Thesis for degree: Licentiate of Engineering

approach often requires detailed kinetic behavior which is close in representation to the
realistic kinetic catalysis.
The heat transport limitation is evaluated first by the larger scale interparticle transport. If the
criterion is fulfilled for the heat transport in the range for the interparticle scale, then there is
no risk for too high temperature gradient across the reactor y-axis for the intraparticle
transport, since R o >> r p and k e approaches the value of λ at low Reynolds numbers. The
interparticle transport criterion for heat transport is considered much stricter than that for
intraparticle transport [49].
The mass transport at the intraparticle scale range is analyzed for the internal diffusion within
the SOFC anode and the Knudsen diffusion is taken under consideration in the calculations.
The effective diffusivity by Knudsen diffusion is defined as [51]:
(3.54)
where d p is the particle diameter and the molecular weight M AB of substance A and B is
defined as in equation (3.11). The effective diffusivity which is based on the ordinary
diffusion is defined here as in equation (3.7). Both of the effective diffusivities are then
averaged as below [51]:
(3.55)
The averaged effective diffusivity is needed in the Thiele Modulus and is here defined as in
[53]:
(3.56)
where r p is the particle radius, k c the mass transfer coefficient, C the concentration, n the
reaction order and D eff the effective diffusivity.
The effectiveness factor is defined in words as [49, 51]:
It is calculated as:
(3.57)
where Φ is the Thiele Modulus. Also, to ensure η ≥ 0.95 it is required that [49]:
(3.58)
34