| Hydrogen-based, sustainable and eco-friendly energy production is being sought as a solution to the current energy and environmental crisis. In both transportation and portable applications, microscale devices are imperative either due to sheer size (e.g., low weight and long duration compared to existing Li-ion batteries) or process intensification considerations. Despite advantages of microdevices, their design is non-trivial owing to the need for thermal management, high catalyst loadings, fast transient responses, non-moving parts, low-pressure drop, etc.; In this thesis, we carry out multiscale modeling of hydrogen producing structured microchemical devices using a hierarchy of chemistry and reactor models to identify critical features for operation and postulate design guidelines. Owing to the simplicity of the catalytic ammonia cracking on ruthenium and its ability to provide CO-free hydrogen, this hydrogen producing route is employed in this work.; In the first part of this thesis, structured (post) microreactors are modeled. A detailed microkinetic model and a one-step rate expression for ammonia cracking are derived for use in computationally expensive computational fluid dynamics (CFD) simulations. CFD model predictions show good agreement with experimental data. Key features, such as mass transfer, pressure drop, catalyst area, and ignition transients are studied. Novel micromixers, based on a flow-driven instability (a Hopf bifurcation), are proposed to overcome limitations of diffusive mixing. The effective diffusivity in structured microdevices is estimated using homogenization theory and incorporated into a 1-dimensional diffusion-convection-reaction model for efficient microreactor optimization.; In the second part of this thesis, multifunctional devices are modeled. Self-sustained operation of a hydrogen producing microdevice is demonstrated via coupling homogeneous propane combustion (exothermic) with ammonia cracking (endothermic). Operating windows are identified using a hierarchy of reactor models (CFD, thermally coupled CSTRs and an overall energy balance) for various materials and flow configurations. Contrary to the macroscale design concept, the choice of flow configuration is governed via materials stability and heat recirculation. High thermal conductivity materials render the choice of flow configuration inconsequential. However, co-current operation is desired since it allows use of a wider range of materials of fabrication. Homogeneous combustion is shown to be rather limiting; catalytic combustion is found to widen the operating window of microdevices. |
