Mass transport in graded monolithic catalysts during gas and multiphase reactions
Ceramic sponges,
also known as open-cell foams,
combine high permeabilities and
large volumetric surface areas with
exceptional heat and mass transport
properties. This unique combination
distinguishes them as promising
monolithic catalyst supports in
fixed-bed reactors for exothermic
processes such as CO2-methanation,
Fischer-Tropsch synthesis (FTS), and
selective oxidations. In previous
projects we demonstrated that the
porosity of sponges can be tuned
locally to control axial and radial
temperature profiles during CO2-methanation
and thus increase space-time-yield
significantly. In this project
we now like to extend the concept of
graded sponges to mass transport
related reaction problems. In a
first step we focus on gaseous mass
transport. Therefore, available
reactor models will be adapted to a
chosen model reaction that suffers
from mass transport limitations. A
comparison of the calculated
concentration profiles to local
concentration maps measured in-situ
with NMR-based (Nuclear Magnetic
Resonance) techniques shall allow
assessing the quality of the model
predictions and giving insight into
the influence of the sponge
structure on gas-phase mass
transport. In addition, local
temperature profiles and integral
analysis of the fluid composition
from bench-scale experiments will
provide further validation. The
model can then be used to predict a
sponge with graded pore size that
will lead to intensified mass
transport and hence to improved
process performance.
In a second step
we like to investigate liquid-phase
transport in solid sponges which is
highly important for FTS as the
developing liquid phase increases
gaseous diffusion within the
reactor. Again, model predictions
will help to estimate the potential
of liquid phase control during FTS,
and guide the development of
tailored sponge structures.
Non-invasive imaging techniques such
as Micro X-Ray Computer Tomography
(µCT) allow to visualize the liquid
phase distribution in solid sponges
to further understand the influence
of specific structure features. In
combination, the model predictions
and the µCT visualizations will
permit to deduce design rules for
advanced liquid control during FTS.
Contact: Thöming , (Kirsch) , since 08/2017 Sadeghi