Microstructured reactors for hydrogen production
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Small scale hydrogen production by partial oxidation (POX) and oxidative steam reforming (OSR) have been studied over Rh-impregnated microchannel Fecralloy reactors and alumina foams. Trying to establish whether metallic microchannel reactors have special advantages for hydrogen production via catalytic POX and OSR with respect to activity, selectivity and stability was of special interest. The microchannel Fecralloy reactors were oxidized at 1000 °C to form a - Al2O3 layer in the channels in order to enhance the surface area prior to impregnation. Kr-BET measurements showed that the specific surface area after oxidation was approximately 10 times higher than the calculated geometric surface area. Approximately 1 mg Rh was deposited in the channels by impregnation with an aqueous solution of RhCl3. Annular pieces (15 mm o.d., 4 mm i.d., 14 mm length) of extruded -Al2O3 foams were impregnated with aqueous solutions of Rh(NO3)3 to obtain 0.01, 0.05 and 0.1 wt.% loadings, as predicted by solution uptake. ICP-AES analyses showed that the actual Rh loadings probably were higher, 0.025, 0.077 and 0.169 wt.%, respectively. One of the microchannel Fecralloy reactors and all Al2O3 foams were equipped with a channel to allow for temperature measurement inside the catalytic system. Temperature profiles obtained along the reactor axes show that the metallic microchannel reactor is able to minimize temperature gradients as compared to the alumina foams. At sufficiently high furnace temperature, the gas phase in front of the Rh/Al2O3/Fecralloy microchannel reactor and the 0.025 wt.% Rh/Al2O3 foams ignites. Gas phase ignition leads to lower syngas selectivity and higher selectivity to total oxidation products and hydrocarbon by-products. Before ignition of the gas phase, the hydrogen selectivity is increased in OSR as compared to POX, the main contribution being the water-gas shift reaction. After gas phase ignition, increased formation of hydrocarbon byproducts upon steam addition leads to a decrease in CO selectivity without any significant change in the hydrogen selectivity. The Rh/Al2O3 foams of the lowest loadings (0.025 wt.%) yield the highest maximum catalyst temperatures, and also display higher propane conversion than the 0.077 wt.% and 0.169 wt.% Rh/Al2O3 for both POX and OSR. However, the oxygen conversion does not reach completion and the hydrogen selectivity is lower, compared to the higher loadings. These effects of loading could be ascribed to differences in particle composition and structure. TPR suggested that a part of the Rh is present as a less reducible phase on the 0.025 wt. Rh/Al2O3 foams as compared to the higher loadings. The Rh/Al2O3/Fecralloy microchannel reactor and the 0.025 wt.% Rh/Al2O3 foams were subjected to experiments with changing residence time. Changing the residence time interval corresponding to 1000 - 2000 Nml/minreactant flow has little influence on conversion and selectivity over the foams, but lowering the residence time below 10 ms (flows higher than 1000 Nml/min) for the Rh/Al2O3/Fecralloy reactor increases the synthesis gas (H2 and CO) selectivity during both POX and OSR. This is probably due to quenching of the gas phase reactions at high linear gas velocity, and suggests that direct formation of hydrogen and CO is part of the reaction scheme. Microchannel reactors thus have potential for isolating kinetic effects and minimising gas phase contributions. The Rh/Al2O3 foams show significant deactivation upon a few temperature cycles under reactant exposure, strongest with steam present in the reactant mixture. FE-SEM analyses confirm that Rh sinter into larger particles upon exposure to reaction conditions, more pronounced when steam is fed. No deactivation is observed for the Rh/Al2O3/Fecralloy microchannel monoliths, despite repeated temperature cycling under POX and OSR reactant exposure. In fact, the selectivity to synthesis gas increases upon time.