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

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

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

Autothermal reforming Background Reforming process of CH4 Reaction △H298θ H2/CO Steam reforming CH4 + H2O = CO + 3H2 206 3 Partial oxidation CH4 + 1 2 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

Background Rh/MgAl2O4 Taken from www.velocys.com 4

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

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

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

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

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

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

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 111.27-50θCO(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 116.12-50θCO(s) (21)CO(s)+O(s)→CO2(s)+Ni(s) 2.0001019 123.6-50θCO(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

Model validation Kinetics Thermodynamics 12

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= 0.025 mm, x = 27.5 mm dcat= 0.100 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

Species distribution 55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3, dcat = 0.100 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

Effects of washcoat thickness 55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3, dcat = 0.025 mm, ε = 0.5, dp = 20 nm, catalyst loading = 4.5-9.1 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

Effects of washcoat thickness 55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3, dcat = 0.025 mm, ε = 0.5, dp = 20 nm, catalyst loading = 4.5-9.1 g/m2 Dramatic increase of effectiveness factor Reforming reaction is intense in the forepart of reactor 16

Effects of washcoat thickness 55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3, dcat = 0.025 mm, ε = 0.5, dp = 20 nm, catalyst loading = 4.5-9.1 g/m2 Catalyst usage: More than 80 percent for washcoat thickness less than 25 μm Damköhler: Mostly in the range of 10-1 - 101 17

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

Effects of washcoat thickness 55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3, dcat = 0.025 mm, ε = 0.5, dp = 20 nm, catalyst loading = 4.5-9.1 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

Effects of washcoat thickness 55 mm × 0.4 mm, 1 atm, 1173 K, 15-104 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

Effects of pore structure 55 mm × 0.8 mm, 1 atm, 1173 K, 62 ms, S/C = 3, dcat = 0.025 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

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

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

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

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 = 4.5-9.1 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

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 = 4.5-9.1 g/m2 Catalyst usage for H2O is worse than CH4 but it is not important because H2O is excessive 26

Effects of washcoat thickness 55 mm × 0.4 mm, 1 atm, 1173 K, 15-104 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