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.
Model description This example investigates the thermal decomposition of methane over a solid Ni-Al 2 O 3 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.
Geometry and Operating Conditions Geometry – 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
Modeling 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. Processes modeled by COMSOL
Modeling Interfaces Reaction Engineering – 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
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.
Result Comparison of concentration transients under two conditions of catalyst: 1) deactivation (unlocked) 2) constant activity 1 (locked)
Result from the coupled model Velocity flow field in the 2D reactor at reacting time 4000 s. Distribution of pressure in the reactor for reacting gas passing through a clean catalyst under transient state at reacting time 4000 s.
Result from the coupled model Concentration distribution of CH 4 and H 2 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