Aqueous-phase reforming: multiphase reaction engineering at the microscale

Hydrogen has been identified as an interesting alternative for fossil fuels. Conventionally,
hydrogen is produced from natural gas or oil, using high temperature and pressure methods such
as steam reforming. In 2002, Aqueous-Phase Reforming (APR) was introduced to reform
oxygenated carbohydrates into hydrogen at more environmental friendly reaction conditions.
In this dissertation, various thermodynamic aspects related to Aqueous-Phase Reforming, as well
as various reactor designs have been discussed. Here, process technological aspects have been
investigated at the microscale using both theoretical and experimental methods, mainly focusing
on the thermodynamics and transport phenomena involved in APR.
The reaction thermodynamics have been evaluated in terms of the enthalpy and Gibbs free energy
of reaction. In addition, the phase state of the reaction mixture at APR reaction conditions has been
studied. To experimentally validate the phase transitions, a high-pressure high-temperature
microfluidic platform has been developed, in which also the boiling mechanisms and the gas/liquid
flow patterns were observed.
Depending on the characteristics, gaseous APR products that form as bubbles on the catalytic
surface may affect transport phenomena. In addition to 2D and 3D-numerical models to investigate
these properties, a transparent microreactor containing hydrophobic micropits has been developed
to aid bubble nucleation and controlling transport phenomena in catalytic multiphase microreactors
in the future.
Introducing a heterogeneous catalyst is essential for APR microreactors. Both spark discharge and
washcoating techniques have been studied for the controlled deposition of catalytic materials in
microreactors. After reaction, the resulting gas/liquid product streams have to be separated, for
which a modular gas/liquid microseparator has been developed. The required low dead volume
and the in- and outlet design have received special attention.
Although APR is already a great improvement over conventional hydrogen production methods,
it is still dependent on a substantial amount of externally supplied heat. In contrast, photocatalytic
reforming (PhCR) of biomass, where biomass is reformed into hydrogen and CO2 using only
sunlight, is an attractive alternative. In parallel, first steps towards studying these aspects with a
microfluidic device were successfully taken.
Finally, the possible commercialization of APR for hydrogen production has been discussed in the
outlook of the thesis.