Low-density Polyethylene Autoclave Research Articles

CFD modeling for the prediction of molecular weight distribution in the LDPE autoclave reactor: Effects of non-ideal mixing

A commercial Low-density polyethylene (LDPE) reactor was modeled to investigate the effects of highly nonlinear hydrodynamics in an industrial-scale autoclave reactor on the polymer Molecular weight distribution (MWD). To reduce the extremely high computational burden of Computational fluid dynamics (CFD) runs and detailed MWD calculations, a combination of the CFD multicompartment model and probability generating function transformation is proposed. The validity of the proposed model was verified by a comparison with experimental data on industrial equipment. The computation time was shown to be only 37% of the actual operation time. The proposed model showed that strong downflow and weak reverse-flow coexisted to form circulation flow between the disks in the autoclave. Polymerization mainly occurred in the strong downflow region of the tubular regime. The circulation flow increased the accumulation of dead polymers to enhance the chain transfer to polymer, which produces long-branched living polymer chains and termination by recombination for a broad MWD with bimodality. Further analysis based on the simulation without the circulation flow showed a narrow MWD and no bimodality. These results indicate that the flow pattern must be properly controlled to produce polymers with desired properties in large-scale polymerization reactors.

110th Anniversary: Nonideal Mixing Phenomena in High-Pressure Low-Density Polyethylene Autoclaves: Prediction of Variable Initiator Efficiency and Ethylene Decomposition

A segregation–backmixing model is developed to simulate the dynamic operation of multifeed high-pressure low-density polyethylene (LDPE) autoclaves, calculate the specific initiator consumption (gr...

Computational Fluid Dynamics Simulations and Experimental Validation of Macromixing and Flow Characteristics in Low-Density Polyethylene Autoclave Reactors

The macromixing and flow characteristics of a low-density polyethylene autoclave reactor stirred with 32 impellers were investigated by experiments and computational fluid dynamics simulations. The simulation results were validated using the experimental data. The mixing curves show an excellent agreement with the experimental results. The flow field, distribution of velocity, and mass flux obtained from the simulations were further analyzed, and the results show that the reactor can be divided into two reaction zones. In each reaction zone, two axial flows form a big circulation loop. The downward flows near the reactor wall decrease along the flow direction, while the upward flows near the shaft increase. In each big circulation loop there are many small radial circulation loops that are similar to those encountered in continuous stirred-tank reactors. Finally, a more accurate macromixing model was proposed, which is helpful to reveal the multizone phenomenon and flow characteristics in reactors.

Unscented Kalman filter based nonlinear model predictive control of a LDPE autoclave reactor

This paper presents a multivariable nonlinear model predictive control (NMPC) scheme for the regulation of a low-density polyethylene (LDPE) autoclave reactor. A detailed mechanistic process model developed previously was used to describe the dynamics of the LDPE reactor and the properties of the polymer product. Closed-loop simulations are used to demonstrate the disturbance rejection and tracking performance of the NMPC algorithm for control of reactor temperature and weight-averaged molecular weight (WAMW). In addition, the effect of parametric uncertainty in the kinetic rate constants of the LDPE reactor model on closed-loop performance is discussed. The unscented Kalman filtering (UKF) algorithm is employed to estimate plant states and disturbances. All control simulations were performed under conditions of noisy process measurements and structural plant–model mismatch. Where appropriate, the performance of the NMPC algorithm is contrasted with that of linear model predictive control (LMPC). It is shown that for this application the closed-loop performance of the UKF based NMPC scheme is very good and is superior to that of the linear predictive controller.

Mixing effects on performance and stability of low‐density polyethylene reactors

AbstractImperfect initiator mixing greatly affects the stability and efficiency of low density polyethylene (LDPE) autoclave reactors. A combined simulation technique utilizing compartment models and computational fluid dynamics extends previous work in the literature by providing a physically detailed picture of imperfect mixing. Analysis indicates that the effective volume for chain propagation in the autoclave reactor can expand and contract in a continuous fashion as operating conditions change. As mixing becomes poor, the effective reactive volume decreases, causing a reduction in initiator efficiency, but an expansion in the stable operation region. Examples demonstrate that accurate prediction of the effective reaction volume is crucial for predicting LDPE autoclave reactor behavior. A new mixing model that represents the feed plume by a series of interconnected tanks with geometrically increasing volumes provides a favorable tradeoff between accuracy and model complexity. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Prediction of Polymer Properties in LDPE Reactors

AbstractSummary: A new analysis tool is presented that uses the governing kinetic scheme to predict properties of low‐density polyethylene (LDPE) such as the detailed shape of the molecular weight distribution (MWD). A model that captures mixing details of autoclave reactor operation is used to provide a new criterion for the onset of MWD shouldering. Kinetic effects are shown to govern the existence of MWD shoulders in LDPE reactors, even when operation is far from perfectly‐mixed. MWD shoulders occur when the mean reaction environment has a relatively high radical concentration and has a high polymer content, and is at a low temperature. Such conditions maximize long chain formation by polymer transfer and combination‐termination, while limiting chain scission. For imperfectly‐mixed reactors, the blending of polymer‐distributions produced in different spatial locations has a small effect on the composite MWD. However, for adiabatic LDPE autoclaves, imperfect mixing broadens the stable range of mean reactor conditions, and thereby increases the possibility for MWD shouldering.Polymer MWD produced in an LDPE autoclave reactor by various kinetic mechanisms.magnified imagePolymer MWD produced in an LDPE autoclave reactor by various kinetic mechanisms.

Modeling LDPE Tubular and Autoclave Reactors

Computational fluid dynamics (CFD) is used in this study to model low-density polyethylene (LDPE) tubular and autoclave reactors. A polymerization reaction model is developed using the method of mo...

Modeling and Simulation of High Pressure Autoclave Polyethylene Reactor Including Decomposition Phenomena.

The aim of the present investigation is the development of a practical model for a high-pressure polyethylene plant. The reactor considered in the present work is an adiabatic slim type autoclave with four zones for free radical polymerization of low density polyethylene (LDPE). A fairly comprehensive, but realistic model is described that has the ability to predict the temperature at each reaction zone, as well as the effects of initiator flow changes. From stability analysis we can identify the range of operating conditions which can effectively be used to prevent runaway reactions, and maximize polymer conversion in LDPE autoclaves.

Dynamic modeling of multizone, multifeed high-pressure LDPE autoclaves

A comprehensive mathematical model is developed to simulate the dy- namic behavior of multizone, multifeed high-pressure ethylene polymerization auto- claves. To describe the complex flow patterns occurring in low-density polyethylene (LDPE) autoclaves, a user-specified multisegment, multirecycle model representation of the actual multizone reactor is established. A general reaction mechanism is em- ployed to represent the kinetics of ethylene polymerization. Dynamic mass, molar species, and energy balances are derived to predict the polymerization rate, monomer conversion, molecular weight developments (e.g., Mn, Mw, long- and short-chain branching), and temperature profile with respect to time and spatial position in the reactor. Detailed results on the start-up and grade transition of a four-zone autoclave reactor are presented and the effects of the macromixing parameters (e.g., number of segments per reaction zone and the total and side external recycle ratios) on the dynamic behavior of the reactor are investigated. It is shown that the model macro- mixing parameters can significantly affect the initiator consumption rate in a reaction zone. The present model is capable of predicting accurately the dynamic behavior of LDPE autoclaves and, thus, can be employed in the design, optimization, and control of these reactors. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2327-2348, 1999

A Study of Macrosegregation in Low-Density Polyethylene Autoclave Reactors by Computational Fluid Dynamic Modeling

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTA Study of Macrosegregation in Low-Density Polyethylene Autoclave Reactors by Computational Fluid Dynamic ModelingGüray Tosun and André BakkerCite this: Ind. Eng. Chem. Res. 1997, 36, 9, 4013Publication Date (Web):September 2, 1997Publication History Published online2 September 1997Published inissue 1 September 1997https://doi.org/10.1021/ie9708438Copyright © 1997 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views162Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (51 KB) Get e-Alerts Get e-Alerts

Simultaneous optimization of mixing patterns and feed distribution in LDPE autoclave reactors

In this work, a model for simultaneous optimization of mixing patterns and feed distribution in LDPE (Low Density PolyEthylene) reactors is presented. The formulation is based on a a novel technique for optimizing mixing patterns in chemical reactors, resulting in an optimal control formulation which is solved as an NLP. The kinetic model is based on representing the molecular weight distribution by statistical moments.

A Study of Macrosegregation in Low-Density Polyethylene Autoclave Reactors by Computational Fluid Dynamic Modeling

Flow and reaction in a typical commercial scale autoclave LDPE reactor were modeled by a three-dimensional computational fluid dynamic (CFD) k-e model in order to shed light on the macrosegregation effects that can occur in these reactors. It is shown that the CFD model predicts significant differences from CSTR behavior. Results are discussed in terms of the effects of macro- and microscale inhomogeneities of concentration and temperature on free radical polymerization kinetics. The observed nonidealities in terms of minima in the initiator consumption curves and multiple steady states are explained on the basis of competing turbulent transport and chemical kinetics. Microsegregation effects are shown to be negligible in comparison to macrosegregation effects. Given the fact that the CFD model is based on reactive scalar and energy balances without adjusted parameters in the three-dimensional flow field of the entire reactor, it is tentatively concluded that commercial-scale LDPE vessel reactors can have significant macrosegregation effects beyond a certain steady-state adiabatic operating temperature that is specific to the initiator being used.

Runaway phenomena in low‐density polyethylene autoclave reactors

AbstractTo study runaway behavior in autoclave low‐density polyethylene (LDPE) reactors, a kinetic model for a perfectly stirred tank reactor is presented. The kinetic model not only includes the standard initiation, propagation, and termination reactions for polymerization, but it also has free radical reactions that describe the decomposition of ethylene ultimately leading to a runaway. Dynamic simulation of the model indicates runaway behavior for the following conditions: excess initiator in feed; feed impurity; feed temperature disturbance; controller failure; and poorly tuned controller. Operating strategies such as mixed initiator feeds and grade transitions are also explored from a dynamic view. Stability analysis indicates safe operating limits for certain variables at typical conditions. The model provides useful insights for preventing runaway reactions in LDPE autoclaves.

Modeling and control of an LDPE autoclave reactor

A two-compartment four-cell model is developed for the adiabetic slim type autoclave reactor for free radical polymerization of low density polyethylene (LDPE). It is possible to determine not only the reactor performance represented by the monomer conversion and the reaction temperature but also the properties of the polymer product characterized by the average molecular weight and the polydispersity. It turns out that the reactor performance predicted is in good agreement with the plant data and the properties of the polymer product are estimated within reasonable ranges of actual values. The steady state multiplicity is found to exist and is examined by constructing the bifurcation diagram. The effects of various operation parameters on the reactor performance and the polymer properties are investigated systematically to show that the temperature distribution plays the central role for the properties of the polymer products. Therefore, it is essential to establish a good control strategy for the temperature in each compartment. The adaptive pole-placement control algorithm is applied to the temperature control of the adiabatic slim type autoclave reactor. The recursive least square method is used for the model identification. To accomplish a satisfactory control, the estimator and controller are initialized during the period of start-up. It is shown that the reactor system can be adaptively controlled by the pole-placement control algorithm, especially when the reactor temperature distribution is changed.

Dynamic flowsheet modeling of polymerization processes using POLYRED

Some of the features of the dynamic flowsheeting package, POLYRED, are described. To demonstrate some of the capabilities of the package, POLYRED is used to formulate models and to estimate model parameters from both batch and continuous process data. Examples include the complications of imperfect micromixing which require mixing models. Both kinetic and micromixing parameters are estimated from data. Results for a low-density polyethylene autoclave reactor are provided as an illustration.

Dynamic modelling and steady-state multiplicity in high pressure multizone LDPE autoclaves

A comprehensive mathematical model is developed to simulate the ethylene polymerization in high-pressure autoclave reactors. Two generalised macromixing models, namely the external recycle and the backflow model are established to describe the complex mixing patterns occurring in multizone, multifeed low density polyethylene (LDPE) autoclaves. According to these models, each zone in the reactor is divided into a sequence of perfectly mixed vessels, called segments. To represent the kinetics of ethylene polymerization a general reaction mechanism is considered and the method of moments is employed to calculate the molecular weight developments. A new technique the so-called ‘numerical fractionation’ is applied to predict the formation of gel and the number chain length distribution in LDPE autoclaves. Finally, a detailed stability analysis is carried out for an LDPE autoclave to identify the number of multiple steady-states.