Multiscale modelling in microstructured reactors applied to environmental catalysis
J. R. Hernández Carucci*, K. Eränen, D. Yu. Murzin and T. O. Salmi
Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre,
Åbo Akademi University, FI-20500, Turku/Åbo, Finland.
*Corresponding author. Tel: +358 2 2154431, Fax: +358 2 2154479, e-mail: johernan@abo.fi
Microstructured reactors open a new perspective to chemical technology and reaction engineering
offering high surface-to-volume ratio, very efficient heat and mass transfer along with intrinsic safety.
Microreactors can be used both as research instruments in laboratory scale to enhance the
development of new catalysts and processes and for the determination of reaction kinetics as well as
real production units in large scale. In the future, dangerous and unstable chemicals are not going to
be transported on the roads, but manufactured on site, where they are needed. With microreactors, fine
chemicals can be produced in a continuous mode, thus enabling stable operation conditions and a high
product quality. It is expected that many chemical processes carried out batchwise today will be
performed in continuous microreactors in the future. On the other hand, growing concern for the
environment and the strict legislation regarding mobile source emissions have led to the emergence of
the research area of environmental catalysis. Biofuels are regarded as an alternative for reducing the
oil dependence in the transport sector. The EU has mandated that biofuels comprise 5.75% of the total
vehicle fuel consumption in the EU by 2010, setting the goal for 2020 to 10%. Since biodiesel
produces more NOx than petrodiesel in combustion, its use as a fuel requires a precise understanding
of NOx reduction. In lean burn engines, selective catalytic reduction (SCR) with hydrocarbons is an
elegant way for addressing this problem.
The preparation of catalytic microreactor elements and the construction of microreactor systems in
laboratory scale have been widely investigated in the literature. However, one of the most powerful
potential uses of the microdevices, i.e., detailed kinetic studies, as well as mathematical modelling of
gas-phase reactions, has not been exploited. To our knowledge, no comprehensive study which
generalizes the mass balance in a microchannel and its catalytic layer, solving the fully-coupled
problem of chemical reactions with a laminar flow model including radial diffusion and diffusion in
the catalyst layer has yet been conducted. With this work we attempt to close this gap: our application
case is the catalytic cleaning of exhaust gases from green fuels over silver/alumina, one of the most
successful catalysts found up to date for this system.
Microstructured platelets (Ø=460 m and depth 75 m) were successfully coated with an Ag/Al2O3
catalyst by washcoating. The coated microplates were tested for the SCR of NOx, using a typical
diesel engine exhaust through the microreactor inlet. Hexadecane, which is regarded as a high-quality
diesel fuel, was used as a model compound for the second-generation biodiesel. Theoretical
calculations in the microchannels were done to describe the concentrations profiles in the bulk phase
and in the catalytic layer. Regardless the thickness of the catalytic layer and depending on the reaction
conditions, diffusion limitation inside the microchannels might play a role in the system. Moreover,
mass transfer limitation from the bulk phase to the surface of the coating could arise, mainly as the
result of molecular diffusion. When studying reacting flows through microchannels, the traditional
approach has been to assume wall-catalyzed reactions. However, when a porous medium acts as a
catalyst (the advantage being its higher surface area and thus higher concentration of active sites), the
mathematical problem becomes a fully coupled system of equations of reaction, convection and
diffusion, which is challenging to solve numerically. We have implemented different levels of
dynamic modelling, coupling the mass balances on the catalytic layer with the one in the
microchannel with external and internal diffusion limitations. The dynamic model consisting of a
system of partial differential equations was discretized using a finite differences formulation with
respect to the spatial coordinate. The resulting ordinary differential equations were solved with
backward difference method to guarantee stability and convergence. The concentration profiles of the
reactants (C16H34, NO) and products (CO, CO2 and N2) of the HC-SCR have been calculated in the
microchannel as well as in the catalytic layer. The outcome indicates that a rigorous model for a
microchannel coated with a catalyst layer of variable thickness can be obtained to evaluate the
reactive flow inside the channels.