Gas Phase Fluidized Bed Reactor Research Articles

CFD MODELING OF POLYETHYLENE GAS PHASE FLUIDIZED BED REACTOR WITH INTERNAL CHILLER

Polyethylene production in a gas phase fluidized bed reactor is exposed to unstable temperature behavior, and if not controlled properly the temperature oscillation can cause polymer melting and potential plant shutdown (Ghasem, 1999). Accordingly, the present work improves the previous simple mathematical model by the CFD modelling that addresses the prediction of ethylene concentration and temperature profile in entire reactor of the fluidized bed reactor employed for polyethylene production. The CFD model considers gas phase molecular diffusion in the axial and radial directions. Results revealed that the size of the internal heat exchanger mainly the exchanger heat transfer interface area has strong impact on the temperature contour inside the reactor. Ethylene feed rate and ethylene concentration, catalyst feed rate effect the temperature profile inside the reactor considerably. The increase in ethylene concentration and catalyst feed rate strongly influences the fluidized bed temperature. As ethylene concentration increases reactor temperature increases. Proper temperature control inside the polyethylene gas phase is essential fluidized reactor to maintain the reactor temperature below polymer melting point and hence long term operation of fluidized bed reactor without reactor shutdown and temperature excursion above the polymer melting point.

Experimental investigation on mechanisms of fine particles generation for the Borealis Borstar multistage olefin polymerization process

ABSTRACTSimultaneous production of large amount of undesired fine particles is a big trouble in the Borstar multistage olefin polymerization process. Aiming at reducing the fine particles, the formation mechanism and formation location of the fine particles were thus studied. First, the influence of catalyst nature was considered. Second, the ash content, bulk density, morphology, and molecular weight distribution of polyethylene with different particle sizes from the small‐scale loop prepolymerization reactor, supercritical loop reactor, and gas phase fluidized bed reactor were studied, respectively. In combination with particle growth model, scanning electron microscope, gel permeation chromatography, and laser particle size analyzer, the particle morphology and growth kinetics were investigated. The results showed that fine particles were mostly generated in the supercritical loop reactor, and were significantly affected by the particle size distribution, residence time distribution, and particle fragmentation of the catalyst. Furthermore, scanning electron microscope images showed the catalysts with low activity tended to generate more fine particles. Based on these results, several strategies for reducing the amount of fine particles were proposed, which could be applied in the industrial process. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46589.

Evaluation of hydrodynamic behavior of the perforated gas distributor of industrial gas phase polymerization reactor using CFD-PBM coupled model

A 2D computational fluid dynamics (Eulerian–Eulerian) multiphase flow model coupled with a population balance model (CFD-PBM) was implemented to investigate the fluidization structure in terms of entrance region in an industrial-scale gas phase fluidized bed reactor. The simulation results were compared with the industrial data, and good agreement was observed. Two cases including perforated distributor and complete sparger were applied to examine the flow structure through the bed. The parametric sensitivity analysis of time step, number of node, drag coefficient, and specularity coefficient was carried out. It was found that the results were more sensitive to the drag model. The results showed that the entrance configuration has significant effect on the flow structure. While the dead zones are created in both corners of the distributors, the perforated distributor generates more startup bubbles, heterogeneous flow field, and better gas–solid interaction above the entrance region due to jet formation.

ARX-NNPLS Model Based Optimization Strategy and Its Application in Polymer Grade Transition Process

Since it is often difficult to build differential algebraic equations (DAEs) for chemical processes, a new data-based modeling approach is proposed using ARX (AutoRegressive with eXogenous inputs) combined with neural network under partial least squares framework (ARX-NNPLS), in which less specific knowledge of the process is required but the input and output data. To represent the dynamic and nonlinear behavior of the process, the ARX combined with neural network is used in the partial least squares (PLS) inner model between input and output latent variables. In the proposed dynamic optimization strategy based on the ARX-NNPLS model, neither parameterization nor iterative solving process for DAEs is needed as the ARX-NNPLS model gives a proper representation for the dynamic behavior of the process, and the computing time is greatly reduced compared to conventional control vector parameterization method. To demonstrate the ARX-NNPLS model based optimization strategy, the polyethylene grade transition in gas phase fluidized-bed reactor is taken into account. The optimization results show that the final optimal trajectory of quality index determined by the new approach moves faster to the target values and the computing time is much less.

Improved single phase modeling of propylene polymerization in a fluidized bed reactor

An improved model for the production of polypropylene in a gas phase fluidized bed reactor was developed. Comparative simulation studies were carried out using the well-mixed, constant bubble size and the improved models. The improved model showed different prediction characteristics of polymer production rate as well as heat and mass transfer behavior as compared to other published models. All the three models showed similar dynamic behavior at the startup conditions but the improved model predicted a narrower safe operation window. Furthermore, the safe ranges of variation of the main operating parameters such as catalyst feed rate and superficial gas velocity calculated by the improved and well mixed models are wider than that obtained by the constant bubble size model. The improved model predicts the monomer conversion per pass through the bed which varies from 0.28 to 5.57% within the practical ranges of superficial gas velocity and catalyst feed rate.

Kinetic modeling of propylene homopolymerization in a gas-phase fluidized-bed reactor

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.

Control of Molecular Weight Distribution and Density of Polyethylene in Gas Phase Reactors

This paper addresses the polyethylene quality transition in gas phase fluidized bed reactors using feedback control. Specifically, a nonlinear model predictive controller is utilized to steer the polymer density and the entire molecular weight distribution to follow predefined targets. The hydrogen and co-monomer inflows are used as the manipulated variables. In this work, the molecular weight distribution is modeled as a function of the reaction kinetics and hydrogen to monomer ratio. The simulation results indicated successful implementation of the control algorithm to attain the desired molecular weight distribution and density, even with the existence of modeling errors. Economically undesirable operations such as excessive purge and/or reduced production rate are avoided by the limited use of monomer and catalyst inflows.

Control of molecular weight distribution of polyethylene in gas-phase fluidized bed reactors

This paper presents a feasibility study of the broadening of polyethylene molecular weight distribution produced using a multisite Ziegler-Natta catalyst in a fluidized-bed reactor. A nonlinear model predictive control algorithm, applied to a validated model of the reactor, is used for the on-line control of the entire molecular weight distribution of the produced polymer. Control of a target chain-length distribution is achieved by selecting a collection of points in the distribution and using them as set points for the control algorithm. An on-line Kalman filter is used to incorporate infrequent and delayed off-line molecular weight measurements. Through simulation the control algorithm is evaluated, under tracking conditions as well as plant-model mismatch. The results demonstrate that the control algorithm can regulate the entire molecular weight distribution with minimum steady state error. However, the efficiency of this approach is highly dependent on the dynamics of hydrogen inside the reactor.

Two-phase modeling of a gas phase fluidized bed reactor for the fluorination of uranium tetrafluoride

A bubbling fluidized bed reactor for the fluorination of uranium tetrafluoride by fluorine gas was simulated employing two-phase models, with the bubble phase assumed to be in plug flow, and the emulsion phase in plug flow (P-P model) and in perfectly mixed flow (P-M model). The model calculations were compared with actual data in term of fluorine conversion. The comparison showed that the P-M model predicted the data satisfactorily. The P-M model was then applied to understand the influence of operating parameters on the reactor performance. A sensitivity study of various operating parameters showed the extent each parameter can influence the rates of fluorine conversion and uranium hexafluoride production.

Origins of Product Heterogeneity in the Spheripol High Impact Polypropylene Process

In the Spheripol process (Basell), high impact polypropylene (hiPP) is produced in two stages in series. First, isotactic polypropylene (i-PP) particles are produced in liquid propylene. These particles are transferred to a gas phase fluidized bed reactor where the elastomeric phase is produced within the isotactic polypropylene. The particulate product obtained in the commercial process is heterogeneous. This heterogeneity may be deleterious for the product performance. In this work, the origins of product heterogeneity were studied combining a detailed characterization of the product sampled from the exit lines of the homopolymerization stage (i-PP particles) and the fluidized bed reactor (hiPP particles) of a commercial unit with a mathematical model of the process. It was found that the experimental results were consistent with equally accessible active sites of uniform activity, the residence time distribution of the catalyst in the different reactors playing the major role in product heterogeneity.

Modeling of a fluidized bed reactor for the ethylene-propylene copolymerization

A mathematical model for the ethylene-propylene copolymerization with a Ziegler - Natta catalyst in a gas phase fluidized bed reactor is presented. The model includes a two active site kinetic model with spontaneous transfer reactions and site deactivation. Also, it is studied and simulated the growth of a polymeric particle which is exposed to an outside atmosphere (monomers concentrations and temperature) that represent the emulsion phase conditions of the reactor. Particle growth model is the basis for the Study of the sizes distribution into the reactor. Two phase model of Kunii - Levenspiel is the basis for the modeling and simulation of the fluid bed reactor, the models developed consider two extreme cases for the gas mixed grade in the emulsion phase (perfectly mixed and plug flow). The solution of the models includes mass (for the two monomers) and energy balances, coupled with the particle growth and residence time distribution models.

Modeling of particle segregation phenomena in a gas phase fluidized bed olefin polymerization reactor

In a gas phase fluidized bed olefin polymerization reactor, it is generally assumed that polymer particles are well mixed and near isothermal reaction conditions prevail. When a highly active Ziegler–Natta catalyst or other type of high activity supported catalyst is used in a gas phase fluidized bed reactor, a certain heterogeneity in the polymer properties is often observed in different size particles. Moreover, particle agglomeration and sheeting phenomena may also occur due to irregular particle growth, internal particle segregation or nonisothermal effect. In many of the past reports, particle residence time distribution has been commonly used as a tool to calculate polymer particle size distribution in a fluidized bed polyolefin reactor. In this paper, a multi-compartment population balance model is presented to directly model the particle segregation phenomena and particle size distribution in a gas phase olefin polymerization reactor. To model the particle segregation effects, size dependent particle transfer constants are employed. The effects of various fluidized bed operating conditions on the particle size distribution are investigated through model simulations.