CFD modeling and simulation of expanded polystyrene pyrolysis in a fluidized bed reactor

Vahid Khebri

doi:10.1515/cppm-2025-0269

Article

CFD modeling and simulation of expanded polystyrene pyrolysis in a fluidized bed reactor

Vahid Khebri

Published/Copyright: January 1, 2026

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From the journal Chemical Product and Process Modeling

Abstract

This study presents a multiphase CFD model, predicated upon the Eulerian–Eulerian framework integrated with the granular kinetic theory, to study expanded polystyrene (EPS) pyrolysis in a fluidized-bed reactor under stable fluidization conditions. The proposed model was validated against experimental data from the literature. Subsequently, the fluidization characteristics and overall gas-particle flow patterns; the temperature profiles and heat-transfer rates; as well as the product distributions and pyrolysis reaction progression within a fluidized-bed reactor were studied in detail, both qualitatively and quantitatively. To deepen the analysis, a comprehensive parametric study was conducted, analyzing the influence of operating temperature, inlet superficial gas velocity, bed particle size, and initial bed height on the yields and compositions of pyrolysis products. The simulation results indicate that operating temperature has the greatest influence on product distribution. For the main product classes, as operating temperature increases, the yield of condensable volatiles diminishes, whereas the yield of non-condensable volatiles rises. Regarding specific products, the yield of styrene – often the target product in EPS pyrolysis – exhibits a non-monotonic trend, initially rising with temperature, reaching a maximum of 61.64 wt% at 600 °C, and then decreasing at higher temperatures. In contrast, inlet superficial gas velocity, bed particle size, and initial bed height are found to have a limited impact on product yields.

Keywords: CFD; pyrolysis; expanded polystyrene; fluidized-bed reactor

Corresponding author: Vahid Khebri, Department of Applied Chemistry Faculty of Chemistry, University of Tabriz, 29 Bahman Street, P. O. Box: 51666-16471, Tabriz, Iran, E-mail: Hejrakkenko@gmail.com

Research ethics: Not applicable.
Informed consent: Not applicable.
Author contributions: The author has accepted responsibility for the entire content of this manuscript and approved its submission.
Use of Large Language Models, AI and Machine Learning Tools: None declared.
Conflict of interest: The author states no conflict of interest.
Research funding: None declared.
Data availability: Not applicable.

Nomenclature

Roman letters

A _j: apparent pre-exponential factor of the j-th reaction, 1/s
C _d: drag coefficient, dimensionless
c _p,q: specific heat capacity of phase q, J/(kg·K)
D _eff,i,q: effective mass-diffusion coefficient of species i in phase q, m²/s
D _i,q: molecular mass diffusivity of species i in phase q, m²/s
d _p: particle diameter in the particle phase, m
E a j: apparent activation energy of the j-th reaction, kJ/mol
e _pp: restitution coefficient for particle-particle collisions, dimensionless
F _r: constant in Eq. (T1-5)
g →: acceleration because of gravitational force, m/s²
g _0,pp: radial distribution function for the particle phase, dimensionless
H _q: specific sensible enthalpy of phase q, J/kg
h _gp: gas-particle volumetric heat-transfer coefficient, W/(m³·K)
I ′: unit tensor
I _2D: second invariant of the deviatoric stress tensor
k _j: rate constant for the j-th reaction, 1/s
k Θ p: diffusion coefficient for particle-phase fluctuation kinetic energy, kg/(m·s)
M _i,g: molar mass of species i in the gas phase, kg/kmol
Mw_pl,U: molar mass per structural unit of the polymer, kg/mol
Nu_p: Nusselt number for the particle phase, dimensionless
P: static pressure shared by all phases, Pa
P _op: operating pressure, Pa
P _p: solids pressure of the particle phase, Pa
P _f,p: solids frictional pressure of the particle phase, Pa
Pr_g: Prandtl number for the gas phase, dimensionless
Pr_T,g: turbulent Prandtl number for the gas phase, dimensionless
R: gas constant, kJ/(mol·K)
Re_p: relative Reynolds number for the particle phase, dimensionless
R _i,q: net generation rate of species i in phase q due to homogeneous reactions, kg/(m³·s)
S _i,q: net generation rate of species i in phase q due to heterogeneous reactions, kg/(m³·s)
S _q: mass source term in phase q due to interphase mass exchange, kg/(m³·s)
S q V → g: momentum source term in phase q due to interphase mass exchange, kg/(m²·s²)
Sc_T,g: turbulent Schmidt number for the gas phase, dimensionless
T _q: absolute temperature of phase q, K
T _0,q: reference temperature of phase q, K
T _GT: glass-transition temperature of the polymer, K
t: time, s
V → p ′: particle fluctuation velocity of the particle phase, m/s
V → q: velocity vector of the particle phase, m/s
V _G: molar volume of glassy amorphous polymer, m³/mol
V _w: molar Van der Waals volume of the polymer, m³/mol
Y _i,q: mass fraction of species i in phase q, dimensionless

Greek letters

α _eff,q: effective thermal diffusivity of phase q, m²/s
α _q: molecular thermal diffusivity of phase q, m²/s
β _gp: gas-particle momentum-transfer coefficient, kg/(m³·s)
γ Θ p: collisional dissipation of particle-phase fluctuation kinetic energy, kg/(m·s³)
Δ H r q: enthalpy source term in phase q due to chemical reactions, W/m³
ϵ _q: volume fraction of phase q, dimensionless
ϵ _p,max: volume fraction of the particle phase at packing limit, dimensionless
Θ _p: granular temperature of the particle phase, m²/s²
κ _g: turbulence kinetic energy of the gas phase, m²/s²
λ _q: thermal conductivity of phase q, W/(m·K)
μ _g: dynamic viscosity of the gas phase, Pa·s
μ _p: shear viscosity of the particle phase, Pa·s
ν _eff,g: effective kinematic viscosity of the gas phase, m²/s
ν _g: kinematic viscosity of the gas phase, m²/s
ν _T,g: turbulent kinematic viscosity of the gas phase, m²/s
ν _B,p: bulk viscosity of the particle phase, Pa·s
ρ _q: density of phase q, kg/m³
ρ _i,p: true density of species i in the particle phase, kg/m³
ρ _pl: density of molten plastic, kg/m³
τ ═ q: stress tensor of gas q, Pa
ϕ _f: angle of internal friction, deg
ϕ _gp: transfer of fluctuation kinetic energy from the particle phase to the gas phase, kg/(m·s³)

Sub/superscripts

eff: effective
f: frictional
g: gas phase
i: species index
j: reactions index
m, n: constants in Eq. (T1-5)
op: operating
p: particle phase
pl: plastic
q: phase index
T: turbulent

Abbreviations

AAE: average absolute error
BC: boundary conditions
BFBRs: bubbling fluidized-bed reactors
CFD: computational fluid dynamics
CVs: condensable volatiles
EE: Eulerian–Eulerian
EL: Eulerian–Lagrangian
EPS: expanded polystyrene
FBRs: fluidized-bed reactors
FKE: fluctuation kinetic energy
HDPE: high-density polyethylene
IC: initial conditions
KTGF: kinetic theory for granular flows
LDPE: low-density polyethylene
MMT: million metric tonnes
MSW: municipal solid-waste
NCVs: non-condensable volatiles
PE: polyethylene
PET: polyethylene terephthalate
PS: polystyrene
TCTs: thermochemical conversion technologies
TFM: two-fluid model
WP: waste plastic

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Supplementary Material

This article contains supplementary material (https://doi.org/10.1515/cppm-2025-0269).

Received: 2025-10-25

Accepted: 2025-12-12

Published Online: 2026-01-01

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Supplementary Material

https://doi.org/10.1515/cppm-2025-0269

Keywords for this article

CFD; pyrolysis; expanded polystyrene; fluidized-bed reactor