1. Modeling transport and reaction in porous catalyst washcoat for steam methane reforming in a microchannel reactor by CFD with elementary kinetics
Chenxi Cao, Nian Zhang, Yi Cheng*
Department of Chemical Engineering,
Tsinghua University, China

2. Outline Background Modeling scheme Results and discussion Conclusions
Microchannel reactors for H2 production: Highly compact and efficient
Modeling scheme
CFD model with elementary kinetics
Transport and reaction in open channel and porous washcoat
Results and discussion
Model validation
Physical fields under baseline condition
Effects of washcoat thickness and pore structure
Conclusions 1

3. Background Industrial process Hydrogen Production Thermochemical
Photo-electrochemical
Electrochemical
Biological
Thermochemical
Thermal decomposition
Oxidative
Electrolysis
Steam reforming
Partial oxidation
Autothermal reforming
Non-oxidative
Catalytic decomposition
Industrial process
Rashmi Chaubey, et al., Renewable and Sustainable Energy Reviews, 2013 2

4. Autothermal reforming
Background
Reforming process of CH4
Reaction
△H298θ
H2/CO
Steam reforming
CH4 + H2O = CO + 3H2
206 3
Partial oxidation
CH O2 = CO + 2H2
-38 2
Autothermal reforming
Depend on feedstock
Conventional fixed-bed reactor
Highly endothermic, high operating temperature
High steam/ methane in feedstock
Resistance in heat transfer
Microchannel reactor
Intensified transfer, faster reactions
Number-up, modularized: flexible production capacity
Compact design, low capital cost 3

5. Background
Rh/MgAl2O4
Taken from 4

6. Background Previous work Lab-scale reactor CFD simulations 5
Zhai, X., et al. International Journal of Hydrogen Energy 2010 and 2011 5

7. Background Reactor simulation is needed
High experimental cost: Materials, micromachining
Difficult direct measurement:
Operating temperature ~1173 K, channel scale < 1 mm
Details at washcoat-scale required: Thickness < 0.1 mm 6

8. Modeling scheme
2-D CFD with elementary kinetics model for SMR in microchannel reactor
Able to account for
Limitation of external diffusion from bulk gas to catalyst surface
Limitation of internal diffusion through catalyst pores
Effects of heat supply and match
Features
Free-fluid region & porous media region
Temperature and composition-dependent physical properties
No lumped heat and mass transfer coefficients
42-step elementary surface reaction network over Ni catalyst 7

9. Governing equations Free-fluid Solid wall Porous washcoat 8
Continuity equation
Navier-Stokes equations
Energy equation
Species equation
Solid wall
Porous washcoat
Forchheimer’s modified formulation of Darcy Equations
Momentum source term
Effective heat conductivity
Permeability
Reaction source terms 8

10. Governing equations Species equation
Average-diffusivity-based description (Bosanquet’s model)
Effective diffusivity
Molecular diffusivity
Knudsen diffusivity
Porosity
Tortuosity
Mean pore diameter
Champan-Enskog
correlation 9

11. Governing equations Reaction source term in energy/species equation
Elementary reaction kinetics
Active metal surface area
porosity
dcat
0.4
16 μm
0.5
19 μm
0.6
24 μm 10

12. Elementary kinetics
Elementary kinetics over Ni-based catalyst for CH4/H2O/H2/CO/CO2 system
Better description of millisecond process
Detailed information of reaction intermediates and surface coverage
Good feedstock adaptability
A/[cm, mol, s]
E[kJ/mol]
(1)H2+Ni(s)+Ni(s)→H(s)+H(s)
1.00010-02
S.C.
(2)O2+Ni(s)+Ni(s)→O(s)+O(s)
(3)CH4+Ni(s)→CH4(s)
8.00010-03
(4)H2O+Ni(s)→H2O(s)
1.00010-01
(5)CO2+Ni(s)→CO2(s)
1.00010-05
(6)CO+Ni(s)→CO(s)
5.00010-01
(7)H(s)+H(s)→H2+Ni(s)+Ni(s)
2.5451019
81.21
(8)O(s)+O(s)→O2+Ni(s)+Ni(s)
4.2831023
474.95
(9)H2O(s)→H2O+Ni(s)
3.7321012
60.79
(10)CO(s)→CO+Ni(s)
3.5631011
θCO(s)
(11)CO2(s)→CO2+Ni(s)
6.447107
25.98
(12)CH4(s)→CH4+Ni(s)
8.7051015
37.55
(13)H(s)+O(s)→OH(s)+Ni(s)
5.0001022
97.9
(14)OH(s)+Ni(s)→H(s)+O(s)
1.7811021
36.09
(15)H(s)+OH(s)→H2O(s)+Ni(s)
3.0001020
42.7
(16)H2O(s)+Ni(s)→H(s)+OH(s)
2.2711021
91.76
(17)OH(s)+OH(s)→H2O(s)+O(s)
3.0001021
100
(18)H2O(s)+O(s)→OH(s)+OH(s)
6.3731023
210.86
(19)C(s)+O(s)→CO(s)+Ni(s)
5.2001023
148.1
(20)CO(s)+Ni(s)→C(s)+O(s)
1.3541022
θCO(s)
(21)CO(s)+O(s)→CO2(s)+Ni(s)
2.0001019
θCO(s)
A/[cm, mol, s]
E[kJ/mol]
(22)CO2(s)+Ni(s)→CO(s)+O(s)
4.6531023
89.32
(23)CH4(s)+Ni(s)→CH3(s)+H(s)
3.7001021
57.7
(24)CH3(s)+H(s)→CH4(s)+Ni(s)
6.0341021
61.58
(25)CH3(s)+Ni(s)→CH2(s)+H(s)
3.7001024
100.0
(26)CH2(s)+H(s)→CH3(s)+Ni(s)
1.2931022
55.33
(27)CH2(s)+Ni(s)→CH(s)+H(s)
97.1
(28)CH(s)+H(s)→CH2(s)+Ni(s)
4.0891024
79.18
(29)CH(s)+Ni(s)→C(s)+H(s)
18.8
(30)C(s)+H(s)→CH(s)+Ni(s)
4.5621022
161.11
(31)CH4(s)+O(s)→CH3(s)+OH(s)
1.7001024
88.3
(32)CH3(s)+OH(s)→CH4(s)+O(s)
9.8761022
30.37
(33)CH3(s)+O(s)→CH2(s)+OH(s)
130.1
(34)CH2(s)+OH(s)→CH3(s)+O(s)
4.6071021
23.62
(35)CH2(s)+O(s)→CH(s)+OH(s)
126.8
(36)CH(s)+OH(s)→CH2(s)+O(s)
1.4571023
47.07
(37)CH(s)+O(s)→C(s)+OH(s)
48.1
(38)C(s)+OH(s)→CH(s)+O(s)
1.6251021
128.61
(39)HCO(s)+Ni(s)→CO(s)+H(s)
50θCO(s)
(40) CO(s)+H(s)→HCO(s)+Ni(s)
4.0191020
132.23
(41) HCO(s)+Ni(s)→CH(s)+O(s)
95.8
(42) CH(s)+O(s)→HCO(s)+Ni(s)
4.6041020
109.97 11
E.S. Hecht, et al., Applied Catalysis A: General, 2005

13. Model validation
Kinetics
Thermodynamics 12

14. Velocity distribution
55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3,
ε = 0.5, dpore = 20 nm, catalyst loading = 6.8 g/m2
dcat= mm, x = 27.5 mm
dcat= mm, x = 27.5 mm
Momentum transfer into porous washcoat
Regular flow
- Free-fluid region: Approximately laminar flow
- Porous washcoat: Plug flow with very small axial velocities (10-3 m/s)
Minor influence of flow condition on reactor performance
- Changes in conversion: less than 0.8% 13

15. Species distribution
55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3,
dcat = mm, ε = 0.5, dp = 20 nm, catalyst loading = 6.8 g/m2
Larger gradients at the front: Faster consumption of H2O
CH4
H2O
Active sites unavailable
Active sites available
Active sites all available H2
Smaller gradients:
Fast diffusion of H2 CO
CO2
Slower formation of CO
Faster formation of CO2
Internal mass transfer resistance > External mass transfer resistance
- Obvious large transverse concentration gradients in porous media region 14

16. Effects of washcoat thickness
55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3,
dcat = mm, ε = 0.5, dp = 20 nm,
catalyst loading = g/m2
Constant space velocity
Not equilibrium-limited
Monotonic decrease for all catalyst loadings and both temperatures
Reactor performance is controlled by both reaction and internal mass transfer 15

17. Effects of washcoat thickness
55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3,
dcat = mm, ε = 0.5, dp = 20 nm,
catalyst loading = g/m2
Dramatic increase of effectiveness factor
Reforming reaction is intense in the forepart of reactor 16

18. Effects of washcoat thickness
55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3,
dcat = mm, ε = 0.5, dp = 20 nm,
catalyst loading = g/m2
Catalyst usage: More than 80 percent for washcoat thickness less than 25 μm
Damköhler: Mostly in the range of 17

19. Effects of washcoat thickness
Effectiveness factor of Ni-based catalyst in fixed bed reactor SR
WGS RM
Ni-based catalyst in microchannel reactor has effectiveness factor up to 100 times than that in fixed-bed reactor 18
G. Pantoleontos, et al., Int. J. Hydrogen Energy, 2012

20. Effects of washcoat thickness
55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3,
dcat = mm, ε = 0.5, dp = 20 nm,
catalyst loading = g/m2
Constant space velocity
H2 yield deceases, while H2/CO increases with washcoat thickness
- the decrease of CO selectivity (not shown) prevails
Thick washcoat is very unfavorable for the purpose of syngas production 19

21. Effects of washcoat thickness
55 mm × 0.4 mm, 1 atm, 1173 K, ms, S/C = 3,
ε = 0.5, dpore = 20 nm, metal content = 10%
Varying space velocity
Contour lines approach vertical
Increasing catalyst loading by using thicker washcoat can allow for higher GHSV when washcoat thickness is less than 75 μm
Washcoat thicker than 75 μm may cause a waste of catalyst 20

22. Effects of pore structure
55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3,
dcat = mm, catalyst loading = 6.8 g/m2
dp = 20 nm
Porosity = 0.5
Weak influence
Stronger effects at smaller pore diameter
High porosity is not necessary
Ensuring a pore diameter no smaller than 8 nm will be beneficial
Pore diameter larger than 50 nm may be not very useful 21

23. Conclusions
A 2-D comprehensive CFD model coupled with elementary kinetics was established to account for transport and reaction in both open channel and porous washcoat
SMR over Ni in microchannel reactor is controlled by both reaction and internal diffusion
The catalyst usage in microchannel reactor is 1-2 orders higher than that in conventional fixed-bed reactor
Increasing catalyst loading by using thicker washcoat is available for washcoat thickness less than 75 μm
Increasing mean pore diameter of catalytic washcoat up to 50 nm will be useful to ensure good accessibility of active sites in catalytic washcoat 22

24. Thank you for your attention!
Acknowledgements
Prof Yi Cheng and the research group
Financial supports from PetroChina
Thank you for your attention! 23

25. Effects of inlet temperature
55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3,
ε = 0.5, dpore = 20 nm, catalyst loading = 6.8 g/m2
Cold feedstock heated up to wall temperature within 5 mm
Inlet temperature is of minor importance for reactor performance
- Changes in conversion is less than 0.2%
- Easy pre-heating 24

26. Effects of washcoat thickness
55 mm × 0.8 mm, 1 atm, 1073 and 1173 K, 62 ms, S/C = 3,
ε = 0.5, dpore = 20 nm, catalyst loading = g/m2
Constant space velocity
Very close to equilibrium at high operating temperature
Less than 2% loss in H2O conversion with differenet washcoat thickness
The H2O is excessive (S/C=3) and insensitive to process changes 25

27. Effects of washcoat thickness
55 mm × 0.8 mm, 1 atm, 1073 and 1173 K, 62 ms, S/C = 3,
ε = 0.5, dpore = 20 nm, catalyst loading = g/m2
Catalyst usage for H2O is worse than CH4 but it is not important because H2O is excessive 26

28. Effects of washcoat thickness
55 mm × 0.4 mm, 1 atm, 1173 K, ms, S/C = 3,
ε = 0.5, dpore = 20 nm, metal content = 10%
Varying space velocity
Greater dependence on washcoat thickness for H2/CO
Dependence on washcoat thickness increases with WHSV
Operation at high WHSV favors thinner washcoat
Using thin washcoat is more important in syngas production at high WHSV 27