(61g) Scale-up of microtechnology for fuel processing applications: comparison between fixed-bed and microchannel reactor systems

Fuel cells have great application potential as stationary power plants, as power sources in transportation, and as portable power generators for electronic devices. Most fuel cells currently being developed for use in vehicles and as portable power generators require hydrogen as fuel. Chemical storage of hydrogen in liquid fuels is considered to be one of the options for supplying hydrogen to the fuel cell. In this case a so-called fuel processor is needed to convert the liquid fuel into a hydrogen-rich stream. Microreactors show large promise in the development of miniature fuel processors for small scale electricity production, where several unit operations can be integrated to form a micro hydrogen producing plant. The question now rises whether microreactors can compete with or even replace current conventional reactors for larger scale fuel processing applications. In order to answer this question a comparative study is presented between between microreactor (in particular microchannel) technology and conventional (in particular fixed-bed) reactor technology for the case of methanol fuel processing. The study is limited to the chemical reactor devices of the methanol fuel processor: a reformer-burner (RefBurn) reactor, with coupling of endo- and exothermal reactions, and a preferential oxidation reactor with integrated heat exchangers (ProxHeatex), with integration of reaction and heat exchange. Detailed system designs are presented for 100 We and 5 kWe power output. The 100 We case is at the scale typical for portable electronics applications, while the 5 kWe case is typical of auxiliary power units, which are used for automotive and domestic power applications. Two RefBurn designs and two ProxHeatex designs are evaluated on four comparison criteria: system volume, insulation volume, system weight, and required catalyst mass. Fixed design critera are used for reactor conversion, maximum reactor temperature, pressure drop, and heat loss. In this way, differences in heat and mass transfer characteristics between both reactor technologies show up as differences in the required catalyst mass and ultimately in the system's size and weight. The conventional fixed-bed ProxHeatex devices at both power outputs consist of three separate units: a fixed-bed reactor and two compact plate-fin heat exchangers, which are the first choice for gas-to-gas heat exchange. The fixed-bed reactors are designed using a two-dimensional pseudo-homogeneous model including heat transfer resistances in the catalyst bed and at the tube walls and including mass transfer resistances in the catalyst pellets. The microreactors at both power outputs are considered as being constructed as a stack of microstructured metal plates. The microreactors are designed using dedicated microreactor models including heat conduction in the microstructured plates and mass transfer limitations in the coated catalyst layer on the microchannel walls. On both power output scales, the microreactor designs outperform the conventional fixed-bed designs, leading to significantly lower reactor volumes and weights. The scaling factors of reactor volume and reactor weight are larger for the microreactor systems as for the conventional fixed-bed systems, indicating that at larger scales the fixed-bed reactors will ultimately outperform the microreactor designs. The study shows that microreactor technology may be a viable alternative for conventional reactors for the design of relatively small-scale reactors in which heat exchange is important. Furthermore, the use of a fuel processor and fuel cell system shows a clear weight benefit over other fuel cell systems in appllications where a large energy storage capacity is required.