Développement d'un réacteur microstructuré basé sur des filaments métalliques catalytiques

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.

Production d'hydrogène dans un réacteur microstructuré

Pierre Reuse

The aim of this work was to operate the steam reforming of methanol and the total oxidation of methanol in a microstructured two-passage reactor. The steam reforming enables the production of hydrogen at relatively low temperatures (250 °C) with only a small amount of carbon monoxide (~ 1%). After a cleaning step (CO preferential oxidation, for example), this hydrogen would be a good feed for a fuel cell to produce electricity for mobile applications. Steam reforming is an endothermic reaction. The energy needed to run this reaction is produced by the total oxidation, a strongly exothermic reaction. To allow an efficient heat transfer between these two reactions, a microstructured reactor was used. These structures, with dimensions in the micron range, are widely known to have far better heat transfer capacity than traditional heat exchangers. Furthermore, these structures have some unique features with respect to classical reactors. A narrow residence time distribution, close to that of a plug flow reactor, can be obtained, keeping selectivity high for complex reactions. Microstructured reactors also have small residence time, allowing very quick variations in the feed and experimental conditions. In chapter 4, the concepts relating to the channel coatings will be presented. A Cu/Zn/Al catalyst provided by Süd-Chemie was transformed into a suspension of nanoparticles in isopropanol by using an attrition grinding apparatus. A thin layer (~5 µm) of catalyst was formed on the microchannel walls. Several tests were performed to characterize the adhesion of this layer. The influence of the layer on the residence time distribution was also investigated; it was possible to maintain Bodenstein numbers as high as 50. Chapter 5 is focused on the steam reforming of methanol, the reaction that generates hydrogen. A kinetic analysis in differential and integral modes established that the rate of the reaction is inhibited by hydrogen, the main product of the reaction; the partial order is - 0.2. The influence of water on the rate of reaction is negligible (partial order of 0.1), and methanol has a positive partial order of 0.7. A kinetic constant of 5.57 · 10-6 mol0.4 · (m3)0.6 · gcat-1 · s-1 at 200 °C and an activation energy of 73.9 kJ · mol-1 complete the description of the kinetics of the steam reforming. Throughout the comparisons, we showed that the catalyst coated in the microchannels has an initial rate for the steam reforming 30% higher than the original catalyst in fixed bed. We can therefore conclude that the catalyst modification procedure does not diminish the catalyst activity. This was also shown by comparing our results with results available in the scientific literature for similar catalysts. CO2 selectivity remains high and constant then decreases as 90% conversion is approached. Working with an excess of water with respect to methanol contributes to keeping the selectivity high. A molar ratio of 1.2 is a good compromise between selectivity and increased energy costs due to the vaporization of the additional water. It's also beneficial to work at conversions below 90% to be in the high selectivity domain and to diminish the inhibiting influence of hydrogen. Finally, we showed that this catalyst deactivates over 100 hours at a temperature of 300 °C. A kinetic model for the deactivation at temperatures between 300 °C and 330 °C was established. In chapter 6 the total oxidation of methanol was studied. A cobalt based catalyst was developed to allow the synchronization of the two reactions temperatures. The minimum temperature for complete conversion for the total oxidation reaction must clearly not exceed an acceptable working temperature for the steam reforming catalyst (i.e. a temperature that results in an acceptable catalyst lifetime). A kinetic analysis determined partial orders for methanol and oxygen as 0.67, the kinetic constant at 230 °C as 5.45 · 10-3 mmol-0.33 · m4 · (gcat · s)-1 and the activation energy as 130 kJ · mol-1. The copper based catalyst used for the steam reforming of methanol was also active for the total oxidation of methanol. Unfortunately the minimal temperature to ensure a total conversion was not compatible with the working temperature of the steam reforming. In chapter 7, the results concerning the coupling of the two previously studied reactions are presented. The numerous fittings and cables necessary for the operation of the reactor didn't allow the reactor to be run in an autothermal mode. Therefore constant, not regulated, heating was used to compensate the thermal losses. By coupling the two reactions and by studying the dynamic response time of the reactor (temperature and conversion of the steam reforming reaction) at the time of stopping and starting the oxidation reaction, we've seen that the temperature increase was around +10 °C within 60 seconds and that the conversion for the steam reforming of the methanol fitted well with the temperature increase. This reactor also has good dynamics and responds quickly to feed variations. When the coupling mode was changed from co-current to counter-current we noticed a significant increase in conversion. Furthermore, selectivity in counter-current mode remained high, even for conversion approaching 100%.

EPFL2003

Compact string reactor for authotermal hydrogen production

Chrystèle Horny, Lioubov Kiwi, Albert Renken

This study addresses the development of a compact reactor for oxidative steam-reforming of methanol (OSRM) to produce hydrogen in autothermal mode for fuel cells. The string reactor uses catalytically active brass wires with a diameter of 500 mu m placed in parallel into a tube. The micro-channels in the reactor for gases are formed between the wires presenting hydrodynamics similar to the one in multi-channel micro-reactors. Due to the high thermal conductivity of brass, the heat generated during methanol oxidation at the reactor entrance is transferred to the zone of the endothermic steam-reforming. The catalysts are prepared by Al-alloy formation on the surface of the brass wires followed by the partial leaching of Al. The catalyst presents a porous layer with the morphology of Raney metals and the chemical composition consistent with the Cu/Zn/Al-mixed oxide. The catalyst surface was additionally modified by incorporating chromium leading to Cr/Cu-spinel. This decreases the degree of the reduction of copper oxide and sintering leading to a stable catalyst. The catalyst was tested in OSRM showing high activity and selectivity to carbon dioxide and hydrogen. The string reactor presents nearly isothermal profile since the temperatures gradient within the reactor length is about 3 K. Micro-structured string reactor presents a short start-up and a fast transient behavior showing a rapid temperature change when adjusting the oxygen amount introduced into the reactor. (c) 2006 Elsevier B.V. All rights reserved.

2007

Développement de réacteurs microstructurés pour la conversion du propane en produits valorisés

Olivier Wolfrath

Industry requires continuously new chemical processes to convert raw materials to valuable products limiting the formation of by-products. Since a few years, an increasing interest has been given to the intensification of chemical processes. This represents a new approach in chemical reaction engineering and calls for a new type of chemical reactors: the microstructured reactors. They are characterized by miniature structures whose components have a sub-millimeter size. The flow channels, for example, have a diameter from ten to several hundred microns. The distinctive feature of these structures is a very large surface/volume ratio, namely between 10'000 and 50'000 m2/m3. In these conditions, heat transfer coefficient is very high, up to 25 kW · m-2 · K-1. It is accepted that the residence time distribution in a reactor strongly influences the product selectivity and yield. In these thin channels, the RTD is particularly narrow due to a short radial diffusion time (ms), allowing a precise control of the residence time. Another advantage of these microstructures is an increase of the inherent safety due to a small amount of the compounds used. However, fabrication of microreactors, especially the introduction of active catalyst in microchannels remains a difficult operation. And integration of microsystems in conventional chemical processes is still a challenge. The aim of this work is first to develop a novel membrane reactor based on a new microstructured catalytic packing in the filamentous form and to apply it to the nonoxydative dehydrogenation of propane. A second objective is to develop a reactor with micro heat exchangers allowing an optimized control of homogeneous reactions. This reactor was tested with the partial oxidation of propene to propylene oxide. The microstructured catalyst above is made of thin and long catalytic filaments placed in parallel with a tubular reactor. This new kind of catalyst is based on silica fibres covered by a γ-alumina layer. The position in parallel of these filaments of 7 μm diameter forces the gases to flow in the fibres direction between the filaments. The hydraulic diameter is about 70 μm. Due to this laminar and regular flow, the pressure drop in the bed was divided by 5 compared to a conventional bed (100-160 μm spheres) and the residence time distribution was particularly narrower. Such novel packing brings new opportunities to the heterogeneous catalysis domain because it can be used for many other reactions improving highly the hydrodynamics and keeping an excellent contact with the reactants. The microstructured catalyst was installed in two concentric zones of a tubular reactor separated by a palladium-silver membrane permeable to hydrogen. This product was removed from the gas phase to shift the thermodynamic equilibrium, enhancing the propane conversion from 22% up to 30%. Another significant advantage in this reactor is the increase of propene selectivity up to 97% in comparison to conventional industrial processes reaching 80-90%. This difference is due to the diminution of hydrogenolysis and hydroisomerization reactions. The dehydrogenation is known to be endothermic. Oxidation of the hydrogen passing through the membrane produced heat for this reaction along the catalytic zone. Moreover, a periodic regeneration by air allowed recovering the activity of the catalyst deactivated by coke formed during dehydrogenation. Periodic reaction operations allowed producing continuously propene with a high selectivity, and regenerating simultaneously the catalyst. The second microstructured reactor was developed integrating two micro heat exchangers at the inlet and outlet of a macroscopic tube. The aim is to heat up very fast the gas and to cool it down as well. Without these exchangers, heating and cooling duration corresponded each to about 20% of the total residence time. The use of the microstructures allowed diminishing significantly these durations (4-5% each). Applied to partial oxidation of propene, this reactor reached a more precise control of reaction temperature and duration. The propylene oxide (PO) selectivity was not increased with this system but, due to the microstructures, it was possible to work at higher temperatures where the reaction becomes very fast, without reaching an oxygen conversion of 100%. Indeed, it is essential to avoid this situation where oligomers and coke form dividing by 2 the PO selectivity. So another new kind of reactor was developed suitable for any fast exothermic reaction whose progression has to be limited because of potential consecutive reactions reducing, for example, the selectivity of an intermediate desired compound.