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An Insight into the Temperature Field and Particle Flow Patterns in a Fluidized Bed Reactor for Non-Pelletizing Polyethylene Process using a 3D CFD-PBM Model Yu Che, Zhou Tian, Zhen Liu, Rui Zhang, Yuxin Gao, Enguang Zou, Sihan Wang, and Boping Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00596 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 19, 2016
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An Insight into the Temperature Field and Particle Flow Patterns in a Fluidized Bed Reactor for Non-Pelletizing Polyethylene Process using a 3D CFD-PBM Model Yu Chea, Zhou Tianb*, Zhen Liua, Rui Zhangc, Yuxin Gaoc, Enguang Zouc, Sihan Wangc, Boping Liua
a
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, P.R.China
b
Key Laboratory of Advanced Control and Optimization for Chemical Processes, Ministry of Education, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, P.R.China c
Daqing Petrochemical Research Center, Petrochemical Research Institute of PetroChina, Chengxiang Road 2, Daqing City, Heilongjiang Province 163714, P.R.China
Corresponding author: Tel: +86-021-64251250 E-mail:
[email protected];
Abstract: This work aims at exploring the temperature and polyethylene (PE) particle flow patterns in pilot-plant fluidized bed reactor via 3D CFD modeling approach. Eulerian-Eulerian model involving ethylene polymerization kinetics is integrated with population balance model to investigate the issues for both traditional pelletizing PE process (TPPP) and non-pelletizing PE process (NPPP). The results show that the regions with large temperature gradients have been observed in top area. The revealed particle flow patterns in two cases are analyzed using four typical flow patterns (Pattern a, b, c, and d) reported in literature. PE particles are vigorously contacted and mixed along the lateral and vertical planes of the reactor in TPPP (Pattern b, c, and d). Solid dispersion is intensely enhanced in the bed vertical direction (Pattern d), and the
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flow patterns show larger circular flow (Pattern a) in radial direction under NPPP due to the larger particle size and gas velocity. Keywords: fluidized bed reactor, 3D CFD model, temperature field, solid flow patterns, population balance
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1 Introduction Fluidized bed reactors (FBRs) are widely applied in polyolefin industry for their capabilities of high heat and mass transfer with uniform particle contacting and mixing state 1-3. Large scale FBRs integrating olefin polymerization always lead to higher bed expansion height and more complicated fluidization behaviors 4-6. The bed-expanding section (BES) is thereby introduced into the apparatuses to improve the particle flow patterns and achieve the desired reactor performance in industry
2, 7
. Meanwhile, new production process
(e.g. non-pelletizing PE process, NPPP) strongly affect the bed temperature field and particle flow patterns, which can lead to obvious differences of the reactor operating status 6, 8. Therefore, exploring the effects of PE production processes and BES region on the bed temperature and particle flow patterns is very essential for the purpose of design, operation and control of large scale FBR under NPPP. FBR is usually influenced by the production process itself and the bed hydrodynamic behaviors9. The chemical reaction efficiencies, transfer properties and energy consumptions always depend on the reactor temperature field and the solid mixing/contacting state, which relies on the particle flow patterns in FBRs. Currently, intensive researches have been conducted to explore the solid flow patterns in lab scale FBRs 1013
. Due to the complicated motions of particles in industrial scale FBRs, the clarification of particle motions
is virtually impossible through experimental methods. Herein, population balance model (PBM) combined with CFD modeling approach has been applied to study the solid particle motions in gas-solid FBRs 4, 5, 14-17. By means of CFD-PBM modeling, plenty of interesting aspects for olefin polymerization FBRs have been involved and performed. Fan et al. applied the coupled model to predict PE particle mixing and segregation behaviors18. Based on the model, Yan et al. developed a multi-scale modeling method, and polypropylene particle flow behaviors had been presented 16. Recently, we predicted the effects of ethylene polymerization and the particle kinetics (growth, aggregation and breakage) on PE particle flow and distribution behaviors using coupled CFD-PBM model in a pilot-plant FBR 6. Besides, rare investigation has been performed to explore the bed temperature field and the solid flow patterns within large scale FBR. Usually, the temperature field and solid polymer particle flow patterns reflect the operation performance of polymerization reactor 19. Fan et al. analyzed the bed hot spots and temperature changes induced by the 3 ACS Paragon Plus Environment
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polymerization reaction20. Chen et al. investigated the temperature field considering propylene polymerization under different operating conditions in a lab scale FBR 21. From the study of a pilot-plant FBR, we drew a conclusion that the bed temperature seriously affected the two-phase volume fractions along the radial/axial directions and the FBR performance 6. Meanwhile, the information of solid flow patterns in FBRs is more crucial for the operation and control of industrial apparatus. Recently, increased attention has been received in investigating the solid flow patterns in gas-solid FBRs through both experimentally and numerically 10, 12, 22-25. Khan et al. reviewed the CFD simulations in FBRs for polyolefin production, and the two-phase flow features have been summarized 26. Nevertheless, the polymer particle flow patterns have not been involved and covered. Numerous studies on bed hydrodynamics and polymerization reaction in FBRs have been concerned, but not much effort devoted to the performance investigation in olefin polymerization reactor. In addition, there is a fast growing interest in developing novel and economic polymer production process, such as NPPP (omitting the pelletizing steps compared to traditional pelletizing PE process, TPPP)7, 27-29
. It should be noted that, BES area with distinct solid motions exists in large scale FBR, exploring the
local and averaged solid flow patterns in the region is the prerequisite to understand the intrinsic mechanisms of solid particle motions. However, the investigations are fairly insufficient for predicting the temperature variation and particle flow patterns, especially in pilot-plant scale olefin polymerization FBR. Moreover, there are no open literatures regarding the effects of NPPP and BES region on the solid flow patterns and properties with either experimental or numerical approach. The current work aims to investigate the reactor temperature field and the polymer particle flow patterns in a pilot-plant ethylene polymerization FBR through a 3D CFD-PBM coupling approach. The bed pressure drop and temperature data are validated by the industrial data. Under TPPP and NPPP, the axial temperature distributions along the bed height direction have been predicted and analyzed. The solid flow patterns are clarified by contrasting the simulated results with the experimental observation four types of flow patterns to obtain the local and averaged solid motions for two processes. In addition, the particle flow patterns have been used to predict the bed operating performance. The time-averaged solid flow properties are applied to evaluate the effects of BES on the solid flow patterns and FBR operation state for both TPPP and NPPP. 4 ACS Paragon Plus Environment
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2 Numerical model description 2.1 Eulerian-Eulerian model with kinetic theory of granular flow In this work, two-fluid model based on the Eulerian-Eulerian approach is employed to describe the bed hydrodynamics. Closures of solid phase are required for the internal momentum transfer in particulate solid phase (solid phase viscosity and pressure gradient). To close the conservation equations, the kinetic theory of granular flow (KTGF) is proposed to cover several modeled terms (stress tensors, solid phase bulk and shear viscosity, radial distribution function, etc.) 30, 31. The continuity, momentum and energy equations for the gas and solid phases and the auxiliary relations are shown in Table S1 in Supporting Information. The
modeling details were shown in the previous work6. The models have been validated using the experimental data in our previous study 7. 2.2 Turbulence model The disperse phase RNG k-ε turbulence model is chosen, and it is identical to that of our previous work 7. According to Hartge et al., the turbulence model gives better agreement with the experimental results than per-phase turbulence model in case of FBR modeling 32. Hence, the disperse phase RNG k-ε turbulence model is selected in the 3-Dimensional numerical simulation based on the coupled CFD-PBM model. 2.3 Drag model shown in conservation momentum equations can be estimated by empirical drag law, it has critical effect on the hydrodynamics of FBRs. The widely used drag correlation models include Gidaspow model, Syamlal-O’Brien model, and Wen-Yu model. Simulations were performed using different drag models 7
, and the significant differences between them were found in the bubble flow status, mean particle
velocity, and solid holdups. It is found that Gidaspow model showed a good agreement with the industrial data7, so it was still used in this work.
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2.4 Particle growth rate According to the expression of polymer particle growth rate proposed by Hatzantonis et al.
33, 34
, the
particle growth rate can be written in term of overall particle polymerization rate ( ), which is the same as that applied in our previous work 6. It can be given as, G =
=
(1)
The polymerization kinetics of gas phase ethylene polymerization over the heterogeneous Ziegler-Natta catalysts has been the subject of numerous studies for the past decades 35, 36. The kinetics scheme comprises of series of elementary reactions including site activation, chains propagation, transfer and deactivation reactions. In order to describe the kinetics of ethylene polymerization in CFD modeling, a kinetics model containing the mainly elementary chain propagation reaction is adopted and the equations are listed as, = ∗
(2)
where is the monomer concentration, 1.0×104 mol·m-3, ∗ is the concentration of catalyst active site, 0.18 mol·kg-1, is the reaction rate constant, and it can be expressed as, $%& ) '& (
= !" #
(3)
2.5 Heat transfer model In gas phase catalytic ethylene polymerization, the rate of energy transfer between gas and solid phases is so important that it can largely influence the temperature distribution of the bed radial and axial directions. The heat transfer model can be obtained as a function of temperature difference, * = ℎ ,- − - / ℎ =
(4)
01' 2' 2 34
(5)
where ℎ (=ℎ ) is the heat transfer coefficient between two phases. 56 denotes the solid phase Nusselt number. The well-known Ranz-Marshall correlation for the dimensionless heat transfer coefficients is applied in the present work, and it can be written as, 56 = 2 + 0.6⁄
>⁄@
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(6)
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