using the new model at high catalyst feed rate. .my (M.A. Hussain). catalyst and triethyl aluminum co-catalyst are charged continuously to the reactor and react with the reactants (propylene and hydrogen) to produce the polymer. The feed gas which comprises propylene, hydrogen and nitrogen, provides fluidization through the distributor, heat transfer media and supply reactants for the growing polymer particles. The fluidized particles disengage from unreacted gases in the disengaging zone. The solid-free gas is combined with fresh feed stream after heat removal and recycled back to the gas distributor. The monomer conversion per pass through the bed can vary from 2% to 5% and overall monomer conversion can be as high as 98% . The polypropylene product is continuously withdrawn from near the base of the reactor and above the gas distributor. The unreacted gas is recovered from the product which proceeds to the finishing area of the plant.
Journal of Polymer Science Part a Polymer Chemistry, 1999
The previously developed kinetic scheme of ethylene polymerization reactions with heterogeneous Ziegler-Natta catalysts (refs 1-3) states that the catalysts have several types of active centers which have different activities, different stabilities, produce different types of polymers, and respond differently to reaction conditions. Each type of center produces a single polymer component (Flory component), a material with the same structure (copolymer composition, isotacticity, etc.) and a narrow molecular weight distribution with Mw/Mn=2.0. This paper examines several features of ethylene polymerization reactions in the view of this mechanism. They include temperature and cocatalyst effects on molecular weight distribution, as well as the effect of reaction parameters (temperature, ethylene and hydrogen partial pressure, -olefin and cocatalyst concentration) on molecular weights of Flory components.
Applied Thermodynamics for Process Modeling in Catalytic Gas Phase Olefin Polymerization Reactors
Macromolecular Reaction Engineering, 2019
The 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.
Optimal Catalyst and Cocatalyst Precontacting in Industrial Ethylene Copolymerization Processes
Journal of Polymers, 2016
In industrial-scale catalytic olefin copolymerization processes, catalyst and cocatalyst precontacting before being introduced in the polymerization reactor is of profound significance in terms of catalyst kinetics and morphology control. The precontacting process takes place under either well-mixing (e.g., static mixers) or plug-flow (e.g., pipes) conditions. The scope of this work is to study the influence of mixing on catalyst/cocatalyst precontacting for a heterogeneous Ziegler-Natta catalyst system under different polymerization conditions. Slurry ethylene homopolymerization and ethylene copolymerization experiments with 1-butene are performed in a 0.5 L reactor. In addition, the effect of several key parameters (e.g., precontacting time, and ethylene/hydrogen concentration) on catalyst activity is analyzed. Moreover, a comprehensive mass transfer model is employed to provide insight on the mass transfer process and support the experimental findings. The model is capable of ass...