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Computational Fluid Dynamics Modeling of Temperature Distribution in Fluidized Bed Polymerization Reactor for Polypropylene Production
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Energy (2017) 000–000 901–906 EnergyProcedia Procedia138 00 (2017) www.elsevier.com/locate/procedia
2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies 2017 International Conference on Alternative in Developing 2017 AEDCEE, 25‐26Energy May 2017, Bangkok,Countries Thailand and Emerging Economies 2017 AEDCEE, 25‐26 May 2017, Bangkok, Thailand
The 15th International Symposium on District Heating and Cooling Computational Fluid Dynamics Modeling of Temperature Computational Fluid Dynamics Modeling of Temperature Distribution in Fluidized Bed Polymerization Reactor for Assessing theinfeasibility using the heat demand-outdoor Distribution Fluidized of Bed Polymerization Reactor for Polypropylene Production temperature function for a long-term district heat demand forecast Polypropylene Production a, Nawaphat Jongpaijita a, Pornchai Bumroongsri *c c a J. Fournierb., B. Lacarrière I. Andrića,b,c*,Nawaphat A. Pinaa, P. Ferrão , Jongpaijit , Pornchai Bumroongsria,* , O. Le Corre
a Department of Chemical Engineering, Faculty of Engineering, Mahidol University, Salaya Campus,Nakhon Pathom 73170 Thailand IN+ Center for Technology Faculty and Policy Research - Instituto Técnico, Av.Campus,Nakhon Rovisco Pais 1,Pathom 1049-001 Lisbon, Portugal Department of Innovation, Chemical Engineering, of Engineering, MahidolSuperior University, Salaya 73170 Thailand b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
aa
Abstract Abstract Propylene polymerization process is one of the chemical processes that take place in the fluidized bed reactor. The gas monomer is fed Abstract through thepolymerization distributor toprocess react with theof solid catalyst processes particles that inside theplace reactor. polymer are gas produced from the Propylene is one the chemical take in theThe fluidized bedparticles reactor. The monomer is fed heterogeneous exothermic Although the fluidized reactor has high heat mass transfer through the distributor topolymerization react with thereaction. solid catalyst particles insidebed thepolymerization reactor. The polymer particles areand produced fromrate, the District networks are limited commonly in the literature as polymerization one of themixing. most effective solutions for decreasing the the thermalheating efficiency is usually due toaddressed theAlthough formation offluidized hot spotsbed and improper In this research, a computational heterogeneous exothermic polymerization reaction. the reactor has high heat and mass transferfluid rate, greenhouse gas model emissions from the building sector. Theseof systems high investments are returned through thefluid heat dynamics the fluidized bedtopolymerization reactor forrequire polypropylene production is developed order to study the the thermal(CFD) efficiency is of usually limited due the formation hot spots and improper mixing. Inwhich this research, aincomputational sales. Due to the changed conditions and building renovation policies, heat demand inincluded the future could decrease, temperature distribution within the reactor. polymer growth rate particle sizeproduction distribution the CFD model dynamics (CFD) model of the climate fluidized bedThe polymerization reactor forand polypropylene is are developed ininorder to study the prolonging the investment return period. development. The heat iswithin generated from highly exothermic polymerization reactionsize anddistribution removed inare theincluded heat exchanger. By proper temperature distribution the reactor. The polymer growth rate and particle in the CFD model The main thisfeed is to assess the feasibility of using the heatinside demand outdoor temperature function for heat demand adjusting thescope monomer velocity, the uniform temperature distribution the –reactor is obtained in heat axial and radial directions. development. The of heat ispaper generated from highly exothermic polymerization reaction and removed in the exchanger. By proper forecast.the The district of Alvalade, located in temperature Lisbon used the as reactor aexternal case isstudy. The is consisted of 665 Additionally, the temperature of the outlet gas stream can be(Portugal), reduced, thuswas reducing the cooling duty. adjusting monomer feed velocity, the uniform distribution inside obtained indistrict axial and radial directions. buildings in bothofconstruction typology. weatherthe scenarios (low, medium, © 2017 Thethat Authors. Published byoutlet Elsevier Ltd. and Additionally, the vary temperature the gasperiod stream can be reduced,Three thus reducing external cooling duty. high) and three district ©renovation 2017 The Authors. Published by Ltd. intermediate, scenarios were developed (shallow, To estimate the error, obtained heat demand values were Peer-review responsibility ofElsevier the Organizing 2017 AEDCEE. © 2017 The under Authors. Published by Elsevier Ltd. Committee ofdeep). Peer-review under responsibility of the scientific committee of the 2017 developed International Conference on Alternative Energy in compared with results from a dynamic heat demand model, previously Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE. and validated by the authors. DThe eveloping Countries and Emerging Economies. results showed that when only weather is considered, thetemperature margin ofdistribution; error couldexothermic be acceptable for some applications Keywords: Computational fluid dynamics; fluidizedchange bed reactor; cooling duty; polymerization reaction. (the errorComputational in annual demand was lower than bed 20%reactor; for allcooling weather scenarios considered). introducing renovation Keywords: fluid dynamics; fluidized duty; temperature distribution;However, exothermic after polymerization reaction. scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1.The Introduction in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1.decrease Introduction renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the Fluidized bed reactors have been extensively used in the olefin polymerization. In gas phase polymerization, the small coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and Fluidizedparticles bed reactors have been used infed thethrough olefin polymerization. In gas polymerization, theoccurs, small catalyst are reacted withextensively theestimations. gas monomer the distributor. As the phase polymerization reaction improve the accuracy of heat demand
catalyst particles are reacted with the gas monomer fed through the distributor. As the polymerization reaction occurs, © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +6-628-892-138 ext 6101 ; fax: +6-624-419-731 . Cooling.
E-mail address:author. [email protected] * Corresponding Tel.: +6-628-892-138 ext 6101 ; fax: +6-624-419-731 . E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review the Organizing Committee 1876-6102 ©under 2017responsibility The Authors. of Published by Elsevier Ltd. of 2017 AEDCEE. Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies. 10.1016/j.egypro.2017.10.133
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Nawaphat Jongpaijit et al. / Energy Procedia 138 (2017) 901–906 Nawaphat Jongpaijit and Pornchai Bumroongsri / Energy Procedia 00 (2017) 000–000
the small catalyst particles are encapsulated by the growing polymer. The fully-grown polymer particles are removed from the bottom of the reactor. The polymerization reaction is highly exothermic so the exothermic heat should be efficiently removed in order to avoid the formation of hot spots. Due to complicated gas-solid interactions, a better understanding of hydrodynamic behavior inside the fluidized bed reactor is required. Computational fluid dynamics (CFD) simulations have received many attentions in simulating the two-phase flow. Behjat et al [1] applied the CFD techniques to investigate the hydrodynamics and heat transfer phenomena in fluidized bed reactors. The Eulerian-Eulerian model was used in the simulation of two-phase flow. It was found that the gas temperature increased with bed height due to the heat of polymerization reaction. Dehnavi et al [2] studied the hydrodynamics and temperature distributions of a two-dimensional (2D) gas-solid fluidized bed reactor. The CFD model in this work was based on a two-fluid model considering both phases to be continuous and interpenetrating. The simulation results showed that the bed temperature gradient in the primary section of the bed was higher than the top. The effects of different drag models were studied. However, the particle growth was not considered in the CFD simulation. Particle growth is one of the important parameters in the simulation of polymerization reactors. In Che et al [3], a 2D pilot-plant fluidized bed reactor for ethylene polymerization was considered in the CFD simulation. The EulerianEulerian model was coupled with a population balance model (PBM) to investigate the effects of particle size distribution. The predicted pressure drop and temperature distribution agreed well with the available industrial data. Akbari et al. [4] studied the hydrodynamics and particle growth in an industrial gas phase fluidized bed polymerization reactor. A 2D CFD-PBM coupled model was developed to illustrate the effect of particle growth rate. The simulation results showed that large particles exist at the bottom of the bed while small particles tend to migrate to the top of the bed. The main assumption for the 2D simulation of fluidized bed reactors lies in the fact that the geometry is axisymmetric. To better simulate the fluidized bed reactors, 3D simulation can be considered. Chen et al. [5] developed a 3D CFD-PBM coupled model to describe the gas-solid two-phase flow in a pilot-scale fluidized bed polymerization reactor. The influences of operational conditions on the temperature filed were studied. The simulation results showed that the temperature of solid phase increased along the axial direction from the bottom to the top of the reactor. The solid temperature near the wall was higher than that in other positions of the reactor. In Li et al. [6], a 3D CFD model coupled with the discrete element model (DEM) was developed to study the heat transfer behavior in a pilot-scale fluidized bed reactor. A constant volumetric heat production was considered in the particle energy equation to mimic the heat of polymerization. It was found that more homogeneous temperature distribution was obtained as the superficial gas velocity increased. In the real plant operation, however, the development of an industrial-scale CFD model for fluidized bed polymerization reactors is required. In this paper, a 3D CFD-PBM coupled model of an industrial-scale fluidized bed polymerization reactor for polypropylene production is developed. The developed model is used to study the temperature distribution within the reactor. The particle growth, aggregation and breakage are included in the developed model. The paper is organized as follows. The geometry and operating condition of an industrial-scale fluidized bed polymerization reactor is presented in Section 2. In Section 3, the modeling equations are described. The simulation results are presented in Section 4. The conclusions are drawn in Section 5. Nomenclature Bag (L; x, t )
birth rate due to aggregation birth rate due to breakage
ps
particulate phase pressure
qi
concentration of catalyst active site
Q gs
heat flux of i th phase intensity of heat exchange between gas and solid phase
coefficients in turbulence model death rate due to aggregation
R gas
gas constant
Dbr (L; x, t )
Rp
polymerization reaction rate
death rate due to breakage
Ti
temperature of i
Ea
activation energy
Bbr (L; x, t )
[C ] *
C1ε , C 2ε
Dag (L; x, t )
r u
th
phase
particle growth rate vector
Nawaphat Jongpaijit and Pornchai Bumroongsri / Energy Procedia (2017) 000–000 Nawaphat Jongpaijit et al. / Energy Procedia 138 00 (2017) 901–906
g
gravitational acceleration
G Gk , m
particle growth rate production of turbulent kinetic energy
vg
r vm vs
gas velocity velocity vector of system m solid velocity
hi
specific enthalpy of i
k
turbulence kinetic energy
Greek notations αg volume fraction of gas phase
kp
propagation rate
αs
volume fraction of solid phase
k 0p
frequency factor of propagation rate
ε
turbulence dissipation rate
K gs
momentum exchange coefficient
µ t ,m
frictional viscosity of system m
Lo
initial particle diameter
σε
granular kinetic theory parameter
Li
th
particle diameter of i size
τg
shear stress of gas phase
L
particle diameter
τs
shear stress of solid phase
ρg
gas density density of system m
monomer concentration
ρm ρs
solid density
mass transfer between gas-solid phases
∆Q rsα
heat of polymerization reaction
th
phase
mk
k
m2
second moment of distribution
[M ] & gs , m& sg m
n(L; x, t ) p
th
moment of distribution
3 903
number density function pressure
2. Geometry and operating condition of the industrial-scale fluidized bed polymerization reactor The sketch of the fluidized bed polymerization reactor is shown in Fig. 1a. The height and diameter of the reactor are 33.9 and 5 m, respectively [4]. The monomer is fed to the reactor at the bottom. The initial bed height is 10 m. From the sketch of the fluidized bed polymerization reactor, the 3D geometry is considered in this work as shown in Fig. 1b. The geometry can be meshed as shown in Fig. 1c for further computation.
Fig. 1. Fluidized bed polymerization reactor (a) the sketch; (b) the 3D geometry; (c) the mesh.
The operating conditions of the fluidized bed polymerization reactor are shown in Table 1.
Nawaphat Jongpaijit et al. / Energy Procedia 138 (2017) 901–906 Nawaphat Jongpaijit and Pornchai Bumroongsri / Energy Procedia 00 (2017) 000–000
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Table 1. The operating conditions of the fluidized bed polymerization reactor. Description Value Unit Description Gas
Value
Unit kg/m 3
Solid kg/m 3
Particle density
910
1.081x10-5
Pa.s
Particle heat capacity
2,104
J/kg.K
Gas heat capacity
1,817
J/kg.K
Initial bed height
10
m
Inlet gas velocity
0.5
m/s
Initial solid volume fraction
0.63
-
Inlet temperature
313
K
Operating pressure
24
bar
Gas density
21.56
Gas viscosity
3. Modeling equations This work is based on a two-fluid model (TFM) coupled with the polymerization kinetics and population balance model (PBM) as shown in Table 2. In TFM, both phases are considered to be continuous and fully interpenetrating. The polymerization characteristics are described by the polymerization kinetics. The population balance model is used to describe the continuously varied particle size distribution. Table 2. The modeling equations [3, 5]. Descriptions
Equations
Eulerian-Eulerian Two-Fluid Model Continuity equation Gas phase Solid phase
Momentum equation Gas phase Solid phase
Turbulent equation
Energy equation Gas phase Solid phase
Polymerization kinetics Population balance model Moment of distribution Particle growth
(
) (
(
)
)
r ∂ α ρ + ∇ ⋅ α g ρ g vg = m& gs ∂t g g r ∂ α ρ + ∇ ⋅ αs ρsvs = m& gs ∂t s s
(
(1)
)
(2)
∂ (αg ρg vrg ) + ∇ ⋅ (αg ρg vrg vrg=) −αg ∇p + ∇ ⋅τ g + K gs (vrs − vrg ) + αg ρg g ∂t r r r ∂ (αs ρs vrs ) + ∇ ⋅ (αs ρsvrs= vs ) −αs∇p + ∇ps + ∇ ⋅τ s + Kgs (vg − vs ) + αs ρs g ∂t
(3) (4)
µ t ,m r ∇ ⋅ (ρ m kv m=) ∇ ⋅ ∇k + G k , m − ρ m ε σε µt ,m ε r ∇ ⋅ (ρmεvm=) ∇⋅ ∇ε + (C1ε Gk,m − C2ε ρ mε ) σε k
(5) (6)
∂pg n r ∂ & gshgs − m & sg hsg αg ρg hg + ∇ ⋅ αg ρg vg hg= −αg + τ g : ∇vg − ∇ ⋅ qg + ∑ Qgs + m p=1 ∂t ∂t n ∂ (αs ρshs ) + ∇⋅ (αs ρsvshs=) −αs ∂ps +τ s : ∇vrs − ∇⋅ qs + ∑(Qsg + m& sghsg − m& gshgs ) + ∆Qrsα ∂t ∂t p=1
(
)
(
[ ]
R p = k p [M ] C , k p = *
)
k 0p
(
(
exp − Ea /( RgasT )
)
)
r ∂n(L; x, t ) + ∇ ⋅ [un(L; x, t= )] − ∂ [G(L)n(L; x, t )] + Bag (L; x, t ) − Dag (L; x, t ) + Bbr (L; x, t ) − Dbr (L; x, t ) ∂t ∂L ∞
mk (x, t ) = ∫ n (L; x, t )Lk dL k = 0,1,..., N − 1
(7) (8) (9) (10) (11)
0
G (Li ) =
3 d (Li ) R p L0 = dt 3ρ s L2i
(12)
Nawaphat Jongpaijit et al. / Energy Procedia 138 (2017) 901–906 Nawaphat Jongpaijit and Pornchai Bumroongsri / Energy Procedia 00 (2017) 000–000
905 5
4. Simulation results In the simulation, the monomer is fed to the fluidized bed reactor at the bottom of the reactor. The contours of solid volume fraction of the polymer particles are shown in Fig. 2. From an initial bed height of 10 m, the bed expands as the time increases until the final bed height of 20 m is reached.
Fig. 2. The contours of solid volume fraction in the fluidized bed polymerization reactor.
Figure 3 shows the contours of the temperature at different values of bed height after the stable bed height has been reached. It can be seen that the values of temperature are uniform indicating that high heat transfer rate can be achieved despite high exothermic heat of polymerization.
Fig. 3. The temperature contours at different values of bed height.
Figure 4 shows the values of bed temperature along the axial distance as the monomer feed velocity is varied. It can be observed that more uniform temperature distribution is obtained as the monomer feed velocity increases.
Fig. 4. The values of bed temperature along the axial distance as the monomer feed velocity is varied.
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Nawaphat Jongpaijit et al. / Energy Procedia 138 (2017) 901–906 Nawaphat Jongpaijit and Pornchai Bumroongsri / Energy Procedia 00 (2017) 000–000
The temperature of the outlet stream at the top of the fluidized bed polymerization reactor must be reduced to 313 K using the external cooling before recycling back to the reactor inlet. Table 3 shows the calculated external cooling duty. It is seen that the external cooling duty is reduced as the monomer feed velocity is increased from 0.3 to 0.5 m/s. Moreover, more uniform temperature distribution is obtained as shown in Fig.4. Table 3. The external cooling duty. Monomer feed velocity (m/s)
External cooling duty per monomer feed (kJ/kg)
0.3
70.86
0.5
63.59
5. Conclusions This paper presents the CFD modeling of the fluidized bed polymerization reactor for polypropylene production. The developed CFD model is based on a two-fluid model (TFM) coupled with the polymerization kinetics and population balance model (PBM). The solid volume fraction and the temperature distribution are computed. The simulation results show that the temperature distribution in the fluidized bed polymerization reactor is uniform indicating that high heat transfer rate can be achieved despite high exothermic heat of polymerization. The external cooling duty in the recycle loop can be reduced by proper adjusting the monomer feed velocity. Acknowledgements This research project was supported by Mahidol University and the Thailand Research Fund. References
[1] Behjat Y, Shahhosseini S, Hashemabadi S H. CFD modeling of hydrodynamic and heat transfer in fluidized bed reactors. International Communications in Heat and Mass Transfer 2008, 35(3), p. 357-368. [2] Dehnavi M, Shahhosseini S, Hashemabadi S H, Ghafelebashi S M. CFD simulation of hydrodynamics and heat transfer in gas phase ethylene polymerization reactors. International Communications in Heat and Mass Transfer 2010, 37(4), p. 437-442. [3] Che Y, Tian Z, Liu Z, Zhang R, Gao Y. A CFD-PBM model considering ethylene polymerization for the flow behaviors and particle size distribution of polyethylene in a pilot-plant fluidized bed reactor. Powder Technology 2015, 286, p. 107-123. [4] Akbari V, Borhani T N G, Shamiri A, Aramesh R, Hussain M A, Hamid M.K.A. 2D CFD-PBM simulation of hydrodynamic and particle growth in an industrial gas phase fluidized bed polymerization reactor. Chemical Engineering Research and Design 2015, 104, p. 53-67. [5] Chen X Z, Luo Z H, Yan W C, Lu Y H, Ng I S. Three-Dimensional CFD-PBM Coupled Model of the Temperature Fields in Fluidized-Bed Polymerization Reactors. AIChe Journal 2011, 57, p. 3351-3366. [6] Li Z, Annaland M V S, Kuipers J A M, Deen N G. Effect of operating pressure on particle temperature distribution in a fluidized bed with heat production. Chemical Engineering Science 2016, 104, p. 279-290.
ScienceDirect ScienceDirect Availableonline onlineatatwww.sciencedirect.com www.sciencedirect.com Available Energy Procedia 00 (2017) 000–000 Energy Procedia 00 (2017) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy (2017) 000–000 901–906 EnergyProcedia Procedia138 00 (2017) www.elsevier.com/locate/procedia
2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies 2017 International Conference on Alternative in Developing 2017 AEDCEE, 25‐26Energy May 2017, Bangkok,Countries Thailand and Emerging Economies 2017 AEDCEE, 25‐26 May 2017, Bangkok, Thailand
The 15th International Symposium on District Heating and Cooling Computational Fluid Dynamics Modeling of Temperature Computational Fluid Dynamics Modeling of Temperature Distribution in Fluidized Bed Polymerization Reactor for Assessing theinfeasibility using the heat demand-outdoor Distribution Fluidized of Bed Polymerization Reactor for Polypropylene Production temperature function for a long-term district heat demand forecast Polypropylene Production a, Nawaphat Jongpaijita a, Pornchai Bumroongsri *c c a J. Fournierb., B. Lacarrière I. Andrića,b,c*,Nawaphat A. Pinaa, P. Ferrão , Jongpaijit , Pornchai Bumroongsria,* , O. Le Corre
a Department of Chemical Engineering, Faculty of Engineering, Mahidol University, Salaya Campus,Nakhon Pathom 73170 Thailand IN+ Center for Technology Faculty and Policy Research - Instituto Técnico, Av.Campus,Nakhon Rovisco Pais 1,Pathom 1049-001 Lisbon, Portugal Department of Innovation, Chemical Engineering, of Engineering, MahidolSuperior University, Salaya 73170 Thailand b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France
aa
Abstract Abstract Propylene polymerization process is one of the chemical processes that take place in the fluidized bed reactor. The gas monomer is fed Abstract through thepolymerization distributor toprocess react with theof solid catalyst processes particles that inside theplace reactor. polymer are gas produced from the Propylene is one the chemical take in theThe fluidized bedparticles reactor. The monomer is fed heterogeneous exothermic Although the fluidized reactor has high heat mass transfer through the distributor topolymerization react with thereaction. solid catalyst particles insidebed thepolymerization reactor. The polymer particles areand produced fromrate, the District networks are limited commonly in the literature as polymerization one of themixing. most effective solutions for decreasing the the thermalheating efficiency is usually due toaddressed theAlthough formation offluidized hot spotsbed and improper In this research, a computational heterogeneous exothermic polymerization reaction. the reactor has high heat and mass transferfluid rate, greenhouse gas model emissions from the building sector. Theseof systems high investments are returned through thefluid heat dynamics the fluidized bedtopolymerization reactor forrequire polypropylene production is developed order to study the the thermal(CFD) efficiency is of usually limited due the formation hot spots and improper mixing. Inwhich this research, aincomputational sales. Due to the changed conditions and building renovation policies, heat demand inincluded the future could decrease, temperature distribution within the reactor. polymer growth rate particle sizeproduction distribution the CFD model dynamics (CFD) model of the climate fluidized bedThe polymerization reactor forand polypropylene is are developed ininorder to study the prolonging the investment return period. development. The heat iswithin generated from highly exothermic polymerization reactionsize anddistribution removed inare theincluded heat exchanger. By proper temperature distribution the reactor. The polymer growth rate and particle in the CFD model The main thisfeed is to assess the feasibility of using the heatinside demand outdoor temperature function for heat demand adjusting thescope monomer velocity, the uniform temperature distribution the –reactor is obtained in heat axial and radial directions. development. The of heat ispaper generated from highly exothermic polymerization reaction and removed in the exchanger. By proper forecast.the The district of Alvalade, located in temperature Lisbon used the as reactor aexternal case isstudy. The is consisted of 665 Additionally, the temperature of the outlet gas stream can be(Portugal), reduced, thuswas reducing the cooling duty. adjusting monomer feed velocity, the uniform distribution inside obtained indistrict axial and radial directions. buildings in bothofconstruction typology. weatherthe scenarios (low, medium, © 2017 Thethat Authors. Published byoutlet Elsevier Ltd. and Additionally, the vary temperature the gasperiod stream can be reduced,Three thus reducing external cooling duty. high) and three district ©renovation 2017 The Authors. Published by Ltd. intermediate, scenarios were developed (shallow, To estimate the error, obtained heat demand values were Peer-review responsibility ofElsevier the Organizing 2017 AEDCEE. © 2017 The under Authors. Published by Elsevier Ltd. Committee ofdeep). Peer-review under responsibility of the scientific committee of the 2017 developed International Conference on Alternative Energy in compared with results from a dynamic heat demand model, previously Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE. and validated by the authors. DThe eveloping Countries and Emerging Economies. results showed that when only weather is considered, thetemperature margin ofdistribution; error couldexothermic be acceptable for some applications Keywords: Computational fluid dynamics; fluidizedchange bed reactor; cooling duty; polymerization reaction. (the errorComputational in annual demand was lower than bed 20%reactor; for allcooling weather scenarios considered). introducing renovation Keywords: fluid dynamics; fluidized duty; temperature distribution;However, exothermic after polymerization reaction. scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the 1.The Introduction in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and 1.decrease Introduction renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the Fluidized bed reactors have been extensively used in the olefin polymerization. In gas phase polymerization, the small coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and Fluidizedparticles bed reactors have been used infed thethrough olefin polymerization. In gas polymerization, theoccurs, small catalyst are reacted withextensively theestimations. gas monomer the distributor. As the phase polymerization reaction improve the accuracy of heat demand
catalyst particles are reacted with the gas monomer fed through the distributor. As the polymerization reaction occurs, © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +6-628-892-138 ext 6101 ; fax: +6-624-419-731 . Cooling.
E-mail address:author. [email protected] * Corresponding Tel.: +6-628-892-138 ext 6101 ; fax: +6-624-419-731 . E-mail address: [email protected] Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review the Organizing Committee 1876-6102 ©under 2017responsibility The Authors. of Published by Elsevier Ltd. of 2017 AEDCEE. Peer-review under responsibility of the Organizing Committee of 2017 AEDCEE.
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 2017 International Conference on Alternative Energy in Developing Countries and Emerging Economies. 10.1016/j.egypro.2017.10.133
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Nawaphat Jongpaijit et al. / Energy Procedia 138 (2017) 901–906 Nawaphat Jongpaijit and Pornchai Bumroongsri / Energy Procedia 00 (2017) 000–000
the small catalyst particles are encapsulated by the growing polymer. The fully-grown polymer particles are removed from the bottom of the reactor. The polymerization reaction is highly exothermic so the exothermic heat should be efficiently removed in order to avoid the formation of hot spots. Due to complicated gas-solid interactions, a better understanding of hydrodynamic behavior inside the fluidized bed reactor is required. Computational fluid dynamics (CFD) simulations have received many attentions in simulating the two-phase flow. Behjat et al [1] applied the CFD techniques to investigate the hydrodynamics and heat transfer phenomena in fluidized bed reactors. The Eulerian-Eulerian model was used in the simulation of two-phase flow. It was found that the gas temperature increased with bed height due to the heat of polymerization reaction. Dehnavi et al [2] studied the hydrodynamics and temperature distributions of a two-dimensional (2D) gas-solid fluidized bed reactor. The CFD model in this work was based on a two-fluid model considering both phases to be continuous and interpenetrating. The simulation results showed that the bed temperature gradient in the primary section of the bed was higher than the top. The effects of different drag models were studied. However, the particle growth was not considered in the CFD simulation. Particle growth is one of the important parameters in the simulation of polymerization reactors. In Che et al [3], a 2D pilot-plant fluidized bed reactor for ethylene polymerization was considered in the CFD simulation. The EulerianEulerian model was coupled with a population balance model (PBM) to investigate the effects of particle size distribution. The predicted pressure drop and temperature distribution agreed well with the available industrial data. Akbari et al. [4] studied the hydrodynamics and particle growth in an industrial gas phase fluidized bed polymerization reactor. A 2D CFD-PBM coupled model was developed to illustrate the effect of particle growth rate. The simulation results showed that large particles exist at the bottom of the bed while small particles tend to migrate to the top of the bed. The main assumption for the 2D simulation of fluidized bed reactors lies in the fact that the geometry is axisymmetric. To better simulate the fluidized bed reactors, 3D simulation can be considered. Chen et al. [5] developed a 3D CFD-PBM coupled model to describe the gas-solid two-phase flow in a pilot-scale fluidized bed polymerization reactor. The influences of operational conditions on the temperature filed were studied. The simulation results showed that the temperature of solid phase increased along the axial direction from the bottom to the top of the reactor. The solid temperature near the wall was higher than that in other positions of the reactor. In Li et al. [6], a 3D CFD model coupled with the discrete element model (DEM) was developed to study the heat transfer behavior in a pilot-scale fluidized bed reactor. A constant volumetric heat production was considered in the particle energy equation to mimic the heat of polymerization. It was found that more homogeneous temperature distribution was obtained as the superficial gas velocity increased. In the real plant operation, however, the development of an industrial-scale CFD model for fluidized bed polymerization reactors is required. In this paper, a 3D CFD-PBM coupled model of an industrial-scale fluidized bed polymerization reactor for polypropylene production is developed. The developed model is used to study the temperature distribution within the reactor. The particle growth, aggregation and breakage are included in the developed model. The paper is organized as follows. The geometry and operating condition of an industrial-scale fluidized bed polymerization reactor is presented in Section 2. In Section 3, the modeling equations are described. The simulation results are presented in Section 4. The conclusions are drawn in Section 5. Nomenclature Bag (L; x, t )
birth rate due to aggregation birth rate due to breakage
ps
particulate phase pressure
qi
concentration of catalyst active site
Q gs
heat flux of i th phase intensity of heat exchange between gas and solid phase
coefficients in turbulence model death rate due to aggregation
R gas
gas constant
Dbr (L; x, t )
Rp
polymerization reaction rate
death rate due to breakage
Ti
temperature of i
Ea
activation energy
Bbr (L; x, t )
[C ] *
C1ε , C 2ε
Dag (L; x, t )
r u
th
phase
particle growth rate vector
Nawaphat Jongpaijit and Pornchai Bumroongsri / Energy Procedia (2017) 000–000 Nawaphat Jongpaijit et al. / Energy Procedia 138 00 (2017) 901–906
g
gravitational acceleration
G Gk , m
particle growth rate production of turbulent kinetic energy
vg
r vm vs
gas velocity velocity vector of system m solid velocity
hi
specific enthalpy of i
k
turbulence kinetic energy
Greek notations αg volume fraction of gas phase
kp
propagation rate
αs
volume fraction of solid phase
k 0p
frequency factor of propagation rate
ε
turbulence dissipation rate
K gs
momentum exchange coefficient
µ t ,m
frictional viscosity of system m
Lo
initial particle diameter
σε
granular kinetic theory parameter
Li
th
particle diameter of i size
τg
shear stress of gas phase
L
particle diameter
τs
shear stress of solid phase
ρg
gas density density of system m
monomer concentration
ρm ρs
solid density
mass transfer between gas-solid phases
∆Q rsα
heat of polymerization reaction
th
phase
mk
k
m2
second moment of distribution
[M ] & gs , m& sg m
n(L; x, t ) p
th
moment of distribution
3 903
number density function pressure
2. Geometry and operating condition of the industrial-scale fluidized bed polymerization reactor The sketch of the fluidized bed polymerization reactor is shown in Fig. 1a. The height and diameter of the reactor are 33.9 and 5 m, respectively [4]. The monomer is fed to the reactor at the bottom. The initial bed height is 10 m. From the sketch of the fluidized bed polymerization reactor, the 3D geometry is considered in this work as shown in Fig. 1b. The geometry can be meshed as shown in Fig. 1c for further computation.
Fig. 1. Fluidized bed polymerization reactor (a) the sketch; (b) the 3D geometry; (c) the mesh.
The operating conditions of the fluidized bed polymerization reactor are shown in Table 1.
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Table 1. The operating conditions of the fluidized bed polymerization reactor. Description Value Unit Description Gas
Value
Unit kg/m 3
Solid kg/m 3
Particle density
910
1.081x10-5
Pa.s
Particle heat capacity
2,104
J/kg.K
Gas heat capacity
1,817
J/kg.K
Initial bed height
10
m
Inlet gas velocity
0.5
m/s
Initial solid volume fraction
0.63
-
Inlet temperature
313
K
Operating pressure
24
bar
Gas density
21.56
Gas viscosity
3. Modeling equations This work is based on a two-fluid model (TFM) coupled with the polymerization kinetics and population balance model (PBM) as shown in Table 2. In TFM, both phases are considered to be continuous and fully interpenetrating. The polymerization characteristics are described by the polymerization kinetics. The population balance model is used to describe the continuously varied particle size distribution. Table 2. The modeling equations [3, 5]. Descriptions
Equations
Eulerian-Eulerian Two-Fluid Model Continuity equation Gas phase Solid phase
Momentum equation Gas phase Solid phase
Turbulent equation
Energy equation Gas phase Solid phase
Polymerization kinetics Population balance model Moment of distribution Particle growth
(
) (
(
)
)
r ∂ α ρ + ∇ ⋅ α g ρ g vg = m& gs ∂t g g r ∂ α ρ + ∇ ⋅ αs ρsvs = m& gs ∂t s s
(
(1)
)
(2)
∂ (αg ρg vrg ) + ∇ ⋅ (αg ρg vrg vrg=) −αg ∇p + ∇ ⋅τ g + K gs (vrs − vrg ) + αg ρg g ∂t r r r ∂ (αs ρs vrs ) + ∇ ⋅ (αs ρsvrs= vs ) −αs∇p + ∇ps + ∇ ⋅τ s + Kgs (vg − vs ) + αs ρs g ∂t
(3) (4)
µ t ,m r ∇ ⋅ (ρ m kv m=) ∇ ⋅ ∇k + G k , m − ρ m ε σε µt ,m ε r ∇ ⋅ (ρmεvm=) ∇⋅ ∇ε + (C1ε Gk,m − C2ε ρ mε ) σε k
(5) (6)
∂pg n r ∂ & gshgs − m & sg hsg αg ρg hg + ∇ ⋅ αg ρg vg hg= −αg + τ g : ∇vg − ∇ ⋅ qg + ∑ Qgs + m p=1 ∂t ∂t n ∂ (αs ρshs ) + ∇⋅ (αs ρsvshs=) −αs ∂ps +τ s : ∇vrs − ∇⋅ qs + ∑(Qsg + m& sghsg − m& gshgs ) + ∆Qrsα ∂t ∂t p=1
(
)
(
[ ]
R p = k p [M ] C , k p = *
)
k 0p
(
(
exp − Ea /( RgasT )
)
)
r ∂n(L; x, t ) + ∇ ⋅ [un(L; x, t= )] − ∂ [G(L)n(L; x, t )] + Bag (L; x, t ) − Dag (L; x, t ) + Bbr (L; x, t ) − Dbr (L; x, t ) ∂t ∂L ∞
mk (x, t ) = ∫ n (L; x, t )Lk dL k = 0,1,..., N − 1
(7) (8) (9) (10) (11)
0
G (Li ) =
3 d (Li ) R p L0 = dt 3ρ s L2i
(12)
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4. Simulation results In the simulation, the monomer is fed to the fluidized bed reactor at the bottom of the reactor. The contours of solid volume fraction of the polymer particles are shown in Fig. 2. From an initial bed height of 10 m, the bed expands as the time increases until the final bed height of 20 m is reached.
Fig. 2. The contours of solid volume fraction in the fluidized bed polymerization reactor.
Figure 3 shows the contours of the temperature at different values of bed height after the stable bed height has been reached. It can be seen that the values of temperature are uniform indicating that high heat transfer rate can be achieved despite high exothermic heat of polymerization.
Fig. 3. The temperature contours at different values of bed height.
Figure 4 shows the values of bed temperature along the axial distance as the monomer feed velocity is varied. It can be observed that more uniform temperature distribution is obtained as the monomer feed velocity increases.
Fig. 4. The values of bed temperature along the axial distance as the monomer feed velocity is varied.
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Nawaphat Jongpaijit et al. / Energy Procedia 138 (2017) 901–906 Nawaphat Jongpaijit and Pornchai Bumroongsri / Energy Procedia 00 (2017) 000–000
The temperature of the outlet stream at the top of the fluidized bed polymerization reactor must be reduced to 313 K using the external cooling before recycling back to the reactor inlet. Table 3 shows the calculated external cooling duty. It is seen that the external cooling duty is reduced as the monomer feed velocity is increased from 0.3 to 0.5 m/s. Moreover, more uniform temperature distribution is obtained as shown in Fig.4. Table 3. The external cooling duty. Monomer feed velocity (m/s)
External cooling duty per monomer feed (kJ/kg)
0.3
70.86
0.5
63.59
5. Conclusions This paper presents the CFD modeling of the fluidized bed polymerization reactor for polypropylene production. The developed CFD model is based on a two-fluid model (TFM) coupled with the polymerization kinetics and population balance model (PBM). The solid volume fraction and the temperature distribution are computed. The simulation results show that the temperature distribution in the fluidized bed polymerization reactor is uniform indicating that high heat transfer rate can be achieved despite high exothermic heat of polymerization. The external cooling duty in the recycle loop can be reduced by proper adjusting the monomer feed velocity. Acknowledgements This research project was supported by Mahidol University and the Thailand Research Fund. References
[1] Behjat Y, Shahhosseini S, Hashemabadi S H. CFD modeling of hydrodynamic and heat transfer in fluidized bed reactors. International Communications in Heat and Mass Transfer 2008, 35(3), p. 357-368. [2] Dehnavi M, Shahhosseini S, Hashemabadi S H, Ghafelebashi S M. CFD simulation of hydrodynamics and heat transfer in gas phase ethylene polymerization reactors. International Communications in Heat and Mass Transfer 2010, 37(4), p. 437-442. [3] Che Y, Tian Z, Liu Z, Zhang R, Gao Y. A CFD-PBM model considering ethylene polymerization for the flow behaviors and particle size distribution of polyethylene in a pilot-plant fluidized bed reactor. Powder Technology 2015, 286, p. 107-123. [4] Akbari V, Borhani T N G, Shamiri A, Aramesh R, Hussain M A, Hamid M.K.A. 2D CFD-PBM simulation of hydrodynamic and particle growth in an industrial gas phase fluidized bed polymerization reactor. Chemical Engineering Research and Design 2015, 104, p. 53-67. [5] Chen X Z, Luo Z H, Yan W C, Lu Y H, Ng I S. Three-Dimensional CFD-PBM Coupled Model of the Temperature Fields in Fluidized-Bed Polymerization Reactors. AIChe Journal 2011, 57, p. 3351-3366. [6] Li Z, Annaland M V S, Kuipers J A M, Deen N G. Effect of operating pressure on particle temperature distribution in a fluidized bed with heat production. Chemical Engineering Science 2016, 104, p. 279-290.