Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Master’s Dissertation Defense
Carlos M. Teixeira
Supervisors:
Prof. José Carlos Lopes
Eng. Matthieu Rolland
17th July 2013
Outline
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
 Introduction
 Objectives
 State of the Art
 Methodology
 Results and Discussion
 Conclusions
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Introduction
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Catalysts performance evaluation
 Performed in units at pilot scale
 The trend is to reduce the size of testing units
(economic and safety reasons)
 Catalyst size remains constant (customer demands)
Consequence
Reactors with low tube-to-particle diameter ratio
(1 < 𝐷/𝑑𝑝 < 5)
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Introduction
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Reactors with low tube-to-particle diameter ratio
 Pseudo Homogeneous Models may not be valid
 Local Phenomena are dominant
 Wall Effect
 Packing Effect
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Introduction
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Example of Packing Effect
Problem Description
0.8
 Packing of eight cylinders with
0.7
different arrangements
 Laminar regime
 Cylinders with constant
concentration in their surface
 Transfer solute to the fluid
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0.5
0.4
0.3
0.2
Particles in contact
the inlet flows through the packing
Cout/Csurface
 Fluid with zero concentration at
0.6
0.1
0
Normalized outlet concentration for the different arrangements
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Outline
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
 Introduction
 Objectives
 State of the Art
 Methodology
 Results and Discussion
 Conclusions
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Objectives
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
 Study the phenomena of single phase fluid flow through
fixed-bed reactors at low particle Reynolds number
 Understand how the packing structure affects the flow
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Outline
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
 Introduction
 Objectives
 State of the Art
 Methodology
 Results and Discussion
 Conclusions
FEUP/IFPEN
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State of the Art
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
CFD Simulation of Fixed-Bed Reactors
Benchmark Method: Lattice Boltzmann
Finite Volume method has been successfully used by
many authors
In most published works, the ratio of tube-to-particle
diameter is low (𝐷/𝑑𝑝 < 10)
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State of the Art
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
CFD Simulation of Fixed-Bed Reactors
Coupling between Hydrodynamics, Heat Transfer and
Chemical Reaction:
Less works on the literature
Applied in small size problems (dozens of particles)
Particle shape: mostly spheres
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Outline
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
 Introduction
 Objectives
 State of the Art
 Methodology
 Results and Discussion
 Conclusions
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Methodology
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Coupling between DEM and CFD
 GRAINS3D (Packing Simulation)
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 PeliGRIFF (Fluid Flow Simulation)
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Methodology
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Grid Refinement Studies
Relative Error in U inlet
1
10-1
0° Re=0.01
0° Re=50
45° Re=0.01
45° Re=50
90° Re=0.01
90° Re=50
10-2
-3
10
10
100
d p/h
1000
Relative error in the inlet velocity as a function of the grid resolution
(ε=0.799, l/dp=1)
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Outline
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
 Introduction
 Objectives
 State of the Art
 Methodology
 Results and Discussion
 Conclusions
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Structured Packed Beds
Unit cell approach
(a)
(b)
A packed bed of simple cubic arrangement of spheres. a) Unit cell b) Alternative
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representation of a simple cubic unit cell.
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Structured Packed Beds of Spheres
Validation Case
Comparison between the simulated dimensionless pressure drop and results from
Hill et al. (2001) for a dilute array of spheres (ε=0.799)
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Direct Numerical Simulation of Fixed-Bed Reactors:
Results and Discussion
Effect of Random Packing
Flow through Structured Packed Beds of Cylinders
Effect of cylinder orientation
100
0°
Dimensionless Pressure Drop, ϕ
45°
90°
10
1
0.1
1
10
100
1000
Redp
Effect of cylinders orientation on dimensionless pressure drop
(ε=0.799, l/dp=1)
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Structured Packed Beds of Cylinders
 Transition from laminar regime to unsteady and chaotic flow
Particle Reynolds number as a function of time for 45º
orientation (ΔP=10 Pa)
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Randomly Packed Beds of Cylinders
Case ID
FBR1
FBR2
FBR3
Nº of particles
540
200
100
0.451
0.444
0.467
Porosity, ε
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Randomly Packed Beds of Cylinders
Simulated Packed Beds
Grid parameters and computing times on 128 processors (𝑅𝑒𝑑𝑝 = 1)
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Randomly Packed Beds of Cylinders
Pressure Drop
Dimensionless pressure drop as a function of porosity. Comparison between
simulations and Ergun correlation predictions (Redp=1).
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Randomly Packed Beds of Cylinders
Spatial Velocity Distribution
Three different zones are identified:
 Recirculation zones in the packing top
and bottom and in the wake of the
particles (with negative velocities)
 High velocity zones where the void
fraction is small and the velocity
increases up to a factor of 15
 Low velocity zones near the particles
surfaces
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Randomly Packed Beds of Cylinders
Statistical Velocity Distribution
1.4
Inlet
Z2
Z3
Z4
Z5
Z6
Z7
Z8
Z9
Z10
Z11
Z12
Z13
Z14
Z15
Outlet
Entire Domain
1.2
P (U z /U inlet)
1
0.8
0.6
0.4
0.2
0
-2
0
2
4
U z/U inlet
6
8
10
Probability density functions of normalized z-velocity in different zones of
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the fixed-bed.
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Direct Numerical Simulation of Fixed-Bed Reactors:
Results and Discussion
Effect of Random Packing
Flow through Randomly Packed Beds of Cylinders
Statistical Velocity Distribution (link with porosity)
0.85
Porosity, ε
0.75
0.65
0.55
0.45
0.35
0
Inlet
0.2
0.4
z/L
0.6
0.8
1
Outlet
Axial average porosity profile
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Randomly Packed Beds of Cylinders
Statistical Velocity Distribution (link with porosity)
Probability density functions of normalized z-velocity for different porosities
(𝑅𝑒𝑑𝑝 = 1)
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Results and Discussion
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Randomly Packed Beds of Cylinders
Statistical Velocity Distribution
Probability density functions of normalized x-velocity for different porosities
(𝑅𝑒𝑑𝑝 = 1)
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Outline
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
 Introduction
 Objectives
 State of the Art
 Methodology
 Results and Discussion
 Conclusions
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Conclusions
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Structured Packed Beds
 The methodology was validated with well-established cases from the
literature
 Dependence of Pressure Drop across Packed Beds of cylinders on its
orientation was studied
 Transition from steady laminar flow to time oscillatory and chaotic flow
was observed at 𝑅𝑒𝑑𝑝 ≥ 60
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Conclusions
Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Flow through Randomly Packed Beds
 Good agreement between Ergun’s pressure drop predictions and
simulation results were found
 Velocity distributions were analyzed and three different zones were
identified
 Velocity distributions appear to follow the average local porosity: the
length to establish the flow is identical to the length to establish the
porosity
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Direct Numerical Simulation of Fixed-Bed Reactors:
Effect of Random Packing
Thank you for your attention
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