Chrystèle Horny
The aim of this work is to develop a microstructured reactor based on filamentous catalysts for the Oxidative Steam-Reforming of Methanol (OSRM), to produce hydrogen as feed for a fuel cell, in an autothermal way. Hydrogen is produced by the methanol Steam-Reforming (SR) reaction. This endothermic reaction requires an external heat source which is, in our case, generated by methanol oxidation. The coupling of these two reactions – SR and oxidation, called oxidative steam-reforming of methanol – is performed in a single reactor. As the oxidation is much faster than SR, it occurs in the first part of the reactor, the SR takes place in the second part. If a conventional fixed bed reactor is used, pronounced axial temperature profiles are developed: a hot–spot due to the exothermicity of the oxidation is generated at the entrance followed by a cold–spot due to the SR. The high temperature may damage the catalyst and the low temperature diminishes the rate of reforming reaction leading to poor reactor performance. Thus the temperature control is crucial. Consequently, a microstructured reactor is used. This kind of reactor has multiple parallel channels with a diameter ranging from ten to several hundreds micrometers. These submillimetric dimensions lead to a high surface to volume ratio and a much higher heat transfer coefficient than in the traditional heat exchangers. These characteristics allow to increase heat exchange between reactions and to avoid hot–spot formation. In this work, brass wires introduced into a macro tubular reactor parallel to the walls are used to create the microstructure. Brass is chosen because of its composition – it contains copper and zinc catalyzing the reforming/oxidation of methanol – and for its high heat conductivity which ensures heat exchange improvements. Moreover the small diameter of reactor channels ensures narrow residence time distribution, leading to high selectivity, and a short residence time, improving reactors dynamic. The characteristics mentioned above are verified in chapter 4 for the reactor developed during this study. The hydrodynamic of this reactor is presented under the influence of wire diameter and catalyst preparation treatment. The flow in the microstructure is close to a plug flow: a Bodenstein number of 105 is obtained for brass wires with a diameter of 480μm. Concerning catalytic treatment, it doesn't appear to modify the hydrodynamic. Compared to a fixed bed reactor, the measured residence time distribution for our microstructured reactor is found to be much narrower; the pressure drops are also smaller. Chapter 5 focuses on the reaction conditions for SR of methanol, the reaction that generates hydrogen. These conditions are determined by using an industrial catalyst (based on copper –zinc – aluminium) and by comparing the catalytic activity measured in our conditions with the ones found in the literature. A molar water to methanol ratio of 1.2 is chosen in order to avoid carbon monoxide production, which is a poison for fuel cells. Chapter 6 deals with the development, the optimisation and the characterisation of brass based catalyst. Brass grids are first tested for SR: alloy composition, type and time of leaching are studied. The optimal catalyst is found to be a CuZn37 grid incorporated with aluminium and treated by an acid leaching during 20 minutes in order to increase its surface area (30m2/g). In order to test brass activity in presence of oxygen, partial oxidation of methanol is first carried out over this catalyst. Then the oxidative steam-reforming of methanol is studied: it is found that the modification of the support and hence of the treatment applied decreases the activity and essentially the stability. Deactivation is attributed to copper particles sintering and to oxidation of the catalyst which is not active for hydrogen production. A screening of additives is performed and it is shown that brass wires incorporated with aluminium and doped with chromium by an impregnation method is active, stable and selective: methanol conversion of 25.3% with a hydrogen selectivity of 42.6% are maintained during more than ten hours. Analyses indicate that this catalyst is not easily oxidised and reduced, due to the presence of spinels on the surface which modify electronic properties of copper. In chapter 7, the OSRM is analysed with focus on reaction mechanism and heat exchange. The reaction mechanism is quite complex due to reactions involved (partial oxidations, total oxidation, decomposition) and to their strong temperature dependence. A production of formaldehyde is observed at low temperature (T < 240°C), whereas at higher temperatures secondary reactions disappear and hydrogen production is initiated. Finally, measurements of the axial temperature profile allow to verify the isothermicity of our microstructured reactor: for a methanol conversion of 43% at T = 262°C a hot–spot of less than 3.5°C is measured and no cold–spot is observed. However, it is shown that the temperature profile is highly influenced by methanol conversion and by the oxygen quantity introduced in the reactor. Temperature variations in the entire reactor are nevertheless much lower than those developed in a fixed bed reactor, which is in agreement with our expectations and corresponds to our pursued objectives.
EPFL2005
