Abstract
The deactivation of catalysts is an important problem in the strongly endothermic steam methane reforming reaction. The local carbon laydown on the catalyst surface may lead to local hot spots, breakage of catalyst particles, and blockage of the reactor tube. Local carbon formation was studied at different operating conditions using particle-scale 3D CFD models of full and hollow cylindrical particles. The results showed that a low steam-to-carbon ratio may cause local carbon formation at high temperature (\gt900K) on the surface of the catalyst particle. The risk of carbon formation was highest at the surface hot spots and inside the catalyst particles where the methane cracking reaction rate exceeded those of the gasification reactions. The internal surface in the 1-hole catalyst particle showed favorable conditions for carbon formation and deposition, similarly to the external surface of the particle. 3D CFD simulations of a 0.76 m length of a full tube of spherical catalyst particles with tube-to-particle diameter ratio 5.96 showed that the rate of carbon formation was much higher next to the heated tube wall and decreased significantly from the tube wall to the tube center.
Acknowledgment
This material is based upon work supported by the National Science Foundation under Grant CTS-0625693. Thanks to Prof. E. Hugh Stitt (Johnson Matthey Technology Centre) and Dr Michiel Nijemeisland (Johnson Matthey Catalysts) for helpful discussions during the performance of this work.
Nomenclature
- cp
specific heat, J/kg·K
- k
thermal conductivity, w/m·K
- keff
effective particle thermal conductivity, w/m·K
- N
tube-to-particle diameter ratio
- ri
reaction rate i, kmol/m3·s
- Re
Reynolds number, ρdpvz/μ
- T
temperature, K
- u
velocity vector, m/s
- vz
superficial bed inlet velocity, m/s
- y+
dimensionless wall coordinate
- Yk
mass fraction of species k
- rc,net
net rate of carbon formation from a steam reforming mixture (kmol/kg-cat/hr)
- Greek Letters
- μ
fluid viscosity, kg/m·s
- ΔHj
heat of reaction j, kJ/mol
- ε
pellet porosity
- ρ
fluid density, kg/m3
- τ
pellet tortuosity
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