Aqueous-Phase Reforming: Multiphase reaction engineering at the microscale

Abstract

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.