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Heat and Mass Transfer in Fixed-bed Tubular Reactor. Junichiro Kugai. 1. Main issue. Continuum plug flow. Uniformity assumption Semi-empirical. Particle-scale. Non-uniform flow Complex. Purpose. - PowerPoint PPT Presentation
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Heat and Mass Transfer in Fixed-bed Tubular Reactor
Junichiro Kugai
1
Main issue
Continuum plug flow
Particle-scale
Non-uniform flowComplex
Uniformity assumptionSemi-empirical
Purpose
• Simulating velocity, temperature, and mass distributions in a catalytic tubular reactor using COMSOL
• Investigating the effect of the flow rate at inlet, heat of reaction, permeability, and porosity on catalytic activity
• Understanding fluid behavior in particle-scale
3
Model and Equations(Axial 2D)
4
A
Fluid flow in porous mediaFluid : Typical syngas Porous media : Catalyst-bed
Axialsym Wall
Outlet
Inlet
EquationsVelocity : Navier-StokesTemperature: Convection-conductionMass (CO conc.) : Convection-diffusion
5
Gas ρ 0.258 kg/m3
η 1.0 e-5 Pa s
Solid ρ 1500 (apparent) ε 0.4 κ 1.0 e-7 m2
CO + H2O CO2 + H2 rate = A exp(-Ea/RT)[CO]0.1[H2O]0.8[CO2]-0.2[H2]-0.6(1-β)
A 1.0 e8 mol/m3-cat/s Ea 70 kJ/mol ⊿H -40 kJ/mol
Gas in solid k 0.76 W/m/K (0.1, 1.2) Cp 316 kJ/K/m3 (2, 525) D 1.5 e-5 m2/s (6 e-5)
Parameters and Boundary Conditions
5
Axialsym
Wall623.2 K
1 e5 PaConvection
0.085 m/s620 K2.542 mol/m3
0.0025m
0.01mOther properties
Reaction rate expression
6
Velocity Temperature CO conc.
Catalyst-bed temperature is determined by the inlet temperatureReaction reaches equilibrium at ~4 mm from the inlet
Distribution of velocity, temperature, and mass
2.54mol/m3
0.6 mol/m3
620 K
623.34 K0.129 m/s
0 m/s
7
Effect of U0 (average linear velocity) on z-velocity
r [10-3 m]
0.005 m/s0.01 m/s
0.05 m/s
0.1 m/s
0.2 m/s
0.4 m/s
8
Effect of U0 (average linear velocity)
z [m]
0.005 m/s0.01 m/s
0.05 m/s 0.1 m/s 0.2 m/s0.4 m/s
z [m]
0.005 m/s
0.01 m/s0.05 m/s
0.1 m/s
0.2 m/s
0.4 m/s
Large difference in behaviorbtw. 0.01 and 0.05 m/s
Temperature
CO conc.
Equilibrium
9
Effect of ⊿H (heat of reaction)
+20 kJ/mol
-400 kJ/mol
Temperature CO conc.
⊿H : Small impact
-80 kJ/mol
Z [m]
10r [10-3 m] r [10-3 m]
Effect of permeability
r [10-3 m]
10-8
10-7
10-6
10-8
10-7
10-6
Temperature CO conc.
Verocity
Homogeneity of fluid
11
Z [m]
PressureP
ress
ure
[Pa]
100001
100000
Κ = 10-8
Κ = 10-7
(ε = 0.2-0.5)
Κ = 10-6
Effect of permeability and porosity
Permeability : Larger impact
Fluid behavior in particle-scale (2D)
Velocity (0~0.22 m/s) CO conc. (2.54~1.67 mol/m3)
13
0.2 0.6 1.0 1.4 1.8 2.2
2.5
0.12
0.05
2.2
Velocity and mass inside a particle
Ve
loci
ty [m
/s]
CO
co
nc. [
mol
/m3]
z [mm]
Conclusion
• An actual WGS reactor was reproduced by the simulation
• Inlet temperature is crucial to the catalyst-bed temperature in the given condition
• Permeability has large impact on homogeneity of fluid in the reaction condition
• Gas diffusion effectively flatten mass distribution from uneven fluid flow
14
15
Effect of porosity
r [10-3 m] r [10-3 m]
r [10-3 m]
0.4
0.3
0.2
0.5
0.2
0.5
Dporous = (ε / τ) Dgas
kporous = kgas ε + ksolid (1- ε)Cpporous = Cpgas ε + Cpsolid (1- ε)
in J/K/m3
Temperature CO conc.
Verocity
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