1. Carbon Deposition in Heterogeneous Catalysis
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2. Background and Motivation
Heterogeneous Catalysis
Characteristics: different phases for reactants and catalyst (gas/liquid and solid);
Processes: porous media flow, chemical reaction, surface chemistry, diffusion, dispersion, etc.;
Rate-controlled: reaction rate constants, catalytic active site.
Some Applications
Reactants (gas)+catalyst (solid): nanofibrous carbon (NFC), exhaust treatment, ammonia synthesis, Sulfuric acid synthesis, etc.;
Reactants (liquid)+catalyst (solid): margarine production.

3. Model description
This example investigates the thermal decomposition of methane over a solid Ni-Al2O3 catalyst. Hydrogen gas and solid carbon are formed as products. Carbon deposits affect the catalyst activity as well as the porosity and fluid flow through the reactor.

4. Geometry and Operating Conditions
Dimension: 2D;
Free channels for inlet and outlet, porous media for middle section;
Size: 1 × 0.4 (m).
Operating conditions
Reaction: atmospheric pressure, temperate between 490—590 oC

5. Modeling Processes modeled by COMSOL Processes modeled by COMSOL
Carbon decomposition chemical reaction;
Catalyst deactivation;
Variable porosity and permeability distribution in a porous media;
Mass transport of reactants along a reactor;
Heat transfer along a reactor;
Momentum transfer along a reactor.

6. Modeling Interfaces Reaction Engineering Transport of Diluted Species
For kinetics investigation
Transport of Diluted Species
For mass transport and reactions
Free and Porous Media Flow
For calculating velocity field
Domain ODE
For calculating varying porosity

7. Results from Reaction Engineering
Concentration transients of methane, hydrogen and deposited carbon as methane decomposes over a Ni/Al2O3 catalyst
Deactivation of catalyst during methane decomposition. The activity of catalyst decreases rapidly at the early stage of reaction, then decreases slowly.

8. Result
Comparison of concentration transients under two conditions of catalyst: 1) deactivation (unlocked) 2) constant activity 1 (locked)

9. Results
Porosity distribution

10. Result from the coupled model
Velocity flow field in the 2D reactor at reacting time s.
Distribution of pressure in the reactor for reacting gas passing through a clean catalyst under transient state at reacting time 4000 s.

11. Result from the coupled model
Concentration distribution of CH4 and H2 along the center line of reactor under fully coupled physics interfaces at different reacting times.
Permeability distribution along the center line of reactor at different reacting times